AAIB Bulletin: 11/2020

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104 © Crown copyright 2020 All times are UTC AAIB Bulletin: 11/2020 G-CSBM AAIB-26778 SERIOUS INCIDENT Aircraft Type and Registration: Reims Cessna F150M, G-CSBM No & Type of Engines: 1 Continental Motors Corp O-200-A Year of Manufacture: 1977 (Serial no: 1359) Date & Time (UTC): 10 July 2020 at 1438 hrs Location: Winchfield, Hampshire Type of Flight: Private Persons on Board: Crew - 1 Passengers - 1 Injuries: Crew - None Passengers - None Nature of Damage: None Commander’s Licence: Private Pilot’s Licence Commander’s Age: 24 years Commander’s Flying Experience: 215 hours (of which 103 were on type) Last 90 days - 23 hours Last 28 days - 17 hours Information Source: Aircraft Accident Report Form submitted by the pilot and further enquiries by the AAIB Synopsis As the aircraft approached Blackbushe Airport the engine lost power and the pilot made a precautionary landing in a field. There was no damage to the aircraft and neither occupant was injured. The engine lost power due to fuel exhaustion. The pilot had used a fuel dipstick through a desire to measure the fuel onboard more accurately, but the dipstick used was not calibrated for the aircraft; this led him to overestimate the fuel onboard. History of the flight The pilot and a friend planned to fly a return trip from Blackbushe Airport (Blackbushe) in Hampshire to Sandown Airport (Sandown) on the Isle of Wight. He was aware that with the two people on board he could not completely fill the fuel tanks as this would put the aircraft above its maximum takeoff weight. He had calculated that he required 16 US gal of fuel for the return trip which included 5 US gal of reserve fuel. Prior to departing from Blackbushe the pilot checked the fuel quantity onboard with a dipstick. There was a wooden dipstick in the aircraft which was marked with a 0, ¼, ½, ¾ and full scale but the pilot found it difficult to see the fuel level on the stick and he wanted a more accurate measurement. Therefore, he found a dipstick which was marked with a more detailed scale, that was easier to read and was similar to one he had previously used on the Cessna 150. Using this he determined there was 9-10 US gal in each tank which he believed was sufficient for his intended trip. 105 © Crown copyright 2020 All times are UTC AAIB Bulletin: 11/2020 G-CSBM AAIB-26778 The flight from Blackbushe to Sandown was uneventful. On the ground in Sandown the pilot re-measured the fuel quantity in each tank using the same dipstick. This showed there was 3-4 US gal in one tank and 9-10 US gal in the other. He was surprised that the tanks were not balanced so contacted another pilot who advised that this was not abnormal for this aircraft type. He was confident he had enough fuel for the return flight plus reserves, so did not refuel. Blackbushe airfield 1 nm Approximate position where the engine lost power Figure 1 Aircraft track approaching Blackbushe Airport The pilot had been trained that the fuel gauges on light aircraft are unreliable, and so did not use them. The return flight was uneventful until the aircraft was approximately 2 nm from Blackbushe. The pilot had just descended to circuit height when the engine started to cough and lose power. He did not know what was wrong with the engine but, being aware that landing options close to the airport were limited if the engine failed completely, he decided the safest option was to make a precautionary landing in a field. He selected a large grass field on his left side and made an 180° turn to position the aircraft on a base leg for the field (Figure 1). The engine continued to run but was still losing power. He landed in the field using full flap and stopped the aircraft (Figure 2). There was no damage to the aircraft and both occupants were uninjured. After landing no fuel could be seen visually in either fuel tank. It was subsequently discovered that the dipstick the pilot had used was calibrated for a Cessna 172 so showed a greater quantity of fuel than was actually present. 106 © Crown copyright 2020 All times are UTC AAIB Bulletin: 11/2020 G-CSBM AAIB-26778 Figure 2 G-CSBM after the precautionary landing Flying club comment The owner of the flying club commented that whilst pilots are trained not to rely on the fuel gauges in light aircraft the gauges on G-CSBM are reasonably accurate. After the incident he sent a message to all pilots at the club reminding them that the fuel dipsticks are different in each aircraft type and to ensure they use the one calibrated for the aircraft they are flying. The flying club uses an electronic aircraft log, so it is not possible for pilots to see when the aircraft was last refuelled and how much flying the aircraft has done since. The owner reported that this was intentional to ensure pilots did not rely on this information and instead measured how much fuel was onboard before each flight. Aircraft information The Cessna 150 has two interconnected fuel tanks, one in each wing. Fuel is gravity fed to the engine (Figure 3). The tanks can hold 26 US gal of fuel but 3.5 US gal is unusable, giving a total usable capacity of 22.5 US gal. Although the fuel tanks are interconnected, it was reported that it is not uncommon for the fuel to be imbalanced after flight. This can be caused by several banked turns in the same direction prior to landing causing the fuel to migrate to one side. 107 © Crown copyright 2020 All times are UTC AAIB Bulletin: 11/2020 G-CSBM AAIB-26778 Figure 3 Cessna 150 fuel system Weight and balance The Cessna 150 maximum takeoff weight is 1,600 lbs (726 kg). G-CSBM basic weight was 526 kg. The pilot calculated that on takeoff from Blackbushe, with 54 kg of fuel and with a combined occupant weight 152 kg, the aircraft weighed 732 kg. Fuel planning and management guidance The CAA skyway code (CAP 1535S1) states that: ‘Fuel gauges in most general aviation aircraft are not very accurate and should