Delta B763 near London on Aug 13th 2017, loss of cabin pressure
Last Update: September 13, 2018 / 14:28:54 GMT/Zulu time
The occurrence aircraft was able to depart London after 21 hours on the ground and positioning to Atlanta as flight DL-9965.
On Sep 13th 2018 the UK AAIB released their final bulletin releasing following comment:
Data from the DFDR showed that during the departure from Heathrow the cabin altitude increased until it reached 15,900 ft when the crew commenced their descent. The flight and cabin crew acted appropriately throughout the emergency and the decision to land slightly overweight was made to ensure prompt medical treatment for a passenger with a medical condition.
The fault on the right engine PRVC would have generated the EICAS message r eng prv, which would have required the crew to turn off the right ECS pack. This action would have caused the left ECS pack to automatically switch to the high-flow mode. It can be seen from the DFDR data, presented at Figure 1, that the left pack automatically switched between low and high-flow mode on two occasions, which is consistent with the crew actioning the QRH. The cabin pressure was also maintained at a satisfactory level during the inbound flight when only the left pack was operating. This indicates that the air conditioning system worked correctly and the flow from the left pack should have been sufficient to pressurise the cabin.
For the cabin pressure to decrease as the aircraft climbed there would have had to have been either insufficient airflow into the cabin or excessive leakage from the cabin. The commander reported that the indicator on the CACP showed that the outflow valve was near to the closed position when they commenced their descent to 10,000 ft. Examination of the aircraft at Heathrow and Atlanta, and inspection and testing of the components removed from the aircraft, could not identify a fault that would have restricted the airflow into the cabin or resulted in an excessive leakage. However, since the components were replaced the aircraft has flown more than 2,000 hours and 240 cycles with no further reported faults with the cabin pressurisation.
The two passenger oxygen masks that failed to deploy were located in separate areas of the aircraft and seat aisles. The aircraft manufacturer was unaware of any other occasions when the masks had failed to deploy on a Boeing 767 aircraft. Should some of the passenger oxygen masks not automatically deploy, then the cabin crew can provide the affected passengers with portable oxygen or mechanically release the oxygen masks.
The AAIB reported that on the previous leg from Atlanta to London the inbound crew had received a "R ENG PRV" message on the EICAS shortly after takeoff, which remained on for the remainder of the flight to London. In London maintenace found the right hand pressure regulating valve worn and replaced it.
While climbing out of London for the occurrence flight the crew again received a "R ENG PRV" message on the EICAS shortly after takeoff. While climbing through FL100 the crew worked the related ENG PRV checklist. The AAIB wrote:
As the warning message was still present after the checklist had been
completed, the crew sent a message to their maintenance staff via the Aircraft Communication Addressing and Reporting System (ACARS) seeking advice. The maintenance staff suggested that the crew should repeat the QRH checklist actions once they reached their cruise altitude, when the required engine power would be lower.
The crew reported that, at the planned cruise altitude of FL320, the cabin pressure was slightly higher than normal but was steady at about 7,000 to 7,500 ft. As they started to action the QRH the audio warning sounded, the master warning light illuminated, the cabin altitude message appeared on the EICAS and the auto inop warning light illuminated on the cabin altitude control panel (CACP). The commander also noticed that the cabin pressure was rapidly rising and recalled it reaching 15,000 ft. He attempted to close the outflow valve manually but noted that the position indicator showed that it was nearly at the closed position.
The AAIB further wrote with respect to the engineering activities:
The operator returned the components that had been removed from the aircraft to an overhaul organisation for examination and testing. The right engine PRVC failed the preliminary inspection (bench check) due to a blown internal diaphragm; this fault would explain the R ENG PRV message on the EICAS. Inspection of the right engine PRV revealed that the position switches, bushings and internal link were all worn. Inspection of the other components did not establish a reason why the cabin failed to maintain pressure when the mode select was in the auto position.
This article is published under license from Avherald.com. © of text by Avherald.com.
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