The Royal Australian Air Force (RAAF) contracted a civilian operator to provide a civil registered Beech 200 Super King Air aircraft, maintenance service, and a check and training service for RAAF aircrew, in accordance with the provisions of the operator's Air Operator's Certificate (AOC).
The flight from Edinburgh, SA, to Oakey, Qld, was conducted as a single-pilot operation. One of the two passengers, who was also a pilot but not qualified to operate the aircraft type, occupied the co-pilot seat. The other passenger was seated in the cabin.
After take-off, as the aircraft climbed through 10,400 ft, the pilot began the 'climb checklist' actions. While performing these checks he received a tracking change instruction from Air Traffic Control (ATC). The passenger in the co-pilot seat noticed that this appeared to temporarily distract the pilot from the checklist as he attempted to reprogram the global positioning system (GPS). The pilot then completed the checklist. During this, the passenger in the co-pilot's seat saw the pilot reposition the engine bleed air switches from the top to the centre positions.
As the aircraft reached the cruise level of FL250, the controller contacted the pilot, indicating that the aircraft was not maintaining the assigned track. The pilot acknowledged this transmission. A short time later the passenger in the co-pilot seat noticed that the pilot was again attempting to program the GPS, and was repeatedly performing the same task. The controller advised the pilot again that the aircraft was still off track, however the pilot did not reply to this transmission. Shortly after this, the pilot lost consciousness.
The passenger in the co-pilot seat took control of the aircraft and commenced an emergency descent. The other passenger then unstowed the pilot's oxygen mask and took several breaths of oxygen from it before fitting it to the unconscious pilot. Neither passenger donned an oxygen mask during the incident.
The controller noticed that the aircraft had conducted an orbit and attempted to contact the pilot, asking him to set the aircraft transponder to 'squawk ident'. The 'ident' signal was received and acknowledged by the controller. The passenger in the co-pilot seat, who had been having some difficulties using the radio, then declared an emergency, indicating that the pilot was incapacitated and that he was conducting an emergency descent.
The controller cleared the aircraft to descend to 5,000 ft and to return to Edinburgh. The clearance was subsequently amended to descend to 6,000 ft due to cloud.
The pilot recovered consciousness during the descent, and once he had regained situational awareness, he noticed that the PASS OXYGEN ON and both BLEED AIR OFF green advisory annunciators were illuminated. These indicators are located on the caution/advisory annunciator panel in the centre instrument sub panel. He also noticed that the engine bleed air switches were selected to the ENVIR OFF position. The pilot reported that he did not see any low cabin pressure warning indications and that the passenger oxygen masks had not deployed.
The pilot then resumed control of the aircraft and carried out an uneventful landing.
The pilot was an experienced RAAF pilot with a civilian flight crew licence, and was qualified to operate the Beech 200 aircraft type. This flight was the first that the pilot had carried out since recently completing type endorsement training.
The passenger in the co-pilot's seat was an experienced RAAF pilot who also held a civilian licence. The passenger in the cabin was an experienced RAAF navigator.
Cabin environment control and oxygen system
The cabin of the Beech 200 was pressurised with environmental air taken from the compressor bleed air outlets of both engines. The bleed air supply was controlled using two three-position switches mounted side by side on the co-pilot's 'environmental' sub-panel. The bleed air switches were of a different shape to most other toggle switches on the instrument panel, which could be discerned by touch. The switches were detented so that they required pulling before changing position (Refer Figure 1).
The switches were placarded bleed-air valve (left and right), and the individual switch positions were (as read from the top selection to the bottom) OPEN, ENVIR OFF, and INSTR & ENVIR OFF. When switched to either the ENVIR OFF or INST AND ENVIR OFF positions, the bleed air valves that controlled the supply of environmental bleed air to the cabin were closed. When switched to the OPEN position, pressurised environmental bleed air flowed to the cabin for air conditioning and pressurisation. The RAAF reported that the switch detents were worn and the switches could be operated without being pulled. The cabin pressurisation controller automatically adjusted an outflow valve in the rear of the cabin to maintain a preset cabin altitude. The pressurisation controller was located on the centre pedestal in the cockpit.
The cabin pressurisation instruments were positioned low on the centre instrument panel, and were partially obscured by the engine and propeller control levers in flight.
The aircraft also had two vent blowers that forced air through underfloor ducts to assist with cabin ventilation. The vent fans were switched on when the aircraft was on the ground to prevent the ducts from overheating. As the aircraft climbed through 10,000ft the aft blower was normally switched off, and the vent blower was normally switched from HI to LOW. The vent fan switches were positioned directly above and below the right bleed air switch on the co-pilot's environmental sub-panel. The switches were of a similar shape to most other toggle switches on the instrument panel, and did not require pulling out of a detent before changing position. The switches were smaller and dissimilar in shape to the nearby bleed air switches.
The sequence of actions detailed in the 'climb checklist' required the pressurisation to be checked before adjusting the airconditioning and aft blower.
A barometric switch inside the pressure hull of a Beechcraft Super King Air 200, controlled the cabin altitude warning system. The switch was designed to activate when cabin altitude exceeded 12,500 ft. When activated the system illuminated the glareshield mounted left and right flashing red master warning lights and the red ALT WARN annunciator on the master warning panel (refer Figure 2). The master warning lights would remain illuminated until pushed to cancel. The ALT WARN annunciator would extinguish following a decrease in cabin altitude to below 12,500 ft. The aircraft had no aural warning device to warn the pilot of a high cabin altitude. The aircraft type had been certified with the foregoing safety equipment installed.
The passenger in the cabin recalled seeing the ALT WARN caption on the warning annunciator panel on the glareshield at the time of the incident, however, none of the occupants recalled seeing or cancelling the operation of the flashing master warning lights.
The aircraft passenger emergency oxygen system used pressurised dry breathing oxygen. A barometric switch, positioned inside the aircraft's pressure hull, would activate at an internal cabin altitude of 12,500 ft. This activated a system that allowed pressurised oxygen to be directed to the mask retaining door actuators, allowing the doors and masks to drop from the overhead panels. The masks would supply oxygen when the mask was pulled to open a valve. In order to function the system had to be armed by the pilot. An override control enabled the pilot to manually operate the passenger emergency oxygen system in the event of an automatic deployment system failure. Pressurisation of the passenger emergency oxygen system was indicated to the pilot by the illumination of the green PASS OXYGEN ON advisory annunciator positioned on the lower instrument panel area, behind the engine and propeller control levers.
Repealed Australian Civil Aviation Order (CAO) 188.8.131.52, issued on 31 December 1965, stated at paragraph 3.0.4, that:
Aircraft which are normally operated under cruise conditions at flight altitudes in excess of 25,000 feet shall be equipped with a device to provide the flight crew with a warning whenever the cabin pressure altitude exceeds 13,000 feet. The warning should not depend on the reading of a gauge.
Note: An aural warning is strongly recommended.
Australian Civil Aviation Order (CAO) 20.4, paragraph 3, stated that:
Oxygen must be stored, and dispensing and control equipment must be installed, on an aircraft in accordance with section 108.26 of the Civil Aviation Orders.
Australian Civil Aviation Order (CAO) 108.26, paragraph 3.1, which was approved on 14 June 1972, stated that:
An oxygen system for an aircraft which is intended for operations at flight altitudes above 25,000 feet shall include a device to provide the flight crew with a warning whenever the cabin pressure altitude exceeds 14,000 feet.
Note: The cabin pressure warning should not depend on the reading of a gauge. An aural warning is strongly recommended.
Amendment 92 of this CAO, approved on 7 July 1987, reflected a change in the cabin pressure altitude at which the device should provide a warning. The amended altitude was lowered to 10,000 ft. While the relevant CAOs strongly recommended an aural warning for cabin pressure altitude, fitment was not mandatory.
The Civil Aviation Safety Authority indicated that CAO 108.26 applied to this aircraft.
The clinical features of acute hypobaric hypoxia include the following:
- impairment of cognitive skills such as judgement, decision-making, memory, self-regulation and self-awareness;
- impaired psychomotor coordination and reaction times;
- restriction of visual field, reduced colour discrimination, reduced auditory acuity and cyanosis; and
- loss of consciousness, finally resulting in death.
Vision is particularly sensitive to hypoxia. With the onset of hypoxia both the rate and the magnitude of decline in vision are greater than the corresponding decline in hearing. Moderate and severe hypoxia causes a restriction of the visual field, with loss of peripheral vision. There may also be a subjective darkening of the visual field. Auditory acuity is also reduced by moderate and severe hypoxia, but some hearing is usually retained even after other senses such as vision are lost.
Reading material recommended by CASA for candidates for the Air Transport Pilot Licence theory examinations, since May 1998, included an academic text by Green, Muir, James, Gradwell and Green (1991), that states:
The alerting function for all important failures should be fulfilled by a [sic] audio warning…
Additional research has also indicated that reaction times to visual indications are shorter when supported by an auditory warning signal (Refer to Attachment A).
RAAF aircrew underwent hypobaric chamber training as part of a normal 3-yearly aviation medical refresher training requirement. The pilot and the passenger in the co-pilot's seat had undergone hypobaric chamber training in 1997. The passenger in the cabin had last undergone hypobaric chamber training in 1992. During this training, personnel were expected to learn to recognise and note their individual symptoms of hypoxia through experience. These symptoms could then be used as an aid to indicate the onset of hypoxia.
Both passengers experienced tiredness and slight nausea during the incident. Neither of these symptoms was recognised by the passengers as indicative of the onset of hypoxia. They had not experienced these symptoms during their RAAF hypobaric chamber training.
Global Positioning System
The aircraft was fitted with an Arnav Star 5000 type GPS. The pilot had received no formal training on this particular type of GPS and was unfamiliar with its operation. This was because the unit was due to be replaced with an updated one.
The civil operator's normal endorsement and check and training syllabus was designed to maximise the performance of pilots undertaking regular flying activities in the civilian environment.
This training initially comprised a ground school on the aircraft type. The candidate would then undertake a minimum of two hours of flight training that covered aircraft familiarisation, normal and abnormal procedures. This training satisfied the requirements for an aircraft type endorsement under Civil Aviation Order 40.1.0. The candidate would then undertake a minimum of 25 hours of further flight training with a training captain. During these flights they would undertake normal commercial activities. This meant that not only did the pilot conduct all the mandatory syllabus requirements for the endorsement, but he would also have received the opportunity to reinforce the sequences of actions necessary for normal operations. This would have reduced the potential for human error in these sequences. The candidate would then be assessed with a 'check to line' theory and flight test. The flight test component would comprise a line flight and a base check that would cover normal, abnormal and instrument flight procedures. The company used a memory based 'flow' sequence of actions to ensure that necessary tasks were completed at each stage of normal flight. A pilot would follow a preset sequence of actions, and would then use a scroll type checklist to double-check that tasks had been correctly actioned.
The nature of military flying operations was not the same as civilian flying operations, and therefore military training focussed more on the needs of military tasks. The training syllabus that was developed for the military pilots was different from the normal syllabus used by the civilian contractor, and was originally set to 15 hours flight time. The military flight-training syllabus comprised a larger section of abnormal and emergency procedures. There was less emphasis on conducting training flights that would accurately reflect the type of operations that would normally be conducted.
The RAAF pilots who would normally have been flying this aircraft were mostly test pilots, and the nature of their operations meant that they needed skills and procedures that would allow them to fly a wide range of aircraft. They normally utilised a checklist at the time of actioning each particular task to ensure that they carried out all required procedures.
The flight procedures for the Beech 200 operations utilised the civil operator's checklist system, however the military pilots also developed their own kneepad-mounted checklist that they used at the time tasks were performed. This was in accordance with procedures that they were familiar with. The kneepad-mounted checklist was used with the agreement of the civil operator's chief pilot, so long as the scroll type checklist was still used in the correct manner.
A part of the kneepad-mounted checklist included the 'climb' checks. These were carried out at the transition altitude (10,000 ft) and included the requirement for the pilot to check that the pressurisation was NORMAL, to turn the aft blower fan to OFF, and for the airconditioning to be adjusted as required.
Following the incident the contracted maintenance facility checked the operation of the cabin altitude warning system. This check only tested the operation of the warning lights and the continuity of the electrical wiring circuit.
During the incident the aircraft passenger emergency oxygen system activated, but the passenger oxygen masks did not deploy. Maintenance engineers examined the aircraft and found that the centre and rear mask retaining doors had been orientated incorrectly. Consequently, the door-mounted release stops were positioned away from their actuator plungers, and could not be contacted by the door actuators. There was a caution in the maintenance manual that highlighted the consequences of incorrect door fitment. The door retaining lanyards were longer than the initial factory fitment, enabling the doors to be fitted 180 degrees from their correct orientation. One door was correctly oriented, but had not deployed because the door actuator was stiff in operation.
The operation of the passenger oxygen system had last been checked, in accordance with the aircraft owner's system of maintenance, on 11 November 1998. No checks or maintenance had been recorded on the system since that time.
Both barometric switches were removed and tested for correct operation. The switches operated normally with no fault found. The maintenance schedule for the aircraft did not require that the switches be tested for correct operation when installed in the aircraft.
Following the reassembly of the cabin altitude warning system and the passenger oxygen system, the aircraft was test flown in both unpressurised and pressurised modes. All systems operated normally throughout the flight.
The chief pilot carried out a further test flight. During this flight, the pressurisation system was turned off to assess whether the change in cabin pressure was immediately detectable by the occupants. The results of the test indicated that the rate of change of pressurisation started gently, and the occupants did not detect the change until the aircraft cabin altitude had climbed 4,000 to 5,000 ft.
The RAAF pilot was not experienced on the type. The additional workload created by instructions from ATC, and from attempting to re-program the GPS at the time when he was completing his climb checks may have captured his attention, thereby reducing his capacity to notice deviations from normal procedure.
Normal procedures included re-positioning blower switches at this stage of the flight. These switches were located very near to the bleed air valve switches, and it is probable that the pilot inadvertently moved both bleed air switches to ENVIR OFF during the climb checks instead of moving the two blower switches. An inadvertent repositioning of the bleed air switches would not be detected by the sequenced monitoring of the pressurisation instrumentation in the climb checklist, as the pressurisation check was before the airconditioning and aft blower checks.
The pilot's performance was progressively degraded due to the effects of hypoxia as the cabin altitude increased.
Safety critical warning systems such as the cabin altitude warning system, need to be sufficiently effective to alert flight crews despite any distractions that may be present at the time. Visual indications supported by auditory alerts have been shown to be more effective in fulfilling this requirement (Refer to Attachment A). CAO 108.26 also strongly recommended an aural warning. The warning system fitted to the aircraft comprised red warning lights on the glareshield. While there was no evidence of failure of the cabin altitude warning system, it did not alert the pilot to the depressurisation in time for him to respond.
The aircraft type had been certified with a cabin altitude warning that was designed to activate at 12,500 ft. This warning system had also been in accordance with the requirements of the Australian CAOs 20.4 and 108.26 current at that time. During the climb there was a limited period when the cabin altitude warning system could have been expected to alert the pilot before the pilot's performance became significantly degraded. Amendment 92 of CAO 108.26 required the warning to activate at 10,000 ft. If the cabin altitude warning operated as required by the amended CAO, the window of opportunity for alerting the pilot would have been increased at a time when the pilot was most able to respond.
The pilot training syllabus was designed in part to meet the perceived needs of military operations. However, the aircraft was being used for operations that were nearer in type to civilian charter. The training syllabus did not provide the same degree of practical reinforcement of normal procedures as was found in the civilian contractor's normal training syllabus. The syllabus therefore did not provide the same tools to enhance resistance to error in normal procedures.
In this type of incident in which the depressurisation was not rapid, the effects of hypoxia gradually develop and difficult to notice. The pilot, having previously checked the cabin pressurisation, had no suspicion that the aircraft was depressurising, and did not associate his inability to master a simple GPS problem with any other aircraft or physiological abnormality.
The pilot and passengers had all undertaken regular hypobaric hypoxia training. Despite this training, they did not identify the onset of the symptoms of hypoxia until one person became unconscious. The training had not provided an effective defence by equipping the flight crew to recognise the onset of symptoms of hypoxia.
The initial factory fitment of the passenger oxygen system mask container doors incorporated short retaining lanyards for the doors. This would have prevented incorrect orientation of the doors during installation. The original lanyards however had been replaced by longer ones, which removed this designed safety feature.
The approved maintenance system only required regular testing of some parts of the automatic oxygen mask deployment system and the cabin altitude warning system. However, not all parts of the systems were required to be tested on a regular basis. A maintenance procedure for a test of the complete systems installed in the aircraft should have indicated that each system would work in flight, however, neither system was required to be completely tested for correct operation in the aircraft.
The maintenance problems found with the passenger oxygen system, and the lack of effective and timely detection of the cabin altitude alert system, were deficiencies that could have resulted in a more serious occurrence. This is especially significant in single-pilot operations. It is likely that the provision of an audible warning device as strongly recommended in CAO 108.26 would have alerted the pilot to the developing pressurisation problem.
- The aircraft cabin altitude warning did not operate at an altitude of 10,000 ft as required by CAO 108.26.
- The modified pilot training syllabus did not give the same level of defence against human error.
- The maintenance systems for automatic oxygen mask deployment and the cabin altitude warning system did not ensure reliable operation.
- Both bleed air switches were inadvertently selected to ENVIR OFF at about 10,000 ft in the climb.
- The cockpit warning system did not adequately alert the pilot to the cabin depressurisation.
- The oxygen mask deployment doors were incorrectly orientated during installation, so that the masks would not automatically deploy when required.
- Hypobaric training did not provide an effective defence to ensure that the pilot or passengers would identify the onset of hypoxia.
As a result of this investigation, a number of safety actions have been undertaken.
Local safety actions
The operator has re-evaluated pilot training procedures, realigning them with the syllabus in the operator's check and training manual.
More stringent currency requirements have now been put in place for single pilot operations by military pilots on this aircraft.
A program has been put in place to conduct regular maintenance of the cabin altitude warning and the supplemental oxygen systems.
The operator has installed an aural warning system in the Super King Air 200 aircraft that was involved in this incident. The operator is also fitting aural warning systems that interface with the cabin pressure altitude warning to the rest of their Super King Air 200 aircraft fleet. These systems are being fitted at an approximate unit cost of $1,000. Consideration is also being given to the installation of similar systems to their other pressurised turbo-prop aircraft.
The Australian Defence Force, Directorate of Flying Safety has also published articles on hypoxia, related to this occurrence, in its 'Flying Feedback' and 'Spotlight' magazines. These magazines are distributed to military flight crew.
The Australian Transport Safety Bureau (formerly the Bureau of Air Safety Investigation) issued interim recommendations, IR19990084, IR19990088, IR19990089, IR19990090, IR19990150, IR19990151, IR19990152, IR19990153, IR19990154 and IR19990155 during the investigation. The responses to these recommendations, without alteration to the text, are included at Annex B.
As a result of this investigation the following recommendations are issued simultaneously with this report.
The ATSB recommends that CASA advise relevant operators of its interpretation of CAO 108.26 in relation to the applicability of the requirements for a device to provide the flight crew of pressurised aircraft with a warning whenever the cabin pressure altitude exceeds 10,000 feet.
The following response was received from CASA on 02 February 2001:
The Civil Aviation Safety Authority accepts this recommendation and is now considering how best to clarify the intent of CAO 108.26, paragraph 3.1, for relevant operators.
ATSB RESPONSE STATUS: CLOSED - ACCEPTED
The ATSB has concerns regarding the ineffectiveness of visual cabin altitude warning systems that are not accompanied by an aural warning. In this incident the inclusion of an audible warning, as strongly recommended in CAO 108.26, may have assisted the pilot to recognise a depressurisation.
The ATSB therefore recommends that CASA mandate the fitment of aural warnings to operate in conjunction with the cabin altitude alert warning systems on all Beechcraft Super King Air and other applicable aircraft.
The following response was received from CASA on 02 February 2001:
The Civil Aviation Safety Authority accepts this recommendation and will move to prepare a regulatory amendment to make it mandatory for pressurised aircraft to have aural cabin altitude alert warning systems. This amendment will follow the normal regulatory development process which, in the first instance, will lead to the circulation of a Discussion Paper. It is anticipated that the paper will be released this month.
ATSB RESPONSE STATUS: CLOSED - ACCEPTED
In accordance with normal procedures the ATSB will continue to monitor CASA's implementation of the recommendations.
Attachment A - Human Factors, Cabin Altitude Alert Warning System
Attachment B - Responses to Previously Issued Recommendations Arising From Occurrence 199902928.
Human Factors, Cabin Altitude Alert Warning System
There are many complex factors associated with this occurrence but one of the human factors issues that the team examined was the cabin altitude alerting/warning system.
The Bureau examined this issue from a human factors perspective rather than as a certification issue. The rationale for recommendation IR 19990153, IR 19990154, and IR 19990155 was as follows:
When designed correctly, auditory warning signals can improve operator performance and reduce accidents (Edworthy, Loxley, & Dennis, 1991). It is important to ensure that appropriate warning system information is presented in a form that individuals or crews can readily understand, and at the right time to facilitate making effective judgements and decisions (Noyes et al., 1995). In particular, ' the alerting function for all important failures should be fulfilled by a [sic] audio warning…' (Green et al., 1991, p. 120). Moreover, auditory warnings should be used when a situation calls for immediate action (Deatherage, 1972; Sanders & McCormick, 1992; Sorkin, 1987). Cabin depressurisation is an example of such an important failure. Furthermore, compliance rates for visual warnings are often low (Edworthy, Stanton, & Hellier, 1995; Wogalter, Kalsher, & Racicot, 1993). However, research has demonstrated that reaction times to visual indications are shorter when supported by an auditory warning signal (Selcon, Taylor, & McKenna, 1995; Stokes & Wickens, 1988). Finally, auditory warnings have an immediacy that may not be apparent with visual warnings and they may also produce higher levels of compliance (Stokes & Wickens, 1988; Wogalter et al., 1993).
Research has demonstrated that vision is particularly sensitive to hypoxia (Fowler, Paul, Porlier, Elcombe, & Taylor, 1985; Ernsting, Sharp, & Harding, 1995). In addition, visual degradation occurs before the auditory modality declines (Brinchmann-Hansen & Myhre, 1989; Fowler, Banner, & Pogue, 1993; Fowler, Elcombe, Kelso, & Porlier, 1987; Fowler, Paul, Porlier, Elcombe, & Taylor, 1985; Green, Muir, James, Gradwell, & Green, 1999; Nesthus, Garner, Mills, & Wise, 1997; Orlady & Orlady, 1999; U.S. Navy Flight Surgeon's Manual, 1993). Moreover, the rate and magnitude of decline of the visual modality in a hypoxic individual is more rapid compared to the auditory modality. Moderate and severe hypoxia causes a restriction of the visual field, with loss of peripheral vision and the development of a central scotoma. There may also be a subjective darkening of the visual field. Auditory acuity is also reduced by moderate and severe hypoxia, but some hearing is usually retained even after other senses such as vision are lost (Ernsting, Sharp, & Harding, 1995).
Therefore, the incorporation of an aural warning to supplement the visual warnings associated with the cabin altitude alert system would provide greater assurance for the integrity of the system.
References and relevant research
Bliss, J, P., Gilson, R. D., & Deaton, J. E. (1995). Human probability matching behaviour in response to alarms of varying reliability. Ergonomics, 38, 2300-2312.
Brinchman-Hansen. O, & Myhre, K. (1989). Effect of hypoxia on the macular recovery time in normal person. Aviation, Space, and Environmental Medicine, 60, 1183-1186.
Deatherage, B. H. (1972). Auditory and other sensory forms of information presentation. In H.P. VanCott & R.G. Kinkade (Eds.), Human engineering guide to equipment design (pp. 123-160). Washington, DC: US Govt. Printing Office.
Edworthy, J. (1997). Cognitive compatibility and warning design. Ergonomics, 1, 193-209.
Edworthy, J., Loxley, S., & Dennis, I. (1991). Improving auditory warning design: Relationship between warning sound parameters and perceived urgency. Human Factors, 33, 205-232.
Edworthy, J., Stanton, N., & Hellier, E. (1995). Warnings in research and practice: Editorial. Ergonomics, 38, 2145-2154.
Edworthy, J., & Stanton, N. (1995). A user-centred approach to the design and evaluation of auditory warning signals: 1. Methodology. Ergonomics, 38, 2262-2280.
Ernsting, J. & Sharp, G. R., revised by Harding, R. M. (1995). Hypoxia and hyperventilation. In J.Ernsting & P.King (Eds.), Aviation medicine (2nd Edition) (pp. 45-59). Oxford, UK: Butterworth-Heinemann Ltd.
Fowler, B., Banner, J., & Pogue, J. (1993). The slowing of visual processing by hypoxia. Ergonomics, 36, 727-735.
Fowler, B., Elcombe, D. D., Kelso, B., & Porlier, G. (1987). The threshold for hypoxia effects on perceptual-motor performance. Human Factors, 29, 61-66.
Fowler, B., Paul, M., Porlier, G., Elcombe, D. D., & Taylor, M. (1985). A re-evaluation of the minimum altitude at which hypoxic performance decrements can be detected. Ergonomics, 28, 781-791.
Green, R. G., & Morgan, D. R. (1985). The effects of mild hypoxia on a logical reasoning task. Aviation, Space, and Environmental Medicine, 56, 1004-1008.
Green, R. G., Muir, H., James, M., Gradwell, D., & Green, R. L. (1991). Human factors for pilots. Aldershot, UK: Ashgate.
Izraeli, S., Avgar, D., Glikson, M., Shochat, I., Glovinsky, M. D., & Ribak, J. (1988). Determination of the 'time of useful consciousness' (TUC) in repeated exposures to simulated altitude of 25,000 ft (7, 620 m). Aviation, Space and Environmental Medicine, 59, 1103-1105.
Letourneau, J. E., Denis, R., & Londorf, D. (1986). Influence of auditory warning on visual reaction time with variations of subjects' alertness. Perceptual & Motor Skills, 62, 667-674.
McCarthy, D., Coban, R., Legg, S., & Faris, J. (1995). Effects of mild hypoxia on perceptual-motor performance: A signal-detection approach. Ergonomics, 39, 1979-1992.
Naval Aerospace Medical Institute. (1991). Physiology of flight. In R.K. Ohslun.
C.I. Dalton, G.G. Reams, J.W. Rose, & R.E. Oswald (Eds.), United States Naval flight surgeon's manual (3rd edition) (http://www.vnh.org//FSManual/01/03Hypoxia.html). Iowa City, Iowa: University of Iowa College of medicine in collaboration with The Bureau of medicine and Surgery, Department of the Navy.
Nesthus, T. E., Garner, R. P., Mills, S. H., & Wise, R. A. (1997, March). Effects of simulated general aviation altitude hypoxia on smokers and nonsmokers (FAA Office of Aviation Medicine Reports FAA-AM-97-07). Washington, DC: FAA.
Noyes, J. M., Starr, A. F., Frankish, C. R., & Rankin, J. A. (1995). Aircraft warning systems: application of model-based reasoning techniques. Ergonomics, 38, 2432-2445.
Orlady, H. W., & Orlady, L. M. (1999). Human factors in multi-crew operations. Aldershot, UK: Ashgate.
Sanders, M. S., & McCormick, E. J. (1992). Human factors in engineering and design (7th Ed). New York: McGraw-Hill.
Satchell, P. M. (1993). Cockpit monitoring and alerting systems. Aldershot, UK: Ashgate.
Selcon, S. J., & Taylor, R. M. (1995). Integrating multiple information sources: using redundancy in the design of warnings. Ergonomics, 38, 2362-2370.
Sorkin, R. D. (1987). Design of auditory and tactile displays. In G. Salvendy (Ed.), Handbook of human factors. New York: John Wiley.
Stanton, N. A., & Edworthy, J. (Eds.) (1999). Human factors in auditory warnings. Aldershot, UK: Ashgate.
Stokes, A. F., & Wickens, C. D. (1988). Aviation displays. In E. L. Wiener & D. C. Nagel (Eds.), Human factors in aviation (pp. 387-431). San Diego, CA: Academic Press.
Takagi, M., & Watanabe, S. (1999). Two different components of contingent negative variation (CNV) and their relation to changes in reaction time under hypobaric hypoxic conditions. Aviation, Space, and Environmental Medicine, 70, 30-34.
Wogalter, M. S., Kalsher, M. J., & Racicot, B. M. (1993). Behavioural compliance with warnings: effects of voice, context, and location. Safety Science, 16, 637-654.
Responses to previously issued recommendations arising from occurrence 199902928
The Australian Transport Safety Bureau classifies the responses to recommendations as follows:
CLOSED - ACCEPTED
ATSB accepts the response without qualification.
CLOSED - PARTIALLY ACCEPTED
ATSB accepts the response in part but considers other parts of the response to be unsatisfactory. However, ATSB believes that further correspondence is not warranted at this time.
CLOSED - NOT ACCEPTED
ATSB considers the response to be unsatisfactory but that further correspondence is not warranted at this time.
The response does not meet some or all of the criteria for acceptability for a recommendation that ATSB considers to be significant for safety. ATSB will initiate further correspondence.
The following interim recommendations were issued during the investigation.
IR19990084, issued on the 28 July 1999
The Bureau of Air Safety Investigation recommends that the Civil Aviation Safety Authority issue a directive for an immediate check of the fitment of passenger oxygen system mask container doors on all Australian Beech King Air B200 aircraft and, all other aircraft similarly equipped.
The following response to IR 19990084 was received from the Civil Aviation Safety Authority on 16 September 1999:
In response to the subject recommendation, CASA has considered the issue of a directive to check the installation of passenger oxygen system mask container doors on all Australian Beechcraft King Air B200 aircraft, and similarly equipped aircraft. The BASI recommendation notes that the maintenance manual has a cautionary note regarding potential for incorrect fitment of the passenger oxygen mask container doors. In view of this, CASA does not consider issue of an Airworthiness Directive to comply with existing maintenance instructions is warranted.
An advisory letter (copy attached) was sent to all Certificate of Registration holders of Raytheon pressurised twin-engine aircraft, in line with the 30 June 1999 interim advice that BASI provided to CASA, to raise awareness of the incident with affected operators of the aircraft. The letter strongly recommended checking each passenger oxygen system mask container door for correct installation, but did not make such a check mandatory. An Airworthiness Advisory Circular, AAC 1-112, was also issued. No further reports of incorrectly installed passenger oxygen masks have been received by CASA.
Further action will be considered when the BASI final report into the incident is made available.
Letter issued by CASA, dated 2 July 1999 to:
Certificate of Registration Holders
All Beech pressurised twin engine aircraft
Relating to: Faulty installation of emergency oxygen system cover plates.
A recent incident involving a Beech 200 aircraft has highlighted a potential safety hazard with the cover plates on the emergency oxygen system. This letter is to draw your attention to the deficiency in order that you may take appropriate actions for the safety of persons flying in your aircraft. Although found on a Beech 200, any Beech aircraft with an emergency oxygen system may be similarly affected.
Following an incident involving the emergency oxygen system, a maintenance investigation was carried out. Although not the primary cause of failure, this investigation found that some of the covers over the passenger mask headliner compartments had been incorrectly installed. If the emergency oxygen system had been activated, automatically or manually, the incorrectly installed covers would not release and the oxygen masks would be unavailable. The operator of the incident aircraft has since inspected four other Beech 200 aircraft. These four aircraft are maintained by a different maintenance organisation. Of the four aircraft, two had oxygen mask covers improperly installed such that they would not be able to operate.
The covers are designed to be pushed open by a plunger which is operated by pressure in the oxygen line. If the cover is installed 180 degrees out of proper position the plunger no longer contacts the striker block fixed to the cover, and the cover remains in place. The Beech 200 maintenance manual notes that caution should be exercised when installing the cover plate. However, when the cover is fitted there are no obvious signs which show that the cover is not properly installed.
When more details are available CASA will contact the manufacturer to determine what further actions may be required to prevent incorrect installation of the covers.
The Civil Aviation Safety Authority strongly recommends an inspection, or test, to ensure that each oxygen mask cover is installed properly as shown in the applicable aircraft maintenance manual at the earliest opportunity. The inspection, or test, should confirm that the striker block in the cover is located below the plunger. If any cover is found to be fitted incorrectly, remove and refit the cover correctly and notify CASA through your nearest district office.
ATSB RESPONSE STATUS: CLOSED-ACCEPTED.
IR19990088, issued on the 28 July 1999
The Bureau of Air Safety Investigation recommends that Raytheon Aircraft issue a directive for an immediate check of the fitment of passenger oxygen system mask container doors on all Beech King Air B200 aircraft and, all other Raytheon aircraft similarly equipped.
Raytheon Aircraft Company response received 11 January 2000.
RAC has published King Air Series Communique 99-005, dated October 1999 (copy enclosed). RAC also published Safety Communique No. 168 (copy enclosed), dated November 1999 and applicable to all Raytheon Aircraft models with deployable passenger oxygen mask systems, to ensure that all operators are aware of the importance of properly installing the doors on the oxygen mask boxes.
Additional Federal Aviation Administration response received 11 January 2000.
Raytheon Aircraft has reported that they do not intend to issue a directive for an immediate check of the fitment of the passenger oxygen system container doors. However, they have published the safety communique noted above. Additionally, Raytheon intends to revise the maintenance manual for the Model 300 series King Airs to provide a cautionary note similar to that provided in the 200 Series maintenance Manual.
ATSB RESPONSE STATUS: CLOSED-ACCEPTED.
IR19990089, issued on the 28 July 1999
The Bureau of Air Safety Investigation recommends that the Federal Aviation Administration issue a directive for an immediate check of the fitment of passenger oxygen system mask container doors on all Beech King Air B200 aircraft and, all other aircraft similarly equipped.
The following response was received from the US Federal Aviation Administration on 11 January 2000.
The Raytheon Aircraft maintenance manual for the Beech Super King Air 200 Series airplanes requires that an oxygen system functional test be performed at the Phase 1 and Phase 3 inspections. This results in an initial inspection at 200 hours and subsequent inspections every 400 hours. During each oxygen system inspection, the operator is required to ensure that the doors on the mask containers open and the masks drop out. Additionally, the Model 200 Series Maintenance Manual cautions operators that the oxygen 'Container door can be positioned 180 degrees off. If this happens, the plunger cannot push the door open when activated.' Other similarly equipped aircraft have the same oxygen system maintenance schedule. The Model 300 Series Maintenance Manual does not have the additional cautionary note.
However, Raytheon has committed to revising this manual to add a similar note to that in the Model 200 manual. To provide an added level of awareness to operators, Raytheon Aircraft has published an article regarding this subject in a King Air Model Communique. The Communique will be mailed to all operators of Raytheon aircraft equipped with auto-deploy oxygen masks to ensure that all operators are aware of the importance of properly installing the doors on the oxygen mask boxes. Considering the maintenance instructions already in place, issuance of an Airworthiness Directive for an immediate check of the fitment of passenger oxygen system mask container doors would not significantly add to the safety of the fleet.
In conclusion, this office recommends the Safety Recommendations be closed. No further ACO action is required or planned.
ATSB RESPONSE STATUS: CLOSED-PARTIALLY ACCEPTED.
IR 19990090, issued on the 28 July 1999
The Bureau of Air Safety Investigation recommends that Raytheon Aircraft examine and implement methods of preventing incorrect passenger oxygen system mask container door fitment as installed in Beech King Air B200 series aircraft, and all other Raytheon aircraft similarly equipped.
Raytheon Aircraft Company response received 11 Jan 2000.
RAC will make a production design change to the B200 oxygen mask containers to provide a method of preventing the doors from being installed incorrectly. RAC will make available through spares, and announce via a Recommended Service Bulletin, the same change to all delivered airplanes. The production design change is tentatively scheduled to be completed by the end of the second quarter of 2000.
RAC has investigated to determine whether the condition referenced in the Interim Recommendation might exist in other Raytheon King Air model airplanes:
- The Model C90A does not have an auto-deploy or drop-down system. It is totally passenger operated (i.e., the passenger opens the door and plugs in the mask).
- The Model B300 is an auto-deploy system made by Puritan Bennett. The lid is permanently attached to the box with two metal lanyards. The lanyards are not long enough to allow the door to be rotated and installed improperly.
- Model 200 serials BB-1 through BB-54 (excluded from the applicability of the B200 system) use the same system as the Model C90A.
- The Model F90 has an oxygen system design similar to the B200. The Interim Recommendation therefore applies to the Model F90 as well. RAC will address the F90 in all corrective actions.
- The Model 100 has an oxygen system design similar to the C90A. It does not have an auto-deploy or drop-down system. It is totally passenger operated (i.e., the passenger opens the door and plugs in the mask).
In addition to the King Air Models, RAC has investigated to determine whether the condition referenced in the Interim Recommendation might exist in other Raytheon model airplanes. The investigation revealed that the condition does not exist.
- The Commuter Airplane Series (Model 1900, 1900C, and 1900D) oxygen box door design does not allow the door to be installed backwards or in any other manner which would prevent the plunger from releasing the door.
- The Beechjet Airplane Series (Model 400, 400A, 400T) design does not permit the oxygen doors to be installed backwards.
- The Starship (Model 2000) does not have the same cover design and cannot be installed backwards. The hinge tabs are on one side, and the plunger interface is on the other. The plunger interface is part of the door. There are no slots for the hinge tabs on the other side of the box, so that the door cannot be installed 180 degrees out. Also, the plunger cannot be inserted into the valve body if the door is backwards.
- The Hawker Airplane Series design does not permit the oxygen doors to be installed backwards.
This information has also been supplied to the U.S. FAA.
Additional Federal Aviation Administration response received 11 January 2000.Raytheon Aircraft has committed to make a production change to the B200 oxygen mask containers to provide a method of preventing the doors from being installed incorrectly. The Model 300 is no longer in production. Raytheon will also make available through spares via a recommended service bulletin the same change to all delivered airplanes. The production design change is scheduled to be completed by the end of the second quarter of 2000.
In conclusion, this office recommends the Safety Recommendation be closed. No further ACO action is required or planned.
ATSB RESPONSE STATUS: CLOSED-ACCEPTED.
IR 19990150, issued on the 7 October 1999
The Bureau of Air Safety Investigation recommends that Raytheon Aircraft develop and publish methods for the in-situ testing of the automatically deployable passenger oxygen activation system and the cabin altitude alert system on Beechcraft aircraft, to ensure complete system operation.
Raytheon Aircraft Company Response dated 10 July 2000
In accordance with Interim Recommendation IR19990150, RAC has reviewed the B200 Maintenance Manual Procedures for Functional Test Procedure of the Oxygen Auto-deploy System, and finds it appropriate for the system. However, there is no functional test for either of the barometric switches (one for the oxygen system and one for the annunciator system) installed in the airplane. RAC will add a procedure to the maintenance manual to functionally check the barometric pressure switches.
RAC reviewed the maintenance manual and found that there is a 'press to test' (which checks the annunciator lights). In the oxygen system functional test procedure, one of the steps is to verify the oxygen indicator light (green, in the caution/advisory panel) is illuminated. These tests meet the certification requirements for the airplane.
Therefore, RAC plans no revisions to the Maintenance Manual with regard to functional test of the oxygen system.
ATSB RESPONSE STATUS: CLOSED-ACCEPTED
IR19990151, issued on the 7 October 1999
The Bureau of Air Safety Investigation recommends that the Civil Aviation Safety Authority reassess the appropriateness of the current maintenance procedures for the testing of automatically deployable passenger oxygen systems and cabin altitude alert systems, to ensure complete system operation.
To date no response has been received from CASA to this interim recommendation.
IR19990152, issued on the 7 October 1999
The Bureau of Air Safety Investigation recommends that the Federal Aviation Administration reassess the appropriateness of the current maintenance procedures for the testing of automatically deployable passenger oxygen systems and cabin altitude alert systems, to ensure complete system operation.
Federal Aviation Administration Response received 31 March 2000.
In assessing Safety Recommendation 99.400, the B200 Maintenance Manual procedures for functional testing of the oxygen auto-deploy system and the cabin altitude alert system were reviewed. The procedures were found to be acceptable with the exception of functional testing of the two barometric pressure switches (one for the oxygen system and one for the annunciator system) installed in the airplane. There is currently no provision to functionally cheek the operation of either switch to ensure that it would provide the required signal at the specified cabin altitude of 12,500 feet. To address this issue, Raytheon Aircraft Company has agreed to revise the affected maintenance manuals to add a procedure to functionally check both barometric pressure switches referred to above.
In conclusion, this office considers the actions identified in this letter to be satisfactory in addressing the safety concern. Raytheon Aircraft Company has committed to making the necessary changes to the maintenance manuals of the affected airplanes to verify that the barometric pressure switches will actuate at the required altitude. Therefore, the Wichita ACO recommends that Safety Recommendation 99.400 be closed.
ATSB RESPONSE STATUS: CLOSED-ACCEPTED.
IR19990153, issued on the 7 October 1999
The Bureau of Air Safety Investigation recommends that Raytheon Aircraft consider the incorporation of an audible warning to operate in conjunction with the cabin altitude alert system on all Beech aircraft so equipped.
Raytheon Aircraft Company Response received 18 July 2000
The 200 Series King Air's Annunciator system consists of a warning annunciator panel with red readouts in the center of the glareshield. Two red master warning flashers are located in the glareshield, one in front of the pilot and one in front of the co-pilot. The altitude warning annunciator triggers the master warning system.
The annunciators are the 'word readout' type. Whenever a fault condition covered by the annunciator system occurs, a signal is generated and the appropriate annunciator is illuminated. If the fault requires the immediate attention and reaction of the pilot, the appropriate red warning annunciator in the warning annunciator panel illuminates and both master warning flashers begin flashing. Any illuminated lens in the warning annunciator panel will remain on until the fault is corrected.
Therefore, in the case of the subject incident, even though the pressurization system was not turned on, the pilot would have been presented with a red flashing light and a red 'ALT WARN' when the cabin altitude exceeded 12,500 feet. These two warnings are more than adequate and meet the certification requirements of the Model B200. There are over 1,600 Model 200 King Airs in operation worldwide with this system installed. Raytheon Aircraft does not believe it is necessary to add aural warning to an already proven visual system.
ATSB RESPONSE STATUS: CLOSED-NOT ACCEPTED
IR19990154, issued on the 7 October 1999
The Bureau of Air Safety Investigation recommends that the Civil Aviation Safety Authority consider the incorporation of an audible warning to operate in conjunction with the cabin altitude alert system on Beech aircraft, and other aircraft so equipped.
The following was issued from the Civil Aviation Safety Authority on the 28 January 2000:
The certification basis for the Beech 200 and similar aircraft, which is accepted by Australia and the Joint Aviation Authorities, requires provision of a warning indication to the pilot when a set pressure differential is exceeded and when the cabin altitude is above 10000 feet. There is no specification of the type of warning system required for Commuter Category aircraft. It should be noted that even for Transport Category aircraft, the warning indication may be 'aural or visual'.
Whilst CASA accepts the Bureau's point that the onset of hypoxia usually degrades visual acuity before hearing, this incident does not provide sufficient justification to mandate retrofitting of audible cabin altitude warning. There have been more than 2000 of the type produced and the design is well proven.
Before imposing such a condition on operators, extensive consultation would need to be undertaken. The Authority will await the outcome of IR19990153 and IR19990155 before contemplating further action on this matter.
The following was issued from the Civil Aviation Safety Authority on the 29 September 2000:
As was indicated to you by letter on 21 January 2000, CASA wished to consider the responses of the aircraft manufacturer (Raytheon Aircraft Company) to IR19990153 and the United States Federal Aviation Administration (FAA) to IR19990155 before contemplating further action on this matter. Now that the ATSB has provided CASA with responses from these organisations we are in a position to comment further.
CASA notes the response of the FAA which includes advice that, although it is recognised that adding an aural warning is a desirable enhancement of the system, requiring such a warning for the existing fleet is not considered necessary to meet the minimum airworthiness standards. This is consistent with CASA's view, first put in an Air Navigation Order (108.26) issued in June 1972 by the then Department of Aviation, which included the following:
Note: '.. The cabin pressure warning should not depend on the reading of a gauge. An aural warning is strongly recommended.'
This recommendation remains current as Civil Aviation Order (CAO) 108.26.
CASA also notes that, in response to IR19990153, Raytheon Aircraft Company states that the warnings provided are more than adequate to meet the certification requirements of the Model B200. The response goes on to say that there are over 1,600 Model 200 King Airs in operation worldwide with this system installed and the company does not believe it is necessary to add aural warning to an already proven visual system.
You have informed us that accident and incident reports currently available to the ATSB from the UK, the United States and New Zealand, relating to some 200 incidents involving turbo prop and piston engine pressurised aircraft, do not contain any reports of failure of the existing warnings to alert the crews to pressurisation failures. The only possible exception is the incident involving VH-OYA on 21 June 1999 (where the alerting system may have failed and the automatic deployment of the passenger oxygen masks did fail), which is the subject of the Interim Report.
CASA therefore believes that there is no valid evidence currently available to support mandating the fitting of an audible warning on pressurised aircraft. CASA recognises that an audible warning is a useful defence mechanism. Safety promotion material will be prepared which will emphasise the position defined in CAO 108.26 strongly recommending an aural warning.
On the basis of the information in the interim report and provided by the ATSB at the meetings on 7 and 15 September, CASA is of the view that a significant factor in the June 1999 incident was the failure of the crew to follow correct operating procedures.
While recognising that physical failures of the aircraft involving the oxygen mask drop down system and the barometric switch associated with the warning system have been addressed, CASA's operational and human factor specialists have expressed concern that the Interim Report on the incident in June 1999 did not address key training, operational and human performance issues.
For example, the ATSB advised that the RAAF crew had used both a civilian and military check list and, apparently, had still failed to set the pressurisation system and had failed to detect that the aircraft was not pressurising as called for in the check list following take-off, and again when passing through 10,000 ft.
ATSB indicated that there had been some discussion with the Defence Forces on this issue and that crew training had been amended to reflect civil requirements. Of course, this does not address the question of whether the civil training requirements are appropriate and effective.
At present, CASA's view is that the training and procedural issues evident in the June 1999 incident were the most significant factors in the events leading up to the pilot's incapacitation, and the physical aircraft failures were the main reason the errors were not picked up earlier.
While it is acknowledged that an aural alarm would provide an additional means of alerting the crew to a depressurisation or no pressurisation, there appears to be insufficient human factors research to indicate that such an alarm would, in isolation, be sufficient to resolve the problem. Improved crew training and adherence to proper operating procedures would appear to offer the most effective way of ensuring the correct operation of all aircraft systems.
OTHER SIMILAR INCIDENTS
At the meeting on 15 September, the ATSB indicated that it was aware of a second incident with a RAAF aircraft since the incident that had resulted in the Interim Recommendation. At the present time, neither the ATSB or the Department of Defence have been able to confirm that there was a second incident. In the event that a second incident did occur, it would be useful to examine the circumstances to determine what lessons need to be learned in relation to crew training and adherence to operational procedures. It would also be useful to ascertain whether the purported second RAAF incident occurred before or after Defence had changed its training for these aircraft.
CASA notes the advice from the ATSB that, to date, no conclusions could be drawn from the preliminary investigation of the Beech Super King Air 200 aircraft in Queensland. CASA has not ruled out the mandating of aural warnings to operate in conjunction with the cabin altitude alert systems on Raytheon King Airs should evidence supporting this action emerge during the investigation, while noting that this requirement would almost certainly have to be extended to apply to all piston and turbo prop pressurised aircraft types. As you know, as part of the industry consultation process, the Authority is required to prepared a Regulatory Impact Statement (RIS). The RIS would have to include a discussion on other options that would be available to address the safety concerns identified by the ATSB. CASA would have to be satisfied on all the evidence available that the fitment of an aural warning device would be the most effective and appropriate way of resolving these safety concerns.
CASA is seeking further advice from the FAA on contemporary human factors research into the issue of aural verses visual alerting systems. We would welcome any further advice that the ATSB has been able to obtain from other sources overseas on this issue.
We regard an audible warning as a good fourth or fifth line of defence, but believe that prevention, via training and promulgating of safety information, is more important than finding another cure.
CASA will convene a series of Major Industry Workshops. At these safety promotion and educational material will be provided to discuss hypoxia and other matters relevant to operation of pressurised aircraft. It is also intended to emphasise operational and training issues to ensure repeat omission of action on checklist items is highlighted and addressed. I believe it is essential that ATSB form part of these workshops to put forward their views and evidence on pressurisation incidents. In this way we can ensure that industry participants are made aware of all the safety issues involved and can also contribute to a debate on the solutions available, including that of mandatory audible warnings.
We would be happy to meet with you again to share our views on these workshops.
ATSB RESPONSE STATUS: CLOSED-PARTIALLY ACCEPTED
IR19990155, issued on the 7 October 1999 The Bureau of Air Safety Investigation recommends that the Federal Aviation Administration consider the incorporation of an audible warning to operate in conjunction with the cabin altitude alert system on Beech aircraft, and other aircraft so equipped.
Federal Aviation Administration response received 31 March 2000.
Safety Recommendation 99.401, which requests consideration of an audible warning to operate in conjunction with the existing visual warning system, has also been reviewed. The existing system utilizes a red 'ALT WARNING' annunciator light. Although no aural tone is present when the red light illuminates, both master warning flashers begin flashing to bring the pilot's attention to the appropriate annunciator. In reviewing this recommendation, the certification basis for the Raytheon Model 200 Series airplanes was reviewed. At the amendment level established in the certification basis, Section 23.841 (f) states: '...an aural or visual signal (in addition to cabin altitude indicating means) meets the warning requirement for absolute cabin pressure limits'. Furthermore, the corresponding Part 25 (Transport Category) requirement (Section 25.841(b)(6)) states: '...an aural or visual signal (in addition to cabin altitude indicating means) meets the warning requirements for cabin pressure altitude limits. Based on the above, the FAA has clearly never specifically required an aural cabin altitude warning. Although it is recognized that adding an aural warning is a desirable enhancement of the system, requiring such a warning for the existing fleet is not considered necessary to meet the minimum airworthiness standards.
In conclusion, this office considers the actions identified in this letter to be satisfactory in addressing the safety concern. The existing visual warning system for high cabin altitude is deemed acceptable. Therefore, the Wichita ACO recommends that Safety Recommendation 95.401 be closed.
ATSB RESPONSE STATUS: CLOSED-NOT ACCEPTED
|Date:||21 June 1999||Investigation status:||Completed|
|Location:||72 km E Edinburgh, Aero.|
|State:||South Australia||Occurrence type:||Incorrect configuration|
|Release date:||07 February 2001||Occurrence class:||Operational|
|Report status:||Final||Occurrence category:||Incident|
|Highest injury level:||None|
|Aircraft manufacturer||Raytheon Aircraft Company|
|Type of operation||Military|
|Damage to aircraft||Nil|