What Are Flaps And Why Are They Important?

Most observant people who have flown on an aircraft may have noticed flaps on the wings being deployed during take-off and landing. However, you might have no idea what their purpose is or how important they are.

TUI Boeing 767 Landing
A TUI Boeing 767 with flaps deployed for landing. Photo Getty Images

The flaps play an essential role in getting an aircraft into the air and keeping it there. They are also a necessary element of making a safe and controlled landing. Here we’ll have a look at what they are and how they work.

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What are wing flaps?

The wing flaps are located on the outer trailing edge of the aircraft’s wing, between the ailerons and the fuselage. Large jet airliners can have as many as three sections to their flaps, which can be extended as required during take-off and landing. Smaller aircraft have flaps of an appropriate size that are attached to the wings by hinges.

Extending the wing flaps increases the curvature, or camber, of the wing surfaces, allowing the aircraft to generate the necessary lift at a slower speed. This reduces the minimum speed, or stall speed, the plane needs to maintain flight safely.

The increased camber produced by the extended flaps also increases drag, which helps to slow the aircraft and allows a steeper approach angle during landing. The length of runway needed for both take-off and landing can be reduced by efficient use of the flaps.

Boeing 757-200
A Boeing 757-200 aircraft landing with wing flaps extended down and the spoilers extended up. Photo: Tony Webster via Flickr

How do wing flaps work?

Wing flaps alter the shape of the aircraft’s wing and divert the air over and under the wing as required. How the flaps are set during deployment determines whether they provide increased lift for take-off or an increase in drag for landing.

The flaps work in conjunction with the plane’s altitude, power and pitch. A safe landing is not dependent on the flaps alone. Pilots need to think ahead, anticipate and make calculated adjustments to allow for wind speed and runway conditions.

During take-off, the flaps are usually partially extended and set between five and 15 degrees, depending on the type of aircraft. For landing, the flaps can be fully extended and will typically be

set at between 25 and 40 degrees. The increase in lift and drag enables a slower and steeper approach for landing before deploying the braking systems.

An additional beneficial effect of extending the flaps is a reduction of the aircraft’s pitch angle, which lowers the nose giving the pilot a better view of the runway.

British Airways, Boeing 747, Early Retirement
A British Airways Boeing 747-400 landing with Fowler flaps extended. Photo: Getty Images

Types of flaps for commercial planes

Among the most commonly used flaps on modern aircraft are slotted flaps. Like other flaps, they increase the camber of the wing, but they also create a slot between the flap and the wing. The slot allows the high pressure from under the wing to flow through onto the upper surface. This delays airflow separation and reduces the drag to produce much more lift.

Large aircraft that need some serious lift utilize Fowler flaps, also known as slotted-Fowler flaps. These flaps extend out on tracks and feature a series of slots that add increased energy to the airflow.

The first stage extension of a Fowler flap produces a significant increase in the lift without increasing the drag much, which is the ideal setting for getting a large jet off the ground. When extending the flaps farther, they move more and more downward to give a significant increase in drag to slow the aircraft down.