History of the Flight
On 9 August 2001 at 2010 hours UTC, a Grob G115 (registration G-BOPU) operated a training circuit detail from Manchester (Barton) using Runway 32. The preflight inspection was carried out by the student. After start, internal and engine power checks were completed, and the aircraft was taxied to the threshold. The fuel gauge indicated full. The student was briefed for a short field take-off and commenced a left hand circuit. As the aircraft positioned downwind, the pre-landing checks were completed. At approximately 800 feet agl, the engine lost power. The instructor immediately checked the mixture control, took control, and transmitted a Mayday call.
The instructor considered a left turn to land back on Runway 09 but assessed the risk of hitting nearby pylons and power cables as too high. He selected a golf course fairway to the west of the airfield as the only suitable landing site, informed the tower, selected full flap, switched off the battery master switch, and completed all emergency checklist items. As the aircraft landed on the fairway, the instructor braked heavily but could not prevent the aircraft from striking a grass bank around the green. The nose gear failed on impact, and the aircraft skidded to a halt. Both occupants vacated the aircraft without injury.
Investigation
Personnel from the maintenance organisation arrived and found the fuel tank empty. However, when the battery master switch was turned on, the gauge indicated full. The annual inspection for the aircraft had been completed that day. The fuel tank had been removed for access but the fuel indicating system was not worked on. After the inspection, the aircraft was refuelled with approximately 30 litres of fuel. The engineer recalled checking for an indication on the fuel gauge but did not note that the full indication did not agree with the amount added. A 20-minute engine ground run was performed.
When the aircraft was returned to service, the instructor performed a thorough external check and took a large sample of fuel by operating the drain. The sample was clear and free from debris, and the fuel contents gauge indicated full. He flew the aircraft in the circuit for one hour and reported that the fuel gauge was not stuck and responded to gusts. On landing, the gauge indicated 'nearly full'.
The aluminium fuel tank has a capacity of 100 litres and is located aft of the seats, beneath the floor of the baggage compartment, preventing visual fuel level checks. The level is sensed by a plastic float in an immersion tube connected to a resistance wire sensor transmitting a signal proportional to fuel quantity, displayed on a gauge on the instrument panel. Operating experience showed the gauging system was historically very reliable.
The maintenance organisation investigated the fuel indicating system after the accident and found no faults. They concluded that the 'full' indication was probably caused by the fuel tank transmitter float sticking at the full position.
Discussion
It was concluded that the engine failed due to lack of fuel. Several opportunities to notice the fuel gauge inaccuracy existed. The fuel tank had been removed during maintenance, and the engineer, when refitting the tank, had not noticed the inconsistency between the full indication and the amount of fuel added. Two instructors had accepted the 'full' indication as correct. It was not common practice for pilots to reconcile previous flight times and fuel burn against fuel remaining and fuel uplift. The flying club had operated two Grob 115 aircraft for 12 years with reliable fuel gauging and no visual means of checking fuel quantity, relying solely on the gauge. The aircraft is not normally refuelled to full due to take-off weight limitations with two medium-build occupants, but on this occasion both instructor and student were of light build.
Approximately 100 examples of this aircraft with this fuel tank configuration have been manufactured. Eight aircraft operating in Australia are equipped with a visual sight gauge. The manufacturer has no record of fuel gauging failures, but Australian operators have reported unreliable indications due to corrosion.
The CAA recognises that fuel gauges fitted to light aircraft can be unreliable and publishes advice to private pilots. In this model, however, it is not possible to check the fuel visually or 'dip' the tank because the fuel tank is located below and forward of the filler cap. The exact amount of fuel can only be confirmed when the tank is full by overfilling and allowing fuel to spill out.
Recommendations
The AAIB issued two recommendations. Recommendation 2002-10: That the Design Authority in Germany, the Luftfahrt Bundesamt, with the assistance of the aircraft manufacturer, Grob Werke GmbH & Co KG, review the reliability of the fuel gauging system on the Grob G115, G115A and G115B aircraft and consider mandating the installation of an additional means by which the fuel quantity can be checked. Recommendation 2002-11: That the CAA encourage flying clubs to have in place an effective procedure that enables pilots to reconcile aircraft fuel state against fuel used and fuel uplifted to act as a back-up procedure for unreliable fuel gauge(s).