History of the flight
On 17 August 2006, at 1200 UTC, a Piper PA-28-161, registration G-BGPJ, was conducting a private flight at Woodvale Airfield, Lancashire. Following an uneventful local flight, the engine stopped during the landing roll. The pilot restarted the engine and taxied the aircraft to the hangar. When the throttle was closed, the engine stopped again. The pilot attempted to restart the engine without success and decided to shut down the aircraft. While discussing the engine failures with the passenger, a passer-by alerted the pilot to flames around the nose landing gear leg. The pilot and passenger rapidly evacuated the aircraft, and the fire was extinguished using a foam fire extinguisher.
Engineering examination
Initial examination by a licensed aircraft engineer revealed moderate to severe fire damage in the lower engine bay area, which had burnt through the engine fuel primer hose and some electrical cable insulation. After replacing the primer hose, the engine started and ran up to full power without problems. However, when power was reduced to idle, the rpm was lower than normal, black puffs of smoke came from the exhaust, and the engine stopped. Fuel was seen running down the back of the carburettor air box. Further engine runs showed normal performance above 800 rpm, but below 800 rpm the engine ran rich, stopped, and fuel was observed from the carburettor air box. The carburettor was removed and sent to the AAIB for examination.
The carburettor, a Marvel-Schebler model MA-4SPA, was tested at an authorised overhaul organisation. The fuel level in the float chamber was well above the maximum allowed, and in certain angular positions fuel poured from the inside of the carburettor bore. Dismantling revealed that one arm of the horseshoe-shaped white hollow plastic float, part number 30-804, was almost completely full of AVGAS. Chemical analysis confirmed the fluid was pure AVGAS. Examination showed a minute area of the plastic hot-weld joint between the float chamber and its lid had not been welded during manufacture, allowing fuel ingress. The gap was so small that only a barely discernible amount of fuel escaped when placed in a vacuum chamber.
Carburettor float history
The float, part number 30-804, was a white hollow plastic design introduced in July 2002 to replace fabricated hollow brass floats due to fuel ingress at soldered joints. After a period in service, it was found that the white hollow plastic floats leaked fuel into the arms around the hot weld joints. Approximately 17,000 of these floats were manufactured. In September 2005, a blue sealed solid float, part number 30-864, was introduced, with no reported cases of fuel ingress as of the bulletin date.
In March 2006, the FAA issued Special Airworthiness Information Bulletin (SAIB) CE-06-33, alerting repair stations and mechanics of service difficulties with certain Marvel-Schebler/Precision Airmotive float-type carburettors. The SAIB noted that fuel leaking into the float, a damaged or worn float, or a damaged or worn float valve could lead to an overly rich mixture and potential loss of power or fire. It recommended operators remove aircraft from service if fuel leakage or poor idle cut-off were evident.
Safety actions
In light of the apparently high rate of safety occurrences with float part number 30-804, the AAIB issued Safety Recommendation 2007-040, recommending that the FAA review the continued airworthiness of the Precision Airmotive float, part number 30-804, fitted to Marvel-Schebler/Precision Airmotive carburettors. Additionally, a similar incident involving a Robin R2120, F-HAPC, in France on 9 September 2006, where the carburettor float was found to contain a large amount of fuel, led the BEA to form safety recommendations to EASA and the FAA regarding withdrawal of this design.