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I found this question. One of the answer tell the flaps are generally retracted after landing. I assume this mean flaps are retracted when the airplane is on the ground between flights.

Flaps are extended for landing and then extended again for take off. Not taking care of their position while on ground (no need of them) and putting them in the required position before take off may save (few) flaps actions (something like putting them from landing to take-off position instead of landing position to fully retracted and then from retracted to take-off position).

Is there some good reason to retract the flaps on ground? Does this good reason varies from a type of aircraft to another?

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    $\begingroup$ flap positions on for landing and takeoff are usually different. $\endgroup$ – ratchet freak Oct 11 '14 at 15:37
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    $\begingroup$ On very light aircraft, the extra flaps might increase lift enough to put excessive strain on the tie-down straps? $\endgroup$ – raptortech97 Oct 11 '14 at 15:56
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    $\begingroup$ I would bet it is for storage purposes. Flaps are more likely to get damaged by some knucklehead walking into them when they are down. Also, it might be because lowered flaps create openings that insects like to get inside and build nests in. Additionally, what raptor said about tie-downs is likely a reason. $\endgroup$ – Bassinator Oct 11 '14 at 16:09
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    $\begingroup$ According to many pilots, the flaps should be retracted on touchdown, or at least before taxiing, especially on windy days, to prevent the aircraft from becoming airborne again. $\endgroup$ – flyingfisch Oct 11 '14 at 16:10
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    $\begingroup$ @raptortech97 not just very light aircraft. Few years ago there was a bad storm at Schiphol and they had to turn many aircraft so the wind wouldn't blow them over. Now imagine if those had had flaps extended, the wind was high enough that it would exceed the takeoff speed of a Fokker 50 with full takeoff flaps. $\endgroup$ – jwenting Oct 12 '14 at 6:21
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A few different reasons:

  1. Good pilots put the aircraft into a well-known (up) configuration after landing, so that the aircraft is ready for use on the next flight.

  2. Flaps down during taxi was a signal to the tower that the aircraft had been hijacked.

  3. Take-off flaps (10 degrees on my airplane) and landing flaps (45 degrees), are nearly always different, so leaving the flaps down after landing at, say, 45, doesn't save any steps for departure. If the pilots forgets to set flaps before takeoff, some aircraft (like mine) will make it almost impossible to depart and climb out on full flaps because of the induced drag, while takeoff with no flaps is possible.

  4. Prevent trucks and people from hitting the flaps while the airplane is parked, especially in the full-down (landing) configuration.

  5. Some pilots employ the practice of retracting the flaps during the rollout because it puts more weight on the wheels, reducing stopping distance.

  6. Flaps down increases the amount of lift generated by the wing, so some aircraft might actually lift off the ground while parked with full flaps in high winds.

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    $\begingroup$ for the reason #3: the sequence 45 degrees --> 0 --> 10 is one step more than 45-->10. And I hope any pilot will check the flaps positions before take off and thus not take off with full flaps. the reasons #2 and #4 seems enough to take care of flaps position $\endgroup$ – Manu H Oct 13 '14 at 11:31
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    $\begingroup$ All of these reasons contribute, but but the first answer is the most correct. Good pilots follow checklists, as per the PPL PTS -- "Throughout the practical test, the applicant is evaluated on the use of an approved manufacturer’s checklist or equivalent" -- because checklists reduce accidents. $\endgroup$ – rbp Oct 13 '14 at 12:49
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    $\begingroup$ Do you have any documentation on #2 ? I'm curious about that :) $\endgroup$ – Quentin H Mar 5 at 8:01
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    $\begingroup$ @QuentinHayot i'm glad you asked! fsims.faa.gov/wdocs/orders/ps_orders/a_7110.49d.htm 6(3)(b) "PILOT SIGNAL (Covert): Leave full flaps down after landing or lower full flaps while on the ground." $\endgroup$ – rbp Mar 27 at 21:36
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You never know what wind speed will hit the aircraft until the crew returns. Also, retracted flaps are much less of an obstacle to careless drivers of airport trucks. This video shows a parked 747 in high winds. Now imagine what would have happened if the flaps had been deployed!

Note that fully extended high lift systems at ground angle of attack can produce five or six times the lift of the wing with flaps retracted. Lowering the trailing edge flaps increases wing area and lowers the zero-lift angle of attack, so at ground attitude the wing is much farther up on the lift curve slope.

In aircraft with manual controls it is also recommended to secure all control surfaces, so they don't float in the wind. Bearings and pushrods could be damaged, and in a careless preflight check might not raise suspicions, but can go on to fail in flight. Better be safe and reduce all chances of wind playing with the aircraft!

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    $\begingroup$ The linked video shows a partially dismantled aircraft, so it may not be representative. But there are also accidents like this ATR72 in Shannon $\endgroup$ – Jan Hudec Oct 11 '14 at 20:02
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    $\begingroup$ Some comments on the video in question suggest that flaps were deployed at the time. I can't tell at all, though. $\endgroup$ – raptortech97 Oct 11 '14 at 20:13
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    $\begingroup$ The video's description says the flaps were deployed, but also that the engines were removed, so the center of gravity is further aft than normal. @raptortech97 $\endgroup$ – Connor Oct 12 '14 at 0:36
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    $\begingroup$ @raptortech97 That appears to correct. If you look at flap track fairings, their angle shows that the flaps are deployed. $\endgroup$ – Farhan Oct 12 '14 at 0:37
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On larger aircraft it would be standard operating procedure to retract the flaps after landing. This is to ensure the crew taking over from you are presented with an aircraft in the correct state in accordance with company SOPs.

Flaps could easily be damaged by ground crew loading and unloading. Also it would be difficult to refuel the aircraft with the flaps/slats extended (Larger Boeing types have the refuel system located at the leading edge underneath a wing).

There are occasions when you would leave the flaps extended, during operations in heavy snow for example - retracting contaminated surfaces may cause damage.

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and then the obvious one... you retract your flaps so you don't keep hitting your head on the flap when you are bent over walking around your plane :)

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    $\begingroup$ Only on a high-wing plane. I did most of my flight training on a DR-400, where you stand on the wing to get in and out of the plane. Before getting out of the plane, you must extend the flaps in order to prevent people from walking on them. $\endgroup$ – usernumber Oct 17 '14 at 21:43
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    $\begingroup$ On a PA28 there's no danger of walking into the flaps, but if you have them extended and try to step on them getting out of the plane they'll continue extending and drop you on the ground :) $\endgroup$ – voretaq7 Oct 19 '14 at 0:58
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On Pipers (Cherokee, Cherokee Six), the flaps had step areas on the inboard ends. When retracted, they were essentially locked in position. When extended to the first notch, they were spring loaded in such a way that they could be pushed down to expose the hinges and linkages for inspection from above. The pre-flight routine would include setting the flaps to 1 notch to facilitate inspection during walk-around, but otherwise keep them retracted until pre-take-off. So a reason for keeping retracted would be to help with boarding.

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