Description

On 20 February 2023, a Fokker 70 (P2-ANT) operated by Air Niugini on a scheduled domestic passenger flight from Port Moresby to Mount Hagen and unable to make an approach on reaching destination was returning to Port Moresby. Only when performing a routine descent check at 10,000 feet was it realised that the landing altitude had not been reset from that for Mount Hagen. An attempt to rectify the problem quickly resulted in mismanagement of the pressurisation control system and serious injury to four occupants and minor injuries to eighteen others.

Investigation      

An investigation into the accident was carried out by the Papua New Guinea Accident Investigation Commission (AIC). Relevant data from the solid-state flight data recorder (SSFDR) was successfully downloaded but data from the solid-state cockpit voice recorder (SSCVR) had been overwritten because it had not been electrically isolated after flight.

The 33-year-old captain, who was acting as pilot monitoring (PM) for the flight, had a total of 8,261 hours flying experience, including 3,098 hours on type. The 38-year-old first officer had a total of 6,454 hours flying experience, including 4,668 hours on type.

The flight had departed Port Moresby almost two hours late for Mount Hagen. On reaching Mount Hagen overhead at 8,000 feet, the crew then found that with work in progress on part of the runway 12 threshold, that runway could not be used because of the “unavailability of PAPI guidance." Conditions for landing on Runway 30 were considered “not suitable due to a reported tailwind” which, when it continued to show no signs that it would become more favourable, prompted a decision to return to Port Moresby.

Descent from FL 310 was commenced with just over 100 nm to run. The crew stated that approaching 10,000 feet (the State Transition Altitude is 20,000 feet) they had run the Approach Check. That's when they found that the Landing Altitude Setting (LAS), which was procedurally required to be set before takeoff from the departure airport, had not been set to sea level as required for the Port Moresby and was still at the 5,500 feet set for Mount Hagen. With the aircraft descending at “more than 2,000 fpm” at the time, it was then recognised that there would not be enough time for the pressurisation system to automatically pressurise the cabin to sea level. The options were assessed as either stopping the descent and continuing on track or deviating from track and maintaining 10,000 feet to allow time for automatic pressurisation to sea level before continuing with the approach.

However, given that either option would delay their arrival, the crew decided to continue the descent and increase cabin pressurisation by manually controlling the rate of change using the quick reference handbook (QRH) procedure for failure of the automatic system. This allowed the direct control of pressurization rate by movement of either a rotary switch or an increase/decrease lever - see below the annotated illustration of the Cabin Pressure Control Panel reproduced in the Investigation Report from the Fokker manual.

The crew stated they had reduced the aircraft descent rate to less than 2,000 fpm and, after selecting the cabin altitude control lever to the down position, they manually increased the cabin rate of descent to between 800 and 1000 fpm. The captain “recalled that throughout this time, they observed no abnormalities with the cabin pressurisation system."

The annotated illustration of the cabin pressure control system as installed on the flight deck overhead system panel (the upper right panel explains use of the manual control lever). [Reproduced from the Official Report]

As the aircraft descended through 8,700 feet, a re-clearance to 3,500 feet was given and acknowledged with further descent clearance to 2,500 feet to establish on the runway 14 instrument landing system (ILS approach) quickly following. FDR data showed that the aircraft was established on ILS final approach for the 14L ILS localizer as it descended through 5,800 feet when only about 10 nm from the runway. Four minutes later, passing 2,500 feet, the landing gear was extended and the crew recalled at this point, “the cabin altitude was at sea level, the cabin rate of change was at zero and the cabin pressure differential was reducing as the aircraft descended."

Based on the procedure being followed and their observations, the crew then set the cabin altitude control lever to the mid position, but as 1,000 feet was approaching, they reported noticing that the cabin differential pressure reading was 3.5 psi and increasing. Since this figure was significant greater than the maximum allowable differential for landing (0.13 psi), they decided to go around and take a hold. The cabin crew subsequently stated that as the go-around commenced, “they started experiencing intense pain in their ears” and saw that the passengers were showing signs of similar discomfort and pain. This was accompanied by a sensation of excessive pressure. Some passengers towards the rear of the cabin began bleeding through their noses and ears, and one passenger vomited blood.

On reaching 2,500 feet, Air traffic Control (ATC) were advised that the go-around had been conducted because of “technical issue” and that the crew wanted to take up a visual hold at that altitude. This was approved, but as the flight was about to enter holding, the senior cabin crew member (SCCM) reported conditions in the cabin to the flight crew. This was acknowledged, and a PA announcement was made to advise that “they would land as soon as the issue was rectified."

The annotated flight track on arrival back in the Port Moresby area. [Reproduced from the Official Report]

An unsuccessful attempt was made to contact the operator’s Maintenance Watch to get technical support to address the increasing cabin differential. With the differential pressure continuing to increase to 6 psi, it was decided to run the QRH Abnormal Procedure for 'Reduced Cabin Pressure Differential’ and this resulted in the differential beginning to decrease. Within ten minutes, ATC were informed that operations were now normal and vectors back onto the ILS approach were requested and provided with a landing on runway 14 then made without further event.

Once parked, a normal disembarkation was carried out with the cabin crew advising the affected passengers to seek assistance from the airline’s Customer Services personnel. When all passengers had disembarked, the cabin crew advised the flight crew that some of the passengers had sustained injuries. However, contrary to applicable procedures after an incident, both pilots reportedly left the aircraft and went to another aircraft which was to be used for their next flight.

The captain was subsequently found to have made an entry in the aircraft Technical Log which read “AUTO PRESS CTRL FAULT." He did not report the actual nature of the pressurisation event to the maintenance team or brief the captain who was taking over the aircraft. The defect entry was cleared with “CPC CHECK NIL FAULT FOUND. BITECHK C/OUT SATIS. REFER AMM 21-31-00-811” and released to service the same day. The captain stated later that “he had incorrectly described the nature of the pressurisation event due to being in a state of confusion."

The injured passengers were attended to by Ground Operations personnel, and 20 of the total of 67 passengers who required further assessment or treatment were taken to the local hospital. One of the two cabin crew declared themselves unwell and was relieved of further duties whilst the other one continued to operate another flight on the same aircraft. Three of the four serious injuries were to two adults diagnosed with “Decompression Injury, Hypertensive Urgency and Hypokalemia” and hospitalised overnight and to a third adult who was diagnosed as having “Otitic Barotrauma” in both ears. The fourth serious injury was to a child diagnosed with "Epistaxis/ Otitic Barotrauma."

The principal Findings of the Investigation into the flight crew’s and specifically the PM captain’s performance in a wider context were identified as related to ‘Flight Operations’, ‘Aircraft Systems’, ‘Human Factors’ and ‘Organisational Aspects’. It was concluded that the captain’s actions and statements indicated that: 

• his knowledge and understanding of the aircraft systems was inadequate and 
• he had lost situational awareness when managing the arrival at Port Moresby and had:
  • decided to follow the Abnormal Procedure for Manual Cabin Pressurisation Control when the automatic mode was functioning normally without any fault.
  • did not fully follow the Manual Cabin Pressurisation procedure and therefore the cabin differential began to increase again to an extent where serious injury was caused to some passengers.
  • decided to follow the Abnormal Procedure for Reduced Cabin Differential Pressure, which is intended to be used in the event of a reduced cabin pressure differential occurring in flight.

Three overall Contributory Factors were identified as follows:

• Organisational factors including multiple changes to flight crew roster and last-minute notification to crew resulted in task saturation and stress and prevented crew situational awareness and good crew resource management and decision making on the day of the event. This resulted in the failure of the crew to set Port Moresby landing altitude when departing the Mt. Hagen overhead.
• Operational and environmental conditions impacted the way crew conducted their operation in Mt. Hagen and in Port Moresby.
• The flight crew did not complete the final step of the Fokker 70 Abnormal Procedures Manual ‘Cabin Pressurisation Control’ instructions, which was to set the manual control lever to the ‘UP’ position before landing to depressurise the cabin and prevent further pressurisation. This oversight meant that the cabin differential began to increase again on final approach, which led to a go-around during which some passengers and cabin crew sustained injuries because of the rapid change in cabin pressure. The flight crew then actioned the Fokker 70 Abnormal Procedures for Reduced Cabin Differential.

Three Safety Recommendations were made as a result of the findings of the Investigation as follows:

• that Air Niugini Limited ensure that in accordance with the operator’s Airport Services Manual, Version 17, subsection 6.18.7.2, the Pilot in Command of an accident flight, either through turbulence or some other cause where an injury occurs to the passengers, should advise the relevant persons of the passengers' names, type of injury and the extent to which medical attention might be required. [24-R07/23-2001]
• that Air Niugini Limited mitigates the risk of flight crew incorrectly or not fully executing procedures in flight by reviewing all flight crew training and competency records to ensure crew are adequately trained and competent in the use of Abnormal and Normal procedures in-flight and understand the systems, performance and operation of the aircraft type endorsed (on their licences). [24-R08/23-2001]
• that Fokker Services (or the OEM) should review the Abnormal Procedure in the Aircraft Flight Manual and other relevant Manuals to ensure that the ‘Manual Cabin Pressurisation Procedure' is reviewed to clarify the final step of the procedure ("BEFORE LANDING: MANUAL CONTROL LEVER UP”) so that the procedure is completed prior to the crew entering the critical phase of final approach to land where further manipulation of the cabin pressurization controller (CPC) may potentially be a distraction. [24-R09/23-2001]

The Final Report was approved on 11 September 2024 and subsequently published online.

Related Articles

• Aircraft Pressurisation System
• Cabin Altitude
• Situational Awareness
• Serious Injury
• Standard Operating Procedures (SOPs)
• Abnormal and Emergency Operations
• Crew Resource Management