Plain English Meaning
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.
Full 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.
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.