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
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.
