liftoff

Liftoff is the precise moment when an aircraft's weight transitions from resting on its landing gear to being fully supported by aerodynamic lift. For fixed-wing aircraft, this occurs when airspeed reaches a value called V2 (takeoff safety speed), typically 10 to 15 percent above stall speed. For helicopters, liftoff happens when rotor thrust exceeds gross weight. The aircraft nose rotates upward (for conventional takeoff) and the main gear breaks contact with the runway or pad.

In maintenance records and flight data, liftoff is marked by discrete sensor signals: weight-on-wheels switches in the landing gear struts open, and if equipped, the air data computer confirms positive vertical velocity. Maintenance technicians distinguish liftoff from rotation (when the nose wheel lifts but main gear remains planted) because they affect different systems. Rotation loads pitch control surfaces; liftoff engages the landing gear retraction sequence and commits the aircraft to flight.

Variants and conditions

Liftoff speed varies by aircraft weight, runway temperature, altitude, and wind. A Boeing 737 at maximum takeoff weight on a sea-level runway lifts off around 150 to 180 knots depending on configuration; a Cessna 172 around 50 knots. Contaminated or soft runways delay liftoff because wheels sink slightly, increasing rolling resistance. Hot days and high-altitude airports reduce air density, requiring longer ground roll and higher true airspeed to reach liftoff.

Premature or delayed liftoff indicates maintenance issues. Landing gear that retracts before rotation causes collapse; nose gear failures prevent proper rotation geometry. Trim tabs rigged incorrectly make reaching rotation angle difficult or impossible within safe runway length. Excessive weight-and-balance drift aft reduces the lift coefficient needed for flight, lowering liftoff speed but increasing pitch control demands.

The term has been standard since powered flight began; early test pilots recorded liftoff as a discreet event in logbooks. Modern glass cockpits log liftoff time and altitude automatically. For certification and investigation purposes, liftoff is the legal boundary: below it, the aircraft is a ground vehicle; above it, the pilot must fly it. Runway excursions, overruns, and aborted takeoffs are classified by whether liftoff occurred.