On a normal flight the flaps that were extended for take-off are retracted gradually after take-off, and those that were extended for landing are retracted after landing. The change is timed by speed: on departure the pilot retracts the flaps only after the aeroplane is climbing steadily at the best-angle or best-rate speed, while on arrival they remain extended until the aircraft is safely on the ground and decelerating. Delaying retraction avoids the sudden loss of lift that could turn a stable climb into a descent or float into an undershoot; once the required speed and obstacle clearance are achieved, the cleaner wing reduces drag for the rest of the flight.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What are retracted flaps?
Retracted flaps refer to a procedure where pilots retract the flaps on an aircraft after takeoff, reducing drag and increasing speed. When the pilot moves the flap handle to the ‘up’ position, the hinged surfaces mounted on the wing trailing edges travel from the extended setting back to the flush, stowed position. In that state they are essentially locked in position when retracted. With the surfaces seated tightly against the wing, the aircraft gains airspeed because the extra drag and added camber are removed. Airflow again follows the original wing contour, so the sudden loss of lift that accompanies full retraction is managed gradually. The airplane does not accelerate toward the ground but instead continues to climb or accelerate as thrust now exceeds drag. A fully retracted flap therefore means that the high-lift devices have been restored to their cruise configuration.
What is the difference between flaps retracted and extended?
The difference between flaps retracted and extended is explained in the table below.
| Flaps Retracted | Flaps Extended |
|---|---|
| No increase in drag | Increase drag |
| No change in lift | Greater lift |
| Higher stalling speed | Lower stalling speed |
| No effect on pitch behavior | Definite effect on pitch behavior |
| No change in wing surface area | Extension of certain types increases wing surface area |
| No slots or gaps | Slotted flaps have slots or gaps |
| No alteration in chord line | Extending flaps increases chord line curvature |
| No broken wing effect | Extending flaps creates a broken wing |
| No asymmetric deployment | Asymmetric flap extension can cause roll |
| No change in approach angle | Flap extension has effect on approach angle |
| No delay in airflow separation | Delayed airflow separation allows higher angle of attack |
| No additional resistance | Enormous drag hampers climb performance |
| No steep descent angle without speed increase | Greater drag permits steep descent angle without airspeed increase |
| No change in spanwise lift distribution | Partial-span flaps alter spanwise lift distribution |
| No outward extension | Flaps extend not only downward but outwards |
| No increased proportion of lift from the inboard half | Inboard half supplies increased proportion of lift |
| No reduced proportion of lift from the outboard half | Outboard half supplies reduced proportion of lift |
When the flaps are retracted, the lift-augmenting devices are stowed inside the wing, so the airplane reverts to its clean, low-drag configuration. Retracted flaps produce less lift and much less drag, which is why extended flaps turn part of the wing into an air brake whereas retracted flaps allow the airplane to accelerate and climb efficiently.
When should the flaps be retracted?
Flaps are retracted only when the aircraft has a positive rate of climb and adequate airspeed above V2, and they remain extended until the aircraft is safely on the ground and decelerating. During cruise they are always stowed so the drag disappears and the pitching moment diminishes.
Retraction normally begins at about 400 ft AGL (121.9 m AGL). Below that height, the reduced camber lets stall speed drop toward 47 kts (87.0 km/h), whereas in clean configuration VS is 47 kts (87.0 km/h) and provides the intended margin. Once the partial-extent flaps are tucked away, the wing is unbroken and parasite drag falls, so straight-and-level flight performance improves without the extra lift devices that affect pitch behaviour and create a broken wing.
When are flaps retracted after takeoff?
Flaps are retracted once the aircraft has passed the initial climb segment, usually a few seconds after liftoff when a positive rate of climb is indicated on the instruments and the pilot calls ‘positive climb’. At that moment the landing gear is retracted immediately after takeoff. Simultaneously, the pilot begins to retract the flaps in scheduled stages so that the flap path meets the clean path to obtain the best possible climb out.
Flap retraction stops the high-lift, high-drag configuration needed during the takeoff roll, and it exchanges that lift for lower drag so that drag-reducing climb rate is no longer a penalty and clean configuration improves climb ability. If the departure is followed by a go-around, the go-around procedure includes reducing flaps to takeoff setting promptly after establishing positive climb. Then the same staged retraction continues once obstacle clearance is assured.
Can a plane take off with flaps retracted?
Yes. If the runway is not short, a Cessna 172S Pilot Operating Handbook recommends 0 degrees of flaps. The aircraft will fly, yet the ground roll lengthens and the climb angle flattens. Retracting flaps too early after takeoff produces the same hazard. Sudden loss of lift makes aircraft accelerate toward the ground. The flight path bends downward until the pilot restores flap or higher airspeed.
Why are flaps retracted after landing?
Flaps are retracted after landing for better braking. Once the aircraft is on the ground the wing is still generating lift, preventing the entire weight from resting on the tires. Retracting flaps after touchdown transfers weight to the wheels and provides better braking performance. Electing to land with partial flaps or even no flaps is a safer option when landing on a long paved runway with strong gusty winds or engine shut down, for partial or no flaps on landing provides greater rudder authority and smoother landings while only a small increase in the landing distance occurs.
Retracting flaps during a power-off 180 landing is necessary. Pilots must review how to fly a power-off 180 landing.In this configuration, electing to land with partial flaps or no flaps is a safer option because greater rudder authority provides better aircraft control.
I understand that retracting the flaps after landing allows for better breaking performance. When the flaps are retracted, the weight is shifted onto the landing gear which aids the craft in slowing down. Thus, retracting flaps allows for better rudder control and smoother landing.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
Why causes a failure in flap retraction?
The main cause for a failure in flap retraction is a weak return spring or binding inside the mechanism. This prevents the drive from pulling the surfaces to zero and is a defect that must be rectified before the next flight.
In the air, the trouble usually starts deeper inside the wing. The flaps are mechanically linked by a torque tube. Pulleys and wires run through turnbuckles, so most GA airplanes have flap interconnects which reduce the risk of asymmetry unless the torque tube itself breaks or a control rod lets go. Wear lets a roller bearing lock up in its track, jamming one flap down while the other rides up, or a linkage malfunction disconnects the chain so that you are left with flaps partially extended. Frayed steel cables or a pulley being stuck in the drum will allow one surface to extend without the other. Whatever the precise failure, the outcome is a split-flap situation: the lowered surface behaves like a giant aileron, demanding opposite aileron toward the stalled side to prevent a sudden roll moment. Such flap asymmetry causes control problems, especially when you land in a crosswind, and the large loss of lift makes the aircraft settle on the runway or stall without warning.
