Scandinavian Airlines Flight 751
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OY-KHO in June 1991.
|Date||27 December 1991|
|Summary||Dual engine failure due to foreign object damage|
Gottröra, Norrtälje Municipality, Sweden |
|Aircraft type||McDonnell Douglas MD-81|
|Aircraft name||Dana Viking|
|Operator||Scandinavian Airlines System|
|Flight origin||Stockholm Arlanda Airport|
|Destination||Warsaw Chopin Airport|
|Injuries||25 (2 serious, 23 minor)|
Scandinavian Airlines Flight 751 is a regularly scheduled Scandinavian Airlines passenger flight from Stockholm, Sweden, to Warsaw, Poland, via Copenhagen, Denmark. On 27 December 1991, a McDonnell Douglas MD-81 operating the flight, registration OY-KHO, piloted by Danish captain Stefan G. Rasmussen (44) and Swedish first officer Ulf Cedermark (34), both experienced pilots with respectively 8000 and 3000 flight hours under their belts, was forced to make an emergency landing in a field near Gottröra, Sweden. Ice had collected on the wings' inner roots (close to the fuselage) before takeoff. It broke off and was ingested into the engines as the aircraft became airborne on takeoff, ultimately resulting in their failure. All 129 passengers and crew aboard survived.
The aircraft was a McDonnell Douglas MD-81, registered OY-KHO, serial number 53003, line number 1844. It made its first flight on 16 March 1991, and was delivered soon after to SAS on 10 April 1991. At the time of the accident, the aircraft had been in service for only 9 months. It was fitted with two Pratt & Whitney JT8D low-bypass turbofan engines.
The aircraft had arrived at Stockholm-Arlanda Airport at 22:09 local time, after a flight from Zürich, on the previous evening and was parked overnight at temperatures of around 0 to 1 °C (32 to 34°F). About 2,550 kilograms (5,620 lb) of flight-chilled very cold fuel remained in the wing tanks. Because of this, clear ice had formed on the upper side of the wings, but was not detected. The aircraft was de-iced with 850 litres (190 imp gal; 220 US gal) of de-icing fluid, but not checked afterwards for remaining ice by the de-icing personnel.
The plane departed from Stockholm at 08:47, on the day of the crash. Shortly after liftoff, pieces of ice slammed onto the fans of both engines, deforming the fan blades sufficiently to disturb the airflow to the compressors. The disturbed airflow caused the compressors to stall and this in turn caused the engines to surge. Because the engines were not throttled down sufficiently, the surges continued. The high loads from repeated engine surges quickly led to the breakup of both engines.
From the pilot's point of view, after 25 seconds of flight, noise, bangs and vibrations caused by No. 2 engine (the right engine) being in surge were first noticed. The flight crew responded by throttling down a little, but an automatic system (ATR, Automatic Thrust Restoration) that had not been described to the flight crew by the Scandinavian Airlines System (SAS), simultaneously increased throttle as a response to the asymmetric engine power and reduced climb rate. As a consequence, the engine surges continued. SAS flight captain Per Holmberg, who was on board as a passenger, noticed the problems early and hurried to the cockpit to assist the crew. Engine No. 1 (the left engine) surged 39 seconds later, and both engines failed at 76 and 78 seconds into flight, at an altitude of 3,220 ft (980 m).
The pilot responded to the loss of both engines by pitching the aircraft down in a dive before leveling it, to try to have it glide the longest possible distance without stalling. The pilots requested a return to Arlanda and attempted the restart procedure, but, with the aircraft emerging from cloud cover at 890 ft (270 m) altitude, they chose a field in the forest, near Vängsjöbergs säteri in Gottröra, Uppland, for an immediate emergency landing.
During the final descent, the aircraft hit several trees, losing a large part of the right wing. It struck the ground tail-first, sliding along the field for 110 metres (360 ft), breaking into three parts before coming to a stop. 25 people were injured, 2 of them seriously, but there were no fatalities. One of the reasons of no fatalities was said to be the brace position that had been instructed by the flight attendants. The tail cone of the plane broke off and the main landing gear of the plane dug marks on the field and sheared off. The plane's nose gear also broke off. The aircraft was determined to be damaged beyond repair and was written off.
The flight crew, and especially Captain Rasmussen, were lauded for the skilled emergency landing in a fast-developing, potentially fatal situation. Rasmussen commented that "few civilian air pilots are ever put to a test of the skills they have acquired during training to this degree" and said he was proud of his crew and very relieved everyone had survived. He chose not to return to piloting commercial aircraft.
Investigation and recommendations
According to the official accident report by the Swedish Accident Investigation Board (SHK), the problem of clear ice formation on the wings in this type of aircraft was a well-known phenomenon at the time of the accident. From 1985 on McDonnell Douglas gave extensive information, including several "All Operators Letters" that dealt with the clear ice problem. In the "All Operators Letter" of 14 October 1986 operators were informed of how the Finnish airline Finnair had solved the problem of discovering clear ice. In 1988 and 1989 McDonnell Douglas arranged "Theme Conferences" dealing with clear ice formation. SAS took part in these conferences.
On 26 October 1991, SAS distributed a "Flight Deck Bulletin/Winterization" to all pilots. It said: "It is the Pilot-in-Charge's responsibility to check the aircraft for any ice or snow that may affect performance" and on the section "Clear Ice" it noted, "Although the awareness within Line Maintenance is mostly good, the responsibility again rests with the Pilot-in-Charge that the aircraft is physically checked by means of a hands-on check on the upper side of the wing. A visual check from a ladder or when standing on the ground is not enough"
Another contribution to the accident was insufficient training of the crew: they were not trained in restoring engine operation after they repeatedly surged. There was no simulator or other training on the engine surging problem. Secondly, they were not informed about a pre-installed automatic thrust system (Automatic Thrust Restoration or ATR). The reason for this lapse of information was that there was no knowledge of ATR within SAS. However, ATR was described in manuals by the aircraft manufacturer which every operator is obliged to know. Even though the system was developed for use in procedures not applied by SAS, a sufficiently careful study of the manuals should have led to SAS noting the system and training its pilots in its function.
The conclusion of the official accident report states:
The accident was caused by SAS's instructions and routines being inadequate to ensure that clear ice was removed from the wings of the aircraft prior to takeoff. Hence the aircraft took off with clear ice on the wings. In connection to lift-off, the clear ice disengaged and was sucked in by the engines. The ice caused damage to the engine fan stages, which led to engine surges, and these triggered irreparable engine failure. The pilots were not trained to identify and eliminate engine surging; furthermore, the Automatic Thrust Restoration system was not known by the pilots, nor noticed by the airline.
In the section "Compressor failures" the report states:
With sufficiently reduced thrust in the right engine and maintained thrust (not increased) in the left engine, the engines would probably not have failed. The aircraft would then have been able to return for landing.
However, the newly installed ATR prevented the pilots from successfully performing the normal remedial measure to halt compressor stall, i.e., throttling back the engines, as the ATR system – designed to prevent pilots using less than normal thrust when climbing out after take-off for noise abatement reasons – restored engine take-off power throttle settings contrary to the pilots' reduced throttle commands. This damaged the engines, until eventually, they failed completely.
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- "Air Traffic Accident on 27 December 1991 at Gottröra, AB county" (PDF) (Official accident report). Report C 1993:57. Case L-124/91. Swedish Accident Investigation Authority. 20 October 1993. Retrieved 3 September 2015.
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- Radio documentary from Swedish Radio P3 about the incident. (Swedish)
- Scandinavian Airlines System (SAS) Flight 751 Accident Animation