Factors That Lead To Fuel Freezing, Part 1

NTSB photo

A laboratory duplication of ice crystals clogging the Fuel-Oil Heat Exchanger on a Rolls-Royce Trent 800 engine is shown in photo from NTSB, Safety Recommendations A-09-17 and A-09-18.

Credit: NTSB

Pilots operating on flights through polar regions need to be aware of the unique issues that may affect flight safety. One of these issues is fuel freezing. Fuel that is exposed to extremely cold temperatures for long hours can chill enough to restrict the flow of fuel to the engine. This can occur through the formation of ice crystals that partially clog a fuel filter, or by the fuel’s viscosity becoming too thick for good “flowability” into the engines.
The phenomenon of ice crystals was brought to the industry’s attention on Jan. 17, 2008, when a Boeing 777-200ER operating as British Airways Flight 38 crashed just short of the runway at London Heathrow Airport after flying a 4,400 nm flight from Beijing. The cruise altitudes for this ultra-long-range flight over Mongolia, Siberia and Scandinavia were between FL350 and FL400, with outside air temperatures between -65 to -74 deg C.  

The flight crew monitored the temperature of the flight while enroute. The fuel temperature never dropped below -34 deg C, still well above its freezing point. However, the fuel did become cold enough for ice crystals to form in the fuel system, obstructing the fuel-oil heat exchanger enough to reduce the fuel flow to the engines when thrust was commanded during the final approach. When the aircraft was two miles from touchdown, the engines failed to respond to the demand for increased thrust from the autothrottles. The flight crew performed admirably in this low speed, low altitude and essentially zero-thrust condition. The aircraft impacted 890 ft. short of the runway.   Remarkably, only one passenger received serious injuries.

The U.S. National Transportation Safety Board (NTSB) investigated a similar event involving the unexpected rollback of an engine of a Boeing 777 on Nov. 26, 2008 while enroute from Shanghai to Atlanta. While cruising at FL390, the flight crew detected that the number 2 engine was operating below the commanded thrust level. The pilots descended the airplane to FL310 and performed the 777 engine response non-normal checklist. The engine recovered and responded normally. The flight continued to Atlanta and landed without further incident. 

Certification authorities and manufacturers monitored these investigations closely because the Boeing 777 had operated within its designed operating parameters throughout these long flights. The extensive investigation included building a mock-up of the jet’s fuel delivery system and subjecting it to the same environmental conditions as the flight experienced.
Jet fuel can contain small amounts of dissolved water. As the fuel temperature drops, the dissolved water can separate out, becoming a potentially serious problem if it freezes in fuel lines or filters. The UK Air Accidents Investigation Branch (AAIB) determined that small quantities of water froze to the inside of fuel lines that ran from the wing tanks to the engines on the British Airways flight. During the final stages of the flight in the warmer temperatures, the command for increased fuel resulted in a surge of soft slush that flowed into the fuel-oil heat exchangers where it froze again. The AAIB identified this problem specific to the design of the Rolls-Royce Trent 800 powerplant. The European Union Aviation Safety Agency subsequently mandated modifications to affected type.
Boeing subsequently issued a flight crew operations manual bulletin addressing the prevention of long-term ice accumulation in the Trent 800 series engines fuel system during extreme cold operations. Flight crews are instructed to follow specific refueling instruction before long-range flights when the ground fuel temperature is below 0 deg C. The bulletin also included a supplementary procedure to assist with clearing accumulated ice in the fuel system if the fuel temperature is below -10 deg C by briefly increasing the thrust of each engine to maximum climb thrust before descent. 

Some business aircraft require the usage of fuel system icing inhibitor as an additive prevents the formation of ice crystals. Fuel system icing inhibitors depress the freezing point of water in the fuel to -43 deg C.   Note that fuel system icing inhibitors must be evenly distributed throughout the fuel supply. It simply can’t be added into a fuel tank after the fuel has been pumped in.  

Fuel Freezing
How often have fuel temperatures become a concern? Major airlines operating flights in polar regions have analyzed the trends in fuel temperatures. Fifty-five percent of United Air Lines’ flights in polar regions had fuel temperatures below -35C.

The descriptive title of “fuel freezing” is a slight misnomer. It gives the false impression that the fuel simply turns into a solid frozen block, the equivalent of water freezing into ice. Jet fuels are a mixture of many different hydrocarbons, each with their own freezing point. Hydrocarbon components with the highest freezing points solidify first, forming wax crystals. As fuel cools, it begins to change from a complete liquid to a liquid with some wax crystals, then to a slush. Don’t confuse this with the formation of fine ice crystals in the fuel.
A fuel’s freezing point defined as: “The temperature at which the wax crystals in the fuel, formed previously when the fuel was cooled, completely disappear when the fuel is warmed.” The important point to remember is that the fuel needs to have the proper viscosity  (i.e., flowability or pumpability) at the engine inlet. The pour point is defined as the lowest temperature at which the fuel still flows, and this will be several degrees warmer than the fuel’s freezing point. 

The fuel in wing tanks exposed during long flights to cold stratospheric temperatures will be cooled to the temperature of the aerodynamic  boundary layer over the wing skin. This temperature is slightly lower than the Total Air Temperature (TAT). Some might wonder why the fuel isn’t chilled to the outside air temperature? The TAT reflects the effect from the impact of high-speed molecules against the wing’s skin, producing a modest amount of aerodynamic heating. Thus, one of the corrective factors in case of excessive fuel cooling is to increase the aircraft’s Mach number to provide more aerodynamic heating. 
In Part 2 of this article, we consider other variables that affect the temperature of the fuel in wing tanks.