why do jet planes increase thrust just before touching down?

Question

pilots of jet airliners throttle back on final descent but then seem to add a short burst of power on crossing the threshold, before throttling back again and then leaving the throttle off to touchdown. Why this burst of power just before the landing flare? Isn't it a waste of fuel?

Answer 1

Some really wild answers, guesses here.
The additional thrust is used to get the nose of the plane up for the flare. It helps to slow the descent rate and keeps the nose wheel up. The object is to touch the wheels down at stall speed. The added thrust gives just enough control so the plane does not drop onto the runway.
Any pilot knows that about flaring before landing.
It is not for safety in a go around.
Boeing pilot?

Get A Grip.

Answer 2

While landing an airplane, especially large jets, the pilot is in a sense balancing the airplane on a thin needle and at the same time trying to thread the needle through a very small hole.

While on final the pilot is providing minimal to zero power because the airplane basically converting gravity, or its downward motion into forward motion. The pilot gets stabilized on what's called a glide slope, which is an imaginary line angle straight out from the runway and angled up about 3 degrees. To hold the glide slope the pilot usually keeps the power in the same spot and uses pitch to control his speed (nose up or down). Well just about when the airplane is crossing the threshold the pilot starts what is called a "flare". He raises the nose of the airplane in order to decrease the rate of descent so that the airplane will hit the ground much softer and so he touches down on the rear two wheels, the "mains". As he raises the noise it increases drag which then slows the airplane down which would then make the airplane fall even faster because you are loosing lifting...so to keep the speed up he adds just a little power. If the pilot does everything right it makes for a really smooth soft landing.

Answer 3

Since you have said that the plane is on the runway and since you compare it to the speed of sound, I suspect that what you are hearing is the effect of thrust reversers on commercial jet aircraft. I do not recall ever intentionally increasing power in the flare, unless I was really dragging it in and then it was a recovery, not a plan for a smooth landing. Thrust reversers continue to pull air in the front of the engine, but it is then vectored radially outward away from the engine centerline. This has the effect of converting all the momentum from the inlet air into a braking load instead of a thrust load. The faster the engine runs, the more inlet air is used for braking and the more effective it is. So the pilot deploys the thrust reversers and then runs the engines up to use the engines as brakes instead of as thrust generators.

Answer 4

It is not revrese thrust. That is only applied afetre the weheels are in contact with the ground.
One of the answereers seems to think that reverse thrust actually reverses the engine! That is so very wrong that the thinking is dangeraous..

The majority of landings at major airports are 'coupled'. This means that the aeroplane's auto pilot is linked to, and follows, guidance beams from the Intrument Landing System. The aeroplane's auto-pilot also knows its height above the runway. At the end of its approach the aircraft must 'flare'. That is, it stops its staight line descent and curves nicely into a horizontal flight ready for touchdown.
Power is needed to control this manouver because it involves lifing the nose slightly.

Answer 5

I don't know if you're thinking of thrust reversers (right after the wheels touch the ground), or in the actual flaring (nose-up attitude before landing).

In flaring, some power may be added to avoid sinking too fast and hitting the runway too hard.

Thrust reversal is just that: as soon as the main gears are on the ground, panels behind the engines deflect the exiting jet of air forwards, to slow the plane down. The engines can rev up considerably (up to about 50% N1, I believe) in this operation.

Answer 6

This is not reverse thrust; reverse thrust is only used once on the ground. The burst of power may be used to keep control over the plane as it gets closer to stalling, or to make sure the plane touches down at an appropriate point on the landing strip.

Answer 7

Because generally planes increase their flaps on approach. each degree of flaps slows the plane and throttle is increased to keep the airspeed at a safe level. Generally at flare, you try to keep the plane in the air at a level "floating." to get a nice landing. so i'm guessing this throttle up was to keept he airplane at a speed where it can float down gently and get a better landing, or to keep the airplane in the air to reach the necesary point on the runway.

Answer 8

So they have power available IMMEDIATELY in case they need it.

In most cases, "spool-up time" (from idle to full throttle) takes several seconds in your typical airliner jet engine. If a sudden aborted landing were necessary, the power to do so would not be instantaneously available without spooling up the engines first.

Answer 9

Throttle is added just before touchdown in case the airplane needs to make a go around. The pilot needs to have the power available at this time. Turbine engines have a lag, also known as "spool up time". These engines cannot give instant power when the throttle is applied like a piston engine. The turbine has to accelerate in order to provide more power and this takes time, usually from 3 to 10 seconds from idle, depending on the engine. On go around, you need the power now. Spooling up the engine on short final has the power at the ready if you need it.

Answer 10

As the aircraft approaches the runway the flaps and other lift enhancing devices are fully extended. This allows the aircraft to fly much slower, but increases the drag. More engine power is needed to keep the plane in flight and controllable.