Li-2 crash near Tashauz Airport

Investigation

The commission established that the crew had promptly detected the drop in revs of the left engine. In the interval while the crew was assessing what was happening and feathering the propeller, the aircraft lost speed and turned left by up to 30-40°, so the crew decided to land on the takeoff heading with a left turn. Finding that flight without descending was impossible, the crew decided to make a forced landing in a field at a chosen site. Seeing at the last stage of the flight that the aircraft was at low height below the calculated descent path and would inevitably collide with obstacles, the crew cleared them by pulling back on the control column. The flight speed then fell below the minimum manoeuvring speed. The aircraft went into a spin attitude and struck the ground with the nose section of the fuselage at an angle of about 90°. The crew did not use the opportunity to land straight ahead immediately after detecting the drop in engine revs, but began making a turn towards the airfield.

Conclusions of the investigation

The immediate cause of the crash was the failure of the left engine in the climb mode up to 100 m and the impossibility, in the resulting situation, of continuing flight without descending.

The crash was facilitated by the complicated conditions (low height, presence of obstacles) when making the forced landing. The premature descent of the aircraft below the calculated path led to the crew, while clearing the obstacles, being unable to cope with piloting the aircraft due to the reduction of speed below the minimum manoeuvring speed.

Contributing causes: insufficient organisation of air traffic control at Tashauz airport — the KDP controller committed violations of radio exchange phraseology, and after receiving the crew's report of the engine failure he did not keep visual watch on the aircraft.

The commission was unable to establish the true cause of the drop in revs of the left engine. The commission suggested that this could have been caused by a short-term over-enrichment of the mixture due to sticking of the needle of the carburettor float chamber, which, based on an analysis of accidents with Li-2 aircraft over the last 10 years, is a common design and manufacturing defect of this unit.

Subsequently GOS NII GA carried out a study of the units of the left engine. It was established that the position of the spool of the 3-way fuel cock relative to the cock body differed from that established by the technical commission. Instead of the 60-70 % stated in the report, the blockage of the cock's inlet opening was 92 %. The opening to the left engine was completely blocked, to the right engine — by 50-60 %. The spring of the 3-way cock's detent could have been compressed to a length of 12.7 mm only if the detent ball was on one of the cams of the cock control shaft, that is, the cock was in an unlocked position.

On the basis of this conclusion, the cause of the failure of the left engine was insufficient fuel supply due to 92 % blockage of the inlet cross-section of the 3-way cock of the fuel system.

Probable causes of the incomplete opening of the cock could have been:

a) failure of the locking mechanism of the fuel cock due to possible shrinkage of its spring;

b) erroneous actions of the flight engineer in operating the aircraft's fuel system in accordance with the Flight Manual (RLE).