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Preheating Fuel May Prevent Ice Blockage

Fri, Mar 20, 2009 — David Evans

Articles, Featured

An urgent safety recommendation has been issued after two airliners had their engines starved of fuel because of ice in the fuel supply lines. The National Transportation Safety Board (NTSB) recommended 11 March 2009 to the U.S. Federal Aviation Administration (FAA) and to the European Aviation Safety Agency (EASA) that engine manufacturer Rolls Royce redesign a key component in the fuel supply piping to reduce the susceptibility to blockage from water ice.

The recommendation stems from two events – an accident and an incident. The accident occurred 17 January 2008 when a British Airways B777 experienced a loss of engine thrust just short of landing at London’s Heathrow Airport after a flight from Beijing. The aircraft pancaked into the ground 1,000 feet shy of the runway. Although the 152 people aboard successfully evacuated the airplane, it was damaged beyond the point of economical repair.

 

The British Airways B777 crashed moments before landing because of ice blocking the fuel lines.

The British Airways B777 crashed moments before landing because of ice blocking the fuel lines.

The UK’s Air Accidents Investigation Branch (AAIB) has on ongoing investigation into the accident, which appears to have resulted from ice blocking the flow of fuel to the engines, right at the moment before touchdown when the pilots called for increased power.

In a September 2008 statement, the AAIB said:

“Extensive data analysis has revealed that not only has there never been a previous occurrence of this type on the Boeing 777, but also that this is the first known occurrence of this nature in any large transport aircraft.”

It is conceivable that no other such incidents were reported, but that does not mean they did not occur, although with less severity than at Heathrow.

In any event, a second “rollback” of engine power occurred 26 November 2008 over Montana, when a Delta Airlines 777 on a flight from Shanghai to Atlanta experienced a loss of power in one of its two engines. The pilots commanded the airplane to descend to 31,000 feet and executed the engine response non-normal checklist that Boeing provided all 777 operators after the British Airways accident at Heathrow.

Engine power was restored and the flight continued to Atlanta. The NTSB has been investigating the incident, and has had an accredited representative participating in the AAIB investigation as well.

We’ve come a long way from the initial AAIB declaration as to the rarity of these ice-blockage events.

“With two of these rollback events occurring within a year, we believe that there I a high probability of something similar happening again,” said NTSB Acting Chairman Mark Rosenker. “We are encouraged to see that [engine manufacturer] Rolls Royce [whose engines were on both the British Airways and Delta Air Lines planes] is already working on a redesign, and we are confident that with the FAA and EASA overseeing the process, this flight safety issue – even one as complex as this – will be successfully and expeditiously resolved.”

From The Times newspaper of London, are cartoon that portrays the situation until a real, final fix comes along.

From The Times newspaper of London, are cartoon that portrays the situation until a real, final fix comes along.

Rolls Royce has indicated that a redesigned fuel system – more specifically, a modified fuel/oil heat exchanger (FOHE) – will be tested, certified and ready for installation within 12 months. The NTSB wants the redesigned unit installed within 6 months of certification.

One wonders why it will take so long. After all, Rolls Royce patented an ice-resistant fuel system in 1980. Not only has the technology been developed, the NTSB has urged a redesign because the interim procedures are problematic:

“While the [interim] procedures may reduce the risk of a rollback in one or both engines due to FOHE ice blockage, they add complexity to flight crew operations, and the level of risk reduction is not well established. And because the recovery procedure requires a descent, the aircraft may be exposed to other risks such as rising terrain or hazardous weather, or the inability to achieve maximum thrust during a critical phase of flight, such as during a missed approach.”

Let’s look at the added complexity, which was outlined in a 29 September 2008 Boeing flight crew operations manual bulletin, issued after the British Airways accident. The bulletin instructed flight crews to perform an ice clearing procedure three hours before descent if the fuel temperature was below -10º C (14º F) by briefly increasing the thrust of each engine to maximum climb power. In addition to this procedure, Boeing added an engine response non-normal procedure that instructed pilots to temporarily reduce fuel flows to moving the thrust levers to minimum idle – the opposite of the foregoing maximum climb power setting – to clear ice from the FOHEs if the engines did not reach commanded thrust.

In the Delta engine rollback, the NTSB determined that the FOHE was restricted by ice for about an hour before the rollback. The NTSB said:

“Both the British Airways and the Delta rollbacks occurred when the measured fuel temperature was about -22º C (-7.6º F). Critical fuel icing temperatures, those between -5º C (23º F) and -20º C (-4º F) are commonly encountered by long-range transport airplanes. Therefore, the Safety Board concludes that there is risk of ice crystal blockage of FOHEs on Trent 800 series engines installed on 777-200s at temperatures commonly encountered by long-range transport airplanes.”

On 23 February 2009, Rolls Royce issued a memorandum to operators stating its intent to redesign the FOHE on Trent 800 series engines. This memorandum came more than a year after the British Airways accident and subsequent testing of the FOHE by AAIB-directed laboratory testing at Boeing. Tests showed that ice crystals from entrained water in the fuel will accumulate, and that this ice can clog up the inlet face of the FOHE.

 

Ice accumulation on the inlet face of a Trent 800 Fuel/Oul Heat Exchanger (FOHE) during testing.

Ice accumulation on the inlet face of a Trent 800 Fuel/Oul Heat Exchanger (FOHE) during testing.

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. In the case of the two rollback events, ice caused the engines in the British Airways and Delta events to be starved of fuel.

One recourse to ice blockages may be to move the FOHE closer to the fuel pumps. This course would involve a fair degree of re-plumbing. Another stratagem may be to install a patent that Rolls Royce received in 1980 but never deployed. The technology, under the heading of “Fuel Systems for Gas Turbine Engines,” was not 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 British Airways and Delta 777s were rare occurrences.

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 [high pressure pump] fuel is passed through the second filter. Ice collects in the first filter until a predetermined pressure drop across it causes all the LP [low pressure pump] 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.”

 

Schematic from 1980 patent showing the fuel system incorporating two filters. One is blocked by ice, so the fuel is flowing through the second to the HP pump. The FOHE is shown as #26 above; the engine burners are shown as #28.

Schematic from 1980 patent showing the fuel system incorporating two filters. One is blocked by ice, so the fuel is flowing through the second to the HP pump. The FOHE is shown as #26 above; the engine burners are shown as #28.

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 may have applied to the two 777s during their layover in China], 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.”

The heart of the system is 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.

The spool valve, showing how it provides fuel from one or the other filter.

The spool valve, showing how it provides fuel from one or the other filter.

 

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,” the patent declares.

Yet a redesigned FOHE seems to be the recourse of choice. One wonders if today, 29 years after the patent, anyone at Rolls Royce remembers the design and why it was thought superior to modifying the FOHE and related plumbing.


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