Swiss A333 near Paris on Nov 21st 2014, loss of cabin pressure
Last Update: October 30, 2018 / 16:29:55 GMT/Zulu time
Incident Facts
Date of incident
Nov 21, 2014
Classification
Incident
Cause
Loss of cabin pressure
Airline
Swiss
Flight number
LX-19
Departure
Newark, United States
Destination
Zurich, Switzerland
Aircraft Registration
HB-JHB
Aircraft Type
Airbus A330-300
ICAO Type Designator
A333
On Dec 9th 2014 the French BEA reported in their weekly bulletin, that the occurrence has been rated a serious incident and is being investigated by Switzerland's SUST.
On Oct 30th 2018 Switzerland's SUST released their final report concluding the probable causes of the serious incident were:
The serious incident is attributed to the failure of the automatic cabin pressure regulation as a consequence of a jammed outflow valve and the fact that the procedure for its manual control was not applied fully.
The following factor was found to be causal:
- The flight crew intuitively initiated an emergency descent, without first having undertaken a structured analysis of the situation.
The following factor contributed to the serious incident:
- The system-related blanking out of the cabin altitude and the differential pressure displays when the cabin altitude reaches below -2060 ft.
The SUST sumamrized the sequence of events:
After an uneventful flight, the amber warning message CAB PR SYS 1 FAULT was displayed in the cockpit at 12:03:12 UTC during the descent from flight level (FL) 370 to FL 310. A single chime sounded a minute later and the amber warning message CAB PR SYS 1+2 FAULT was displayed simultaneously.
Subsequently, the flight crew decided to request an immediate further descent from Reims control and sent an urgency message (Pan Pan). The flight crew don their oxygen masks, initiated an emergency descent and informed the cabin crew of this. A short time later, the flight crew reported Mayday to air traffic control and received clearance to descend to FL 150.
The flight crew were of the view that the oxygen masks in the cabin had been released. Shortly afterwards, the commander (57, ATPL, 17,985 hours total, 4,191 hours on type) initiated the corresponding ECAM procedure which, among other things, requests that the cabin altitude is to be controlled manually. The flight crew briefly discussed the displayed cabin altitude and found it to be okay. Approximately five minutes later, the commander mentioned that the cabin altitude was no longer being displayed.
The flight crew therefore decided to land as quickly as possible and to fully open the outflow valves manually during the approach at an altitude of approximately 4000 ft. This was to ensure that the cabin doors could be opened after landing. The commander subsequently informed the line maintenance department in Zurich that they no longer had the cabin altitude and differential pressure displays. At 12:11:47 UTC, the flight crew informed air traffic control that they would cancel Mayday.
After a frequency change to Zurich’s approach control, the flight crew was given clearance for an instrument approach to runway 14 with the aid of radar vectoring. Shortly before landing, the copilot (42, ATPL, 6,399 hours total, 739 hours on type) noticed that the cabin altitude was being displayed again. At 12:38:08 UTC, the aircraft HB-JHB touched down on runway 14.
The passengers and the crew disembarked the aircraft normally. There were no injuries.
The SUST reported following ACARS messages were received from the aircraft:
12:03 UTC: AFT OUTFLOWVALVE
12:03 UTC: CAB PR SYS 1 FAULT
12:04 UTC: AFT OUTFLOWVALVE
12:04 UTC: CAB PR SYS 1 + 2 FAULT
12:10 UTC: CPC 1
12:17 UTC: CAB PR SAFETY VALVE OPEN
A following functional test found no anomalies. The Cabin Pressure Controller 1 (CPC1) was removed for further testing, a few days later CPC2 was also removed from the aircraft. Both CPCs passed the tests by the manufacturer.
According to the Quick Access Recorder the ouflow valve had "jammed during the closing process in a position of approximately 7 degrees. This almost corresponded to the closed position." A visual inspection of the outflow valve did not show any anomaly and passed standard testing by the manufacturer. An extended functional test was therefore conducted. The SUST summarized:
Both CPCs were connected to the so-called base tester (see figure 7). In this test system, the aircraft’s inputs (ADIRS, FMGC, etc.) are simulated and the outputs are displayed on a screen. The test outflow valve was used as aft outflow valve. The forward outflow valve was simulated. Both CPCs were connected to special equipment with which the cabin pressure (Pcabin) could be simulated. The external pressure (Pambient) was simulated by the base tester. The pressure conditions were set to those of the serious incident.
The aft outflow valve got jammed in a position close to closed, in order to force a loop closure failure. Both CPCs entered standby fault (failure state) status successively, which was consistent with expectations. This chronological course corresponded almost exactly to the QAR recordings.
On the base tester display, the Arinc 429 digital output signals of both CPCs were observed. All showed the status ‘invalid status’ (ssm = 00).
The SUST analysed:
By 12:03:12 UTC, the requested position of 3.59° and the actual position of 7.61° of the aft outflow valve created a discrepancy which led to the deactivation of CPC 1 and the activation of CPC 2. Because the corresponding monitoring function in CPC 2 also found a discrepancy (set point = 1.41°, actual value = 7.18°), this also turned itself off at 12:04:12 UTC. Subsequently, the aft outflow valve remained at approximately 7° and the forward outflow valve at approximately 1.5°. This discrepancy is difficult to see on the analogue display on the CAB PRESS page (see annex 8). In the absence of manual operation of the outflow valves by the crew, the cabin pressure was subsequently able to build up steadily. During the descent, the cabin differential pressure initially dropped. After reaching a flying altitude of 10 000 feet, the cabin differential pressure increased rapidly during level flight and reached values of 8.6 PSI and higher (see annexes 1 and 5).
To avoid overloading the airframe, the A330 in question is equipped with two safety valves. These are located on the pressure dome at the rear of the aircraft (see annexe 6). These valves operate autonomously. The safety valves open at a cabin differential pressure between 8.75 PSI and 8.95 PSI. In the period between 12:16:43 UTC and 12:24:47 UTC, safety valve 1 opened three times. This was displayed to the crew as warning message SAFETY VALVE OPEN on the E/WD, accompanied by a single chime and the illumination of the master caution warning lights. The switching status of the safety valve was schematically evident on the CAB PRESS page. After the crew had opened both outflow valves manually at
12:32:28 UTC, the cabin differential pressure fell rapidly, and at 12:36:02 UTC reached a value of zero at a flying altitude of 3000 feet.
As mentioned in chapter 1.16.2, both CPCs and the aft outflow valve were removed after the flight and tested by the equipment manufacturer. No malfunctioning of the equipment could be determined. It cannot be ruled out that a foreign object was located between the aft outflow valve’s frame and a valve flap and so led to the jamming. By manually opening the aft outflow valve, this foreign object could eventually have been dislodged and removed itself. However, during a test on the ground, the cabin pressure system functioned properly again.
Displays in the cockpit
The crew mentioned in their report that after switching the cabin pressurization system to manual mode, the cabin altitude and cabin differential pressure displays had failed.
In order to verify this, the equipment manufacturer carried out measurements on the cabin pressurization system (see chapter 1.16.2.4). These revealed that the stated parameters are no longer displayed at a cabin altitude below -2060 feet due to technical reasons. In the present case, a cabin altitude of -10 000 feet was reached. According to the CVR recordings, the crew first mentioned the loss of said displays as the cabin altitude reached a value of -2680 feet.
The fact that the commander informed the copilot at 12:06:58 UTC that the cabin altitude was 2000 ft, and that the copilot asked the commander with surprise at 12:08:47 UTC whether he had seen that the cabin altitude was now in the minus range and that the cabin was “pumping up”, dispels any doubts that the cabin altitude as well as the differential pressure were displayed up to that time.
By the time the cabin altitude had again increased to above -2000 feet (see annexe 1), it was mentioned that the displays were available again.
Hence, it was not a technical malfunction. With appropriate operation of the cabin pressurization system in manual mode, such low cabin altitude levels should not be reached.
The SUST analysed human and operational factors:
The discussion in the cockpit indicates that in this phase the commander still thought that the oxygen masks had been released in the cabin. Thus, he did not reduce the cabin sink rate because he would have had to open the outflow valves manually which ran contrary to the perceived decompression. The decision not to adjust the outflow valves was possibly supported by the fact that when descending at FL 220 at 12:06:58 UTC, a cabin altitude reading of approximately 2000 ft had been taken. This altitude did correspond to the tabular value, the cabin sink rate of 1250 ft/min, however, was still almost 1000 ft/min above the recommended value.
Shortly afterwards, the copilot asked the commander whether he had noticed that the cabin altitude was now in the negative range and the cabin pressure was increasing further. It is doubtful whether the commander consciously processed this question as he was distracted by the communication from air traffic control only seconds later. The displayed cabin altitude of -480 ft and the cabin sink rate of 1200 ft/min, together with the copilot’s remark, would have been a further indication that the differential pressure was high and could have been reduced by opening the outflow valves.
At 12:10:51 UTC when the commander ascertained that the cabin altitude was no longer displayed, it was already at an attitude of -2680 ft according to the recordings. Due to the system design, the analogue cabin altitude is blanked out on the CAB PRESS page below -2060 ft, and the digital display is replaced by amber crosses (see chapter 1.16.2.4 and annex 1 and 4). Consequently, the flight crew no longer had any information about the cabin altitude and differential pressure. It is understandable that the flight crew were anxious under these conditions and did not change the position of the outflow valves. The flight crew’s decision, to land as quickly as possible and to manually open the outflow valves at 4000 ft QNH, was therefore appropriate for the situation.
A little later, the commander contacted the line maintenance department in Zurich and described the problem. A solution could not be found. Towards the end of this approximately five-minute-long conversation, the copilot notified the commander that the safety valve had opened. The copilot thereby showed a good overview: he had assumed the control of the aircraft along with handling of the communication with air traffic control and this contact was intensive in this phase of the flight. No more than half a minute later, the copilot added to his comment that the cabin had „pumped up” and that therefore the safety valve had opened. This would have been a further indication towards the problem with the differential pressure and the cabin altitude. There was no response to the copilot’s observation.
In summary: after a cabin pressurization problem had arisen, the flight crew decided on an emergency descent without prior, structured analysis. Thus, the failure of the automatic cabin pressurization control system led to a challenging task for all parties. Control of the cabin altitude in manual mode would, on the other hand, only have led to a slight increase in the flight crew’s workload.
Aircraft Registration Data
Incident Facts
Date of incident
Nov 21, 2014
Classification
Incident
Cause
Loss of cabin pressure
Airline
Swiss
Flight number
LX-19
Departure
Newark, United States
Destination
Zurich, Switzerland
Aircraft Registration
HB-JHB
Aircraft Type
Airbus A330-300
ICAO Type Designator
A333
This article is published under license from Avherald.com. © of text by Avherald.com.
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