Definition
The angle, measured in degrees, to which the wing flaps are extended downward from their retracted (faired) position. Flap deflection changes the wing's camber and, at larger angles, its frontal area, altering lift and drag characteristics.
Plain English
How far down the flaps are lowered, measured in degrees from their stowed position. A small deflection means the flaps are barely down; a large deflection means they are lowered a lot.
Context Anchor
Used when discussing flap settings during takeoff, approach, landing, and go-around planning.
Derivation
Deflection' comes from the Latin 'deflectere,' meaning 'to bend down or aside.' In aviation, it describes how far a control surface has been moved from its neutral position — here, how far the flap has been bent downward.
Why Pilots Care
Different deflection angles produce different results. Small deflections (around 10–20 degrees) add lift with little drag — useful for takeoff. Larger deflections (30–40 degrees) add a lot of drag with less additional lift — useful for steeper approaches and slower landings. Choosing the right setting affects climb performance, approach angle, and landing distance.
Intuition Check
Flap deflection is not the lift produced by the flap. It is the amount the flap is moved, usually stated as an angle such as 10 degrees or 30 degrees.
Example Sentence 1
On short final, the pilot selected full flap deflection to steepen the approach and reduce landing speed.
Example Sentence 2
Too much flap deflection during takeoff increased drag and lengthened the ground roll.