How Flying Works
What Flaps and Slats Actually Do
On 4 April 1979, a TWA Boeing 727 cruising at 39,000 feet near Saginaw, Michigan suddenly rolled and dived. It dropped roughly 34,000 feet in 63 seconds before the crew regained control at around 5,000 feet. The cause was traced to a single leading-edge slat, the No. 7, stuck out in the extended position at cruise speed. A device meant only for low speed had deployed at high speed, on one wing only. All 89 people on board survived, but the aircraft was badly damaged. No incident shows more clearly why flaps and slats matter. A wing shaped for a 500mph cruise is the wrong shape for a 150mph landing, and these devices exist to change that shape safely, on command, only when it’s needed.
Two devices, two edges, two jobs
A wing’s cruise shape is designed for speed and fuel economy, not for flying slowly. Slow down too much in that shape and the airflow peels away from the wing, and the aircraft stalls. That’s a real problem, because takeoff and landing are exactly when an aircraft flies slowest. Flaps and slats solve it from opposite edges of the wing. Captain Joe’s video “How Do FLAPS EXTEND on Airplanes?” walks through how. Flaps, on the trailing (back) edge, slide down and back. That increases both the wing’s area and its curve (called camber): more lift, at the cost of more drag. Slats, on the leading (front) edge, slide forward instead and open a narrow gap between the slat and the wing. That gap doesn’t add lift the way a flap does. It feeds fresh energy into the air flowing over the top of the wing, keeping it attached at a much steeper angle than the wing could otherwise hold before stalling.
Try it
Deploy the flaps and slats
Slats slide forward and down from the front edge; flaps slide back and down from the rear. Together they make the wing bigger and more curved, so it can make enough lift at a much lower speed. Pick a setting to see the stall speed fall. The price is extra drag, which is why landing flap is too draggy for takeoff. Illustrative numbers.
Why takeoff and landing use completely different settings
Takeoff and landing want different things from the same devices, which is why the flap settings look so different. On takeoff the priority is lift without extra drag. The engines are already working near their limit to speed the aircraft up, so a moderate setting of about 5-15 degrees adds enough lift to get airborne at a safe speed without fighting drag all the way down the runway. On landing, drag stops being the enemy and becomes useful, because slowing down is the whole point of the approach. Flaps commonly go to 25-40 degrees, visibly further out and down than on takeoff. They add the lift needed to fly safely at low speed and the drag that helps bring the speed down.
What TWA 841 actually changed
The National Transportation Safety Board’s official finding named the No. 7 slat staying extended, together with the captain’s untimely control inputs against the roll, as the probable cause. The Board found no mechanical failure that explained the slat extending. The captain and the Air Line Pilots Association disputed that, arguing a mechanical fault, not crew action, was to blame, and the disagreement has never been fully settled. Both sides agreed on the danger. A high-lift device extending on one wing only, at a speed it was never designed for, is close to the worst case the whole system exists to prevent. It doesn’t just add lift; it adds it on one side, rolling the aircraft instantly. Modern flap and slat systems are designed against exactly this, with dedicated sensors and protection logic that watch for one-sided deployment before it can develop into anything like what TWA 841’s crew faced.
The other leading-edge device: the Krueger flap
Slats aren’t the only way to change a wing’s front edge. Some aircraft use a Krueger flap instead, named after the German engineer Werner Krüger who developed it.
A slat moves the leading edge itself forward and down. A Krueger flap leaves the leading edge alone. It is a panel stored flat on the underside of the wing that swings down and forward on a linkage, forming a new, rounder nose in front of the old one.
The effect is similar: more curve (camber) and a bigger effective wing, so the air stays attached at the slow speeds of takeoff and landing. Krueger flaps are simpler and lighter in some places, especially close to the fuselage, where the wing is thick.
The Boeing 707 and 747 used them on parts of the leading edge. The 727 combined Krueger flaps inboard with slats outboard. Look under the wing of a 747 on approach and you may see these panels hanging down near the fuselage, ahead of the main wing.
Sources & Further Reading
Common Questions
Frequently Asked Questions
Why don't planes just use flaps all the time?
Because extra lift comes with extra drag, and at cruise that drag is pure waste. It would burn far more fuel for nothing, since the wing already makes plenty of lift at cruise speed. Flaps and slats are for the slow ends of a flight, takeoff and landing. For the rest of it they fold away flush with the wing.
What's the actual difference between a flap and a slat?
Where they are and what they do. Flaps sit on the trailing (back) edge of the wing and extend down and back. That adds wing area and curvature: more lift, and more drag. Slats sit on the leading (front) edge and extend forward, opening a slot that keeps the airflow stuck to the wing at a much steeper angle than it could otherwise manage. They delay a stall rather than adding lift directly.
Why does landing use so much more flap than takeoff?
Takeoff wants plenty of lift with little drag, because the engines are already working hard to speed the aircraft up. Flaps are set to a moderate 5-15 degrees. Landing wants lift and lots of drag, because the drag helps slow the aircraft. Flaps commonly go to 25-40 degrees, visibly further than on takeoff.
Could a slat problem like TWA 841 happen on a modern aircraft?
The slat problem on that 727 has been addressed by decades of later design and maintenance rules. Modern aircraft also carry systems that detect and prevent flaps or slats deploying on one side only, before it can become uncontrollable. It’s exactly the kind of one-off lesson aviation is built to learn permanently.
Last reviewed: October 2026. Facts that change, such as fleets, airspace and airline ownership, are re-checked on a schedule. Spotted an error? Email contact@skyplus.news and we’ll fix it. See how we check facts.