How can a plane spiral downwards uncontrolled instead of gliding straight ahead?

Question

There have been cases (example) where a plane seems to spiral downwards. Shouldn't it glide?

I always thought you'd have a reasonable chance of landing a plane even without engines if you find some space to land on.

Answer 1

An aircraft flies because it has forward speed through the air. The movement of air over the wings produces lift that holds up the plane. But there's a certain minimum speed any given aircraft has to have to keep the wings lifting. The physics of lift are complicated, but they depend on the air making its way over the wing in a relatively smooth and even way.

When the speed drops too low (or, technically, the angle of attack becomes too high), the wings can enter a stall, where the airflow separates from the wing and becomes chaotic. The wing stops producing lift. There are ways to manage a stall; the pilot needs to drop the nose, pick up speed, get the air moving over the wings properly again, and then bring it back to a more normal flight attitude.

Normally, if a plane loses the engine(s), it does indeed glide for quite a while, provided the pilots react correctly and let the nose go down enough to maintain that all-important speed. However, there can be other issues. For example, ice on the wings can ruin the aerodynamic flow and cause a stall in unexpected situations.

Under some circumstances, a stalling aircraft can get into a worse condition called a spin. That's what it sounds like you're describing -- a spin is a nose-down corkscrewing movement while the wings are stalled. It's caused by one wing stalling harder than the other, which makes the airplane enter an uncontrolled roll as it drops. Spins are harder to escape than a normal stall (and there are actually several different kinds of spin).

So, yes: If an airplane loses the engine(s), a pilot has a good chance of landing relatively safely, provided there's an open area within range to land on. A stall or spin is not an inevitability, nor even likely, but if a lot of things go wrong at the same time it could lead to a situation that the pilots may not be able to get out of.

Answer 2

If a plane loses all sources of thrust and only has its kinetic energy left, why wouldn't it glide? Answer to that would be that it does climb, but not forever. Wings stall when the speed becomes too low. If the stall is too deep, it can be very difficult to recover from and a stall can develop into a spin.

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How a Spin Happens:

Stall: A spin begins with a stall, this happens usually cause the plane is going too slow or the wing airfoil is offended by something like icing. If this stall happens unevenly between the wings, a spin can develop.

Yaw and Roll: When one wing stalls before the other, the aircraft begins to yaw (rotate horizontally) and roll (tilt) towards the stalled wing. The differential lift and drag between the two wings create this rotation.

Autorotation: As the aircraft continues to yaw and roll, it enters a state of autorotation, where the stalled wing continues to descend, while the other wing, which is still generating some lift, keeps the aircraft in a continuous rotation. The plane spirals downward in a corkscrew path.

Descent: In a spin, the aircraft loses altitude rapidly as it continues to rotate around its vertical axis. The descent rate is very high, making spins extremely dangerous if not recovered quickly

Answer 3

Once a stall develops into a spin, the airflow over the wings is so badly perturbed that a glide cannot be established. The pilot must immediately begin a procedure called spin recovery. This involves using the controls to tip both wings into a position where normal airflow can be re-established over them so that they can "start flying" again.

While the spin recovery is taking place, the plane is still dropping out of the sky which means a full spin recovery might consume one thousand feet or so of altitude. This means that if the plane is less than 1000 feet above the ground, there's not enough altitude to recover and the plane spins into the ground.

Every different plane design has its own spin/stall characteristics and its own published procedure for recovery, which the pilot must be able to execute flawlessly when needed, since there might not be time enough for a second try.