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Planes these days use fly by wire systems while older ones had a mix of hydraulics with electronic assistance like the MD-11 (And maybe 737-800?).

But what would happen if you lost the electronic systems and you had to fly manually? I remember an incident of this happening in a MD-11 and pilots using raw muscle power to operate the flying surfaces (And using something like 10 kilos of force only to keep the yoke in place) or putting 10 kilos of force to the yoke constantly (My memory isnt exactly bad. It was a months ago)

So How much raw muscle power is needed to fly an MD-11 or an 737-800 without assistance for 10 minutes and then land? Through two pilots or as a single person.

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    $\begingroup$ It's not an answer, as you seem to be only asking about the MD-11 or 737, but the A-10 had "a mechanical system as a back up if hydraulics are lost. Flight without hydraulic power uses the manual reversion control system; pitch and yaw control engages automatically, roll control is pilot-selected. In manual reversion mode, the A-10 is sufficiently controllable under favorable conditions to return to base, though control forces are greater than normal." $\endgroup$ – CalvT Jun 11 at 12:36
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    $\begingroup$ Lots of airplanes used hydraulics only to provide boost to the existing mechanical controls, like car power steering. Probably the first mass produced airplane to do this was the P-38L's hydraulically boosted ailerons. It could actually outroll single engine fighters at high speed because full aileron was available with little effort by the pilot, whereas the FW190 guy had both hands on the stick trying to get max roll rate. In that case if you lose the hydraulics you are back to manual controls, like losing your power steering. $\endgroup$ – John K Jun 11 at 16:19
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    $\begingroup$ The 737 has manual reversion (except on the rudder), allowing control without hydraulics, along with its predecessors the 707 and 727. These are the only Boeing jets designed this way. $\endgroup$ – user71659 Jun 11 at 17:58
  • $\begingroup$ @user71659 how strong someone must be to fly at near stall speeds (Because I think you need more muscle power to fight higher winds) with manual reversion? $\endgroup$ – Delta Oscar Uniform Jun 11 at 18:01
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Only small planes can fly without power actuation. The largest size airliner that can be flown manually would be about a B737, which actually has manual back-up for the elevator & ailerons as in this answer. The B737 uses a balance tab to assist in lowering the hinge moments during manual actuation.

In a much larger aeroplane like the DC-10, there is no practical amount of reducing hinge moments with servo tabs or other means, and flight control surfaces are deflected using hydraulic power only. This is schematically as follows:

own work, some decades old

When the pilot moves the stick, they deflect an input linkage which opens the servo valve of one or more hydraulic actuators. These then deflect, moving the output end, thereby closing the servo valve again. As long as there is enough total hydraulic pressure to overcome the aeroforce hinge moments, the actuator moves the control surface without any effort from the pilot required at all. In fact this would be so very light that artificial feel springs are mounted.

However, in some circumstances the actuators cannot overcome the control hinge moments, for instance due to:

  • Having to fight one of the other actuators, for instance because of a hardcover servo valve failure;
  • Hydraulic pressure loss in one or more of the actuators;
  • Mis-trim and high airspeed cause surface blow-down, as in several recent crashes.

Note that in this case, when the pilot pulls on the stick, they first deflect the servo valve until it hits the stop, and this now becomes the fixed hinge point of the input linkage. Further stick deflection results in direct manual surface deflection, against the hinge moments caused by the aeroforces and against oil still slushing around in the hydraulic system.

Like mentioned, in large aircraft with no measures to lighten hinge moments, the flight crew cannot practically control the flight path using manual force. A DC-10 did manage to actually land after total hydraulic failure by using differential engine thrust, not manual actuation.

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  • $\begingroup$ The 707 and/or DC8 had cable and pulley flight controls. I'm pretty sure the big four engine piston liners did as well. The old flying boats certainly did. $\endgroup$ – acpilot Jun 11 at 15:51
  • $\begingroup$ One way to tell non-hydraulic flight controls at a glance is the existence of tabs and mass balances, and offset hinge lines (like a B-17's rudder). $\endgroup$ – John K Jun 11 at 16:13
  • $\begingroup$ What happens in practical terms is the depressuized surface will deflect under air loads until the servo valve bottoms out, then the surface will start pushing on the cable control circuit. It might move about half way to full travel up or down before bottoming out the servo (the sag you see when the controls are unpowered on the ground). In flight it would do the same thing, float down under airloads until it starts pushing on the control circuit as the servo bottoms out. You'd be able to resist the last half of travel perhaps before the PCUs themselves get to their travel limits. $\endgroup$ – John K Jun 11 at 20:42
  • $\begingroup$ @JohnK No that is not what happens. The servo valve must “bottom out” first before the surface can start moving. The actuator itself does not bottom out before the surface stop. The surface will be at a position where the hinge moment equalises the internal resistance of the hydraulic circuit, wherever that is. You can push both ways and try to resist (which will be futile if designed as a fully hydraulic circuit). $\endgroup$ – Koyovis Jun 12 at 2:01
  • $\begingroup$ @acpilot yes, same thing except that a cable can only transmit tension forces and you need a set of 2 of them for full control. $\endgroup$ – Koyovis Jun 12 at 2:03
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You can't. Most of these airplanes are flown with hydraulically powered control surfaces with no mechanical input possible from the cockpit flight controls. The flight controls just operate servo control valves in the hydraulic actuators, like the bucket on a front end loader but a little fancier. If it's FBW, the FBW system does the same thing at the actuators using torque motors operated by the FBW computers to drive the servo control valves.

There will be two, or three, hydraulic actuators driving each control surface. The flight controls in the cockpit just control the extension/retraction of the actuators. Control feel in these airplane comes from spring devices in the control circuit to simulate "air loads". There is no direct connection between the pilot and the dynamic forces acting on the control surface. If you lose all of the hydraulic actuators, you lose the control surface.

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  • $\begingroup$ There is no feedback from aeroforces - if there is hydraulic pressure. If no hydraulic pressure, there is feedback through the servo valve deadband. $\endgroup$ – Koyovis Jun 11 at 1:36
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    $\begingroup$ This answer seems to ignore the MD-11 part of the question. Searching for "MD-11 fly-by-wire" finds several sources saying either that the MD-11 is not fly-by-wire (here, here) or that it has FBW only in some modes (here). $\endgroup$ – T.J. Crowder Jun 11 at 7:01
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    $\begingroup$ The MD 11 guys were not able to displace the flight controls. They were controlling pitch and yaw with thrust alone. $\endgroup$ – John K Jun 11 at 12:28
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    $\begingroup$ @T.J.Crowder This answer also seems to be ignoring the 737 part of the question, which actually does have direct mechanical backup. $\endgroup$ – reirab Jun 11 at 17:55
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    $\begingroup$ @Koyovis After the engine failure, UA 232 was controlled solely by differential throttle. Part of their problem was the fact that the initial failure happened while the aircraft was making a right turn, leaving the rudder was deflected slightly to one side; they had to fight that steering force the whole way because they were unable to move the control surfaces without hydraulics. Their flight path has a few loops in it where they basically drifted off course and decided it was easier to go with the rudder and turn all the way around to the right than to fight it by turning left. $\endgroup$ – anaximander Jun 12 at 8:38
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As a complement information to Koyovis answer, please note

On large airplanes, it is rather a « servo tab » that will do the job you are asking for, in case of hydraulic failure rather than assisting the normal force that moves the elevator, it becomes the sole force that makes the elevator move. Therefore during normal flight it acts like a balance tab, in case of failure it becomes the sole active force. Like the balance tab, the servo tab moves in the opposite direction of the flight control’s trailing edge.

Please refer to the following website:

https://www.flight-mechanic.com/flight-controls-tabs/

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  • $\begingroup$ Servo tab can be a mechanism, it means that the pilot deflects the tab only, which then deflects the surface. A balance tab is mechanically geared to the surface, pilot deflects the surface directly and the balance tab moves as a result of surface deflection. $\endgroup$ – Koyovis Jun 12 at 2:08

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