Fuel mismanagement

Citation

(1998), "Fuel mismanagement", Aircraft Engineering and Aerospace Technology, Vol. 70 No. 3. https://doi.org/10.1108/aeat.1998.12770cab.047

Fuel mismanagement

Fuel mismanagement

Keywords Aircraft, Engines, Safety

A Cessna 421 aircraft was on a private flight from Elstree to Shobdon in Hertfordshire, UK. The meteorological forecast indicated that a warm front was approaching Southern England from the south-west and conditions were generally deteriorating. The visibility on departure from Elstree at 14:37hrs was greater than 10km with a broken cloud base at 2,500ft. When the aircraft arrived at Shobdon the visibility was estimated to be 3-4km in light drizzle with a cloud base at approximately 1,200ft, and the surface wind was 090°/5kt. The first radio contact between the aircraft and Shobdon was made at about 15:02 hrs when the pilot called to say that he was inbound from Elstree. In response to this call he was passed the airfield details. The pilot later called when approaching Leominster and subsequently called downwind for runway 09 which has a right hand circuit. The operator of the ground to air facility at Shobdon saw the aircraft on the downwind leg abeam the tower at what appeared to be a normal circuit height. He did not observe the aircraft downwind but shortly afterwards he heard a brief and indecipherable radio transmission which sounded like a scream. This same transmission was heard by an aircraft enthusiast who was monitoring the radio transmissions on an "airband" radio. The radio operator repeatedly attempted to make contact with the aircraft but to no avail and so he instructed an aircraft refueller to inform the emergency services that an aircraft had crashed.

Aircraft fuel system

The fuel system on this aircraft consisted of a main tank and an auxiliary tank associated with each engine; the fuel selector also allowed fuel from a main tank to be fed to the opposite engine. The main tanks were located in each wing tip and the auxiliary tanks were in the main wing structure outboard of each engine. The useable fuel was 189 litres (50 US gallons) in each main tank and 182 litres (48 US gallons) in each auxiliary tank; thus the total fuel capacity was 742 litres (192 US gallons).

On the day prior to the accident the aircraft had been refuelled with 60 litres of fuel into each main tank. There was no record of the fuel tanks having been completely full in the recent past. With the errors involved in attempting to produce accurate estimates of the fuel consumption since then, the resultant figures are likely to have been highly inaccurate; therefore, the exact fuel status prior to and after this refuelling is unknown. The pilot flew to Ostend, Belgium, for an overnight stay, a flight time of 51 minutes. While at Ostend he did not refuel, although he did buy two litres of engine oil. On the return flight to Elstree he was airborne for 1 hour 46 minutes after which he refuelled with 50 litres into each auxiliary tank. The aircraft then crashed 45 minutes after take-off from Elstree. Using a fuel flow of 110 litres per hour for these three flights, and allowing for each take-off and climb to 5,000 feet with the subsequent circuit to land, a fuel consumption of about 445 litres is calculated.

Medical aspects

The post-mortem examination on both pilots did not reveal any indications that drugs or alcohol had played any part in the event; however, both pilots had considerable pre-existing disease. The pilot-in-command had high blood pressure, which was being treated, but there was some doubt about his compliance with his treatment. There was also evidence of coronary artery disease and he had previously suffered a heart attack. The Australian pilot did not have a valid medical certificate and therefore was not permitted to fly as a pilot. His medical certificate had not been valid beyond 10 December 1996 since he had not complied with the requirements of the Office of Aviation Medicine, Civil Aviation Safety Authority of Australia. He was a very large man, 195cm tall and weighing 148kg. It was not possible to define the precise role, if any, of the medical condition of either pilot in this accident. However, with a large person in the left-hand seat it would have been difficult for either pilot to monitor and operate the fuel selector switches, which were located on the cockpit floor between the seats.

Engineering information

Both propellers had separated as they struck the ground. The left-hand propeller blades were relatively undamaged and did not show any chord-wise scoring indicative of rotation; likewise there was no blade tip damage. Note: the propeller is geared to rotate at 75 per cent of the engine speed and will not windmill at low aircraft speed. The right propeller showed signs of rotation and had light damage to two blades; there was a small amount of forward curl on the tip of one of the blades consistent with engine power at impact.

Recollections of the survivor

At the time of the accident the sole survivor had been seated behind the pilot in the right-hand seat and was facing aft with his lap strap secured. He suffered significant injuries to the head and was not interviewed until five weeks after the accident at which stage he was still affected by trauma. He had approximately 1,000 hours' flying experience of which about 500 hours were on twin engine aircraft. Most of this flying had been completed at Elstree and he had known the pilot in command of the accident aircraft for about eight years.

The survivor has very little firm recall of events on the day of the accident and was very keen to differentiate between those memories that were definite and those that were less so. However, he did have some vague recollections of the flight which are summarised.

He believes that the weather at Elstree was cloudy prior to departure but that it was a reasonable day for flying. His next recollection was that while downwind at Shobdon the left engine surged and then stopped. The pilot-in-command (right-hand seat) then tried to crossfeed fuel to the left engine; he does not recall which tanks were in use. At this stage the pilot in the left seat was flying the aircraft. The survivor believes that he looked out of the window and saw that they were flying very low over a rural landscape with some trees and open farmland. The weather was overcast with grey clouds and it was raining. The speed was low, the stall warning was operating and then the aircraft suddenly entered a spiral to the left. He had little recollection at all of the impact but while in hospital he had told his mother that he remembered somebody screaming; however, at the time of the interview he could no longer remember this event. He also had the impression that there was not very much fuel on board and that the intention had been to refuel at Shobdon.

Some elements of this recollection can be verified. His description of the weather at Elstree and Shobdon are correct as is the stage of flight and the countryside in which the aircraft crashed. His memory of flying low is substantiated by the two witnesses one of whom saw the aircraft roll to the left. Furthermore, the engineering evidence demonstrates that the left propeller was stationary at impact and that there was very little fuel in the aircraft.

In summary, examination of the engines showed that they had both been mechanically and electrically capable of running; however, at impact the left engine was stationary. It was also likely that there was very little fuel on board the aircraft at the time of the accident. It is therefore probable that mismanagement of the fuel system caused the left engine to stop. The eyewitness accounts are consistent with the behaviour of a twin engine aircraft that has suffered a failure of one engine and is flown below its mimimum control speed for flight on one engine. With a low power setting on the right (live) engine the speed was allowed to reduce further until the left wing stalled. There was then insufficient height available to regain control of the aircraft.

ReferenceAAIB Bulletin, January 1998.