Scandinavian Airlines Flight 751
Encyclopedia
Scandinavian Airlines Flight 751 was a regularly scheduled Scandinavian Airlines flight from Stockholm
, Sweden
to Copenhagen
, Denmark
and Warsaw
, Poland
that crashed on 27 December 1991. The McDonnell Douglas
MD-81 was piloted by Danish
captain Stefan G. Rasmussen
and Swedish
first officer Ulf Cedermark. Ice had collected on the wings prior to take off, which broke off and was sucked into the engines when the aircraft became airborne. After losing both engines, the pilots were forced to make an emergency landing in a field near Gottröra
, Sweden.
The incident is known as Gottrörakraschen (English:
Gottröra crash) in Sweden.
on the previous evening and was parked overnight at temperatures of around 0 to 1 °C. About 2550 kg 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 liters of de-icing fluid, but not checked afterwards for remaining ice by either the de-icing personnel or the captain.
At lift-off and rapidly after it, 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 engine 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.
The airliner
was piloted by Danish
captain Stefan G. Rasmussen
and Swedish
first officer Ulf Cedermark. It was headed to Warsaw
, Poland
through Copenhagen
, Denmark
.
From the pilot's point of view, after 25 seconds of flight, noise, bangs and vibrations caused by no. 2 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 SAS, simultaneously increased throttle as a response to increasing altitude. As a consequence, the engine surges continued. An 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 surged 39 seconds later, and both engines failed at 76 and 78 seconds into flight, at an altitude of 980 meters.
The pilot responded to the loss of both engines by pitching the aircraft down in a dive before leveling it, to try and have the aircraft glide the longest possible distance without stalling
. The pilots requested a return to Arlanda and attempted the restart procedure, but with the plane breaking through the cloud cover at 270 meters the pilot chose a field in the forest, near Vängsjöbergs säteri in Gottröra
, Uppland
, for an immediate emergency landing.
During the final descent the plane hit some trees, losing a large part of the right wing. It impacted the ground tail-first, sliding along the field for 110 m, breaking into three parts before coming to a stop. Twenty-five people were injured, two of them seriously, but there were no fatalities. One of the reasons for the lack of fatalities was said to be the brace position that had been instructed by the flight stewardesses.
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.
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 Oct 1986 operators were informed of how 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 (ATR for Automatic Thrust Restoration). 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 builders 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' 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.
's episode "Pilot Betrayed", along with interviews of the some of the crew, and the poignant resignation of Pilot Rasmussen.
Stockholm
Stockholm is the capital and the largest city of Sweden and constitutes the most populated urban area in Scandinavia. Stockholm is the most populous city in Sweden, with a population of 851,155 in the municipality , 1.37 million in the urban area , and around 2.1 million in the metropolitan area...
, Sweden
Sweden
Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....
to Copenhagen
Copenhagen
Copenhagen is the capital and largest city of Denmark, with an urban population of 1,199,224 and a metropolitan population of 1,930,260 . With the completion of the transnational Øresund Bridge in 2000, Copenhagen has become the centre of the increasingly integrating Øresund Region...
, Denmark
Denmark
Denmark is a Scandinavian country in Northern Europe. The countries of Denmark and Greenland, as well as the Faroe Islands, constitute the Kingdom of Denmark . It is the southernmost of the Nordic countries, southwest of Sweden and south of Norway, and bordered to the south by Germany. Denmark...
and Warsaw
Warsaw
Warsaw is the capital and largest city of Poland. It is located on the Vistula River, roughly from the Baltic Sea and from the Carpathian Mountains. Its population in 2010 was estimated at 1,716,855 residents with a greater metropolitan area of 2,631,902 residents, making Warsaw the 10th most...
, Poland
Poland
Poland , officially the Republic of Poland , is a country in Central Europe bordered by Germany to the west; the Czech Republic and Slovakia to the south; Ukraine, Belarus and Lithuania to the east; and the Baltic Sea and Kaliningrad Oblast, a Russian exclave, to the north...
that crashed on 27 December 1991. The McDonnell Douglas
McDonnell Douglas
McDonnell Douglas was a major American aerospace manufacturer and defense contractor, producing a number of famous commercial and military aircraft. It formed from a merger of McDonnell Aircraft and Douglas Aircraft in 1967. McDonnell Douglas was based at Lambert-St. Louis International Airport...
MD-81 was piloted by Danish
Denmark
Denmark is a Scandinavian country in Northern Europe. The countries of Denmark and Greenland, as well as the Faroe Islands, constitute the Kingdom of Denmark . It is the southernmost of the Nordic countries, southwest of Sweden and south of Norway, and bordered to the south by Germany. Denmark...
captain Stefan G. Rasmussen
Stefan G. Rasmussen
Stefan G. Rasmussen is a Danish former pilot who captained the crash-landing SAS flight 751 on December 27, 1991. There were no fatalities in the crash, and Rasmussen received much recognition for his handling of the incident...
and Swedish
Sweden
Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....
first officer Ulf Cedermark. Ice had collected on the wings prior to take off, which broke off and was sucked into the engines when the aircraft became airborne. After losing both engines, the pilots were forced to make an emergency landing in a field near Gottröra
Gottröra
Gottröra is a village in Norrtälje Municipality in the province of Uppland, Sweden. Several hundred ancient monuments are registered in Gottröra.In 1991 the Scandinavian Airlines Flight 751 had to make an emergency landing near Gottröra.-References:...
, Sweden.
The incident is known as Gottrörakraschen (English:
English language
English is a West Germanic language that arose in the Anglo-Saxon kingdoms of England and spread into what was to become south-east Scotland under the influence of the Anglian medieval kingdom of Northumbria...
Gottröra crash) in Sweden.
Accident
The aircraft had arrived from ZürichZürich
Zurich is the largest city in Switzerland and the capital of the canton of Zurich. It is located in central Switzerland at the northwestern tip of Lake Zurich...
on the previous evening and was parked overnight at temperatures of around 0 to 1 °C. About 2550 kg 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 liters of de-icing fluid, but not checked afterwards for remaining ice by either the de-icing personnel or the captain.
At lift-off and rapidly after it, pieces of ice slammed onto the fans of both engines
Jet engine
A jet engine is a reaction engine that discharges a fast moving jet to generate thrust by jet propulsion and in accordance with Newton's laws of motion. This broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, pulse jets...
, 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 engine surge
Compressor stall
A compressor stall is a situation of abnormal airflow resulting from a stall of the aerofoils within the compressor of a jet engine. Stall is found in dynamic compressors, particularly axial compressors, as used in jet engines and turbochargers for reciprocating engines.Compressor stalls result in...
. 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.
The airliner
Airliner
An airliner is a large fixed-wing aircraft for transporting passengers and cargo. Such aircraft are operated by airlines. Although the definition of an airliner can vary from country to country, an airliner is typically defined as an aircraft intended for carrying multiple passengers in commercial...
was piloted by Danish
Denmark
Denmark is a Scandinavian country in Northern Europe. The countries of Denmark and Greenland, as well as the Faroe Islands, constitute the Kingdom of Denmark . It is the southernmost of the Nordic countries, southwest of Sweden and south of Norway, and bordered to the south by Germany. Denmark...
captain Stefan G. Rasmussen
Stefan G. Rasmussen
Stefan G. Rasmussen is a Danish former pilot who captained the crash-landing SAS flight 751 on December 27, 1991. There were no fatalities in the crash, and Rasmussen received much recognition for his handling of the incident...
and Swedish
Sweden
Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....
first officer Ulf Cedermark. It was headed to Warsaw
Warsaw
Warsaw is the capital and largest city of Poland. It is located on the Vistula River, roughly from the Baltic Sea and from the Carpathian Mountains. Its population in 2010 was estimated at 1,716,855 residents with a greater metropolitan area of 2,631,902 residents, making Warsaw the 10th most...
, Poland
Poland
Poland , officially the Republic of Poland , is a country in Central Europe bordered by Germany to the west; the Czech Republic and Slovakia to the south; Ukraine, Belarus and Lithuania to the east; and the Baltic Sea and Kaliningrad Oblast, a Russian exclave, to the north...
through Copenhagen
Copenhagen
Copenhagen is the capital and largest city of Denmark, with an urban population of 1,199,224 and a metropolitan population of 1,930,260 . With the completion of the transnational Øresund Bridge in 2000, Copenhagen has become the centre of the increasingly integrating Øresund Region...
, Denmark
Denmark
Denmark is a Scandinavian country in Northern Europe. The countries of Denmark and Greenland, as well as the Faroe Islands, constitute the Kingdom of Denmark . It is the southernmost of the Nordic countries, southwest of Sweden and south of Norway, and bordered to the south by Germany. Denmark...
.
From the pilot's point of view, after 25 seconds of flight, noise, bangs and vibrations caused by no. 2 engine being in surge were first noticed. The flight crew responded by throttling
Throttle
A throttle is the mechanism by which the flow of a fluid is managed by constriction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases , but usually decreased. The term throttle has come to refer, informally and incorrectly, to any mechanism by which...
down a little, but an automatic system (ATR, Automatic Thrust Restoration) that had not been described to the flight crew by SAS, simultaneously increased throttle as a response to increasing altitude. As a consequence, the engine surges continued. An SAS flight captain, Per Holmberg, who was on board as a passenger, noticed the problems early and hurried to the cockpit
Cockpit
A cockpit or flight deck is the area, usually near the front of an aircraft, from which a pilot controls the aircraft. Most modern cockpits are enclosed, except on some small aircraft, and cockpits on large airliners are also physically separated from the cabin...
to assist the crew. Engine no. 1 surged 39 seconds later, and both engines failed at 76 and 78 seconds into flight, at an altitude of 980 meters.
The pilot responded to the loss of both engines by pitching the aircraft down in a dive before leveling it, to try and have the aircraft glide the longest possible distance without stalling
Stall (flight)
In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded...
. The pilots requested a return to Arlanda and attempted the restart procedure, but with the plane breaking through the cloud cover at 270 meters the pilot chose a field in the forest, near Vängsjöbergs säteri in Gottröra
Gottröra
Gottröra is a village in Norrtälje Municipality in the province of Uppland, Sweden. Several hundred ancient monuments are registered in Gottröra.In 1991 the Scandinavian Airlines Flight 751 had to make an emergency landing near Gottröra.-References:...
, Uppland
Uppland
Uppland is a historical province or landskap on the eastern coast of Sweden, just north of Stockholm, the capital. It borders Södermanland, Västmanland and Gästrikland. It is also bounded by lake Mälaren and the Baltic sea...
, for an immediate emergency landing.
During the final descent the plane hit some trees, losing a large part of the right wing. It impacted the ground tail-first, sliding along the field for 110 m, breaking into three parts before coming to a stop. Twenty-five people were injured, two of them seriously, but there were no fatalities. One of the reasons for the lack of fatalities was said to be the brace position that had been instructed by the flight stewardesses.
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.
Investigation and recommendations
According to the official accident report, the problem of clear ice formation in 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 Oct 1986 operators were informed of how Finnair
Finnair
Finnair Plc is the flag carrier and largest airline of Finland, with its headquarters on the grounds of Helsinki Airport in Vantaa, Finland, and its main hub at Helsinki Airport. Finnair and its subsidiaries dominate both the domestic and international air travel markets in Finland. The largest...
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 (ATR for Automatic Thrust Restoration). 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 builders 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' 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.
Dramatization
The accident was featured in Air Crash InvestigationMayday (TV series)
Mayday, also known as Air Crash Investigation in the United Kingdom, Australia and Asia and Air Emergency and Air Disasters in the United States, is a Canadian documentary television programme produced by Cineflix investigating air crashes, near-crashes and other disasters...
's episode "Pilot Betrayed", along with interviews of the some of the crew, and the poignant resignation of Pilot Rasmussen.
External links and references
- Swedish Accident Investigation BoardSwedish Accident Investigation BoardThe Swedish Accident Investigation Board is a Swedish government agency tasked with investigating all types of serious civil or military accidents that can occur on land, on the sea or in the air...
(SHK)- Final report - Translated from the original Swedish by Tim Crosfield M.A., at the request of the SHK. (Archive)
- Final report - The original version. (Archive)
- Radio documentary from Swedish Radio P3 about the incident. (Swedish)