L2012-05 Forced Landing of an Ultralight Aircraft at Hyvinkää on 26 August 2011

An Ikarus C 42 B ultralight aircraft, registration OH-U478, made a forced landing in a field at Hyvinkää on 26 August 2011 at 16:06. The engine stopped on a training flight at approximately 160 m AGL when the aircraft was in a landing pattern. The aircraft sustained no damage during the landing. The flight instructor and the student pilot escaped without injury.

The flight commenced from Helsinki-Malmi airport at 15:30, heading for Hyvinkää aerodrome where it arrived at approximately 16:00. The plan was for the student pilot to make 6-7 practice landings at Hyvinkää. This was his third cross-country training flight with an instructor. Whereas the first approach and landing were uneventful, during the second approach the engine RPM suddenly decreased to idle at approximately 160 m AGL when the aircraft was on the downwind leg. The power lever, however, was still at cruise power. The flight instructor took over the flight controls. Within a few seconds the engine stopped. The flight instructor realised that they would not make it to the aerodrome and so he steered the aircraft north towards fields and made a suc-cessful emergency landing.

The engine was visually inspected, but no abnormalities were found. The engine started normally and numerous test runs were carried out. The fuel system was inspected and pump pressures were measured. The mechanical fuel pump pressure was correct (0.3 bar) and the electric fuel pump pressure was the minimum permissible (0.15 bar). At first nothing out of the ordinary was found when the ignition systems were being tested. However, after several test runs during an ignition test it was discovered that ignition circuit B was inoperative. This fault appeared sporadically and on several test runs did not materialise at all. Nevertheless, because circuit A was operative, the engine ran almost normally.The electronic module of circuit B was replaced with another unit known to operate normally.

During one test run an ignition malfunction was also detected in circuit A. The malfunction in this circuit appeared less often than the one in circuit B. The electronic module of circuit A was also replaced. Following a test flight the aircraft was approved for flight operations.

After that the engine operated faultlessly for 60 flight hours until one training flight when intermittent malfunctioning occurred in one of the ignition circuits. Following this, the entire ignition system was removed, placed on an ignition test bench and tested for approximately six hours at varying RPM. The ignition system worked flawlessly during the test run, after which it was reinstalled on the aircraft. Test runs at varying engine RPM continued for approximately 1.5 hours. Most of the test was, however, run at cruise power RPM. The ignition system worked normally. Since the fault was not found, the entire ignition system was replaced with another system known to operate normally and the aircraft was approved for flight operations.

The engine worked perfectly for approximately 100 flight hours until one time during taxiing when it stopped four times and would not restart. The flight instructor and the student pilot pushed the aircraft back to the hangar.

The entire fuel system was inspected and partly replaced. The mechanical fuel pump was opened at which time it was discovered that the suction valve cover was slightly off of its correct position over the intake port. Hence, the pump was not generating pressure.Sideways scoring was visible on the valve cover which indicated that the pump had periodically functioned normally and periodically operated either partially or not at all.

The electronic modules that were assembled in the engine at the time of the forced landing were sent to the German and Austrian air accident investigation authorities for testing under their su-pervision. The modules were functionally tested at the engine manufacturer’s laboratory. Prior to testing both electronic modules were X-rayed and the images were analysed. The electric con-tacts of some of the components in the electronic module of circuit A were found to have too little soft solder in them. Quick Electrical Tests were performed on both electronic modules. The electronic module of circuit B failed its first test, but it functioned normally during the two following tests. No faults were discovered during the thermo cycle test. The electronic module of circuit A passed both tests flawlessly.

The engine stoppage was caused by a damaged suction valve in the mechanical fuel pump and the subsequent fuel feed failure. Low fuel pressure in the electric fuel pump was a contributing factor.

It is likely that neither of the ignition systems’ intermittent malfunctions caused the engine to stop.

During the investigation engine manufacturer published an information letter where it orders temperature sensitive label to be installed on electronic module. Based on this letter Finnish Transport Safety Agency issued an airworthiness directive, where installation of temperature sensitive label is mandatory to all electronic modules of Rotax 912 –engines, which are not certi-fied. During the investigation BRP-Powertrain (Rotax) published a line maintenance manual (ver. 3 /1.9.2012), where replacement of the fuel pump is mandatory in every 5 years.

The Safety Investigation Authority issues one safety recommendation. It is recommended that Deutscher Aero Club e.V. (ultralight aircraft type certificate approval organization in Germany) advises the aircraft manufacturer to comply with the engine manufacturer’s instructions and rec-ommendations as regards the construction of fuel systems.

L2012-05 Report (pdf, 0.32 Mt)