Cityjet RJ85 near Dublin and Paris on Jul 1st 2016, loss of cabin pressure
Last Update: April 4, 2018 / 16:33:15 GMT/Zulu time
The French BEA reported on Jul 21st 2016 based on information received from Irish AAIU, that the aircraft suffered a high cabin altitude warning and an emergency descent while already in French Airspace. The AAIU rated the occurrence a serious incident and is investigating.
On Apr 4th 2018 the AAIU released their final report concluding the probable cause of the serious incident was:
Failure of the aircraft to correctly pressurise after take-off due the inadvertent selection of the ram air switch to OPEN, combined with an incorrectly fitted ram air non-return valve.
There are no design features that preclude an incorrect installation of the ram air non-return valve.
The AAIU complained that neither CVR nor FDR had been preserved, however, commented that the data of the flight were provided by the operator's flight data monitoring (FDM) system.
The AAIU reported that climbing through FL100 the crew performed a cabin pressurization check and found cabin altitude and pressure within normal range, however, noticed the airflow coming through the air conditioning vents was "quite weak". Climbing through FL160 a white cabin pressurization advisory message illuminated, the crew worked the "pressurization controller fault" checklist. At FL200 an amber caution advising of a pressurization failure illuminated, the cabin altitude continued to climb. The crew levelled off at FL220 and worked the "pressurization controller failure" checklist requiring to switch the cabin pressurization control to manual, however, the crew did not regain control of the cabin pressure. The cabin altitude was climbing through 9500 feet at that point, the crew donned their oxygen masks and initiated an emergency descent to FL100. At FL100 the crew worked the "Emergency Descent After Pressurization Failure" checklist and was instructed to select the ram air switch to OPEN and discovered the switch was already set to OPEN. Because the cabin pressure differential had already decreased to 0.5psi the crew selected the ram air switch to SHUT and regained control of the cabin pressure and returned the system to automatic control. The crew continued the flight to destination without further incident.
Maintenance tested the pressurization system and found no anomaly.
The AAIU wrote: "Subsequently, the Flight Crew did not recall touching the switch at any stage before the occurrence and could not explain how the ram air switch came to be in the OPEN position. A different crew had operated the aircraft on the previous flight on the day of the occurrence. The Operator informed the Investigation that the crew who operated the previous flight did not experience any problems with the pressurisation system. The Operator also advised that no maintenance was carried out during the turnaround in LFPG and that Ground Servicing personnel would normally not have entered the cockpit at this time."
The AAIU analysed:
An optional ram air system, which provides aircraft ventilation in unpressurised flight, was fitted to all Avro RJ aircraft in the Operator’s fleet. The system can also assist with the clearing of smoke or fumes from within the aircraft. The normal position for the ram air switch is SHUT and it should only be selected to OPEN when instructed by an abnormal/emergency checklist such as the ‘Emergency Descent after Pressurization Failure’ checklist or the ‘Smoke, Fumes or Fire’ checklist.
The flight crew who operated the aircraft on the previous flight did not report any problems with the pressurisation system, indicating that the ram air switch was likely in the correct (SHUT) position while the aircraft was airborne during that particular flight leg. The Flight Crew who operated the aircraft on the occurrence flight did not recall touching the switch at any stage before the occurrence and could not explain how it was in the OPEN position. The aircraft’s ‘Normal Checklist’ does not contain an explicit requirement to check the position of the ram air switch before flight. However, it is contained in the expanded pre-flight checklist. The Investigation notes that the FCOM requires the flight crew to confirm the switch positions of the ‘AS REQD’ items.
The positions of the switch and valve are not recorded on the FDR nor do they form part of the Operator’s FDM data set. Consequently, the Investigation was unable to determine exactly when the ram air switch was selected to OPEN. The unguarded ram air switch is in close proximity to the switches for the air conditioning packs and is of the same shape and size as these switches, and may have been inadvertently disturbed when the post-flight checks were being completed after landing, or when pre-flight checks were being performed before the occurrence flight. It is also possible that the switch was inadvertently disturbed during access to, or egress from the cockpit when the flight crews changed over before the occurrence flight. The Operator advised that no maintenance was carried out during the turnaround in LFPG and that Ground Servicing personnel would normally not enter the cockpit at this time.
The operator's maintenance system automatically issued three work orders to inspect the air recirculation non return valves (NRV) left hand, right hand and aft. However, the aircraft was not equipped with an aft recirculation NRV, the part numbers on that work order actually referenced the ram air NRV. The work orders were carried out by one technician who first inspected the left hand, then right hand recirculation NRV and that aft NRV.
The AAIU analysed:
The same technician also certified the work order that was raised for NRV S/N 22867-7, which the work order stated, was located in the “AFT” position. However, the action taken section of the work order stated:
Aft NRV (cabin and flight deck supply ducts) removed, cleaned, inspected and refitted. Satis [satisfactory]. Ref AMM 21-50-54-201.
The AMM reference used was appropriate for an inspection of an NRV fitted to the cabin and flight deck supply ducts. This suggests that the technician, upon locating NRV S/N 22867-7, may have thought that due to the layout of the ducts at the rear pressure bulkhead, the NRV belonged to the normal cabin and flight deck supply. Section 21-50-54 of the AMM indicates the direction of airflow at the NRV installations in the cabin and flight deck supply ducts (Section 1.7, Figure No. 4); however, because this AMM section is not applicable to the ram air NRV, it does not indicate the direction of airflow at this valve. Notwithstanding this, the direction of airflow is embossed on the body of the ram air valve.
Nevertheless, it is still possible that the technician misunderstood the direction of the airflow due to the layout of the ducts and intentionally refitted the NRV in the orientation consistent with this misunderstanding; component design is such that it does not preclude the incorrect installation of a ram air NRV.
No defects were recorded as a result of the maintenance inspection of the left hand recirculating NRV and the NRVs fitted to the cabin and flight deck supply ducts, indicating that the technician (who also installed the ram air NRV) correctly installed these valves. It is therefore equally possible that the technician correctly understood the direction of airflow at the ram air NRV, but unintentionally installed it the wrong way round.
It is essential that scheduled maintenance task requirements are clearly and accurately described to help ensure that the correct work is carried out. The Operator advised the Investigation that it has amended the aircraft’s AMP to include a scheduled inspection of the ram air NRV, which will include a requirement for an inspection during reinstallation by a second technician to ensure the correct orientation of the NRV following its installation. Consequently, no Safety Recommendation is made regarding the incorrect maintenance task description.
This article is published under license from Avherald.com. © of text by Avherald.com.
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