Quite often I see aircraft parked at the gate with rudder deflected to the max. I see this either with Boeing and Airbus aircraft so it must have been done on purpose. I know that there are dampers connected to the yaw, or are those not powerful enough to keep the rudder straight e.g. undeflected?

  • $\begingroup$ I don't think this was discussed, but also sometimes deflecting the controls in a certain direction with relation to the wind while the aircraft is stationary helps to keep a sudden gust of wind from flipping or turning the aircraft. This is mostly done on GA aircraft as they are lighter in weight. $\endgroup$
    – and_also
    Oct 7 '16 at 4:47
  • $\begingroup$ I don't have any sources on this so I'll leave it as a comment, but I once heard that large aircraft have a rudder outfitted with a spring (to supress flutter). When there are no aerodynamic forces the spring force is to only force present, deflecting the rudder. $\endgroup$
    – ROIMaison
    Oct 7 '16 at 7:45
  • $\begingroup$ Related: What is the difference between reversible and irreversible controls? $\endgroup$
    – ymb1
    Oct 7 '16 at 13:52
  • $\begingroup$ Yaw damper is connected primarily to the lateral accelerometer, so if the ground is sloping even a bit, it should actually deflect the rudder. However that would interfere with control on the runway, so it is never active on the ground anyway. $\endgroup$
    – Jan Hudec
    May 29 '17 at 16:51

The control surfaces of airliners are not connected to the pilots' control via cables; they are operated by a hydraulic pump. When the engines are shut down, there is no hydraulic pressure in the pipes, and the control surface is free to move.

The rudder is moved from the center position by wind. If you observe the gates, you should note that all airplanes have their rudder deflected to the same direction. In a perfect no-wind scenario, the rudder would stay neutral.

There are no gust locks which the pilot must arm before leaving the cockpit on Boeing aircraft (and I presume Airbus too). Gust locks are, however, common in general aviation aircraft. They can also be found on some small business jets.

The reason being, control surfaces on small aircraft work "both ways" - if you move the elevator by hand, the yoke in the cockpit will move as well. These are called "reversible flight control systems". Therefore, when the control surfaces are moved by wind, the entire system - yoke, cables, wheels, bearings etc. are all moved back and forth. This brings a significant amount of wear and stress to the system. Strong wind may also cause the surfaces to deflect past their designed maximum, causing damage.

The control surfaces on airliners are much heavier and much stronger. Also, they are not "directly" connected to the cockpit's controls - when there is no hydraulic power, pretty much every component is disconnected. Unless you're facing gale force winds, it is safe to just let them swing free with wind and gravity.

  • 1
    $\begingroup$ Could you elaborate on why there is this difference between commercial and general aviation? $\endgroup$ Oct 6 '16 at 14:49
  • 3
    $\begingroup$ On some Boeing aircraft such as the 737, the flight controls do in fact deflect when the surfaces deflect with the wind (or jetblast). While the surfaces are heavy enough that they aren't going to be banging around the way a light aircraft's surfaces would, there IS deflection in the cockpit. Which is why, at the gate, one electric hydraulic pump is sometimes left on so that the yoke doesn't randomly move fore/aft when you're sitting in (or climbing into) the seat. $\endgroup$
    – Ralph J
    Oct 6 '16 at 15:38
  • $\begingroup$ On most tri-cycle gear general aviation aircraft the rudder pedals are used for both the nosewheel steering on the ground and yaw control in the air by moving the rudder. Since they are directly connected, when tied down the nosewheel doesn’t move and the rudder stays in place. (There are exceptions, aircraft like the Grumman Tiger and Cirrus SR22 have castering nosewheels that are not controlled by the rudder pedals. Turns are made with differential braking.) $\endgroup$
    – JScarry
    Oct 6 '16 at 15:39

I have no experience on Boeing or Airbus airplanes but the most likely cause would be gust locks installed on the aircraft.

  • Some aircraft, like a C-172, have a pin that locks the aileron and elevator in place. Other airplanes have an externally mounted device locking the rudder in place.

C-172 Gust Lock http://www.weekendcfii.com/photos/c172preflt/C172_gust_lock.jpg

  • Other aircraft have a strap that goes around the flight controls and rudder. The strap is tightened and the controls are locked in a fully deflected state.

Learjet Gust strap http://archive.da.aero/images/intelligence/content/learjet/200911-old_gust_lock.jpg

  • 5
    $\begingroup$ It would be good to elaborate on why the gust lock is needed. I assume to stop controls flapping around and potentially damaging themselves or something else? $\endgroup$ Oct 6 '16 at 14:45
  • 1
    $\begingroup$ Is this because on GA aircraft they are directly connected to the steering column e.g. the wind would move the stick inside the cockpit, possibly harming an occupant? $\endgroup$ Oct 6 '16 at 14:50
  • 5
    $\begingroup$ Its because uncontrolled slamming of the surfaces damages the control surfaces, linkages, hinges, etc. It really isn't going to do much to anything in the cockpit since there are physical stops for the control column. Some wind can be strong enough to snap control cables or stress them beyond design limits. $\endgroup$
    – Ron Beyer
    Oct 6 '16 at 16:02

When the aircraft is parked there's no hydraulic pressure, which means that the rudder will be deflected by external forces, ie wind.

The ailerons and elevators might get deflected by the wind or their own weight as well.

  • 1
    $\begingroup$ I'm not sure that this is accurate. Many aircraft have gust locks to avoid this because wind movement can cause damage (and has even been attributed to crashes). $\endgroup$
    – Ron Beyer
    Oct 6 '16 at 13:31
  • 2
    $\begingroup$ @RonBeyer Most larger aircraft don't have gust locks since the control surfaces are not directly linked to the control inputs in the cockpit (heck, they might not be connected at all in aircraft with FBW system) $\endgroup$
    – DeepSpace
    Oct 6 '16 at 15:22
  • 2
    $\begingroup$ Damage to things in the cockpit isn't the point, the control surfaces waving around in the wind damages the linkages at the surfaces themselves. You don't want them just waving around. Hydraulically controlled surfaces don't need one, not because they just let them flap around, but because a depressurized hydraulic controller doesn't allow movement. $\endgroup$
    – Ron Beyer
    Oct 6 '16 at 16:00
  • 1
    $\begingroup$ @RonBeyer The top voted answers disagrees with you, at least with regard to large commercial jets. $\endgroup$ Oct 6 '16 at 17:44
  • $\begingroup$ @CareyGregory No, it doesn't. $\endgroup$
    – Vladimir F
    Oct 7 '16 at 16:38

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