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 the Finnish flag carrier 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.
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 setting in contrary to the pilots' reduced throttle commands. This effectively destroyed the engines, until eventually, they failed completely.
Read more about this topic: Scandinavian Airlines Flight 751
Famous quotes containing the words investigation and:
“There is no one kind of thing that we ‘perceive’ but many different kinds, the number being reducible if at all by scientific investigation and not by philosophy: pens are in many ways though not in all ways unlike rainbows, which are in many ways though not in all ways unlike after-images, which in turn are in many ways but not in all ways unlike pictures on the cinema-screen—and so on.”
—J.L. (John Langshaw)