In the small planes I fly, it is called a "Gust Lock", and consists of a small metal bar that fixes the yoke in position, so that the yoke cannot move (left/right, or forward/back), and by association, the ailerons and elevator cannot move either.
As the name implies, this is for dealing with Gusts of wind... a sudden bit of wind over an unlocked surface can move the surface to its extreme position until it hits a mechanical stop. If this happens repeatedly, say, over a stormy night, the repeated banging into a mechanical stop can create wear and tear on the surface, the control cables, or the yoke in the cockpit.
For gliders (probably other aircraft as well), there are two reasons to fix ailerons, elevator (and rudder) positions:
You want to avoid uncontrolled (potentially damaging) sudden movements caused by (repeated) wind gusts.
If you choose or allow unlucky positions/configurations, your plane might exhibit unwanted positional initiative (e.g. an aileron might motivate one wing to lift unexpectedly, hitting heads, tarmac or other obstacles). In mildly gusty conditions, wings are often temporarily fixed by additional weights (usually old tires or similar) to avoid that, too. As mentioned in the comments, this is not sufficient for anything more than average winds.
Other folks have already addressed why we want to lock the flight controls when we leave the aircraft - "to prevent damage from gusts of wind", and "to keep the plane from doing interesting things in a strong breeze".
To the question of the "best" position to lock controls in, for light aircraft the general consensus is that the rudder and ailerons should be locked in the neutral position - In this position any wind blowing over the wings will cause them to lift uniformly, putting tension on the tiedown ropes holding the aircraft to the ground. (If the ailerons are locked at an angle one wing will generate more lift than the other, allowing one of the tiedown ropes to go slack.)
Elevators are a subject of a little more nuance and debate: It's generally considered best to lock the elevator in the "nose down" position (because any gusts of wind over the tail will push the nose down, rather than encouraging the nose to lift, this also means that those gusts will apply tension to a tie-down rope located at the tail rather than causing it to go slack).
On some aircraft locking the elevator in the nose-down (or nose-up) position opens up spaces that birds may try to nest in (for example, check out the tail of this Grumman) - some pilots prefer to lock the elevator of these aircraft in the neutral position to discourage avian visitors.
Generally it's better to lock the controls than leave them unlocked to bang around in the wind, so on some light aircraft not equipped with control locks pilots use the seatbelt to hold the controls in place. This often results in a "sub-optimal" control position (elevator fully nose-up, ailerons full-deflection left or right), but because the wind speed at which control surfaces may bang against their stops is usually lower than the speed at which the aerodynamically unfavorable control positions will substantially affect the aircraft this is a "lesser of two evils" approach.
After a discussion in chat about control surface locks I was wondering why they need to be locked in the first place. Why do they need to be locked? What is the best position to lock them in? up, down, neutral.
With all the shiny new glass cockpits it would seem that the days of the spinning mechanical gyro (and associated tumbling due to gimbal lock) should be over: Sparing everyone the boring math it should suffice to say that solid-state gyros can be engineered and built in such a way that gimbal lock is impossible, but I'm not certain that's how they're actually designed. Do modern AHRS systems with solid-state gyros (or replacement electronic horizons like the RC Allen 2600 series) still suffer from gimbal lock, or do they provide true 3-dimensional freedom? I'm interested primarily
computer either aggregates multiple inputs or a pilot can press a "priority button" to lock out inputs from the other side-stick. On US flight 1549, the CVR transcript shows that Sully hit the Priority T/O button, after the co-pilot (Skiles) handed over control of the aircraft: 15:27:23.2 - Sully: My aircraft. 15:27:24.0 - Skiles: Your aircraft. 15:27:26.5 - FWC: Priority left. I'm curious as to why there's a need for this button at all? Here's a short video on YouTube, demonstrating this.
How do flying wings, like the B-2 Stealth bomber, actually keep themselves from yawing out of control without a vertical stabilizer? For the record, I assume this has to be a simple mechanics... WWII. They didn't have flight control computers back then, and the only control complaints I recall them having is that early versions had a tendency to flip over backwards when approaching stall speeds, well, that and the ground effects were pretty strong. But, no mentions of going into flat spins when going into hard maneuvers (that I recall). So how do they control that Y axis on flying wings
Every time I travel on an airplane, the stewardess forces me to shut down my iPad. I just lock it, and start using it again 5 minutes later. I wonder why this is? Is it because I have to have my hands free if the airplane crashes? Or is there any other logical explanation?
Provided an aircraft with a fly-by-wire system, there are basically two possible choices when it comes deciding how to let the pilots interface with it: rate control / attitude hold: a deflection of the stick will command a certain rate, releasing it will make the system maintain the current attitude. See the Airbus Normal control law. direct control: a deflection of the yoke will directly translate to a deflection of the surfaces, mimicking the "old" mechanical control setup. It is my understanding that this is the design choice of Boeing in its new aircrafts. I do not wish to discuss
Inspired by this question. My knowledge concerning helicopters is quite limited: what is auto-rotation? are there other "rotations" possible? in what do they differ?
examples....). That is, until I ran across this little tidbit in the Air Traffic Control Order while researching another question: 4-8-6. CIRCLING APPROACH a. Circling approach instructions may only be given for aircraft landing at airports with operational control towers. So then the question becomes, why do they have circling minimums at non-towered airports?? No tower here. ATC can't clear me to circle. Why do we have circling minimums??
I know that for land aircraft and seaplanes that they require separate endorsements to fly them. However, for the case of amphibians, what do you need to fly one? Do you need to have another, completely different endorsement, or just a seaplane and land endorsements? What about if you always fly it on water or land?
Autopilots used in piston GA usually do not have throttle control. They only manage the control surfaces. However to trim an aircraft one needs to play on both throttle setting (and more physically, thrust) and control surface deflections (aerodynamic forces). What happens if the autopilot cannot trim the aircraft due to the propulsion settings? Is there any alert from the autopilot for the pilot?