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MadsDad
26th Apr 2005, 19:28
Talking to a pilot of my acquaintance recently and he mentioned a landing he had done with a hydraulic failure which meant he had no nose-wheel steering. He also mentioned 'things got quite exciting for a while'. Thinking about it after though I started wondering what happened to the steerable wheels under those circumstances. I can think of four options:-

1. The wheel will lock into position, pointing 'straight ahead'.

2. The wheel will lock into position in whatever orientation it happens to be when the failure occurs (which could be at right angles to the aircraft).

3. The wheel is allowed to pivot freely on the landing strut, with the wheel centre directly below the strut.

4. The wheel is allowed to pivot freely round the landing strut but the wheel centre is offset (like an old-fashioned chair castor).

Of the four I would think that :-

1. this would be best provided you can make a dead straight approach with no offset (for instance for a crosswind). Any offset though and there will be a problem.

2. This sounds like a method of getting dead quickly and spectacularly.

3. Similar to 2, but a bit more slowly.

4. this would work since the wheel will pivot until it behind the landing strut relative to the direction of the landing and will probably continue pointing in that direction (although you would presumably get some steering effect while it is sorting itself out).

I would suspect that 4. would be the normal method, on the 'least bad under most circumstances' rule but can anyone enlighten me on what is actually used?

gas path
26th Apr 2005, 19:57
Basically the nose wheel will castor. The (horizontal) centreline of the axle is behind the vertical centreline of the oleo.
As the nose leg oleo extends after takeoff a pair of steering cams in the leg ensure that the nose wheels straighten out and they will remain like this until the oleo is compressed again on landing.
With a hydraulic failure the alternate extension system when selected will unlock the doors and the gear uplock which will then freefall and lock into position, assisted by a good old fashioned spring! There will be some fluid trapped in the steering metering valve and jacks, this acts as a shimmy damper.
Juditious application of brakes and reverse thrust keeps everything nice and tidy on the rollout, and with differential braking one could even 'steer' the a/c to the ramp.

MadsDad
26th Apr 2005, 20:30
Many thanks GP.

I even guessed the right option for once.

The Ugly Fend Off
27th Apr 2005, 16:56
Good reply, it will castor. But on some aircraft instead of the 'good old spring' we have a nice pressurised nitrogen store which blows it all down. A bit of alternate braking is an option but if you have pretty crap brakes the cable is always the safer option. Then you can rely on the good old gingers to come along with a bright yellow towing truck to take you home!

IFixPlanes
27th Apr 2005, 18:43
Hi MadsDad
#4 is correct. ;)
The AMM A320Fam says:
...The sliding tube assembly, which includes the wheel axle, is inclined 9 degrees forward. Because of this the wheels return freely to the center position...

Ingo

Piltdown Man
29th Apr 2005, 11:59
MadsDad: You also might like to know that when you land you also have a wopping great big ruddery thing that will work down to 60kts (airspeed), differential brakes on wheels which can be used (ish) to provide (a sort of) steering, reverse thrust, which again can be used differentially and also, a reduced crosswind limit would normally apply - thus making the task of steering less onerous. However, given the choice, I would prefer working hydraulics!