Investigation Summary
The accident was caused by a phenomenon known as 'cold-soaked fuel frost.' The aircraft had arrived from Zurich the previous night with a large amount of very cold fuel in its wing tanks. While parked overnight in temperatures near freezing, this chilled fuel caused moisture to freeze into a layer of clear ice on the upper wing surfaces. During takeoff, the flexing of the wings caused this ice to shatter and fly into the intake of the Pratt & Whitney JT8D-217C engines. This led to engine surging and damage. Unbeknownst to the pilots, the aircraft was equipped with an Automatic Thrust Restoration (ATR) system, which automatically increased engine power in response to the surging, accelerating the engines' destruction.
Final Conclusions
The Swedish Accident Investigation Board (SHK) concluded that the primary cause was the failure to detect and remove clear ice from the wings. Contributing factors included the lack of pilot training regarding the ATR system, which SAS had not informed its crews about, and the pilots' lack of awareness of the engine surging symptoms. The successful outcome was attributed to the pilots' skill in gliding the aircraft and the assistance of a non-duty SAS captain, Per Holmberg, who entered the cockpit to assist with emergency procedures.
Video Analysis
The Miracle of Gottröra | SAS Flight 751 — A detailed look at the events of Scandinavian Airlines Flight 751, where a dual engine failure led to a miraculous crash landing in a Swedish forest.
Watch on YouTube ↗If playback is blocked, the owner has disabled embedding — use the link above.
Systems & Failure Modes
Automatic Thrust Restoration (ATR)
A system designed to automatically increase thrust in the remaining engine if one engine fails; however, it increased power to the failing engines, causing them to disintegrate.
Pratt & Whitney JT8D-217C
The rear-mounted turbofan engines which suffered catastrophic internal damage after ingesting clear ice.
Interesting Facts
- 01The aircraft was named 'Dana Viking'.
- 02The flight was a multi-leg journey from Stockholm to Warsaw via Copenhagen.
- 03Captain Stefan G. Rasmussen had 8,000 flight hours; First Officer Ulf Cedermark had 3,000.
- 04The aircraft was nearly brand new, having been manufactured earlier in 1991 with only 1,608 airframe hours.
- 05The Automatic Thrust Restoration (ATR) system was designed to increase thrust if an engine failed during takeoff, but in this case, it exacerbated the damage to the surging engines.
Safety Actions & Advisories
ATR System Awareness
Airlines were required to ensure pilots were fully briefed on the existence and operation of automatic thrust systems like ATR.
Clear Ice Detection
Enhanced procedures for 'hands-on' checks of wing surfaces were implemented to detect clear ice that is invisible to the eye.