What happened
Following an earlier investigation into an accident involving a Gardan GY80-180, registration G-AWAC, at Compton Abbas Airfield, new information has come to light regarding the aircraft's fuel system configuration. The aircraft, which was operated with a rear fuel tank installed, was subject to a previously unidentified technical instruction.
The investigation
Subsequent to the publication of the initial accident report, the AAIB identified the existence of Socata Service Bulletin (SB) No 28, dated November 1967. This bulletin addressed a potential vulnerability in the fuel system design. Specifically, the bulletin noted that if the fuel cock was set to a position combining multiple tanks—such as "Left + Right" or "Left + Right + Rear"—air could potentially enter the fuel lines if one of the tanks became empty.
To mitigate this risk, the Service Bulletin instructed that the specific markings for these combined tank selections be obscured using either a plate or tape, effectively preventing the pilot from selecting those settings. While the physical positions of the fuel cock remained functional, the visual indicators were to be rendered unusable.
Findings
Investigation into the maintenance history of G-AWAC established that the aircraft's serial number (234) fell outside the range of aircraft requiring the modification at manufacture. However, the aircraft was equipped with an optional rear tank, and the safety measures intended to prevent the use of certain fuel cock settings were not in place. It appears that any tape or plate previously used to obscure the markings had become detached. The aircraft owners were reportedly unaware of the requirements set out in the Socata Service Bulletin.