The US Federal Aviation Authority have now issued an Airworthiness Directive (AD) concerning the problems with ice in the fuel system of Boeing 777-200 and 300 airplanes fitted with Rolls Royce Trent 800 engines. The AD requires the crew to monitor the fuel temperature within 3 hours of top of descent and if it is below a certain temperature they must increase the engine power to prevent the formation of ice in the main tank fuel supply system. They can do this by stepping up to a higher altitude or increase the airspeed. There is no requirement to carry out this procedure if the aircraft is not fitted with Trent 800 engines.

There is also a procedure for the circulation of the fuel on the ground by a certified mechanic to prevent the formation of ice.

This AD is a result of the problems encountered in January 2008 by a British Airways Boeing 777 which failed to respond to a command for increased thrust while the aircraft was on approach to London Heathrow airport and landed short of the runway. the interim report suggested ice formed in the main fuel tank supply system during low fuel flow conditions in cruise flight. This ice then dislodged and caused a restriction in fuel supply during high flow conditions during approach.

No doubt there will be more measures to prevent the formation of ice from the engine and aircraft manufacturers but in the mean time the FAA are counting on this procedure to prevent the recurrence of the fuel restriction and loss of engine power.

British Airways Boeing 777 Crash at Heathrow

Ice has always been the enemy of aircraft, it builds up on the wings and reduces lift and it lies on the runway and reduces braking and directional control when landing, but now there is a new hazard. After nine months of investigation the Air Accidents Investigation Branch (AAIB) have decided the cause of the British Airways Boeing 777 landing short at Heathrow and crashing was due to ice in the fuel. All the passengers and crew escaped the aircraft with only minor injuries apart from one man who broke his leg.

It has been known for years that there is water in aviation fuel and when the temperature drops, due to the aircraft operating at altitude, ice crystals form in the fuel. There have been chemical and mechanical solutions to this problem and it is a largely unknown problem in large commercial aircraft.

The fuel passing through a Trent 800 engine like the one that was powering the BA aircraft goes to an oil/fuel heater in the engine fuel system. This takes hot oil from the engine to a matrix of small pipes and passes fuel around the pipes on its way to the fuel control unit and the burners. This is normally sufficient to melt any ice that may be in the fuel so it will not interfere with the operation of the engine. The theory of the investigators is that ice formed before the engine oil/fuel heater and reduced the amount available for the engines to use.

The High Pressure fuel pumps that are downstream of the oil/fuel heater showed evidence of cavitation. This is a condition that happens when the impellor of the pump has an insufficient fuel supply to boost to a higher pressure and mechanical damage is caused to the pumps. Clearly there was low fuel supply to the engines but where was the restriction? Because of filters, low-pressure pumps and valves that are all in the fuel supply system it has not yet been determined as to where the problem lies.

Why it occurred is also a mystery as there has never been a case of fuel starvation due to icing in the fuel system of a commercial airliner. The Flight Data Recorder has shown that within 6 seconds both engines reduced power to a value below that needed to maintain flight and the automatic systems and the pilots took the required actions to supply more fuel but to no avail as it could not get to the High Pressure Pump.

The low fuel flow at cruise power settings on an especially cold, long flight may have caused a build up of ice in an area in the fuel system which, with the high fuel demand in the landing phase of flight, detached from its location and caused the restriction. This theory has not yet been proven despite extensive and imaginative testing. The reason it is being put forward is every other scenario has been eliminated. As this is a preliminary report we will have to wait some time, maybe years, before the final report is issued and the cause is known. In the mean time the FAA and the manufacturers of the engine and aircraft are going to look at ways to mitigate the effects of long, cold flights on the fuel system.