Aviation safety investigations & reports

Boeing Co 737-33A, VH-CZQ

Investigation number:
Status: Completed
Investigation completed


The Boeing 737-33A aircraft, registered VH-CZQ, departed Townsville at 1805 EST on a scheduled passenger service to Brisbane. Soon after levelling at flight level (FL) 330, the flight crew observed that the master caution light, the cabin pressurisation auto-fail light and the pressurisation standby light had illuminated. The crew completed the non-normal procedure and, as cabin pressurisation was being maintained, they decided to continue the flight to Brisbane.

At about 1909, the flight crew experienced physiological sensations, which indicated that the flight deck was depressurising. The crew donned their oxygen masks and the copilot noticed that the cabin rate of climb indicator was displaying a rate of climb of 4,000 fpm. Shortly afterwards, as the cabin altitude climbed through 10,000 ft, the crew observed that the master caution light and passenger oxygen `on' light had illuminated and heard the cabin altitude warning horn. The flight crew completed the non-normal procedure for a rapid depressurisation and emergency descent and advised air traffic control that the aircraft had left the cruising level due to a depressurisation. The pilot in command advised the cabin crew on the public address system (PA) of the descent.

The cabin in-flight service had been completed and the four flight attendants were at the front of the aircraft when the oxygen masks dropped from overhead passenger service units (PSU) throughout the cabin. When the oxygen masks dropped, the cabin supervisor returned to his seat at the forward left entry door, donned an oxygen mask and removed the PA handset from its receptacle. Another flight attendant also returned to his crew seat at the forward left door and donned an oxygen mask. The two remaining flight attendants, standing at the front of the aircraft when the oxygen masks dropped, returned to their crew seats at the rear left entry door.

Some passengers were assisted by a flight attendant as she walked to the rear cabin. When the flight attendant reached the rear crew seat she found that the crew oxygen masks had not automatically deployed from the overhead stowage. She released the masks by activating the manual release on the overhead panel, and was seated just as the second flight attendant reached the crew seat.

The flight attendants reported that the floor angle during the emergency descent seemed no different than a normal descent and they did not notice any movement of cabin or galley items. One flight attendant reported that she did not feel in danger. Her ears were `popping', but there were no indications of depressurisation such as those taught in emergency procedures training. The flight attendants reported that they `weren't short of breath, felt [they] had time, there were no objects flying in the cabin or cabin misting'.

The flight data recorder information showed that at 09:08:55, while in cruise at FL330, the cabin altitude warning activated indicating that the cabin altitude had exceeded 10,000 ft. Thirty-six seconds later at 09:09:31, the emergency descent was initiated with the thrust levers being closed and the autopilot level change mode being selected. The pitch attitude then decreased from 3.2 degrees nose up to 4.4 degrees nose down and the aircraft began to descend at a rate of 4,600 feet per minute. At 09:11:01, the speedbrake was deployed as the aircraft descended through FL276 at a rate of 4,500 feet per minute, with a pitch attitude of 3.7 degrees nose down.

The maximum nose-down pitch attitude during the emergency descent was 4.4 degrees for a 6-second period as the aircraft descended through FL 285 at 5,000 feet per minute. By the time the aircraft reached FL199, the pitch attitude had reduced to 1.4 degrees nose down, with a rate of descent of 3,400 feet per minute. By way of comparison, the pitch attitude during a normal descent of a B737-300 series aircraft is about 2 degrees to 3 degrees nose down above FL260, reducing to about 1 degree nose down during the latter part of the descent.

When the aircraft levelled at 10,000 ft, the pilot in command advised the passengers by PA that oxygen masks would no longer be required, and instructed the cabin crew to commence follow-up actions. The cabin supervisor went to the flight deck to confirm instructions and advise the pilot in command of the conditions in the cabin. The flight attendants reported that, although all passengers were calm, three or four passengers needed reassurance, but additional oxygen was not required. The flight crew continued the flight to Brisbane at 10,000 ft and landed at 1953 without further incident. There were no injuries.

The flight crew later reported that they felt a slight movement on the rudder pedals on two occasions during the descent into Brisbane. The crew considered the rudder pedal movements to be minor and did not take any further action. About the same time, the crew also noticed that the map display on the Electronic Horizontal Situation Indicator (EHSI) was incorrect. The aircraft position depicted on the EHSI differed from the position indicated on the other navigation instruments.

Operator's Operations Manual

Volume B3 of the operator's Operations Manual contained information about the safety equipment and procedures. Section 5.1 included information about cabin depressurisation and actions to be taken by the flight crew and cabin crew following a depressurisation event. The manual used the term decompression when referring to depressurisation.

The manual stated that there were two types of decompression, gradual and rapid (explosive). The information provided in section 5.1 related primarily to the rapid type of decompression. The manual stated that `the angle in the cabin will become very steep as the aircraft descends at an approximate rate of 6000 ft per minute (normal rate of descent is approximately 2000 ft per minute)'. The manual also contained a warning that required the immediate use of oxygen by all crewmembers following the deployment of drop out oxygen masks in the passenger cabin. The manual advised cabin crew that the effects of a decompression event on the aircraft were:

  1. Sudden boiling of liquids;
  2. Loud noise as air escapes;
  3. Air becomes thin, cold and dry;
  4. Fog forms in cabin (should not be confused with smoke);
  5. Dust and objects blown about;
  6. Smoke alert devices in toilets may be activated; and
  7. Flight crew may initiate an emergency descent causing the angle in the cabin to become very steep.

Aircraft examination

Following the incident, the electrical/electronic (E/E) bay was inspected and water was found to be dripping from the forward galley floor into the bay. There was also evidence of moisture leakage under the forward passenger door and service door. Moisture stains were found on the racks and ducting within the bay. The inspection also revealed that the moisture shroud was missing from above the E1 rack in the forward part of the bay.

A number of avionics units were removed from the aircraft, including the pressurisation controller, yaw damper coupler, auto-throttle computer and two stall warning computers. These units exhibited evidence of water damage, including moisture staining of component casings and corrosion of connector pins. There was no record of the maintenance staff finding moisture contamination in the E/E bay prior to the incident.

The operator stored beverages in polystyrene ice containers that were stowed in the forward galley. The containers were partially filled with crushed ice that melted during flight. The flight attendants on the incident flight did not recall any water spillage occurring from the ice containers, nor did they recall any turbulence that may have caused a spillage. The flight attendants reported that on previous flights on various aircraft there had been spillage from the ice containers, usually during landing.

Component testing

The manufacturer of the pressurisation controller inspected the unit and subjected it to a series of tests. A visual inspection of the outer case of the unit and the circuit boards did not find any damage or anomalies and a series of functional tests did not reveal any faults. Although there were indications of moisture staining on the outer case, there was no evidence of any residual damage to the unit due to water contamination. The manufacturer was aware of other instances where water had entered pressurisation controllers and caused problems in one or more of the operational modes.

An inspection of the yaw damper coupler found that the electrical filter was shorting out. After the filter was replaced the unit operated satisfactorily. A dent was also found on the top cover of the outer housing of the coupler, but it did not affect the operation of the unit. An inspection of the rudder power control unit found that the transfer valve was unserviceable due to an open electrical circuit.

Aircraft information

The aircraft was acquired from another Australian operator during October 2001 and underwent a pre-delivery inspection prior to commencing passenger operations on 1 November 2001. The operator's contracted maintenance provider conducted the inspection that was intended to ensure compliance with airworthiness directives and service bulletins.

The previous operator had purchased the aircraft from the United Kingdom and it was entered onto the Australian civil register on 3 December 2000. Between December 2000 and January 2001 the aircraft underwent modification and heavy maintenance work at an overseas engineering facility before entering revenue operations in Australia. The modifications included the removal of an airstair from under the forward passenger entry door. During that work the airstair drip pan and the cloth moisture shroud were removed from the E/E bay.

The documentation covering the removal of the airstairs specified that the moisture shroud was to be replaced following the modification work. However the shroud was not installed because the kits were temporarily unavailable from the manufacturer. One of the operator's engineers, authorised by the Civil Aviation Safety Authority to approve a design modification or repair, assessed that the absence of the moisture shroud would not affect the safety of the aircraft. On 29 January 2001, the engineer approved an amendment to the engineering release that permitted the installation of the shroud within 12 months of receipt of the parts.

The aircraft was subsequently operated on Australian domestic passenger services between February and September 2001 without the moisture shrouds being fitted. An order was placed with the manufacturer for the shroud kits and the delivery of the kits was due in November 2001. The moisture shrouds were not fitted to the aircraft when it was delivered in October 2001 to the current operator, who was unaware that the shrouds had not been fitted to the aircraft.

During the investigation, the aircraft manufacturer provided the following advice:

`Boeing advises that these shrouds are required in order to ensure the airworthiness of the airplane. 737 airplanes should not be used for revenue flight with these moisture shrouds not installed.'

`The 737 MMEL/DDPG does not provide any relief for these items to be removed from the airplane. Furthermore, the purpose of these shrouds is to protect the electronic equipment from moisture ingress. There have been several reports of 737 airplanes experiencing uncommanded flight movement due to moisture ingress into certain electronic components in the E/E Bay.'

On 18 April 2002, another Australian registered Boeing 737-33A, VH-CZR, sustained a depressurisation incident during a scheduled passenger service from Auckland to Christchurch, New Zealand. The incident occurred soon after top of descent when the crew selected the pressurisation system to the standby mode. The aircraft had a history of pressurisation controller problems when being operated in the AUTO mode, until approximately three weeks prior to the incident when the controller was replaced. The pressurisation controller fitted to the aircraft at the time of the occurrence failed during post incident testing, as the selection of standby mode resulted in the cabin outflow valve moving to the fully open position.

VH-CZR had a similar service history as CZQ, having been purchased by the same previous operator and undergoing similar airstair removal modifications at the overseas engineering facility prior to entering service in Australia. The aircraft also had not been fitted with moisture shrouds following the removal of the airstairs and the moisture shield over the E1 rack was not installed on the aircraft at the time of the occurrence in New Zealand.


The reason for the cabin depressurisation was likely to have been due to the moisture shrouds not being fitted after the removal of the airstairs. This permitted the ingress of water into the E/E bay and the pressurisation controller, resulting in a malfunction of the operating modes of the unit. The electrical faults found in the yaw damper coupler electrical filter and the rudder power control unit may have led to the rudder pedal movement detected by the flight crew during the descent into Brisbane.

Extensive research has shown that the effect of oxygen deprivation can be insidious and, as such, cabin crew may not be able to correctly judge their oxygen intake. Research conducted by the US Federal Aviation Administration (FAA) Civil Aeromedical Institute indicates that physical activity such as that performed by a cabin crewmember will significantly shorten the time of useful consciousness during an aircraft depressurisation. Based on that research, the FAA's recommended procedures for cabin crew during depressurisation are to immediately don the nearest oxygen mask, sit down, or grasp a fixed object and hold on in order to brace themselves until given clearance to move about the cabin by the flight crew.

The operator's emergency procedures for immediate action for cabin crew following depressurisation reflected that advice. However, in this occurrence, two cabin crew, while returning to the rear crew seats, assisted some passengers before taking oxygen themselves. A further delay to oxygen intake occurred as oxygen masks, that had failed to deploy automatically above the rear crew seat, had to be manually released before use.

Some cabin crew appeared to have judged that the angle of the cabin during the descent was not very steep. The operator's emergency procedure manual referred only to a `very steep' angle of descent and other more severe characteristics of depressurisation. The manual did not discuss the possibility of an emergency descent that may be less than `very steep' or indications that may be less severe than those associated with a rapid or explosive depressurisation. This may have led some cabin crew to believe that in the absence of other characteristics associated with depressurisation, they could safely assist passengers while moving to the rear of the aircraft before using crew oxygen. Remaining where they were, using oxygen until advised by the flight crew that a safe level had been reached, may have been a safer practice than moving through the cabin to reach the rearmost crew seats before taking oxygen.

Significant Factors

  1. The aircraft systems faults reported most likely arose as a result of moisture ingress into a number of electronic components located in the aircraft E/E Bay.
  2. Moisture ingress was possible due to moisture shields not being fitted in the E/E Bay following the modification, by the previous operator, to remove the airstairs from the aircraft.
  3. The moisture shields were deemed by the aircraft manufacturer to be mandatory equipment.
  4. The cabin floor angle during the emergency descent was reported as being similar to a normal descent and this may have led the cabin crew to assess that it was safe to return to their crew seats at the rear of the cabin during the emergency descent.

Safety Action

Local safety action

Following the occurrence, the forward galley and forward toilet drain lines were inspected and no defects were found. The operator subsequently removed the forward galley and resealed the floor in the forward vestibule area under the galley and adjacent to the forward doors before re-installing the galley components. The operator's maintenance provider designed and manufactured an approved aluminium moisture shroud that was fitted in the E/E bay on 11 December 2001. Functional checks on the avionics units were carried out and the aircraft was returned to service on 13 December 2001.

The operator advised that a safety article would be distributed to cabin crew with specific feedback in regards to this occurrence. An operations manual revision would be initiated to include a discussion about varying emergency descent profiles and aircraft effects. The operator also advised that this information would be included in initial and recurrent cabin crew training.

General details
Date: 02 December 2001   Investigation status: Completed  
Time: 1910 hours EST    
Location   (show map): 19 km SE Thangool, (NDB)    
State: Queensland   Occurrence type: Air/pressurisation  
Release date: 11 February 2004   Occurrence category: Serious Incident  
Report status: Final   Highest injury level: None  

Aircraft details

Aircraft details
Aircraft manufacturer The Boeing Company  
Aircraft model 737  
Aircraft registration VH-CZQ  
Type of operation Air Transport High Capacity  
Damage to aircraft Nil  
Departure point Townsville, QLD  
Departure time 1813 hours EST  
Destination Brisbane, QLD  
Crew details
Role Class of licence Hours on type Hours total
Pilot-in-Command ATPL 6000.0 21500
Co-Pilot/1st Officer ATPL 12.0 7600
Last update 13 May 2014