jagged edge. A sharp and irregular edge on a piece of metal, wood, or plastic material.
jam acceleration (gas turbine engine operation). Rapid movement of the power control lever of a gas turbine engine which calls for maximum acceleration of the engine.
jamming (radar and radio). Electronic or mechanical interference used to disrupt the display of aircraft on radar or the transmission/reception of radio communications or navigation information.
jam nut. A thin nut screwed down on top of a regular nut to lock it onto the bolt. The jam nut keeps the regular nut from backing off. A jam nut is sometimes called a check nut.

JAN specifications (joint Army-Navy). Specifications for technical equipment that have been established by a joint board of the United States Army and Navy. These specifications describe the equipment in detail. They are usable for civilian equipment as well as for equipment built specifically for the military.
JATO (jet assisted takeoff). A system of auxiliary thrust used for heavily loaded aircraft to assist their takeoff. JATO units are small rocket engines fastened to the aircraft structure when a difficult takeoff is to be made.
The JATO units are fired the moment the aircraft rotates for takeoff, and the boost from the rocket engines provides the additional thrust needed to get the aircraft airborne. JATO is more properly called RATO for Rocket Assisted TakeOff.
javelin. The streamlined wood or metal rod that is secured at the intersection of the landing and flying wires of a biplane. Javelins prevent the wires from vibrating in flight.
jerry can. A flat-sided five-gallon can, used for carrying fuel. Jerry cans are made so they stack together and fit in special racks on vehicles.
jet (fuel metering system component). A special threaded plug with a specifically designed hole drilled through it. The diameter and length of the hole and the approach angles to the hole determine the amount of fuel that can flow through the jet for a given pressure drop across it.

Jet-A fuel (gas turbine engine fuel). A kerosine-type fuel used in most commercially operated jet-propelled aircraft. Jet-A is similar to military JP-5 fuel. Its flash point is between 110° and 150°F, and its freezing point is –40°F.
Jet-A1 fuel (gas turbine engine fuel). A kerosine-type fuel similar to Jet-A fuel except that it contains additives that decrease its freezing point to –58°F. Jet-A1 fuel is used in aircraft that remain at high altitudes for long periods of time. Jet-A1 fuel is similar to NATO JP-8 fuel.
jet assisted takeoff. See JATO.
Jet-B fuel (gas turbine engine fuel). A gasoline-type fuel used in most military jet-propelled aircraft. Jet-B fuel has a flash point of 0°F and a freezing point of –76°F. Jet-B fuel is similar to the military JP-4 fuel.