FAR 91.151 dictates how much or how little fuel is required for VFR flight, expressed in time not gallons. Airplanes need 30-minutes reserve for daytime VFR and 45 minutes at night. Helicopters can squeak in with as little as a 20-minute buffer. What term tells ATC that you're not declaring an emergency but have reached a low enough fuel level that any undue delays might just trigger an emergency?
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