Depressurisation involving a Fokker 100, VH-NHC, 167 km south-south-east of Geraldton Airport, Western Australia, on 10 August 2020

On 10 August 2020, a Fokker 28 Mk 0100 (Fokker 100), registered VH‑NHC (NHC) was being operated by Network Aviation on behalf QantasLink as scheduled flight QF1618 from Perth to Geraldton, Western Australia. On board were five crew and 57 passengers. For this flight, the captain was the pilot flying (PF) and the first officer was the pilot monitoring (PM).^[1]

The pre-flight crew briefing identified the possibility of encountering rain showers and turbulence en route, requiring intermittent illumination of the seat belt signs. The flight crew also discussed the option to conduct an area navigation (RNAV) approach into Geraldton. The flight departed at 0717 Western Standard Time,^[2] which was about 5 minutes behind schedule due to a period of heavy rain delaying the boarding of passengers.

Following a normal departure and climb, with a slight diversion to avoid weather, NHC levelled at flight level 260.^[3] At 0738, after about 8 minutes in the cruise, the flight crew received a master warning alert consisting of a red light and a triple-chime. At the same time, an ‘excessive cabin altitude’ warning and associated emergency procedure displayed on the multi-function display unit (MFDU). The excessive cabin altitude emergency procedure for the flight crew required them to:

• fit oxygen masks
• establish flight crew communication
• descend.

The flight crew donned their oxygen masks, established effective communication with each other and then, at 0739, commenced the procedure for an emergency descent.

The cabin supervisor observed the seat belt sign illuminate and, believing it may be indicating possible turbulence, instructed the other cabin crew to secure the galley. At about this time, one of the cabin crew reported hearing an unusual sound from the flight deck. The cabin supervisor identified the sound as consistent with the flight crew using oxygen masks and directed the cabin crew to prepare for possible decompression procedures. One cabin crew member went to the rear of the aircraft and one remained at the forward station, with the cabin supervisor. The cabin crew then secured themselves in their seats. Shortly after, the flight crew conducted a cabin announcement (PA) ‘attention cabin crew, descent, descent, descent’.

After about 30-60 seconds, when the cabin oxygen masks did not deploy as was expected, the cabin supervisor contacted the flight crew to inform them of the situation. The flight crew reported looking at the cabin altitude indication, and noted it was increasing but had not yet reached the altitude where the masks would automatically deploy. In order to minimise communication with the cabin, during a period of high workload, they elected to manually deploy the oxygen masks.

Once the masks had deployed, the cabin crew commenced their aircraft decompression procedure whereby they direct the passengers to ‘fit oxygen and tighten seat belts’. The forward cabin crew member reported that, when they pulled on the lanyard to initiate the oxygen flow, one of the masks separated and fell to the floor. They fitted the remaining two masks however, they believed there was no oxygen flow. During this time, the flight crew conducted a PAN PAN call^[4] and continued with their emergency descent procedure.

At 0743 the excessive cabin altitude warning extinguished and the flight crew levelled NHC out at an altitude of about 9,000 ft. The PM made a PA to the cabin that oxygen was no longer required. Due to their proximity and desire to avoid a return flight at low level through showers and possible turbulence, the flight crew elected to continue to Geraldton. A short time later, the PF made a PA and advised the cabin the reason for the emergency descent and that they would be continuing to Geraldton. The cabin crew checked the passengers for any injuries, or the need for further oxygen, and then prepared the cabin for landing.

The aircraft landed at Geraldton at 0804, followed by a normal taxi and shutdown. Prior to disembarking the aircraft, the PF stood at the front of the cabin, further detailed the event and offered that passengers could approach any flight, cabin or ground crew if they had any questions or concerns. There were no injuries to crew or passengers.

Post-flight maintenance

A post-flight maintenance inspection identified that an insulation blanket had become wedged in the secondary (air) outflow valve, affecting pressurisation (Figure 1).

Figure 1: Insulation blanket caught in the outflow valve

Source: Network Aviation, annotated by ATSB

1. Pilot flying (PF) and pilot monitoring (PM) are procedurally assigned roles with specifically assigned duties at specific stages of a flight. The PF does most of the flying, except in defined circumstances; such as planning for descent, approach and landing. The PM carries out support duties and monitors the PF’s actions and aircraft flight path.
2. Western Standard Time (WST): Coordinated Universal Time (UTC) + 8 hours.
3. Flight level: at altitudes above 10,000 ft in Australia, an aircraft’s height above mean sea level is referred to as a flight level (FL). FL 260 equates to 26,000 ft.
4. PAN PAN: an internationally recognised radio call announcing an urgency condition which concerns the safety of an aircraft or its occupants but where the flight crew does not require immediate assistance.

Pressurisation system

The cabin altitude (pressure) indications are located in the flight deck, in an overhead panel (Figure 2). Cabin altitude is regulated by the primary and secondary outflow valves, which are located at the pressure bulkhead at the rear of the forward cargo compartment. The secondary outflow valve is located alongside the primary outflow valve. The valves operated together to control cabin altitude, in either automatic or manual mode. For this flight, the cabin altitude would have been at about 4,000 ft, for the corresponding cruise altitude of FL 260. An excessive cabin altitude warning activates at 10,000 ft.

Figure 2: Cabin altitude indicator

Source: Network Aviation, annotated by ATSB

The ingested insulation blanket prevented the modulation of one of the outflow valves, affecting cabin pressure control. This resulted in a gradual cabin altitude increase, until the flight crew were alerted, once the cabin altitude had exceeded 10,000 ft.

A review of the flight data for this flight identified the time and aircraft altitude when the excessive cabin altitude warning activated and deactivated however, the recorded flight data did not include any cabin altitude parameters. Therefore, the maximum cabin altitude (pressure) could not be determined.

Oxygen system

The oxygen system in the Fokker 28 Mk 0100 (Fokker 100) consists of three sub‑systems. The flight crew oxygen system was available when required and was a standalone system of storage and distribution, which supplied oxygen for up to three crew. The portable oxygen system, which consisted of a cylinder and mask, was located near the forward cabin crew station. It was typically administered by cabin crew and was available to anyone on board. The primary oxygen system was a ‘fixed drop-down’ system and provided oxygen to the passengers and cabin crew.

There were drop-down systems located above every seat group, in the toilets and above the cabin crew seats. Each location consisted of a chemical oxygen generator, supplying the drop-down masks. A spare mask was available at each location. That is, three masks were available above each dual-seat group, each cabin crew station and in the toilets. Four masks were available for each 3-bay seat group. The forward cabin crew system also had a ‘pull’ lanyard extension, to enable the seated cabin crew to reach the masks, as the unit was not directly overhead (Figure 3).

The chemical oxygen generator consisted of a sodium chlorate core, along with some other trace elements. Application of heat, via the firing mechanism, initiated a chemical reaction that resulted in the production of oxygen. Once the chemical reaction has been commenced, it could not be stopped and oxygen supply generally lasted 10-20 minutes. This allowed sufficient time for the aircraft to descend to an altitude where oxygen was no longer required, typically at, or below, 10,000 ft.

In the Fokker 100, the drop-down masks deploy automatically in the event of the cabin altitude exceeding 14,000 ±500 ft. The flight crew can also manually deploy the drop-down masks at any point. Network’s procedures for flight above FL 250 provided flight crew discretion of cabin oxygen deployment between 10-14,000 ft cabin altitude.

Whether automatically or manually deployed, the drop-down overhead panel will open, and the masks will partly drop, being held up by lanyards. When any mask is pulled toward the individual, the lanyard pulls a safety pin at the firing mechanism, initiating the oxygen generation. The bag may or may not inflate, depending on the individual’s breathing rate. However, an in‑line green indicator, located in the tube of each mask, confirms the flow of oxygen.

Figure 3: Cabin oxygen system

Source: Network Aviation, annotated by ATSB

When asked how they assessed that the forward cabin crew oxygen system was not working, the cabin supervisor advised:

• they could not feel any airflow
• the mask did not inflate
• they could not hear any airflow.^[5]

Post-occurrence maintenance included replacement of oxygen generators where passengers had been located and refitting of the system on unoccupied seats, where the generator had not been activated. Network’s maintenance department advised that the forward crew oxygen generator had been activated, evidenced by the configuration of the firing mechanism and colour-change of the chemical indicator dots (Figure 4).^[6]

Figure 4: Oxygen generator firing pin and activated forward cabin crew generator

Source: Network, annotated by ATSB

With regard to the detached oxygen mask, Network advised it was not able to determine if the mask had separated due to it being pulled, or if it had not been connected at last maintenance. Irrespective of the detached mask, Network also advised that oxygen should have flowed to the remaining masks in that unit. Further, the cabin crew had the option to use any spare passenger masks.

Explosive, or rapid, decompressions (a rate greater than 7,000 ft/min) are very rare. The majority of decompression events are ‘gradual’, stemming from issues such as leaking door seals, incorrect system mode selection by flight crew and mechanical-related problems. In many cases, the oxygen system is not required.

AR-2008-075-1 Staying safe during an aircraft depressurisation (passenger information bulletin) and AR-2008-075-2 Aircraft depressurisation (cabin crew information bulletin) were published by the ATSB to provide comprehensive information regarding depressurisation and oxygen system use.

Insulation blanket information

The insulation blankets installed on the Fokker F100 are made from glass fibre with a foil outer covering. They are located throughout the fuselage and provide thermal insulation^[7] and acoustic damping. Reinforcement strips are fitted on the underside, to prevent sagging of the insulation blanket against the aircraft skin, where moisture may become trapped. The insulation blankets are fixed to the airframe structure via plastic ties (Figure 5 and Figure 6). The aircraft maintenance manual (AMM) tasks 25-53-00-000-814A Remove the insulation blanket assemblies and 25‑53‑00‑400‑814-A Install the blanket assemblies, provided detailed information on removal, inspection and installation of the insulation blankets.

Insulation blankets were available from the manufacturer, with each being assigned a part number, to suit each location. Alternatively, Service Letter 293 (SL 293) permitted local manufacture, detailing manufacture process and installation procedures. SL 293 advised that the insulation blankets were to be installed in accordance with ‘AMM Task 25-28-00-400-814A and/or with double-sided tape’ (of a defined specification). SL 293 did not reference the requirement for the reinforcement strips.

Figure 5: Insulation blanket location and installation

Source: Network Aviation, annotated by ATSB

Figure 6: Typical insulation blanket installation using the plastic ties

Source: Network Aviation, annotated by ATSB

Maintenance procedures

NHC had recently undergone heavy maintenance at Fokker Services Asia (FSA), in Singapore.^[8] The aircraft returned to line on 28 July 2020 and had flown 12 sectors at the time of the occurrence. The heavy maintenance check included internal zonal inspection of the forward cargo compartment (job instruction card (JIC) 062110-00-01) and visual inspection of specific structural locations nearby, and including, the outflow valve area (JIC 533005-00-01). Both JICs advised that linings and sound proofing^[9] was to be ‘removed as required’ to gain access to the inspection locations.

Post-occurrence actions

On 11 August 2020, the flight crew of another Network Fokker 100, reported a slower than expected rate of cabin altitude (pressure) reduction during descent. When the engineers gained access to the outflow valve area, they noted the insulation blanket had migrated and was covering the secondary outflow valve, with parts of the blanket ‘starting to migrate toward the valve opening’. Troubleshooting identified the defect was associated with the outflow valve, which was replaced however, the event also identified another insulation blanket that was incorrectly secured.

On 12 August 2020, following the identification of the incorrectly installed insulation blankets on two aircraft, Network initiated a fleet-wide inspection of insulation blankets located in the area of the primary and secondary outflow valves. The inspection identified that only 1 of their 17 aircraft had the insulation blankets correctly installed, with two aircraft having no insulation blankets fitted. Another aircraft was undergoing heavy maintenance at FSA and already had the insulation blankets removed, so its status could not be determined.

Due to unavailability of the plastic ties and replacement insulation blankets, Network received approval from Fokker Services to temporarily install the blankets under the outflow valves with a defined double-sided tape. These blankets were to be inspected fortnightly, until the replacement blankets and plastic ties were available.

Following notification of the issue from Network, FSA conducted an investigation into how the insulations blankets came to be installed without the required plastic ties. They identified that the JICs for the zonal inspections did not reference the associated AMM tasks for removal, inspection and installation of the insulation blankets.

Flight and cabin crew communications

A sterile flight deck environment incorporates procedures for safety critical phases of flight, such as take‑off and landing, when non-essential activities and communications are not permitted. While it is primarily focused on communication between flight crew members, it also applies to cabin crew contact with the flight deck. In this occurrence, the depressurisation did not occur during a sterile flight deck period.

Network’s emergency procedures manual stated that, communication of safety-related information in an emergency should be ‘clear, concise and direct’.

The flight crew reported that, after the ‘descent’ cabin announcement (PA), they would not expect to hear from the cabin until after the ‘oxygen no longer required’ PA. The cabin supervisor advised that the cabin crew procedure was to instruct the passengers to fit oxygen and tighten seat belts, once the masks had deployed. In addition, cabin crew were trained to contact the flight deck if the oxygen had not deployed after 3 minutes following the descent announcement. The cabin crew reported that, as they did not have any cabin altitude information and the masks had not deployed as anticipated, they elected to contact the flight crew to inform them of the situation.

Previous occurrences

A search of the Civil Aviation Safety Authority (CASA) Service Difficulty Report and ATSB databases, between 1 January 2000 and 1 October 2020, did not identify any occurrences of an insulation blanket being ingested in an outflow valve of a Fokker 100. A search of the ATSB database identified an insulation blanket-associated depressurisation occurrence involving a Boeing 737, in 2018. The cargo flight, with three persons on board, identified the pressurisation issue on climb. The crew donned oxygen and conducted an emergency descent. The aircraft then returned to the departure airport and for a normal landing.

5. Following the occurrence, the rear cabin crew reportedly advised they knew their mask was working as they could ‘hear’ the oxygen flowing.
6. Heat is generated during the oxygen generation chemical reaction, which will change the indicator dots from white to black.
7. Thermal insulation includes preventing the cargo area and components of the aircraft, such as the water system, from freezing during flight.
8. Fokker Services Asia is part of Fokker Services, which is the current Type Certificate Holder for this aircraft type.
9. The terms soundproofing and insulation blankets are interchangeable for this aircraft type.

Introduction

During cruise, the flight crew were alerted to increasing cabin altitude and subsequently initiated an emergency descent, to an altitude of about 9,000 ft, as per their procedures. The crew then elected to continue to their destination, avoiding a low altitude return flight in poor weather, and conducted a normal landing. Post-occurrence inspection identified that an insulation blanket had been ingested into one of the outflow valves, affecting the aircraft’s ability to maintain cabin altitude (pressure).

This analysis will discuss the maintenance procedures and practices that resulted in the insulation blanket not being correctly secured to the aircraft structure and the communication between the cabin and flight crew.

Maintenance procedures

The aircraft had recently undergone heavy maintenance checks, which included zonal structural inspections. The job instruction cards (JICs) detailed the requirements for the structural inspections and advised that the insulation blankets could be ‘removed as necessary’ but there was no reference to the aircraft maintenance manual insulation blanket removal and installation procedures.

From the available information, it was not possible to determine how long the insulation blankets had been unsecured. However, based on the maintenance history, they are unlikely to have been installed correctly when the aircraft returned to service following the most recent heavy maintenance checks.

The majority of the aircraft’s insulation blankets are located behind panels and covers where, movement in the event of not being correctly installed, is generally hindered. In contrast, the insulation blankets located in the same area as the outflow valves, are subject to the varying rates of airflow required to modulate cabin pressure and, if not correctly secured, are free to migrate.

Service Letter 293 did not identify the need for reinforcement strips during manufacture and had offered double-sided tape as an alternative way of securing the insulation blankets to the aircraft structure. While not directly linked with this occurrence, the inconsistency between the service letter and the AMM increased the risk of incorrect installation of insulation blankets. Further, insulation blankets without the reinforcement strips not only allowed them to sag, and come in contact with the aircraft skin, but the increased flexibility meant they were more likely to be ingested into an outflow valve.

In this occurrence, it could not be determined if the insulation blanket was original from the factory or locally manufactured, as per the service letter. Whichever the case, it was apparent that the insulation blanket had not been correctly secured, highlighting the importance of consistency across documentation, to reduce the risk of misinterpretation.

Industry best practice recommends that, when removing a part or component, to not presume it had been correctly installed previously. In all cases, the relevant maintenance documentation should be referred to, ensuring the part or component is being installed to the current specifications.

Flight and cabin crew communication

With the introduction of reinforced cockpit doors, it has been recognised that this has had the effect of introducing an additional psychological barrier between flight crew and cabin crew. There has been a history of misunderstanding and hesitancy by cabin crew of informing flight crew of critical and sometimes life-threatening situations occurring in or external to the cabin of the aircraft.^[10]

In this occurrence, the action of the cabin supervisor to question a possible safety issue was in line with industry expectations and procedures. While they may have queried the lack of oxygen masks sooner than as per their training, it was still in line with safety best practice. Further, there was no requirement by the flight crew to respond if they were in a critical stage of flight.

10. FAA AC 120-48A ‘Communication and Coordination Between Flight Crewmembers and Flight Attendants’, dated 27 January 2020.

From the evidence available, the following findings are made with respect to the depressurisation and emergency descent involving a Fokker 100, registered VH-NHC, 167 km south‑south‑east of Geraldton Airport, Western Australia, on 10 August 2020.

Contributing factors

• While the manufacturer's instructions for the zonal inspections detailed that installation blankets could be removed 'as necessary', they did not reference the insulation blanket installation procedure. This resulted in insulation blankets not being secured to the structure. [Safety issue]
• An insulation blanket became wedged in the secondary outflow valve, affecting aircraft pressurisation and resulting in the requirement for an emergency descent.

Other factors that increased risk

• The instructions for local manufacture and installation of insulation blankets were not consistent with the aircraft maintenance manual procedures, which increased the risk of migration and ingestion into an outflow valve.

Other findings

• The cabin supervisor had no way to determine that the cabin altitude had not yet passed the threshold for oxygen mask deployment. Therefore, their action in choosing to contact the flight deck during the emergency descent, to inform the flight crew that the masks had not deployed, was consistent with safety best practice.

Maintenance Publications

Safety issue number: AO-2020-041-SI-01

Safety issue description: While the manufacturer's instructions for the zonal inspections detailed that installation blankets could be removed 'as necessary', they did not reference the insulation blanket installation procedure. This resulted in insulation blankets not being secured to the structure.

Sources of information

The sources of information during the investigation included the:

• Flight crew and cabin supervisor
• Network Aviation
• Civil Aviation Safety Authority
• Fokker Services
• Fokker Services Asia

References

Australian Transport Safety Bureau 2008, Staying safe during an aircraft depressurisation, Safety publication AR-2008-075(1).

Australian Transport Safety Bureau 2008, Aircraft depressurisation, Safety publication AR‑2008‑075(1).

Submissions

Under section 26 of the Transport Safety Investigation Act 2003, the ATSB may provide a draft report, on a confidential basis, to any person whom the ATSB considers appropriate. That section allows a person receiving a draft report to make submissions to the ATSB about the draft report.

A draft of this report was provided to the flight crew and cabin supervisor, Network Aviation, Civil Aviation Safety Authority, Fokker Services, Fokker Services Asia, Onderzoekbaad (Dutch Safety Board) and the Transport Safety Investigation Bureau of Singapore.

Submissions were received from:

• Network Aviation
• Civil Aviation Safety Authority
• Fokker Services
• Transport Safety Investigation Bureau of Singapore.

The submissions were reviewed and, where considered appropriate, the text of the report was amended accordingly.