6
$\begingroup$

I understand form documentation that in normal law, pedal input is not necessary to keep coordinated flight. It is merely said that using the pedals is not necessary. Later in the documentation it is said that the law for the yaw axis is either normal, alternate or direct (depending on the condition). For the normal law, automatic functions are implemented (yaw damping, coordinated flight). What happens when the pilot provides a non-neutral command on the pedals? Such information is well documented for pitch an roll control (command of load factor and roll rate), but not for yaw.

On the A380, the documentation states that, "A pedal deflection results in a proportional sideslip and bank angle". I found no such precision for the A320, but it seems reasonable to assume the pedals command a sideslip angle.

In normal and alternate law on a A320, do the pedals command a sideslip? If not, what do they command?

$\endgroup$
4
  • 2
    $\begingroup$ Comment, not answer - I’m not entirely sure. I think on any Airbus before A380, rudder pedal directly commands rudder surface deflection (i.e. direct law). If you look at front-on views of the aircraft, you‘ll notice A380 and A350 have beta vanes (sideslip sensors) which are a necessity if you wish to implement a sideslip control loop. Earlier Airbus types don’t have that so cannot have a sideslip control law (but what they actually have I‘m not sure, as I said). $\endgroup$ Mar 29 '19 at 20:39
  • $\begingroup$ @CptReynolds I'm seeing conflicting sources on how beta is sensed on the A320. One source notes that the A320 has AOA3 at an angle from the horizontal plane of the aircraft. Thus by comparing AOA3 versus the other two AOA sensors, which are on the plane, you can get beta. Another paper says beta is from lateral acceleration, i.e. ball in a tube. Note that beta is displayed on the PFD as the split triangle, so there definitely is beta sensing. $\endgroup$
    – user71659
    Mar 30 '19 at 5:28
  • 1
    $\begingroup$ @user71659 Isn’t split triangle simply lateral acceleration (ball in tube), which is related but not equal to beta? $\endgroup$ Mar 30 '19 at 9:19
  • $\begingroup$ @CptReynolds No. From the FCOM: "This trapezoidal index moves beneath the roll index. On ground, it represents the lateral acceleration of the aircraft. In flight, it shows sideslip (as computed by the FAC)." $\endgroup$
    – user71659
    Mar 30 '19 at 16:28
3
$\begingroup$

You are correct. On all Airbus FBW aircraft, the rudder pedals normally control the sideslip angle, beta. See this presentation for a source, slides 3, 5-6.

As a note, on pre-A345/A346 models, the rudder pedals are mechanically connected to the rudder. However, the FBW system adds in an additional input, via the yaw damper servos, to drive the rudder to the position the FBW control loops want.

Additional details can be found in Advances In Aircraft Flight Control, M. B. Tischer, page 90:

Finally, it was decided that the rudder pedals would command a combination of sideslip and roll angle to restore some of the conventional aircraft behavior.

Figure 5 shows that the rudder command is a gain-scheduled proportional control based on sideslip, yaw rate, roll rate, and roll angle. The gain scheduling is based on airspeed and high-lift configuration.

$\endgroup$
8
  • 1
    $\begingroup$ I think your source on slide 3 actually shows that automatic rudder control is for some roll/beta law, but „manual“ (pedal) input is direct rudder input added to the automatic rudder deflection? $\endgroup$ Mar 29 '19 at 21:22
  • 1
    $\begingroup$ @CptReynolds I explained that in my answer. Generally the action of the pedals through the mechanical link will move the rudder to close to the position the FBW control loops want. The FBW system then adds or subtracts the remainder, via the YD servos and a summing mechanism, to get the rudder position that is desired by the beta law. Thus when the FBW system is working, the rudder is under its control, but when the system fails and stops providing inputs, the mechanical link has full control. $\endgroup$
    – user71659
    Mar 29 '19 at 21:31
  • 1
    $\begingroup$ sorry, you need to change this answer. While not specifically stated so in the docs, rudder is always in direct on A320: when flying single engine you constantly need to adjust rudder pedal deflection (and/or rudder trim) for any speed change, or any thrust change. This would not be the case if pedals would command beta angle. $\endgroup$
    – Radu094
    Jun 4 '19 at 1:32
  • $\begingroup$ @Radu094 See my references. I stand by this answer. In OEI, the beta command doesn't have enough authority for thrust asymmetry, so the aircraft enters a stable turn. $\endgroup$
    – user71659
    Jun 4 '19 at 1:33
  • $\begingroup$ I have read you references, and appreciate your effort, but the conclusion is incorrect. Rudder is in direct. I am typerated on A320 $\endgroup$
    – Radu094
    Jun 4 '19 at 1:39
0
$\begingroup$

The rudder deflection is the result of the sum of 2 inputs: the pedals and the yaw damper.

Turn coordination is controlled by the yaw damper actuator and has no return to the pedals. Therefore during a turn the pedals won’t move.

What happens when the pilot provides a non-neutral command on the pedals?

If the pilot acts on the pedals during a turn, the yaw damper action will automatically be reduced to maintain a correct coordination, until the yaw damper action is cancelled. Beyond that during a turn the yaw damper will not do an opposite correction, i.e. if the pilot action is beyond the required amount for a correct coordination.

During a leveled flight an action on the pedals is similar to an action on the rudder trim.

$\endgroup$
3
  • 1
    $\begingroup$ I fail to see what your answer adds to the existing ones. Please elaborate those points. $\endgroup$
    – Manu H
    Jun 3 '19 at 20:45
  • $\begingroup$ @Manu H, to the contrary of what is said in the comments « The FBW system then adds or subtracts .........., to get the rudder position that is desired by the beta law », if this was true, no side slip should occur whatever the rudder position done by the pilots. In the simulator during a turn if you push slightly the rudder during the turn, the turn keeps centered, but beyond a position the excessive input shows on the side slip, thus during a turn the yaw damper actuator adds the missing amount, but if the pedals input is excessive the yaw damper will not subtract the excess. $\endgroup$
    – user40476
    Jun 4 '19 at 9:00
  • $\begingroup$ please made it clear into the answer and not in the comments. $\endgroup$
    – Manu H
    Jun 5 '19 at 4:23

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.