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.
Recorded information
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
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.
