Skywork D328 near Zurich on Mar 14th 2012, loss of cabin pressure
Last Update: December 31, 2014 / 15:08:58 GMT/Zulu time
Incident Facts
Date of incident
Mar 14, 2012
Cause
Loss of cabin pressure
Airline
Skywork Airlines
Flight number
SX-600
Departure
Bern, Switzerland
Destination
Vienna, Austria
Aircraft Registration
HB-AES
Aircraft Type
DORNIER 328
ICAO Type Designator
D328
A replacement de Havilland Dash 8-400 registration HB-JIK reached Vienna with a delay of 2 hours.
Germany's BFU reported in their monthly bulletin Swiss BFU is investigating the occurrence, Germany's BFU participates in the investigation representing the state of manufacture.
On Dec 31st 2014 the Swiss SUST (former Swiss BFU) released their final report concluding the probable causes of the serious incident were:
The serious incident is attributable to the fact that when the commercial aircraft took off, the forward outflow valve was not completely closed and the cabin altitude became excessive.
The failure of the crew not to notice an incorrectly set operation element for controlling the cabin pressure control system was identified as a direct cause of this serious incident.
The following factors contributed to the occurrence of the serious incident:
- The control of the forward outflow valve, which is provided for in manual mode, also functions in automatic mode.
- The position of the forward outflow valve is not displayed to the crew.
Although the following factor did not directly cause the serious incident, in the context of the investigation it was identified as a risk factor:
- The commander and flight attendant could not understand each other via the interphone.
The captain (40, ATPL, 4,011 hours total, 535 hours on type) was pilot flying, the first officer (31, CPL, 9,345 hours total, 345 hours on type) was pilot monitoring during a normal departure. While climbing through FL100 the crew performed the FL100 checklist, the commander noticed that the cabin altitude indicated 3000 feet while she was used to see 1600 feet at that point, the cabin climb rate was normal however. The first officer recalled that he saw the cabin climbing at 50 feet per minute while he was used to see 0 working that checklist, he did not consciously perceive the cabin altitude. The crew continued the climb to FL270, the aircraft levelled off on autopilot. About a minute after levelling off the crew received a CABIN ALT caution message and a triple chime, the captain noticed the cabin altitude at 9600 feet and called "Cabin Altitude", both pilots donned their oxygen masks. The commander turned on the fasten seat belt signs, announced "Cabin Crew at Station" and asked the first officer to declare emergency, then initiated an emergency descent. After the Mayday had been radioed and the crew had received unrestricted clearance to descend the captain also assumed radio communication and instructed the first officer to work the related checklists, the captain noticed the cabin altitude at 10,500 feet, the first officer at 11,500 feet.
In the meantime cabin crew had interrupted preparation upon some unusual lateral movement of the aircraft prompting her to expect something was amiss, then heard the call "cabin crew on station". As the descent was not very steep and oxygen masks were not released she did not think there had been a decompression. She tried to contact the cockpit via Interphone but did not understand the captain, realised however that the crew had donned their oxygen masks.
The captain assessed that all doors were closed, she could however not be certain about the condition of the airframe and therefore decided to reduce the speed for the remainder of the flight returning to Bern. The first office, while working the checklists, switched the cabin pressure control to manual and opened the outflow valves at 5000 feet QNH, the cabin descent rate adjusted to the aircraft's descent rate.
The aircraft overflew Zurich Airport on the way back, ATC offered a diversion to Zurich, however, as there were no vibrations the crew continued to return to Bern.
The flight attendant, still unable to establish contact with the cockpit, had made her own assessment of the situation, briefed the passengers on the brace position and prepared the cabin for the arrival also telling passengers that the flight crew was still busy and would report later. Some time later the cockpit door opened and she noticed the flight crew had removed their oxygen masks, the captain briefed the flight attendant about the decompression, a normal landing was to be expected.
The aircraft landed safely back on Bern's runway 14.
According to the flight data recorder the cabin reached a maximum altitude of 11,700 feet.
The SUST analysed that there had been no technical defect or malfunction which caused the serious incident.
With reference to the statements of captain and first officer about their recollection of cabin altitude readings while working the FL100 checklist the SUST analysed: "This statement was based on the experience of the commander, as the operator's Bern-Belp homebase, from which approximately 50% of flights took off, has a reference altitude of 1673 ft. If after taking off from Bern-Belp the cabin altitude is read while working through the FL 100 checklist, this value must correspond approximately to the aerodrome elevation, since according to the differential pressure build-up, the cabin altitude begins to rise, and did begin to rise and in this case did so at exactly this point. The statement of the copilot that the cabin rate was practically zero at this checklist point corresponds to experience for the same reason."
The SUST analysed with respect to the cabin altitude rise: "The continuous rise in the cabin altitude leads to the conclusion that the forward outflow valve was not entirely closed at the time of take-off. This does not correspond to the copilot's statement that he had checked that the MAN CAB ALT control knob had been in the DN position when he went through the checklist during cockpit preparation for the flight (cf. chapter 1.17.1.2). A complicating factor in this context is that the MAN CAB ALT control knob is active regardless of the selected mode (automatic or manual) and therefore the forward outflow valve does not automatically close when switched to auto mode. Since the cabin pressure altitude could be managed in MAN mode (cf. point 6 in the respective checklist in Annex 5), as the copilot stated, it can be concluded that the forward outflow valve worked properly but was not fully closed at the very beginning. Adversely in this context is the fact that the position of the forward outflow valve is nowhere displayed. The crew can therefore only indirectly determine whether the forward outflow valve is closed via the position of the MAN CAB ALT control knob. Only the cabin altitude, cabin pressure differential and cabin rate are indicated on the corresponding system page (cf. figure 2, chapter 1.6.3). If, as in the serious incident which is the subject of the investigation, the rate of climb is normal, it is only possible to recognise that the forward outflow valve is open if, as the checklist demands, the following two values are checked beside the cabin rate of climb: "Cabin Altimeter indicates correct cabin altitude for ambient altitude. Cabin Differential pressure is increasing." It should be noted that this review is practically impossible for the crew, because it occurs in a flight phase in which these values are constantly changing and there is a lack of specific information for comparison."
The SUST analysed human and operation aspects: "For the flight preparation, the copilot worked according to the FLIGHT DECK PREPARATION CHECKLIST. In the case of the checkpoint regarding the setting of the MAN CAB ALT control knob, he had to examine factors including whether this was in the full left / DN position (cf. chapter 1.17.1.2) in order to ensure that the forward outflow valve was closed. The copilot stated that he had examined it. The investigation came to the conclusion that this valve was not fully closed. This means that the MAN CAB ALT control knob was not in the full left / DN position. However, the position of the forward outflow valve is not recorded on the flight data recorder. It seems plausible for the following reason that the control knob, not being in the full left / DN position, was not noticed during cockpit preparation: In normal operation, this control knob is not used before, during, or after the flight. It is a commonly observed psychological phenomenon that the position of such a control element is often overlooked when working through a checklist, as it is unconsciously assumed that it is in the correct position. Tests in the operator’s simulator also indicated that during cockpit preparation, many of the tested pilots did not notice that the MAN CAB ALT control knob had not been set to the full left / DN position and was therefore incorrect."
The SUST analysed that the actions of the flight crew, cabin crew as well as ATC following the cabin altitude alert were appropriate and safety conscious, the actions of ATC were also called "forward-thinking" and the flight attendant's behaviour was lauded as prudent.
Aircraft Registration Data
Incident Facts
Date of incident
Mar 14, 2012
Cause
Loss of cabin pressure
Airline
Skywork Airlines
Flight number
SX-600
Departure
Bern, Switzerland
Destination
Vienna, Austria
Aircraft Registration
HB-AES
Aircraft Type
DORNIER 328
ICAO Type Designator
D328
This article is published under license from Avherald.com. © of text by Avherald.com.
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