Jet A freezes at -40°C, Jet A-1 at -47°C; both points are far below the temperatures normally reached in cruise, so aircraft fuel generally does not freeze at high altitudes. Even when the thermometer falls toward the fuel's freezing threshold, continuous pumps keep the liquid moving and mixing, stopping ice from forming in one corner of the tank before the rest. Should water droplets remain in the lines, fuel heaters add a second safeguard, preventing them from crystallising and blocking filters.
Although the fuel itself stays liquid, its flammability window is narrow: the lower limit for Jet A is roughly 38°C (100°F) in open air, while ignition begins near 300°C and open-air burning can reach 1,030°C; under ideal adiabatic conditions the flame peaks at 2,230°C. Because the freezing point is so low, the chief risk is not solidification but the reduced availability of fuel when tanks approach these limits on ultra-long polar routes.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
Is airplane fuel flammable?
Airplane fuel is classified as combustible, not flammable. Airplane fuel is not highly flammable like gasoline due to its high flash point which is far above gasoline's -43°C (-45.4°F). bThe higher flash point reduces the risk of accidental ignition, so a passenger jet is safer than a gasoline-powered car in terms of flammability.
How flammable is airplane fuel in practice? Inside a tank, flammability depends on temperature of the liquid fuel and the tank ullage. Whenever fuel temperature is sufficiently high, the fuel-air mass ratio in the ullage space climbs above the lower flammability limit of 0.03. Ratios between 0.040 and 0.072 sit in the flammable range. The center wing tank of TWA 800 held such a mixture, showing that, under those conditions, the vapor was ignited by a spark, flame, or hot surface. Outside these specific tank conditions jet fuel ignites only when the engine's design deliberately atomizes and heats it. Left alone it is not inherently flammable.
Is airplane fuel explosive?
Airplane fuel is not explosive by itself. The liquid fuel will not explode unless it first evaporates and mixes with air in the correct proportion. When a tank is partially empty, the space above the liquid - called the ullage - contains fuel vapor. If this vapor mixes with air so that the fuel-to-air ratio lies between 0.6% and 47 % by volume, the mixture becomes potentially explosive. Outside these limits the blend is either too lean or too rich to ignite, so maintaining sufficient fuel in the center wing tanks keeps the vapor concentration below the lower explosive limit and greatly lowers the risk. An explosion still requires an ignition source like a spark, arc, static discharge, or a hot surface inside the tank. Without that source, even an ideal mixture will not explode. Flight tests and accident data show that explosions have occurred in Boeing 747, 727, and Tu-154M center or wing tanks, yet the worldwide frequency is estimated at only one event every five years without preventative measures. Designers reduce the hazard by using high-flash-point fuels like JP-5 and JP-7, by adding static dissipaters and corrosion inhibitors, and by venting the ullage to keep the vapor-air ratio outside the flammable range during ascent, cruise, and descent.
Does airplane fuel generate heat? Jet fuel is a highly refined combustible liquid and does not spontaneously generate heat. Heat enters the fuel from the surrounding structure, from the airframe, or from the sun, and this external warming raises the liquid temperature. As the temperature rises, more fuel evaporates and the concentration of heavy fuel vapor - whose density is 5.7 times that of air - increases inside the tank. If the vapor-air mixture reaches the flammable range and an ignition source is present, the energy released during rapid combustion is experienced as an explosion. Thus, jet fuel does not create heat on its own but absorbs heat from its surroundings , and that heat sets the stage for evaporation, vapor-air mixing, and, under precisely defined conditions, an explosive event.
How hot does airplane fuel burn?
Jet A and Jet A-1, the two most common airplane fuels, burn at different temperatures. Before any ignition occurs, the fuel must first vaporise and reach its auto-ignition point. For Jet A this occurs at temperatures as low as 210°C (410°F) when the air-fuel ratio is favourable. Jet B, a wider-cut fuel, self-ignites at about 260°C (500°F). Flash-point and hot-surface limits are even lower: Jet A and Jet A-1 flash at or above 38°C (100°F), and AGARD studies recommend keeping all surfaces that contact Jet A below 240°C (464°F) to prevent hot-surface ignition. In flight, fuel-tank temperatures at cruising altitude normally stay between 38°C and 54°C (100°F and 130°F), well below ignition thresholds but warm enough to keep the fuel fluid at high altitude.
Under full power the fuels burn at approximately 980-1,500°C (1,796-2,732°F), and in modern high-performance engines the combustion gas approaches 2,000°C (3,632°F). The theoretical maximum adiabatic flame temperature for Jet A-1 is 2,230°C (4,050°F), while the visible flame sheet in the combustor can register about 1,980°C (3,600°F) under ideal conditions.
Does airplane fuel freeze?
Jet A-1, the grade used on every long-range passenger jet, freezes at -47°C (-52.6°F). That figure is the specification freezing point, the temperature at which solid hydrocarbon crystals first appear. The fuel is already wax-laden and sluggish a few degrees higher, so the true limit for safe operation is nearer to -40°C (-40°F). A fuel temperature sensor mounted in each tank lets the flight crew watch the number fall in real time. If the indicated value approaches the warning line, the crew requests a descent or a lower-latitude track while the crystals are still few and small.
The crystals themselves do not have to blanket the whole tank to stop an engine. British Airways Flight 38, descending into London Heathrow, lost thrust on both engines when a handful of ice crystals - grown from entrained water already near 0°C (32°F) - broke free from the full-length ducts and lodged in the narrow fuel-inlet elbows. The sudden blockage starved the engines of flow, proving that a few grams of ice have the same effect as a full freeze-up. Operators therefore treat -47°C (-52.6°F) not as a theoretical laboratory value but as the border that must never be crossed in cruise, let alone on approach.
To stay on the warm side of that border, modern jets borrow heat from the very engines they feed. Bleed air and hot oil are run through plate-and-fin heat exchangers and the warmed fuel is returned to the tanks, raising the bulk temperature many degrees above the ambient skin temperature. Routing supply lines through wheel wells or wing leading-edges adds a second, passive layer of protection, while the fuel itself, acting as a heat sink for hydraulic fluid and generator coolers, keeps moving and therefore keeps mixing, delaying local cold spots where crystals seed.
How does airplane fuel not freeze?
Airfuel fuel is prevented from freezing using additives. Additives lower the freezing point far below the temperature found at cruise altitude, so the fluid itself is engineered to stay liquid. Aircraft design keeps that liquid in motion. Pumps constantly circulate fuel through the system and motion prevents molecules from organizing into ice. While the fuel travels, it is warmed. Some lines are routed through areas that generate heat like the engine. Other designs run hydraulic lines close to the fuel so that the heat will help slow down potential freezing. A heat exchanger can also use fuel to cool engine oil, and the same loop returns warmed fuel to the tanks. Waste heat from other services warms the fuel further, and insulation surrounding the tanks prevents low external temperatures from affecting the fuel inside. Finally, pressurization prevents condensation inside the fuel tank from freezing, eliminating ice crystals that block fuel flow.
