Planes use flaps to land and to take off. So why do they retract them while they are on the ground, ie why not just leave them deployed?


Planes use flaps to land and to take off. So why do they retract them while they are on the ground, ie why not just leave them deployed?

In: Engineering


Because you have people and things moving on/around the wings when the plane is grounded. Leaving the flaps open means you take the risk that something breaks them or something to fall inside the wing an damage something. You don’t want to take off only to find out that, crap, you can’t close the flaps because there is a screwdriver in it, and now you tried to close it the thing broke down.

Less possibility of foreign object damage from ground based things getting into the wings/flap mechanisms.

The settings for the flaps are different for takeoff as well as landing, so some adjustment is going to have to take place anyway.

While damage risk is also a reason, the biggest is actually because they cause increased drag.

Basically they help with landing and taking off, but they also slow the plane down. Jet fuel is expensive so efficiency is important.

People keep mentioning the possibility of damage to extended flaps (which I’m sure is a secondary reason), but the main reason is to keep the plane on the ground. Flaps are designed to create significantly more lift at low speeds at the expense of increasing drag. They deploy them on takeoff and landing so the plane can transition to and from flight at lower ground speeds, which reduces stress on the airframe and landing gear and is just easier for the pilot to manage.

If you leave flaps deployed on the ground you risk gusts of wind lifting one or both wings while the aircraft is parked. That’s bad. Since they also increase drag (especially in landing configuration) in windy/gusty situations they also increase the forces on the brakes and chocks while parked. The risk of the aircraft moving uncontrollably, tipping, flipping, etc is all significantly increased with the flaps down on the ground.

So they retract them when they don’t need them.

All the reasons everyone has given, but also: taxiing with your flaps down at the very least used to be a signal for hijacking. It possibly still is. I know if you for any reason are going to be taxiing with your flaps down you must notify the tower.


Seems like no one actually can explain Flaps that well. As a pilot we use flaps for takeoff because it increases our lift on the ground.

In the air, flaps produce a lot of drag. You retract them to reduce drag and go faster.

When you’re on approach, you put the flaps down to slow you down. It gets the airplane slower. And it helps significantly on reducing your landing distance since it’s a lot of drag.

Depending on the manufacturer, some require flaps up when taxiing through slush/snow, as well as during and after de-ice/anti-ice sprays. I imagine it’s to keep contaminants out of the flap tracks. And yeah I guess there’s that point about highjacking, although just because they’re down, doesn’t mean they’re automatically gonna send SWAT. I imagine it’s usually associated with a 7500 code or Mayday. But mostly, I would just say it’s good airmanship. Use something, then put it away.

The only exception I can see is for short field landing techniques. The goal is to immediatly take the weight off the wings and into the tires so you can dive on the brakes. Lifting the flaps right at touchdown can help the weight transfer.