Network Australia F100 near Geraldton on Aug 10th 2020, loss of cabin pressure
Last Update: January 22, 2021 / 10:05:17 GMT/Zulu time
Australia's TSB opened a short investigation into the incident and reported: "The engineering inspection revealed an insulation blanket had become lodged in the pressurisation outflow valve, which prevented the aircraft from pressurising."
On Jan 22nd 2021 the ATSB released their final report concluding the probable causes of the incident were:
- 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.
- 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.
The ATSB reported the captain was pilot flying, the first officer pilot monitoring. During preflight briefing the possibility of rain and turbulence enroute had been identified possibly requiring intermittent seat belt signs while enroute.
The ATSB summarized the sequence of events:
Following a normal departure and climb, with a slight diversion to avoid weather, NHC levelled at flight level 260. 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
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 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.
The ATSB stated: "A post-flight maintenance inspection identified that an insulation blanket had become wedged in the secondary (air) outflow valve, affecting pressurisation."
The ATSB reported:
NHC had recently undergone heavy maintenance at Fokker Services Asia (FSA), in Singapore. 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 proofing9 was to be ‘removed as required’ to gain access to the inspection locations.
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.
The ATSB reported:
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.
The ATSB analysed:
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.
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.
Aircraft Registration Data
This article is published under license from Avherald.com. © of text by Avherald.com.
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