United B764 near Zurich on Aug 23rd 2017, cabin did not pressurize
Last Update: July 2, 2018 / 21:19:58 GMT/Zulu time
The flight was cancelled.
The occurrence aircraft returned to service on Aug 25th 2017.
On Oct 9th 2017 Switzerland's SUST reported that the cabin pressure did not build as expected prompting the flight crew to initiate an emergency descent. The aircraft received minor damage. The occurrence was rated a serious incident and is being investigated by the SUST.
On Jul 2nd 2018 the SUST released their final summary report releasing following findings:
The aircraft was examined after landing and the following was ascertained:
- The equipment cooling overboard exhaust valve was fully open.
- The electrical connector on the equipment cooling overboard exhaust valve was unplugged.
- The air duct connected to the equipment cooling overboard exhaust valve was severely damaged (see Illustration 1).
- No fragments of the damaged air duct were found in the aircraft.
- The rotary switch on the equipment cooling panel in the overhead panel was in the STBY position.
Further evaluations revealed that, during pre-flight preparation, the flight crew had noticed the status message FWD GND EXH VAL on the EICAS. Two technicians from SR Technics, who possessed the necessary licenses required for maintenance work on a B767, subsequently carried out an error analysis and concluded that AVS/IFE inboard/outboard exhaust valve 1 was defective. This valve is part of the alternative ventilation system (AVS) and the cooling system for the inflight entertainment system (IFE) on the ground respectively. In consultation with United Airlines’ maintenance control centre (MCC) it was decided that the aircraft could be given clearance in accordance with the minimum equipment list (MEL), which stipulates the deactivation of this valve.
In the office, one of the two technicians subsequently studied the procedure for deactivating the valve described in the MEL. In addition to pulling the appropriate circuit breaker, the electrical connector to the valve should be pulled and secured in accordance with operating procedures, and the valve should be fully opened manually. Additionally, a sticker should be placed on the equipment cooling panel in the cockpit to make the flight crew aware that the valve has been deactivated.
The technician subsequently returned to the aircraft without taking a written form of the operating procedures described in the MEL with him. Following that, he deactivated the equipment overboard exhaust valve instead of AVS/IFE inboard/outboard exhaust valve 1. He did not apply the appropriate warning sticker in the cockpit. The flight crew did this themselves before departure.
During this time, the second technician remained in the office and finished administrative duties.
During the investigation carried out by the STSB, it was established that the term AVS/IFE inboard/outboard exhaust valve was not used consistently in the aircraft maintenance manual (AMM) and in United Airlines’ MEL: in some places the term AVS/IFE inboard/overboard exhaust valve was used.
The SUST analyzed and concluded:
The equipment cooling overboard exhaust valve closes automatically when the rotary switch on the equipment cooling panel is in the AUTO position and both engines are running whilst on the ground or when the aircraft is in the air. Because the electrical connector for the valve had been mistakenly unplugged by the technician, the valve remained fully open, and remained so even after the flight crew had set the switch to the STBY position during the flight.
The air duct to the equipment cooling overboard exhaust valve is not designed to withstand larger pressure differences as the valve is designed to normally be closed during flight. In the present case with an open valve, the air in the air duct could escape into the atmosphere unhindered. The crew stated that the pressure in the cabin stabilised upon switching the selector switch from AUTO to STBY and that the cabin altitude began to slightly decrease again. This can probably be attributed to the fact that in the new configuration the cabin air was no longer being actively conveyed across the aircraft by a fan.
During the continued climb, the pressurisation system was no longer able to compensate for the loss of pressure occurring due to the open equipment cooling overboard exhaust valve, and as a result the cabin altitude began to rise again. At a flying altitude of approximately 19,500 ft, the ‘cabin altitude’ warning light came on, which meant that the cabin altitude had now increased above 10,000 ft. At this time, the crew noticed that the problem with the pressure in the cabin persisted and consequently initiated an emergency descent.
It seems obvious that the air duct was not able to withstand the increased strain resulting from the occurring pressure difference and therefore burst.
The flight crew’s decision to continue the climb along the intended flight path to a flying altitude of over 10,000 ft AMSL, even though pressurisation of the cabin was not ensured according to the processed checklist, was not in accordance with the operating procedures and was risky.
The mix-up by the technician, who deactivated the wrong valve in the open position, was the cause of the serious incident. The approach of working from memory, without taking a written form of the operating procedures described in the MEL with him, was identified as a contributory factor during the investigation.
This article is published under license from Avherald.com. © of text by Avherald.com.
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