18
$\begingroup$

I am currently involved in a University level modelling excercise which looks to investigate the handling qualities of a novel nose wheel steering system.

As part of this study, I am interested in understanding what are the most challenging taxi manoeuvres that a commercial airline pilot would typically execute, both regularly, or in special circumstances.

Some specifics for the question:

  • I am primarily interested in answers from pilots of Single Aisle sized airliners (A320 family, 737 series, etc.), although general cases are also welcome.
  • I am interested in the kinds of manoeuvres which place the highest demand on the NWS system in terms of responsiveness, max rate demand, etc.
$\endgroup$
  • 1
    $\begingroup$ @Mark's comment on one of the answers suggests researching causes of taxiing accidents or overruns may be a good place to look for more ideas on what can be done to improve nose wheel steering systems. $\endgroup$ – Todd Wilcox Aug 24 '18 at 21:50
19
$\begingroup$

"Most challenging" is a matter of opinion, but if I were to pick, it is probably the 180 degree turn.

The purpose of the maneuver is to turn the aircraft around and face the opposite direction. The technique is to position the aircraft to one side of the runway (taxiways are generally too narrow for this), then apply maximum steering angle, minimum thrust as well as differential braking (i.e. braking on the inner set of wheels only).

The challenge is that the nose wheel, outer set of wheels, outer wingtip and the tail each have their own turning radius. It is made even trickier when the nose wheel is a good 20~30 feet behind the cockpit. The pilots will be above the runway grass while the nose wheel is still on the pavement.

This maneuver is challenging enough that most aircraft manuals I've seen (e.g. B737, B777, A320) dedicate a good 10~20 pages just for this. Here is an example diagram for the B737-600:

enter image description here B737 FCOM Volume 2, Section 1.10.7

This maneuver is mostly executed on small airports where there is no taxiway connecting to the end of the runway. For example, the only taxiway connection to the runway might be at the runway's middle length. A plane that wishes to use the full runway length to takeoff will have to turn onto the runway, taxi down half its length to the end, then execute a 180. On large airports this is rarely an issue, although a 180 might be useful when part of the taxiway / runway is blocked due to an incident.

$\endgroup$
  • 3
    $\begingroup$ From an engineering perspective, the highest risk is probably any type of turn where side loads, or shear loads, end up applied to the nose gear (forcing a turn too tight). Whatever new design OP has in mind would probably need to account for whatever worst-case tolerance is required in the case of pilots running too much thrust accidentally in a turn. $\endgroup$ – J... Aug 24 '18 at 19:41
  • 1
    $\begingroup$ This matches up with my impression based on reports on AvHerald: most low-speed off-runway incidents occur when a large airplane turns around at the end of a runway. $\endgroup$ – Mark Aug 24 '18 at 21:37
  • $\begingroup$ Kevin, with an A380, does this "ever happen" ? I guess it is literally possible but would it just be something that is never done? $\endgroup$ – Fattie Aug 25 '18 at 13:18
  • 2
    $\begingroup$ @Fattie I vaguely remember an incident where an A380 en route to Sydney has to land at an alternative airport somewhere on an island in the Pacific where the runway was wide and long enough to land... but then the aircraft got stuck trying to do a 180 degree turn at the end of the runway. $\endgroup$ – jcaron Aug 26 '18 at 13:59
  • $\begingroup$ A good example of a busy airport where this happens a lot is London City. They have to do it at the end of the runway (before take-off or after landing), and also do a similar manoeuvre when parking at the gate (though I don’t remember if it’s a full 180?), as they obviously arrive nose-in, but turn around to park nose out. $\endgroup$ – jcaron Aug 26 '18 at 14:02
13
$\begingroup$

My experience is on the CRJs up to the CRJ900. I'd say the biggest challenge is going on the tiller at the end of the landing roll, and maneuvering on the ramp doing tight turns. You are using a "steering wheel" with a really fast ratio compared to a car, no tactile feedback (just a light centering spring generally), and a small tire footprint below you, or on a bigger airplane, behind you, that has to change direction of a lot of mass.

Mainly, it's avoiding turning the nose wheel faster than it can deal with the moving mass. You have to learn to manage the rate of steering angle input vs rolling speed to minimize the side loads, which can really wear out nose wheel tires, or worse, causing side scrubbing of the tire (which you know is happening because of the vibration coming up through the floor).

Worst case I would say would be exiting a runway to a taxiway with a 90 degree turnoff and you've been braking hard to make that taxiway and maybe you are still a tiny bit fast when you get there and try to crank the tiller anyway. Or trying to make a rally sharp pivot turn and starting to turn while rolling too fast, or turning the tiller too fast. You can use brake to help the nose wheel with the turn, but that is kind of a intuition thing as well.

A great NW steering system feature could be something like a side load sensor that would adjust the gain of the steering control and back off on the sensitivity if sideloads got too high. Kind of an anti-skid system for steering.

$\endgroup$
  • 5
    $\begingroup$ I'd have some trepidation about a system that decided I had turned too sharply, too fast, and limited the nose wheel movement. Might be the difference between scraping the nose tires and getting the nose gear stuck in the grass. $\endgroup$ – nexus_2006 Aug 24 '18 at 18:21
  • 4
    $\begingroup$ I agree with you @nexus_2006, however, such a system may be the difference between getting stuck in the grass and snapping the nose gear due to to high a side loading. I'd rather be stuck in the grass in that situation... $\endgroup$ – FreeMan Aug 24 '18 at 19:24
  • 4
    $\begingroup$ I have to admit I would too. It would be a way to cope with ham-fisted pilots, like the ones who get on the brakes really hard then let them off suddenly on the rollout. Like how ABS on cars is to cope with people who don't know how to use brakes. $\endgroup$ – John K Aug 24 '18 at 19:34
  • $\begingroup$ ABS, that's exactly what that last paragraph reminded me of. I imagine doing something like that on NW steering of a large plane is a heck of a lot more difficult to design correctly than ABS on a car though... $\endgroup$ – Mast Aug 25 '18 at 18:15
  • $\begingroup$ Re ABS, I don't want to suggest that anti-skid on airplanes is for the ham fisted. Without anti skid just about everybody would blow tires all the time because it's extremely difficult to manually modulate brake pressure to avoid locking wheels when using hard braking with no A/S. But I would take care to bring on maximum braking as gently as I could and try to plan ahead and ease the brakes off gradually at the end of the rollout, if pax was aboard, so they don't get jerked around. $\endgroup$ – John K Aug 25 '18 at 23:01

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.