ANZ B733 enroute on Aug 30th 2013, loss of cabin pressure

Last Update: August 25, 2016 / 16:57:19 GMT/Zulu time

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Photo of ANZ Air New Zealand ZK-NGI, Boeing 737-300 — ANZ Air New Zealand ZK-NGI, Boeing 737-300 (Photo credit: Aero Icarus / Flickr / License: CC by-sa)

Incident Facts

Date of incident
Aug 30, 2013

Classification
Incident

Cause
Loss of cabin pressure

Airline
ANZ Air New Zealand

Flight number
NZ-414

Departure
Wellington, New Zealand

Destination
Auckland, New Zealand

Aircraft Registration
ZK-NGI

Aircraft Type
Boeing 737-300

ICAO Type Designator
B733

An ANZ Air New Zealand Boeing 737-300, registration ZK-NGI performing flight NZ-414 from Wellington to Auckland (New Zealand) with 76 passengers and 5 crew, was enroute at FL330 a few minutes prior to top of descent when the crew initiated an emergency descent due to the loss of cabin pressure, the passenger oxygen masks were released. The aircraft levelled off at 7000 feet and continued to Auckland, where the aircraft landed safely.

New Zealands TAIC have dispatched two investigators on site to examine the aircraft.

The airline reported the aircraft suffered a gradual loss of cabin pressure, the passenger oxygen masks were automatically released.

On Aug 25th 2016 the TAIC released their final report concluding the probable causes of the serious incident were:

- The cause of the loss of cabin pressure was not determined.

- An intermittent defect within the air-conditioning and pressurisation system could not be excluded as the cause of the loss of cabin pressure.

- The pilots did not follow exactly the emergency checklist actions, which increased the risk of an action being omitted or a required sequence of actions being altered.

- The actions of some of the cabin crew during the incident showed that their emergency training had not sufficiently stressed the importance of sitting down and fitting a mask without delay, and had not allowed for a range of scenarios or adequately familiarised the crew with the oxygen equipment.

The TAIC reported the aircraft had already commenced descent towards Auckland descending through FL300 when the cabin altitude warning occurred, the cabin pressure indicator showed the cabin pressure was quickly decreasing. The crew confirmed the air conditioning systems were properly configured with no fault indications, then donned their oxygen masks and initiated the emergency descent procedure. The lead flight attendant reported the passenger oxygen masks had deployed and checked he pilots had donned their masks. The captain, pilot monitoring, silently worked the related checklists but omitted the step to deploy the passenger oxygen masks as the announciator had already been illuminated.

The TAIC wrote: "The cabin crew were preparing the cabin for landing when the masks deployed. FA1 was at about mid-cabin, moving forward; one attendant (FA3) was in the rear galley; and the third (FA2) was pushing a cart to the rear galley (see Figure 2). FA1 continued walking to her normal crew station at the front left entry door and fitted a mask. She said the green flow indicator was visible but the attached reservoir bag was not inflating. She was later uncertain whether oxygen had been flowing. FA1 remained standing by her seat, observing the cabin, until the pilots announced that masks could be removed."

The TAIC reported the cabin turned hot, passengers noticed a chemical smell and the smoke detectors triggered in the aft lavatory first, then forward lavatory and continued throughout the remainder of flight although cabin crew actioned the related checklists.

Following landing, while taxiing to the apron, the "equipment cooling OFF" illuminated, the crew switched to the alternate cooling source, the indication persisted however.

The flight attendants were unable to open the doors because the cabin was still slightly pressurized. The flight crew depressurized the aircraft manually.

The TAIC reported the captain (pilot monitoring) had accumulated more than 10,800 hours of total flight experience, 3,500 hours thereof on type. The first officer, also in the ranks of a captain, pilot flying, had accumulated more than 12,000 hours of flight experience with 5,200 hours on type.

A post flight inspection of the aircraft showed both air conditioning controllers were operating normally. There had been a "inflow/leakage fail" message recorded about one minute prior to the loss of cabin pressure, the TAIC explained: "That status meant there was a low inflow of air or excessive air leakage, or possibly both conditions."

Checks for leaks showed the leakage rate was within permitted limits however close to the maximum.

Both cabin pressure controllers were replaced as precaution.

The outflow valves, that were recorded properly closed in flight, were replaced as a precaution, however, were found acceptable during manufacturer tests, which could not explain the rate of leakage encountered in flight.

The safety pressure relief valves were replaced, checked and were found they did not meet the specifications in both at what pressure they opened at and outflow rate, the manufacturer confirmed that finding but stated this was normal according to the age of the relief valves. There was no evidence either of the valves had opened during the incident flight. The relief valves could not explain the depressurization.

Following the replacements another cabin leak test was performed with better results than the first test. The aircraft underwent a test flight and was returned to service.

The TAIC wrote in their analysis:

According to Boeing’s description of the pressurisation system, the ‘Auto fail’ annunciator on the overhead panel should illuminate if the pressure controller senses a sustained cabin rate of climb greater than 2,000 ft per minute. That light did not illuminate during this incident, and the pressure controller did not record that it had come on. This suggested that the high climb rate observed by the pilots was of very short duration. Both digital pressure controllers tested satisfactorily after the incident, but both were replaced as a precaution.

No individual component of the cabin pressurisation system was identified as having caused the loss of cabin pressure. The outflow valve, selector panel and cabin pressure controller all have internal, built-in test functions that report any faults affecting the control of cabin pressure, whether intermittent or constant18, to the controller, which records these faults to non-volatile memory. The non-volatile memory data for this incident was examined and no pressurisation system failures were recorded. The tear-down examination of those components revealed no faults. Experience has shown that older pressurisation system components can have intermittent faults that do not show during subsequent testing. A combination of components with low or marginal performance could have caused the incident, but no such combination was identified. Boeing submitted that the cabin pressure continuing to decrease when the outflow valve was closed indicated a root cause that was related to insufficient cabin air inflow or excessive fuselage leakage.

The TAIC analysed:

A loss of cabin pressure is one of the few in-flight conditions that require an immediate, memorised response by pilots. The usual training for this event is a simulated rapid decompression while at cruise altitude. Pilots frequently practise this exercise so the memorised actions counter the surprise and workload of an actual emergency, which could otherwise lead to their making errors. However, this incident differed from the practised scenario because the loss of cabin pressure occurred when the aeroplane was already in a normal descent. It was conceivable that the different circumstances interfered with the pilots’ recall and performance of the memorised actions.

Both pilots fitted their oxygen masks immediately after they recognised the loss of cabin pressure. However, their actions varied from the published procedures in the following areas:

- no cabin announcement was made to warn the cabin crew of the emergency descent
- the flight deck ‘passenger oxygen’ switch was not selected on
- the subsequent checklist reviews were not conducted out loud.