Report on serious incident en-route Oslo-Trondheim, Norway, on 25 September 2014 to British Aerospace ATP, SE-MAF operated by West Air Sweden AB

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Description

A cargo plane from West Air Sweden AB got control problems while en route. The problems started with a loss of airspeed in icing conditions. The crew activated the aircraft's system for de-icing, however, the speed continued to drop. Suddenly the aircraft started buffeting and made uncontrolled attitude changes. The crew disconnected the autopilot, picked up speed in a descent and regained control. The cruising flight level was reduced by 2000 ft, and the ice disappeared. The loss of altitude was not critical in relation to terrain.

The flight continued to the destination without further problems. Subsequent investigations revealed a technical fault in the de-icing system. This was, however, not significant for the course of events.

Findings made during the investigation indicate that the loss of speed was caused by a combination of icing and mountain waves. The airspeed dropped below the recommended speed in icing conditions, and the plane was about to stall. The phenomenon was well known before this incident occurred, and was described in Aircraft Flight Manual as well as in other documentation. Despite this, the investigation revealed shortcomings in the Operator’s knowledge in this area. A potential for improvement in documentation from the type certificate holder was also identified.

Safety recommendation

Safety recommendation SL 2015/09T

Ice accretion on the wings can cause the ATP aircraft to stall prematurely, before the stick shaker warns the crew. Accidents have occurred on similar aircraft types when crew members have failed to recognise natural buffeting as a warning of an imminent stall. The AIBN believes safety margins can be strengthened by making ATP pilots aware of the applicable recommended minimum speeds in icing conditions. In addition to planned and initiated measures like improved descriptions in manuals and training programs, the AIBN believes that values for recommended minimum speeds in icing conditions should be easily available for pilots in the cockpit.

The Accident Investigation Board Norway (AIBN) recommends that BAE Systems consider including recommended minimum speeds in icing conditions in the official Speed Cards for ATP, or in another suitable manner that makes the information easily available for reference in the cockpit.

Safety recommendation SL 2015/10T

Ice accretion on the wings can cause the ATP aircraft to stall prematurely, before the stick shaker warns the crew. Accidents have occurred on similar aircraft types when crew members have failed to recognise natural buffeting as a warning of an imminent stall. The AIBN believes safety margins can be strengthened by making ATP pilots aware of the applicable recommended minimum speeds in icing conditions. In addition to planned and initiated measures like improved descriptions in manuals and training programs, the AIBN believes that values for recommended minimum speeds in icing conditions should be easily available for pilots in the cockpit.

Should the type certificate holder BAE Systems not implement measures to comply with this recommendation above, (cf. AIBN's safety recommendation 2015/09T), the AIBN recommends that the operator does so on its own initiative.

The Accident Investigation Board Norway (AIBN) recommends that West Atlantic Group finds a suitable way to make minimum speeds in icing conditions for ATP easily available in the cockpit.

Location
En-route Oslo-Trondheim, Norway
Occurrence date
25.09.2014
Aircraft
British Aerospace ATP
Registration
SE-MAF
Meteorological conditions
IMC
County
Oppland
Type of occurrence
Serious incident
Type of operation
Commercial, other
Category of operation
Heavy, aeroplane (> 10 000kg)
Aircraft category
Landplane, Multi-engine, Turboprop/turboshaft
FIR/AOR
ENOS (Oslo ATCC)
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