What happened
On 26 February 2000, a Sukhoi SU-26M, registration RA44531, was conducting a flight test at White Waltham Airfield to verify the operation of the aircraft's belly fuel tank. The flight was intended to test the fuel system functionality with the external tank installed, following recent modifications to the aircraft.
After taking off using fuel from the main tank, the pilot reached level flight and began a sequence of switching between the various fuel sources. While the pilot was transitioning between the M2 tank and the belly tank, the engine began running roughly. In an attempt to rectify the issue, the pilot re-selected the main tank and then the M2 tank, but the engine continued to malfunction. As the engine power dropped significantly, the pilot realized the aircraft could not return to the airfield and opted for an emergency landing in a field located 90 degrees to the left of the flight path.
Although the touchdown was smooth, the soft, wet ground caused the landing gear to sink. This led to the aircraft overturning onto its back. The pilot sustained 1 minor injury and was able to exit the wreckage without assistance.
The investigation
Investigators examined the fuel system architecture of the Sukhoi SU-2lyM, which utilizes a complex arrangement involving a main tank, an M2 tank, an extra fuselage tank, and a belly tank. The investigation focused on the transition between these tanks and the behavior of the fuel priming system.
It was established that the maintenance organization's engineers had previously explained the fuel tank arrangement to the pilot. However, the investigation noted that when switching between different fuel tanks, it can take approximately 10 to 15 seconds for air to be purged from the system. Furthermore, while the priming system can assist in this transition, incorrect use of the pump handle can result in neat fuel being pumped directly into the cylinders.