The Embraer Brasilia aircraft was being operated on a Regular Public Transport flight from Dili, East Timor to Darwin NT and was levelling off to cruise at Flight Level (FL) 210.
Shortly after, and when the aircraft was approximately 90 NM SW of Dili, the flight crew realised that the cabin air pressure was changing. The change was confirmed by reference to the cabin altitude and cabin rate of climb indicators. The cabin altitude was rising at an increasing rate, with the rate of increase quickly exceeding the instrument's full-scale deflection of 2,000 ft/minute.
The crew immediately commenced a high-speed descent and, because they were reacting to the increasing cabin altitude ahead of the aircraft's warning systems, did not don the supplemental oxygen masks. The aircraft was descending at a rate of 3,000 ft/min. While on descent, the crew continued to monitor the cabin altitude indicator.
As the cabin altitude exceeded 10,000 ft, the aircraft master caution warning and the cabin altitude warning chimes activated. At that stage, the aircraft was passing FL140 and continuing to descend at about 3,000 ft/min. With approximately one minute remaining before reaching 10,000 ft, the crew again decided against using the supplemental oxygen masks.
The crew continued the descent and appeared to regain some control of the cabin altitude by using the cabin pressurisation controller in the "manual" mode which maintained a cabin altitude of approximately 8,000 ft. They levelled the aircraft at FL112, the lowest safe altitude for this route segment, and continued to their planned destination. The flight attendant informed the pilots that during the descent the supplemental oxygen masks in the main cabin had automatically deployed.
No injuries were reported as a result of the incident.
Maintenance staff inspected the aircraft and replaced the pressurisation controller. The pressurisation system performed normally on the next scheduled flight. However, on the following sector, the system again malfunctioned. ATSB investigation report BO/200003725 refers.
Flight data recorder
Analysis of the flight data recorder indicated that the aircraft reached top of climb at 0930 UTC (Coordinated universal time) and was maintaining FL 210. At 0944, the aircraft began to descend. The descent profile indicated that the descent was conducted at a rate of about 2,100 ft/min. At 0949, the aircraft levelled off at FL113.
Cabin altitude warning system
The aircraft was equipped with a cabin altitude warning system. In the event of the cabin altitude exceeding 10,000 ft, a 3 chime aural alert and a voice "cabin" warning sounded. In addition, a red "cabin alt" warning light would illuminate on the main annunciator panel and the red "master warning" light would flash. The system activated as designed.
Supplemental oxygen system
A supplemental oxygen system was installed for use by the crew and passengers in the event of a failure of the cabin pressurisation system. It was a conventional high-pressure gaseous storage system, which distributed low-pressure oxygen to the crew and passenger breathing masks. The passenger masks were stored in overhead dispensing units. The supplemental oxygen system operated normally and the masks automatically deployed when the cabin altitude exceeded 14,000 ft.
Hypoxia describes the physiological condition where insufficient oxygen is available to meet the needs of the body. The condition is particularly significant because of the rapid rate at which symptoms can manifest themselves and the variation in the onset of symptoms between individuals. A person suffering the effects of hypoxia could experience a range of symptoms capable of adversely affecting their ability to safely operate an aircraft. These symptoms include impairment of mental performance, loss of judgement, vision impairment, memory loss, reduced levels of awareness and muscular impairment.
The effects of hypoxia may be such that the person is unable to recognise the symptoms or identify that their level of performance has been impaired.
Individuals experiencing the effects of hypoxia can have difficulty in completing even simple tasks. The severity of these symptoms depends on many factors, including the altitude to which the individual is exposed, the duration of the exposure and individual physiological differences. Untreated, hypoxia can result in loss of consciousness and death.
Depressurisation events and response procedures
The US Federal Aviation Authority Civil Aeromedical Institute (CAMI) classified any occurrence of decompression as significant if the cabin altitude exceeded 14,000 ft, the cabin masks were deployed, or if the occurrence resulted in injuries. The incident satisfied two of those criteria.
A rapid depressurisation, as defined by CAMI, occurs when the cabin altitude increases by more than 7,000 ft/min. There was no evidence that during the incident the change in cabin altitude exceeded that rate.
The crew had planned to cruise at FL210. For flights in "pressurised aircraft engaged in flights not above FL250", the Civil Aviation Safety Authority required that supplemental oxygen be used by all flight deck crew "at all times during which the cabin altitude exceeds 10,000 ft". "A crew member (not being a flight crew member on flight deck duty) ... must use supplemental oxygen at all times during which the cabin pressure altitude exceeds Flight Level 140." CAO 20.4 refers.
The aircraft must also carry sufficient supplemental oxygen for passengers as specified in CAO 20.4 Subsection 7.5.
"Supplemental oxygen for passengers
7.5 A pressurised aircraft to which this subsection applies that is to be operated above 10,000 feet flight altitude must carry sufficient supplemental oxygen:
(a) where the aircraft can safely descend to Flight Level 140 or a lower level within 4 minutes at all points along the planned route and maintain Flight Level 140 or a lower level for the remainder of the flight - to provide 10% of the passengers with supplemental oxygen for 30 minutes or 20% of the passengers with supplemental oxygen for 15 minutes; and
(b) where the aircraft cannot safely descend to, or maintain, Flight Level 140 or a lower level in accordance with subparagraph (a) - to provide each passenger with supplemental oxygen for so much of the flight time above Flight Level 140 that exceeds 4 minutes duration and to provide 10% of the passengers with supplemental oxygen for 30 minutes or 20% of the passengers with supplemental oxygen for 15 minutes."
The Quick Reference Handbook (QRH) used by the crew included a checklist for use following an illumination of the cabin altitude warning light. That checklist did not include a requirement for the crew to don oxygen masks but contained checklist items to establish control of the cabin pressure utilising the manual pressurisation controller. The QRH also included a checklist for rapid depressuriastion that required the crew to immediately don oxygen masks and commence an emergency descent.
The crew was unaware of the nature of the pressurisation problem and consequently, unaware of the possibility that the remaining cabin pressure could be suddenly lost. Such an eventuality would have immediately exposed them to environmental conditions where they could have experienced symptoms of hypoxia.
During the descent, the aircraft cabin altitude exceeded 14,000 ft and the passenger oxygen masks automatically deployed. It was not possible to estimate the maximum cabin altitude experienced nor the length of time that the cabin altitude exceeded 10,000 ft. The passenger oxygen masks had most probably deployed during the first 3 minutes of the descent from cruising altitude.
The crew did not use oxygen at any stage during the descent, despite the cabin altitude exceeding 14,000 ft. Due to the apparently short time interval involved and the moderate cabin altitude attained, it is unlikely that the crew experienced any significant symptoms of hypoxia.
The discretionary use by crews of supplemental oxygen following activation of the cabin altitude warning system did not recognise that, in some circumstances, the crew's decision making may have already been impaired and that the response to the warning should take such a scenario into account. The aircraft manufacturer's QRH checklist (following an illumination of the cabin altitude warning light) did not include a checklist item for the crew to don oxygen masks, potentially exposing them to the effects of hypoxia while performing the checklist items.
Due to the insidious nature of hypoxia and the potentially rapid onset of symptoms, any depressurisation event could be critical for flight safety and could result in crew incapacitation. In such circumstances, the precautionary use of supplemental oxygen is essential.
The Australian Transport Safety Bureau is currently investigating a safety deficiency relating to manufacturer's checklists.
Any safety output issued as a result of this analysis will be published on the Bureau's website www.atsb.gov.au.
|Date:||30 August 2000||Investigation status:||Completed|
|Time:||1145 hours UTC|
|Location:||167 km SE Dili, Aero.|
|Release date:||20 March 2002||Occurrence class:||Technical|
|Report status:||Final||Occurrence category:||Incident|
|Highest injury level:||None|
|Aircraft manufacturer||Embraer-Empresa Brasileira De Aeronautica|
|Type of operation||Air Transport Low Capacity|
|Damage to aircraft||Nil|
|Departure point||Dili, EAST TIMOR|
|Departure time||0914 hours UTC|
|Role||Class of licence||Hours on type||Hours total|