The FOHE serves the dual purpose of cooling the engine lubricating oil and, in the process, warming the fuel such that ice does not affect the downstream components.

The blockage causing the momentary interruption of fuel flow and the failure of the engines to respond to the command for increased thrust in the moments before landing, when additional power was necessary to compensate for flaps and landing gear deployment. (See Aviation Safety & Security Digest, ‘Ice Blocked The Heat Exchanger, Leading to Crash,’ home page.)

The United States patent, Number 4201044, was issued to engine manufacturer Rolls-Royce Ltd., whose Trent 800 engines were on the accident aircraft, in May 1980. The technology, under the heading of “Fuel Systems for Gas Turbine Engines,” was apparently never deployed, but this is understandable. Relatively few jetliners were operating in arctic skies at that time, and the kind of extreme low temperatures experienced by the accident airplane, both in Beijing and on the flight to London, are rare occurrences.

In addition, constant angle of descent approaches to landing were not the norm in the 1980s, when the traditional step-down approach to the airfield was used. The new constant angle of descent provides less time for the fuel to be heated by the ambient air as it moves from the LP pump to the FOHE and ultimately to high pressure (HP) pump – which is the stage at which the fuel pressure is raised and the fuel is atomized for injection through the burners inside the engine’s combustion chamber.

The patent explains how ice is prevented:

“A fuel system is provided with two filters between the LP pump and the HP pump, both of which are used (in parallel) when the temperature of the fuel is above 0ºC [32ºF]. When the temperature of the fuel drops below 0ºC, all the LP fuel is passed through the first filter, and a small supply of warm HP fuel is passed through the second filter. Ice collects in the first filter until a predetermined pressure drop across it causes all the LP fuel flow to be switched to the second filter. The first filter is then de-iced by a small supply of warm HP fuel. This cycle is repeated continuously while the temperature of the LP fuel is below 0ºC.”

The technical description concludes that the system “is particularly compact and hence has a low cost and low weight compared to more conventional heating systems for preventing the formation of ice in the fuel.”

According to the technical description:

“A particular problem encountered by aircraft operating in climates where the temperature drops below 0ºC is the formation of ice in the fuel. This is unavoidable since water becomes entrained in the fuel in warmer climates such as by condensation of damp air in the fuel tanks of the aircraft [the situation that applied to the accident airplane’s layover in Beijing], or even in storage tanks on the ground. This water content does not present a problem when the temperature is above the freezing point, but below freezing point ice crystals form in the fuel and tend to block fuel filters, particularly at high fuel flow rates, such as when the aircraft is taking off, or otherwise using high power and consequently large amounts of fuel [after considerable time at idle power, the engines on the accident airplane were commanded to increase thrust just before touchdown].”

The heart of the system as a so-called “spool valve,” which switches the flow of fuel when a predetermined quantity of ice has accumulated in one of the filters, which is then thawed as the spool valve directs the flow of fuel to the other filter.

The spool valve has three positions. The first position is used when the main fuel flow is delivered to both filters, which is the case at temperatures above 0ºC. The second and third positions are utilized when the fuel temperature drops below 0ºC, and the fuel is delivered to one or the other of the two filters. The system measures the pressure drop across each filter, and at a predetermined setting the spool valve will move from its second to its third position, and vice versa.

Using hot oil, as in the FOHE, or using air or electrical heaters, tends to result in bulky, heavy units “since they must be capable of heating all the fuel flow to above 0ºC to melt the ice in the fuel.”

It should be mentioned that the purpose of the FOHE is primarily to cool engine oil, not to heat the fuel. The interim AAIB report indicated that ice accumulation was likely to have interposed between the LP pump and the FOHE. The solution to such blockages may be to move the FOHE closer to the pumps. This course would involve a fair degree of re-plumbing. Placing the two filters and the spool valve between the LP pump and the FOHE will not be trivial, either, but seems a valid stratagem to insure a lack of ice in the fuel flow.

The installation would not be needed on all aircraft of a particular model. Rather, it would only have to be fitted to those aircraft flying polar routes.