Created Monday, Nov 22nd 2021 13:37Z, last updated Monday, Nov 22nd 2021 13:37Z
An Air Niugini Fokker 100, registration P2-ANF performing flight PX-121 from Wewak to Port Moresby (Papua New Guinea) with 43 passengers and 5 crew, was climbing through about FL300 to cleared FL350 about 70nm west of Madang (Papua New Guinea (PNG)) when the initiated an emergency descent due to the loss of cabin pressure, the passenger oxygen masks were released. The aircraft diverted to Madang for a safe landing.

PNG's Accident Investigation Commission (AIC) rated the occurrence a serious incident and opened an investigation.

On Apr 17th 2020 Papua New Guinea's Accident Investigation Commission (PNG's AIC) released their preliminary report reporting the aircraft was climbing shortly after takeoff when the #1 engine's (RR Tay 650, left hand) bleed system indicated a fault. The crew switched the system off and on, which cleared the fault and the system began to work again normally again. The aircraft was climbing through FL290 out of Wewak, already deviating around weather, when the aircraft entered cloud, the captain (ATPL, 11,936 hours total, 4,976 hours on type), pilot flying, activated the engines' (Tay 650) anti-ice systems. About 3-4 seconds later the #1 engine bleed fault indication illuminated, another 3 seconds later the #2 engine bleed fault indication illuminated. The crew worked the checklists "Double Bleed Fault", both pilots donned their oxygen masks and switched both bleed air systems off. About 5 seconds after both bleed air systems were switched off the master caution activated, the captain levelled the aircraft off just above FL300, a PAN was transmitted and the crew initiated a rapid descent. When the aircraft descended through FL290, the excessive cabin altitude warning activated, the crew continued the rapid descent and worked the checklists "Excessive Cabin Altitude" and "Emergency Descent". The aircraft levelled at 10,000 feet about 35mn south west of Madang. The aircraft entered a hold a Madang and landed on Madang's runway 25 about 32 minutes after the bleed air fault indications activated and about 18 minutes after the aircraft had levelled at 10,000 feet. There were no injuries.

The first officer (CPL, 4,993 hours total, 1,876 hours on type) was pilot monitoring.

On Nov 22nd 2021 the PNGAIC released their final report concluding the probable causes of the serious incident were:

The loss of cabin pressure inflight was due to the simultaneous faults with the Pressure Regulating and Shut-Off Valve (PRSOV, SOPRV) and Shut-Off and Temperature-Modulating Valve (SOTMV) within the engine bleed systems.

The aircraft had recurrent defects on the bleed systems. The investigation found that no proper fault isolation and troubleshooting were applied to identify and address the underlying causes of the faults prior to the occurrence.

Moreover, due to the intermittent characteristics of the faults, the system tended to operate in a normal manner when subject to tests and resets done either by maintenance or flight crews whenever those faults appeared, which difficulted a more in-depth maintenance assessment and action.

The PNGAIC analysed that the aircraft was not equipped with an automatic tape player to sound an automatic decompression warning to the cabin, the flight crew also did not make such an announcement, cabin crew therefore believed the fasten seat belt signs and no smoking signs indicated turbulence ahead. Some time later however the cabin crew heard an announcement "Emergency Descent" and conducted their according tasks.

The PNGAIC analysed aircraft maintenance:

The investigation found that there were irregularly intermittent faults in the bleed air systems of the aircraft, observed in a number of opportunities well before the day of the occurrence and had been identified by the Operator as recurrent defects. Particularly, these faults reoccurred earlier on the 18 March 2020, and during the operation in which this serious incident took place.

In the context of the occurrence, as the SOTMVs were not functioning according to the engine RPM and the SOPRVs were regulating above the upper limit, bleed air may have been supplied in excess and at high temperature, causing the overheat switch to close the SOPRVs and SOTMVs and with this, discontinuing the supply of bleed air to the aircraft, leading to the increase in cabin altitude that originated the cabin depressurisation event.

The investigation identified that as the faults were occurring at irregular intervals, and flight crews and maintenance were conducting tests and resets of the system when they occurred, which temporarily solved the issues and allowed the continuation of flights or the release of the aircraft back to service, no additional fault identification or troubleshooting procedures, nor effective follow-up, were conducted by the Operator to identify and address the underlying causes of these faults.

Moreover, it was only during the post-occurrence maintenance actions that a number of components were found defective in both engine bleed systems and, in particular, the SOTMVs and SOPRVs, which were then replaced.

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