KLM Royal Dutch Airlines - Koninklijke Luchtvaart Maatschappij

On the basis of the evidence available and the investigation of possible serious threats to safety, the Board cannot establish the cause of the accident with certainty. Moreover, investigation of the possibility that the cause of the accident may have been a bomb explosion has yielded no conclusive facts to support such a hypothesis. The statements in the press to this effect have either been based on misunderstandings or else were tendentious. On the other hand, the Board attributes a high degree of probability to hypothesis that the cause of the accident is related to 'overspeeding' of one of the outboard propellers resulting from oil pollution after a gear had been damaged when the supercharger of the corresponding engine was accelerated (shifted). This probably took place close to the time of the accident. The overspeeding of the propeller, owing to the obstruction of metal particles in the regulator valves may have been such as to cause the propeller pitch to decrease and the propeller could not be feathered. The condition might provoke a flight disturbance which could be corrected only by prompt and powerful handling of aileron and rudder controls. in view of the rapidity with which this defect develops and taking into account that recognition of the nature of this malfunction requires a certain time, it is not always possible for the crew to intervene early enough to restore conditions of controlled flight. With respect to the presumed cause of the accident the Board has no grounds to suppose that the occurrence of the presumed malfunction might be attributable to neglect on the part of maintenance personnel or to incorrect measures taken by the crew or that the crew had been at fault in piloting the aircraft after the malfunction had occurred.

As a result of the foregoing it is concluded that the explanation of the accident lies either in an error on the part of the pilot or in a technical failure. The Board was unable to state whether the accident could have been caused by a combination of both pilot error and technical failure. The Board was of the opinion that low runs should not be made by aircraft on scheduled services. The magnitude of the danger that is inherent in the proximity of the ground on take-off and landing should not be increased unnecessarily by flying at low altitude.

- Failure of the captain to correlate and interpret his instrument indications properly during flap retraction, resulting in necessary action not being taken in sufficient time. This failure was partially accounted for by the effect on instrument indications of inadvertent and unexpected gear re-extension,
- Loss of aircraft performance due to inadvertent landing gear re-extension,
- The captain failed to maintain sufficient climb to give him an opportunity of meeting unexpected occurrences.

Investigations were unable to determine the exact cause of the accident. Nevertheless, a number of hypothesis as to the cause of the accident were developed. Some possibilities considered were:
- overheating of the electric system with heavy smoke development,
- explosion of one of the high pressure bottles,
- failure of a cockpit window,
- failure of the automatic pilot system.
However, no hypothesis could be formulated in which all occurrences and evidence could be made reasonably acceptable. Therefore, in November 1955, after a 15-months' period of intensive investigation, the conclusion had to be drawn that the cause of the accident could not be established.

The exact cause of the accident could not be determined with certainty. During the extensive technical investigation no facts were found to be true for a technical failure of any part of the aircraft before the accident. The damping of the cable tensioners did not fully meet the specifications however, the Board considered that this could hardly have affected steerability. The ascent was somewhat flatter than normal and possibly created the rapid execution of the commands "flaps up" and "first power reduction" provide a greater condition steering movement required than the captain was used to. That need not be serious as long as the captain retained the confidence he had could bring back the plane in ascending flight. All in all, the investigation did not provide a clear cause for the captain Of course it is important that one of the conclusions of the Council was that “On the basis of the foregoing, the Board, together with the preliminary investigator, is of the opinion that in this case does not warrant the application of any sanction against the person is present.

Failure of a propeller blade during flight shortly before landing at Don Mueang Airport. This caused the loss of number three engine and fire to break out in the engine nacelle which could not be extinguished. When landing, the right main landing gear collapsed as a consequence of the fire.

The exact cause of the accident could not be determined with certainty. However, it is possible that the crew continued the approach below the minimum descent altitude to maintain a visual contact with the ground.

The Board was of the opinion that two successive errors of judgment were committed by the pilot. These were as follows:
- He initiated a landing procedure at an aerodrome with which he was not acquainted, in weather conditions which were lower than the minimum prescribed for landings on that aerodrome,
- Upon arrival over the aerodrome, he flew at too low an altitude over a terrain which he must have known to include a hill which constituted a hazard for his aircraft, which hill he was not able to see, on account of the poor visibility and low cloud.
The Board further considered that the following factors contributed towards the accident to a considerable extent:
- Air Traffic Control did not advise the pilot to delay his landing until the weather conditions had improved, or otherwise to divert to another aerodrome,
- Air Traffic Control designated a runway for the landing which necessitated the aircraft venturing low over dangerous terrain as indicated above.

As only few debris were found, investigations were unable to determine the exact cause of the accident. Nevertheless, it was the opinion of the experts that the loss of control may have been caused by a failure of the automatic pilot system.