Definition
A military-specification turbine engine fuel that is a wide-cut blend of kerosene and gasoline fractions. JP-4 has a lower flash point and higher volatility than civilian Jet A, with a freezing point near -72°C, which made it suitable for cold-weather and high-altitude military operations. It was the primary U.S. Air Force jet fuel from the 1950s until it was largely replaced by JP-8 in the 1990s due to JP-4's flammability hazards.
Plain English
An older type of jet fuel used mainly by the military. It was a mix of kerosene and gasoline, which made it light and good for cold conditions but easier to ignite than regular jet fuel.
Context Anchor
Seen in aircraft fuel specifications, military aircraft manuals, fueling records, and maintenance references for older turbine-powered aircraft.
Derivation
The 'JP' stands for Jet Propellant, a U.S. military fuel-naming series. The number 4 simply marks its place in the sequence of specifications (JP-1, JP-2, JP-3, JP-4, etc.) as fuels were refined and standardized over time.
Why Pilots Care
If a pilot is operating or maintaining an older military or surplus aircraft, knowing the fuel specification matters: JP-4 burns and vaporizes more readily than civilian Jet A, which affects starting behavior, cold-weather performance, and ground handling safety.
Intuition Check
Do not assume JP-4 is the same as Jet A. Both are jet-engine fuels, but JP-4 is lighter, more vapor-forming, and must be treated as its own approved fuel grade.
Example Sentence 1
The aircraft's original manuals specified JP-4, but modern operators typically run it on Jet A with approved adjustments.
Example Sentence 2
Because JP-4 evaporates more readily than heavier fuels, crews follow extra fire-safety steps during refueling.