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Interim Recommendation issued to: Civil Aviation Safety Authority

Recommendation details
Output No: IR19990154
Date issued: 07 October 1999
Safety action status:
Background: Why this Interim Recommendation was developed

Output text

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.

As a result of the investigation into this safety deficiency, the Bureau simultaneously issues the following interim recommendations:

IR19990150

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.

IR19990151

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.

IR19990152

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.

IR19990153

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.

IR19990155

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.

Initial response
Date issued: 28 January 2000
Response from: Civil Aviation Safety Authority
Response text:

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 10,000 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 2,000 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.

Further correspondence
Date issued: 29 September 2000
Response from: Civil Aviation Safety Authority
Response status: Monitor
Response text:

AUDIBLE WARNINGS

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.

OPERATIONAL FACTORS

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 ACTIONS

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.

Further correspondence
Date issued: 20 October 2000
Response from: Civil Aviation Safety Authority
Response status: Closed - Partially Accepted
Response text:

In 1999 ATSB issued Interim Recommendation IR19990154. CASA responded to that Recommendation on 28 January 2000. That response began by stating:

"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."

This statement is misleading and deserves clarification. The certification basis for the Beech 200 and many other aeroplanes with a maximum take-off weight not exceeding 12,500 lb (5670 kg) is Part 23 of the Federal Aviation Regulations of the USA. In February 1977, at amendment 23-17, Part 23 was amended to state that aircraft with pressurised cabins must have a visual or aural warning of depressurisation when the cabin pressure altitude exceeds 10,000 ft. Prior to amendment 23-17 Part 23 stated that a visual or aural warning must be provided but did not nominate a cabin pressure altitude at which the warning must activate.

The Beech 200 is certificated in the USA to Part 23 at amendment 23-9. Therefore in the USA no Beech 200 aircraft is required to have a warning of cabin depressurisation at 10,000 ft. All Beech 200 aircraft are equipped with a visual warning which activates at a nominal cabin pressure altitude of 12,500 ft.

ATSB response:

The following letter was sent to the Civil Aviation Safety Authority on 2 November 2000:

Thank you, for your letter dated 13 October 2000, indicating what you considered to be `errors of fact' that you would like to see corrected in the CASA response to the ATSB Interim Recommendation IR19990154.

The ATSB has no problem including your letter as an addendum to the original response; however, some of the comments in your letter may be misleading and you may want to correct these before the CASA response is finalised.

In paragraph two you indicate that the "ATSB advised that the RAAF crew had failed to set the pressurisation system and failed to detect the aircraft was not pressurising". This is not correct. The ATSB advice to you in the text of the interim recommendation, IR19990154, stated that following the pilot in command regaining consciousness `It was then found that both bleed air switches were in the "environment off" position and that the aircraft was not pressurised'. At that early stage of the investigation the facts were not fully known. The investigation report, currently at the interested parties stage, reflects the advice that Defence has given you. Specifically that the pilot was seen to select the bleed air switches to "environment off" at the transition altitude by the passenger in the co-pilot's seat.

Regarding your comments at paragraph four, the ATSB was verbally notified of a second incident alleged to have occurred during December 1999, just prior to the meeting between the ATSB and CASA on 15 September 2000. As you mentioned in your letter of response dated 29 September 2000, this was communicated to CASA at the 15 September meeting. A copy of the Department of Defence Air Safety Occurrence Report was subsequently logged into the ATSB system on 18 September 2000, and reported in the ATSB Weekly Summary on 27 September 2000. This summary was sent to your office in accordance with the normal procedure.

In view of the above you may wish to amend your input before the CASA response is finalised.


ATSB Note: CASA did convene a series of industry workshops. Further correspondence on this issue was then taken up under recommendation R20000288. Please refer to that record for more information.

 
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Last update 01 April 2011