Why do they have elevators in such a position as if the joystick is pushed forward, even though there's nobody inside the cockpit?
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17$\begingroup$ Gravity? ........ $\endgroup$– TomMcWOct 9, 2019 at 15:35
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77$\begingroup$ They're depressed because they would rather be flying. $\endgroup$– Pete BeckerOct 10, 2019 at 12:26
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5$\begingroup$ Is there any trend or habit for pilots to push the stick forward to make exiting the aircraft easier? $\endgroup$– FreiheitOct 10, 2019 at 16:12
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10$\begingroup$ FWIW, while I won't dispute the accepted answer for this particular case, you will find that for a lot of smaller airplanes -- e.g. 4- to 6-seater piston-engine general aviation airplanes -- you'll see the elevator in this sort of fixed position as a result of a "control lock" installed inside the cockpit on the controls themselves, which intentionally puts the control surface in this position to guard against gusts of winds (both to keep the control surface from flapping in the wind, as well as to ensure that lifting forces generated by the wind push in the least-disruptive direction) $\endgroup$– Peter DunihoOct 11, 2019 at 0:12
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4 Answers
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It generally means that the hydraulic actuator (power control unit) driving the surface has an "idle" facility that allows fluid to move internally between the two sides of the actuator piston, or just circulate in the pressure/return lines, and when unpressurized it acts more or less like a hydraulic damper even though the input spool valve is at its "null" (no command) position.
The result is that when unpressurized it's free to move (although damped) and if gravity wants to pull the surface in one direction it'll do so. As soon as hydraulic pressure is applied, it'll snap back to whatever position is being called for by the input system.
Sometimes you have PCUs that are "non-idling" and when unpressurized they are hydraulically locked which is only broken when there is a control input to the controlling spool valve. Such surfaces stay where they are even when the hydraulic pressure has bled off, unless you move the controls to break the hydraulic lock.
On the CRJs, the elevator PCUs are idling, and the elevators sag when unpressurized. The aileron PCUs however are non-idling, and the ailerons don't sag when unpressurized. If you try to move an RJ's aileron when its hydraulics are off, it is totally rigid.
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10$\begingroup$ It's probably also worth mentioning that putting the centre of gravity of the elevator (taileron?) behind the pivot is also a deliberate choice. It helps to reduce flutter and smooths the response to turbulence. $\endgroup$ Oct 10, 2019 at 12:40
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3$\begingroup$ Not quite like that. For unpowered surface you DON'T want the surface's CG aft of the hinge line because this encourages flutter,being a complimentary oscillation between the control surface and the wing it's attached to, and usually requires balancing the surface to put the CG at or forward of the hinge line. However, when the surface is hydraulically driven, you can take credit for the rigidity, and when unpressurized, the damping ability, of the hydraulic actuator(s) to prevent the mutually reinforcing oscillation of flutter from getting started, so they get away without balancing. $\endgroup$– John KOct 10, 2019 at 15:28
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1$\begingroup$ Canadair Regional Jets? None of those look like passenger planes. google.com/search?q=aircraft+CRJ $\endgroup$– Foo BarOct 10, 2019 at 15:56
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2$\begingroup$ If the airplane has a single actuator driving a critical surface, and that actuator becomes depressurized, yes of course. Civilian transport airplanes have to have redundant actuators to eliminate single-points-of-failure like this. Military airplanes may or may not. $\endgroup$– John KOct 10, 2019 at 17:38
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2$\begingroup$ Just an example of an idling vs non idling actuator driven control surface. I used those as examples because I know the systems and most people have observed these airplanes on the ramp if they've ever traveled by air. Military aircraft operate with similar components but different and often less stringent redundancy requirements. $\endgroup$– John KOct 10, 2019 at 18:36
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@John K's answer is perfect. However, in other mechanical systems such as elevators, fork-lifts, factory machines, etc and also NON hydraulic systems (and also possibly Jets) it is designed to be so so that when the machine is idle/switched-off the system is in a 'non-stressed' state or in a 'safe state'.
The keywords are 'design' and 'requirement'.
As @Makyen 'requests clarification' - lifts (elevators) have a safety feature for power-loss/(electronics) failure conditions. The battery backup brings the lift to a safe 'landing position'and stops and opens the doors {if the lift was moving during a power loss} (the CPU, in modern lifts decides on the best location). Lifts also have a mechanical power independent system that locks the lift mechanically on a safe position which can be the closest floor. Lifts have many safety features. But my point is, to the original question, there is a design requirement which is well planned and thought of rather than a 'random' choice. Similarly for jets the requirement, as someone else pointed out is (possibly) to keep the jets on the ground during heavy winds/storms, but the design also takes into consideration idle stress, energy consumption, etc- a win-win situation.I would reliably assume that fighter jets have much, much more redundant/safety systems to maintain reliability and serviceability.
As for fork-lifts, I am not talking about hold-shutdown positions. There is a training requirement and also storage zone requirement for forklifts. It would be unwise to store a forklift/crane in a loaded/stressed position due to safety requirements, etc (just like when the jets are 'stored').
Ref: Outage safety also 'informative'
FYI: I am ISO-9001 (1993/1994) certified/trained in service engineering. Also, at that time I was acquinted with one of the Airbus-A340 designers - my favourite plane (the 747 gets an honourable mention). My favourite jet is the MIG-21 (the F-16 gets an honourable mention) (although there are newer ones that are better). My uncle flew for the RAF during WW2 and another was a mechanic for the F-4, etc (received a lot of knowledge from them)...
Trust this helps.
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2$\begingroup$ It does help indeed, welcome to the community! :) $\endgroup$– gsamarasOct 11, 2019 at 17:54
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$\begingroup$ Do you really think elevators are designed to gradually move to some equilibrium location when the power is lost? That would be completely unsafe. They are designed to stay at the same location as they were when the power was lost. Forklifts, also, are designed to hold the current position of the hydraulically operated parts when power is lost/turned off. To do otherwise would be a safety concern. The only forklifts I've seen that didn't hold position were know to have hydraulic leaks. If you meant something other than the opposite of what I've described, then please edit to clarify. $\endgroup$– MakyenOct 11, 2019 at 20:01
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1$\begingroup$ They put fork lifts like that to avoid wear on return stop valves and other hydraulic line components. Resting against the ground is the only way to ensure that the system is de-pressurized and not resting on a normally-closed return valve etc. Splitters for instance, do not like backpressure differences. (a splitter splits oil volume equally to two lines when operating - if you want to lift two points with equal rate but unequal weight, for instance.) $\endgroup$– StianOct 11, 2019 at 20:13
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This is a safety feature. You don't want the airplane to blow over in a strong wind. With the elevators down (as you see them in the pictures), a gust from the front will push the plane's nose down and keep the main wing from generating lift. A gust from behind will push the nose up, but in this case, the main wing will present its top surface to the gust, and will push the airplane back down. Either way, you're safe.
@zymhan: My main source is my flight training. I was taught not only to park the aircraft with the controls locked in a position that kept the aircraft from blowing away, but also to move the control surfaces while taxiing to keep wind from getting under the wings. Taxiing into the wind, we keep the elevator down (pushing the nose down); turning a corner so the wind was from abeam, we deflect the upwind aileron up (pushing that wing down); etc.
Small general aviation planes like I fly can easily be picked right up and flipped over by a strong gust. Even if you've got them tied down, you never want to risk one of your knots coming out or an old rusty clip breaking, leaving your airplane free to go tumbling across the airfield. Leaving controls in the right place takes some of the stress off the tie-downs. Every little bit helps.
Modern fighters and larger aircraft won't just up and blow away unless you get into a serious storm, although serious storms do happen, especially on aircraft carriers at sea. The greater risk to large aircraft is more prosaic: you don't want them to tip backward and knock their tails against the tarmac. At this point, we're beyond the scope of my pilot training and into the realm of my aeronautical engineering degrees. Aircraft landing gear is designed to place nearly all the weight on the main gear. The mains are located just slightly aft of the c.g. so that on a hard landing, they take most of the shock, and the nose doesn't snap down violently onto the nose gear. This lets you design the nose gear to be as light as possible, and the structure that supports the main gear as short and light as possible. However, with not much weight resting on the nose gear, the drawback is that it doesn't take much wind to lift the nose up and bang the tail against the ground. Many planes have tailstands to prevent this. But again, parking the plane with elevators down is just that much extra insurance.
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8$\begingroup$ This is a very interesting point, but do you have any sources for it? $\endgroup$– zymhanOct 10, 2019 at 16:26
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3$\begingroup$ Fighter jets may be stored on the edge of aircraft carriers, in bad weather. That means it is very important - so I think the feature exists and is intentional. $\endgroup$ Oct 11, 2019 at 4:20
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3$\begingroup$ @Volker: While that's good logical thinking, I doubt it applies to jets on aircraft carriers. Six to eighteen chains tie down the aircraft, depending on weather and ship maneuvers. $\endgroup$– NavOct 11, 2019 at 14:43
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4$\begingroup$ Saying that wind would 'push the nose down' is a bit misleading anyway - what it will do is lift the tail upwards, which doesn't sound like a desirable outcome to me. $\endgroup$– MikeBOct 11, 2019 at 15:11
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2$\begingroup$ Actually high winds can snap chains. Happened to a buddy of mine. Heck of a way to loose an aircraft. He now prefers steel tie downs to aluminum. $\endgroup$ Oct 11, 2019 at 15:44
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Same principle in small aircraft as all aircraft control column locks hold them in place All explained above by other posts it is not a fashion or pilot habit it is part of post flight check list ensure control lock is in place
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1$\begingroup$ It's not a control column lock, as explained in the other answers. $\endgroup$ Oct 11, 2019 at 15:08