front-to-back ratio (semiconductor characteristic). A rough measurement of the condition of a semiconductor diode. When the resistance of the diode is measured with an ohmmeter, it should have a high resistance one way (the junction is reverse-biased), but when the leads are reversed, the resistance should be low (the junction is forward-biased). A high front-to-back ratio normally indicates a good diode.
frost (meteorology). Ice crystal deposits formed by sublimation when the temperature and dew point are below freezing.
frustrum. The portion of a cone or a cone-shaped solid from which the top has been removed. The top and the bottom of the frustrum are parallel surfaces, with the top surface smaller than the bottom.
FSDO (Flight Standards District Office). An FAA field office serving an assigned geographical area, staffed with Flight Standards personnel who serve the aviation industry and the general public on matters relating to the certification and operation of air carrier and general aviation aircraft.
FSDO activities include general surveillance of operational safety, certification of airmen and aircraft, accident prevention, investigation, enforcement, etc.
FSS (Flight Service Station). Air traffic control facilities which provide these services:
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 Pilot briefing

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Enroute communications

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 VFR search and rescue services

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 Assistance to lost aircraft and aircraft in emergency situations

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 Relay ATC clearances

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 Originate Notices to Airmen

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Broadcast aviation weather and NAS information

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 Receive and process IFR flight plans

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Monitor navaids
Some flight service stations perform these services in addition:

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Provide Enroute Flight Advisory Service (Flight Watch)

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Take weather observations

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 Issue airport advisories

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Advise Customs and Immigration of transborder flights

fuel. Any material that can be burned to release its energy. Gasoline, kerosine, coal, wood, and natural gas are all forms of fuel.
fuel-air mixture ratio. The ratio of the number of pounds of fuel to the number of pounds of air in the mixture burned, in an internal combustion engine.
fuel boost pump (aircraft fuel system component). An auxiliary fuel pump used in an aircraft fuel system to provide fuel pressure for starting the engine, for use as an emergency backup in case the fuel pump fails, and to transfer fuel from one tank to another.
fuel control unit (gas turbine engine). The fuel metering device for a gas turbine engine that senses the variables of power lever position, pressure, speed, and temperature in the engine. The fuel control unit then meters the correct amount of fuel to be sent to the burners for the existing conditions.