The speed at which a large commercial airplane lands is not a single, fixed number but a calculated target speed that changes before every flight. This speed, known as touchdown speed, is the velocity at which the wheels physically touch the runway surface. For a typical passenger jet, this speed ranges from approximately 140 to 170 knots (160 to 195 miles per hour). The necessary landing speed is dynamically computed by the flight crew based on current aircraft conditions and the environment. The goal is to maintain a safe margin above the minimum flying speed while minimizing speed to ensure the shortest possible stopping distance.
The Critical Speeds During Approach and Touchdown
The final approach is governed by precise speeds to ensure a safe descent. The most fundamental speed is the Reference Landing Speed (Vref), the target speed the aircraft maintains as it crosses the runway threshold. Vref is calculated to provide a safety margin above the aircraft’s stall speed (Vso) in the landing configuration.
Vref is typically set at 1.3 times the stall speed, ensuring the wings produce sufficient lift to prevent an aerodynamic stall near the ground. For example, a medium-sized jet might have a Vref of 135 knots, while a heavy jet could be closer to 155 knots.
The actual target speed flown is V approach or V fly, which is Vref adjusted for wind. Pilots commonly add a wind correction factor, often half the steady headwind component plus the full gust factor. This added speed compensates for sudden decreases in headwind, such as wind shear, which could drop the plane’s speed too close to the stall margin.
Once over the runway, the pilot initiates the flare, a gentle pitch-up that slows the rate of descent. The actual touchdown speed is usually slightly lower than Vref, perhaps five to ten knots less, as the aircraft’s lift is reduced just before contact. This calculated system ensures the aircraft lands at the slowest possible speed while maintaining control and lift.
Key Variables Determining Landing Velocity
Landing velocity is not fixed for a specific aircraft model; it is recalculated before every landing based on dynamic variables. The greatest factor is the aircraft’s weight at the time of landing. A heavier aircraft requires more lift to stay airborne, and since lift is a function of speed, a higher landing weight necessitates a higher Vref and touchdown speed.
For instance, a Boeing 737 landing at its maximum weight might require a Vref of 145 knots. If the aircraft has burned off most of its fuel, the Vref could drop to 130 knots. Pilots must check the landing weight and recalculate Vref for each arrival.
Environmental factors like wind conditions also influence the target approach speed. A headwind component reduces the aircraft’s ground speed relative to its airspeed, allowing the aircraft to fly slower over the ground while maintaining the required airspeed over the wings. A strong headwind shortens the distance needed to stop after touchdown. Conversely, a tailwind increases the ground speed for the same indicated airspeed, requiring a longer runway.
Another consideration is density altitude, which combines the effects of air temperature and pressure on air density. On a hot day or at a high-elevation airport, less dense air reduces the lift the wings can generate. To compensate, a higher True Airspeed is necessary to achieve the required Indicated Airspeed (Vref) and maintain lift.
How Aircraft Decelerate After Contact
Once the wheels touch the runway, the aircraft must rapidly dissipate the kinetic energy it carries to slow down safely. This deceleration phase relies on a coordinated sequence involving three primary systems: aerodynamic braking, reverse thrust, and mechanical wheel braking.
The first system is aerodynamic braking, which begins the moment the main wheels touch down. Ground spoilers, large panels on the upper surface of the wings, automatically deploy upwards. This deployment increases drag and destroys the remaining lift the wings are producing. Destroying the lift transfers the aircraft’s full weight onto the landing gear, allowing the wheels to achieve maximum braking friction.
Almost simultaneously, the flight crew activates the thrust reversers, mechanisms built into the engine nacelles. These systems redirect the powerful exhaust airflow forward, creating a significant braking force. On high-bypass turbofan engines, cascade-type reversers slide back to reveal vanes that force the fan air forward. Thrust reversers are most effective at high speeds, providing a large portion of the initial deceleration force.
The final braking system involves the mechanical wheel brakes, which are similar to a car’s disc brakes but larger. These brakes are paired with an advanced anti-skid system that modulates pressure to prevent the wheels from locking up. The mechanical brakes take over as the primary stopping force once the aircraft slows down, usually below 60 or 70 knots, bringing the machine to a complete stop or taxi speed.