What happened
On 7 February 1999, a Tipsy Nipper T.66 Series 2, registration G-ATBW, was conducting a private flight from Stapleford Tawney Airfield in Essex. After completing an initial 35-minute flight earlier in the day, the pilot prepared for a second flight, estimating a remaining endurance of approximately 2 hours 15 minutes based on the main tank capacity and the assumption that fuel would transfer from auxiliary wingtip tanks.
Roughly 10 minutes after departing for the second leg, the pilot opened the transfer cock to allow fuel to flow from the wingtip tanks into the main tank. During the flight, the pilot monitored the fuel level using a sight-gauge tube. However, while on the downwind leg of the landing circuit, the engine suffered an abrupt loss of power. As the engine failed completely, the pilot was unable to reach the runway and instead performed an into-wind forced landing in a nearby field.
The landing was heavy due to an upsloping terrain and a high rate of descent. Upon touchdown on the soft ground, the left main and nose landing gear collapsed, causing the aircraft to tip onto its nose before settling with a wingtip on the ground. The pilot sustained minor injuries, but the aircraft suffered a broken propeller, landing gear collapse, and fuselage deformation.
The investigation
The AAIB examined the fuel system, which had been modified to include two auxiliary tanks in the wingtips. These tanks were designed to feed fuel via gravity into the main tank. The investigation focused on the fuel level indication system, which consisted of a clear plastic tube sight-gauge. At the time of the accident, this gauge lacked calibrated markings or a clearly defined 'empty' position.
Investigators found that while the pilot believed fuel was transferring from the wingtip tanks, the main tank was found to be nearly empty following the accident. Although 10 litres of fuel had been placed in the wingtip tanks at the start of the day, only 7 litres remained in the left wingtip tank after the crash. The investigation also reviewed the pilot's calculations regarding fuel consumption and the reliability of the transfer process.
Findings
- The engine failed because the main tank ran out of fuel.
- The pilot's estimation of fuel endurance was inaccurate because he assumed sufficient fuel transfer from the auxiliary tanks, which did not occur as expected.
- The fuel level indication was unreliable because the sight-gauge tube had no calibrated markings and the 'empty' position was not clearly marked.
- The pilot used the bottom edge of the wing spar as a zero datum for fuel readings, whereas the actual zero point was a different mark on the tube.
- Previous flow tests indicated that a significant amount of fuel could remain unusable in the wingtip tanks after transfer, a fact the pilot was unaware of.