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 dove — dropping roughly 34,000 feet in 63 seconds before the crew regained control at around 5,000 feet. The cause traced back to a single leading-edge slat, the No. 7, becoming isolated in an extended position while the aircraft was flying at cruise speed — a device meant to work only at low speed, suddenly deployed at high speed, on one side of the wing only. All 89 people aboard survived, but the aircraft suffered real structural damage, including a detached flight spoiler. That single incident is the clearest possible illustration of why flaps and slats matter as much as they do: a wing shaped correctly for 500mph cruise is a genuinely different, worse shape for 150mph landing, and the entire point of these devices is to change that shape safely, on command, only when needed.
Two devices, two edges, two jobs
A wing's cruise shape is optimized for speed and fuel efficiency, not for flying slowly. Slow down too much in that shape and the airflow over the wing separates and the aircraft stalls — which is a real problem, because takeoff and landing are exactly when an aircraft is flying its slowest. Flaps and slats exist to solve that, from opposite edges of the wing. Captain Joe's video “How Do FLAPS EXTEND on Airplanes?” walks through the mechanism directly: flaps, on the trailing (back) edge, extend downward and backward, which increases both the wing's surface area and its curvature (camber) — more lift, at the cost of more drag. Slats, on the leading (front) edge, extend forward instead, opening a narrow slot between the slat and the wing. That slot doesn't add lift the way a flap does; it re-energizes the airflow passing over the top of the wing, keeping it attached at a much steeper angle of attack than the wing could otherwise sustain before stalling.
Why takeoff and landing use completely different settings
The two phases want different things from the same devices, which is why flap settings visibly differ between them. On takeoff, the priority is lift without unnecessary drag — the engines are already working near their limit to accelerate the aircraft, so a moderate flap setting, typically 5-15 degrees, adds enough lift to get airborne at a safe speed without fighting extra drag the whole way down the runway. On landing, drag stops being the enemy and becomes genuinely useful: slowing the aircraft down is the entire point of the approach, so flaps commonly extend to 25-40 degrees, visibly further out and down than on takeoff, adding both the lift needed to fly safely slow and the drag that helps bring the speed down.
What TWA 841 actually changed
The National Transportation Safety Board's official finding pointed to isolation of that single slat as the probable cause, though the flight crew and the Air Line Pilots Association disputed the specific mechanism, arguing a mechanical fault rather than crew action was responsible — a disagreement that was never fully resolved. What both sides agreed on was the danger itself: a high-lift device extending on one wing only, at a speed it was never designed for, is close to the worst-case scenario the whole system exists to prevent, because it doesn't just add lift, it adds it asymmetrically, rolling the aircraft the moment it happens. That's exactly the failure mode modern flap and slat systems are engineered against, with dedicated sensors and protection logic specifically watching for asymmetric deployment before it can develop into anything close to what TWA 841's crew had to recover from.
Sources & Further Reading
Common Questions
Frequently Asked Questions
Why don't planes just use flaps all the time?
Because more lift comes bundled with more drag, and at cruise speed that drag is pure waste — it would burn far more fuel for no benefit, since the wing already generates plenty of lift at cruise speed without changing shape. Flaps and slats exist specifically for the slow-speed ends of a flight, takeoff and landing, and retract flush against the wing for the rest of it.
What's the actual difference between a flap and a slat?
Position and job. Flaps sit on the trailing (back) edge of the wing and extend downward and backward, increasing both wing area and camber — more lift, more drag. Slats sit on the leading (front) edge and extend forward, opening a slot that keeps airflow attached to the wing at a much higher angle of attack than it could otherwise manage, which delays a stall rather than adding lift directly.
Why does landing use so much more flap than takeoff?
Takeoff wants high lift with low drag, since the engines are already working hard to accelerate — flaps are set to a moderate 5-15 degrees. Landing wants high lift and high drag, because the extra drag actually helps slow the aircraft down — flaps commonly extend to 25-40 degrees, visibly further than on takeoff.
Could a slat problem like TWA 841 happen on a modern aircraft?
The specific mechanical isolation the NTSB pointed to has been addressed through decades of subsequent design and maintenance requirements, and modern aircraft carry systems specifically to detect and prevent asymmetric flap/slat deployment before it becomes controllable. It's exactly the kind of single-incident lesson aviation is built to absorb permanently rather than repeat.