How fast do planes fly?

Step 1 of 8 · Explain

Cruise: about 830 km/h, Mach 0.78

In the model, an A320 of 64 tonnes cruises 11 km up at 829 km/h. At that height the air is −56.5 °C, and in cold air sound travels more slowly: 1,062 km/h instead of 1,225 km/h at sea level. So the same 829 km/h is Mach 0.78 up there. That is why pilots and flight plans state the cruise speed as a Mach number rather than in km/h.

In steady cruise the engines push exactly as hard as the drag holds the plane back, 40 kN in the model, about two thirds of the thrust lever. The speed through the air is what the wing and the engines feel. The number on the cabin's flight map is something else, the speed over the ground (see below).

Step 2 of 8 · Try it

Why airliners don't fly faster

The engines aren't the limit. Over the curved top of the wing the air speeds up, and near Mach 0.8 it goes faster than sound in places. Shock waves form there, and the drag starts to rise steeply; aerodynamicists call this drag divergence. In the model at 11 km the A320 meets 40 kN of drag at 829 km/h, 50 kN at 900 km/h and 73 kN at 950 km/h, almost twice as much as in cruise for 15 % more speed.

Flying faster would therefore cost a lot of fuel for little time saved. To hold its height at 900 km/h the model plane burns 311 kg of fuel per 100 km instead of 273 kg, at 950 km/h 434 kg. At full thrust up there, about 56 kN, it can't hold its height above about 920 km/h. The A320's certified maximum is Mach 0.82, about 870 km/h (Airbus A320 family specifications). Airlines choose a cruise speed just below the drag rise.

Flying lower doesn't help: the air is denser down there, and the same 829 km/h meets 92 kN of drag at sea level, more than twice as much as at 11 km.

Step 3 of 8 · Predict

Speed alone doesn't lift a plane off

A common idea is that a plane simply lifts off by itself once it is going fast enough, and that the nose only comes up afterwards to climb. Some also picture the engines at full power pushing the plane up. Both miss what happens at the wing.

On the runway an A320's wing meets the air at only about 1°, with all wheels on the ground. In the model, with takeoff flaps and full thrust at 280 km/h, the speed at which it normally lifts off, the wing then makes only 382 kN of lift against 628 kN of weight, about 61 %. Kept like this, the plane would need about 370 km/h to lift off, and more than 2 km of runway instead of about 1.1 km. The engines push forward, along the runway; the wing has to turn that speed into lift.

Step 7 of 8 · Explain

Rotation: the nose comes up and the plane lifts off

What lifts the plane off is the pilot raising the nose. Pilots call it rotation, and the speed at which they start it VR. The elevator at the tail pushes the tail down and the nose up, and the wing meets the air more steeply: each degree adds about 40 kN of lift at 280 km/h. At about 8° the A320's wing makes 670 kN, more than its 628 kN weight, and the plane leaves the runway. The model lifts off when the lift is 5 % above the weight, at about 278 km/h.

There is a limit: above about 11° the tail would hit the runway, a tail strike. So rotation has to come at the right speed, not too early. After lift-off, with the angle held at 8°, the plane settles at about 270 km/h and climbs about 16 m/s, after about 1.1 km of runway.

Step 6 of 8 · Try it

Weight and flaps decide the takeoff speed

A heavier plane needs more lift, and at the same angle that means more speed. In the model, with takeoff flaps and rotated to 8°, the A320 lifts off at 246 km/h at 50 t, 278 km/h at 64 t and 307 km/h at its maximum of 78 t. The full plane also needs more runway: about 1.8 km instead of 0.7 km for the light one. That's why pilots work out the takeoff speeds before every flight from the actual weight.

Flaps and slats make the wing more curved and larger, so it lifts enough at a lower speed. Without them, the same plane at 280 km/h and 8° makes only 443 kN of lift; it would need about 340 km/h and 1.7 km of runway. With takeoff flaps it lifts 670 kN. The landing setting lifts even more, 942 kN, but doubles the drag: once it has settled after lift-off, the plane climbs only about 10 m/s instead of 15–16. So airliners take off with a middle setting and save the full flaps for landing.