The Embraer Brasilia aircraft was being operated on a Regular Public Transport flight from Dili, East Timor to Darwin NT. Shortly after the aircraft had levelled off at FL230, the pilot in command became aware of cabin air pressure changes. That was verified by reference to the cabin rate of climb indicator, which indicated that the cabin altitude was increasing at a rate of approximately 500 ft/min.
The crew reported that they attempted to regain control of the cabin pressurisation system by switching from the automatic to the manual pressurisation controller. When that action did not rectify the problem, the crew immediately commenced a descent. The rate at which the cabin altitude was increasing was not excessive and the crew elected to perform a normal descent. While on descent, the cabin altitude rate of climb suddenly increased to about 1,000 ft/min. The rate of pressure loss was not uncomfortable and the aircraft was rapidly approaching FL140. The crew did not don supplemental oxygen masks during the initial descent.
The flight attendant was at the rear of the aircraft when the passenger oxygen masks deployed. At almost the same time the interphone sounded and she moved to the front of the aircraft to answer the call (there was no cabin interphone at the rear of the aircraft). The flight attendant did not use any of the spare passenger masks as she moved forward through the cabin and did not don her mask after arriving at her crew station. The pilot in command advised her that there was a slight depressurisation problem and that they had commenced a descent to 10,000 ft. The pilot warned her that the passenger masks may deploy and she advised him that they already had. The flight attendant was then instructed to get the Emergency Procedures card and perform the emergency briefing. The flight crew were not wearing supplemental oxygen masks when she spoke to them at that stage of the descent.
The crew reported that they donned oxygen masks later during the descent after the passenger oxygen masks automatically deployed. The flight attendant did not don a mask at any stage of the descent. She reported that some passengers also did not use the masks after they deployed, or after being instructed to do so during the emergency briefing.
The crew established the aircraft in visual meteorological conditions, with fine weather forecast for the planned route. The aircraft landed with about 1,100 lb of fuel, which was within the standard company reserves for depressurised operations. No injuries were reported as a result of the incident.
The aircraft sustained a similar pressurisation problem 12 hours earlier. Following that incident maintenance crews replaced the pressurisation controller and returned the aircraft to service. The aircraft then completed one sector without incident.
In response to the second occurrence, the rear cargo compartment door seal was inspected and found dislodged from its retaining rail. The damage was assessed to have most likely occurred during the loading and unloading of passenger baggage and freight.
After reinstallation of the door seal in the retaining rail, the aircraft was test flown and returned to service without re-occurrence.
Analysis of the flight data recorder indicated that the aircraft reached top of climb (FL231) at about 2347 UTC (Coordinated universal time) and almost immediately commenced a descent. The recording indicated that the descent was conducted at a rate of approximately 2,600 ft/min and that the aircraft reached FL140 at 2351. The aircraft maintained that altitude for approximately 5 minutes before continuing a 500 ft/minute descent to maintain a cruise altitude of 10,000 ft.
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 would sound. 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 flight crew masks were of the quick donning type. The passenger masks were stored in overhead dispensing units positioned in the ceiling of the passenger cabin. An altimetric switch ensured automatic deployment of the oxygen masks whenever the cabin altitude exceeded 14,000 ft. Manual deployment of the masks could also be performed from the cockpit. The supplemental oxygen system operated normally during the incident flight 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. Those 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 those symptoms depends upon 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 occurrence satisfied two of those criteria.
A rapid depressurisation, as defined by CAMI, occurs when the cabin altitude increases by more than 7,000 ft/minute. There was no evidence that during the occurrence the change in cabin altitude exceeded that rate.
The crew had planned to cruise at FL230. 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 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 depressurisation that required the crew to immediately don oxygen masks and commence an emergency descent.
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 probably deployed during the first 4 minutes of the descent from FL 230.
The aircraft cabin altitude during the occurrence was potentially critical in terms of possible hypoxia. However, the crew was unaware of the nature of the pressurisation problem and, consequently, unaware of the possibility that the remaining cabin pressure could suddenly be lost. Such an eventuality would have immediately exposed the passengers and crew to environmental conditions where they could be expected to experience symptoms of hypoxia.
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 flight attendant did not don an oxygen mask during the incident. When she was briefed by the pilot in command, the flight crew were not wearing masks and that possibly influenced her behaviour in regard to the use of supplemental oxygen. The failure of some passengers to don a mask may have been influenced by their observation of the flight attendant not using one.
The procedures permitting discretionary use of supplemental oxygen following activation of the cabin altitude warning system did not recognise that, in some circumstances, the crew's decision-making may already have been impaired. The response to such a warning should take that factor 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.
Maintenance action immediately following the first incident flight did not correctly identify why the pressurisation system failed and the aircraft was dispatched with the original defect unrectified. The loss of cabin pressure was most likely due to the dislodged rear cargo door seal.
Local safety action
Following this occurrence, the operator's baggage handling staff were briefed on the importance of the aircraft cargo door seals and the potential for them to be damaged during loading operations.
ATSB safety action
The Australian Transport Safety Bureau is currently investigating a safety deficiency relating to aircraft 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:||2345 hours UTC|
|Location:||83 km SE Dili, Aero.|
|Release date:||20 March 2002||Occurrence class:||Operational|
|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||2330 hours UTC|
|Role||Class of licence||Hours on type||Hours total|