C11/2002L Ultralight aircraft accident in Viitasaari on 16 November 2002

On Saturday 16.11.2002 at approximately 14.15 - 14.50 Finnish time there was an aircraft accident at Viitasaari airfield in which an ultralight Rans Courier S-7 L aircraft crashed on a test flight. The pilot was injured in the crash. The Accident Investigation Board Finland initiated an investigation n:o C 11/2002 L on 18.11.2002 and appointed investigator Juhani Hipeli investigator-in-charge and investigator Seppo Pulkkinen member of the commission.

The pilot started the take off from the beginning of runway 34 with the aircraft in a clean configuration. His take off technique was to do the take off roll in clean configuration to better the ground acceleration. As he approached the rotation speed of 55 MPH he meant to choose the trailing edge flaps down one notch (10°). At the time of the crash the trailing edge flaps were, however, in the full down position (40°) and the selecting lever in the equivalent third notch. It is probable that the pilot chose the flaps in the full down position at take off. When the aircraft was nearly over the end of the runway the engine suddenly died. After this the aircraft very rapidly decelerated to the stalling speed or near it. The aircraft configuration, deceleration and centre of gravity being clearly in front of its forward limit caused together a strong pitch down movement and led to a rapidly steepening dive. The pitch of the aircraft was no longer under control. The aircraft hit the ground in a left spiralling motion in an approximate 50-60° dive and banked approximately 30° to the left.

When the engine was studied it was found out that as the engine had been modified from automobile use to aircraft use an essential change had been made to the fuel system. To avoid vapour lock the fuel pump is equipped with a return pipe which leads to the fuel tank in the automobile engines. The fuel pressure of the pump rises when the carburettor float chamber is full and the excess fuel flows back to the fuel tank via the return pipe. The fuel pump operates all the time when the engine is running and the continuous fuel flow cools the pump. In the accident aircraft the fuel return pipe had been joined to the fuel suction pipe at an approximate distance of 10 cm from the fuel pump. This construction circulates fuel around the pump, return pipe and suction pipe and causes the fuel temperature to increase. This increases vaporisation and the pump output decreases. The fuel flow from the fuel tank decreases and stops completely when a vapour lock is created. When the fuel flow stops the engine dies. The vapour lock can best be avoided by using the correct fuel system composition. The vapour lock can also be avoided by adding an electric pump to the system to create extra fuel pressure and decrease vaporisation. This would probably inhibit the vapour lock from developing in the first place.

The cause of the accident was the dying of the engine during take off and the subsequent loss of control. The probable cause of the dying of the engine was a generation of a vapour lock in the fuel system and stopping of the fuel flow. The vapour lock developed because the fuel vapours were able to accumulate in the fuel pump and the fuel suction pipe due to the incorrect installation of the fuel return pipe. A contributing factor to the incorrect installation of the fuel return pipe was a complete failure of the supervision of the engine modification. The loss of aircraft control and crash was contributed by the fact that the aircraft pitch angle was not fully controllable due to the centre of gravity being too forward.

The investigation commission recommends that the Finnish Civil Aviation Administration would augment its guidance material dealing with the supervision of private built aircraft. It is recommended that an expert reviews an engine modification and gives a statement of it every time a non-aircraft engine is modified to aircraft use. The commission recommends that the real airworthiness monitoring capacity of the first inspections of the experimental aircraft and their equipment is improved. It is also recommended that the Finnish Aeronautical Association would pay special attention to the building abilities of the experimental aircraft builders when it processes their building licences.

C11/2002L Report (pdf, 1.08 Mt)