On 18 March 2020, a Fokker 100 (P2-ANF) being operated by Air Niugini was on a scheduled domestic passenger flight from Boram to Port Moresby in day VMC when bleed air malfunctions were annunciated during the climb. Approaching FL300 enroute to an intended cruise altitude of FL350, a rapid rise in cabin altitude occurred and both bleed air systems failed prompting both pilots to don oxygen masks. As an Emergency Descent was commenced, the rapid rise in cabin altitude resulted in an Excessive Cabin Altitude Warning. A PAN was declared and taking account of terrain, a diversion to Madang was completed without further event.
The annotated ground track of the flight (Port Moresby is indicated as Jackson International). [Reproduced from the Official Report]
The event was reported to the Papua New Guinea Accident Investigation Commission (AIC) the following day and an Investigation was immediately commenced. Recorded data from the FDR and CVR were successfully downloaded but relevant CVR data had been overwritten. Recorded ATC communications data were also available.
Both pilots were Papua New Guinea nationals. It was noted that the 39 year-old Captain, who had been PF for the flight, had a total of 11,936 hours flying experience including 4,976 hours on type and 9,404 hours in command (all types). He had been employed by Air Nuigini as a pilot for 13 years. The 36 year-old First Officer had a total of 4,993 hours flying experience of which 1,876 hours were on type and had been employed by Air Nuigini as a pilot for 9 years. Three cabin crew were on board and the 26 year-old SCCM had “over three years cabin crew experience”.
The Captain stated during interview that after departing from Boram, the crew had responded to a left engine bleed fault annunciation by resetting the system in accordance with the applicable QRH Checklist. As the fault indication did not reappear, it was decided to continue the flight as planned climbing to FL 350 on an almost direct track to Port Moresby.
Twelve minutes after takeoff whilst passing FL 230, Moresby Centre was asked for permission to deviate up to 15 nm either side of track due to convective weather ahead and this was approved. Approaching FL 290, both engine anti-icing were momentarily activated as the aircraft encountered cloud but on clearing cloud a few seconds later they were deactivated. As the aircraft continued climbing, a left engine bleed fault light was again annunciated followed after about three seconds by a right engine bleed fault indication. The MFDU displayed the ‘Double Bleed Fault’ Checklist, the Captain levelled off at FL300 and both pilots donned their oxygen masks as required.
Half a minute later, in response to an observed rapidly rising (2000 fpm) cabin altitude, the Captain initiated a rapid descent and altered course towards the coast to avoid high ground in the Mount Wilhelm area. The First Officer stated that as the descent was continued, the Captain switched off both engine bleed systems in order to reset them in accordance with the Double Bleed Fault Checklist but before he had switched them back on, an ‘Excessive Cabin Altitude’ warning was annunciated at a recorded FL 293 and the corresponding Checklist was displayed on the MFDU. At this point, the crew recalled noting that the cabin altitude had exceeded 14,000 feet. The crew stated that they had switched on the fasten seatbelt/no smoking signs and whilst the First Officer made an ‘Emergency Descent’ PA and manually deployed the passenger cabin oxygen masks, the Captain had commenced an emergency descent by setting the thrust levers to flight idle and (not explained) retracting the speed brakes.
As the aircraft passed FL 240, the Captain declared a PAN to Moresby Centre and advised of the emergency descent and added that they were 63 nm from waypoint MUDIX (located at Goroka) and was instructed to advise when operations returned to normal. Moresby Centre then contacted Madang FIS of the flight’s PAN status so that they could notify any potentially affected traffic in their area but were advised that they were unaware of any.
Seven minutes after the descent from FL300 had begun, the aircraft was levelled at 10,000 feet and the ‘Excessive Cabin Altitude’ warning ceased. The crew removed their oxygen masks and after initially deciding to divert to Nadzab, they re-assessed the weather conditions in that area and decided instead to divert to Madang. The Madang TWR was then contacted and appraised of the flight’s intentions. The crew then reported having reviewed the checklists which had been displayed on the MFDU during the emergency phase of the flight and completed the remaining items on the ‘Emergency Descent’ and ‘Double Bleed Fault’ procedures. During the latter, they were able to successfully reselect both engine bleed systems.
The Madang overhead was reached 5½ minutes later and after two holding patterns during which the cabin crew were instructed to carry out their follow up duties and the pilots reviewed the QRH ‘Manual Cabin Pressurisation Control’ procedures, a descent and positioning for runway 07 was commenced. Six uneventful minutes later, the aircraft landed at Madang for a normal disembarkation.
Why It Happened
The Operator’s maintenance records following the emergency diversion indicated that during the troubleshooting process, both Shut Off Temperature Modulating Valves (SOTMV) and both Pressure Regulating Valves (PRSOV) were replaced in accordance with the AMM after testing showed that they were not reliably sustaining their required performance.
These component changes led to the cessation of the intermittent bleed air system faults which had previously been occurring at irregular intervals. However, it was concluded that “well before” the investigated event, there had been recurrent defects in the aircraft’s bleed air system but no meaningful attempt at fault identification or use of troubleshooting procedures to resolve this. Overall it was concluded that no effective process to identify and address the underlying causes of these faults had been followed.
During the Investigation, it was also found that eight passenger oxygen masks which had not activated properly were subsequently not reported as defective by the cabin crew and the resulting absence of corresponding maintenance records meant that it was not possible to determine the cause of the defective oxygen units.
The Causes of / Contributing Factors to the occurrence were identified as follows:
- The loss of cabin pressure in flight was due to simultaneous faults with the PRSOVs and SOTMVs in the engine bleed air system.
- The aircraft had been having recurrent defects in its bleed systems but no proper fault isolation and troubleshooting was applied to identify and address the underlying causes of the faults prior to the occurrence.
- Due to the intermittent characteristics of its faults, the bleed air system tended to operate in a normal manner when subjected to both function tests by maintenance and to resets done by flight crews whenever such faults appeared, which made more in-depth maintenance assessment and action difficult.
Two Safety Recommendations were made as a result of the Investigation as follows:
- that Air Niugini should ensure the use of its cabin defect report forms is reinforced for cabin crew to ensure that all cabin defects are recorded and reported in a timely manner to enable and facilitate maintenance actions.
- that Air Niugini should ensure that in their Fokker 100 series aircraft which are not fitted with tape recorders or if the automatic announcement is inoperative or fails to work, the appropriate announcement will be made in accordance with the Operators Safety and Emergency Procedures Manual, when applicable.
The Final Report was approved for release on 22 November 2021 and subsequently published.