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-   -   B747 Taxi & Turning Operations (https://www.pprune.org/tech-log/560141-b747-taxi-turning-operations.html)

NSEU 12th Jun 2015 08:20


The other day we had the body gear steering inop. In this case, the taxiing it was only a slight bit more difficult for the turn even shallow 90 degree turns but when a sharp 90 degree turn was made, a lot of that creaking noise was heard(more than normal) and was felt through the tiller.
During sharp turns, even with body gear steering working, rubber is left on the tarmac as the parts of the aircraft travel through different arcs, at different speeds, causing sideways skidding/rubbing. Body gear steering is designed to reduce this effect, but it can't eliminate it all.

Of course, with the system off, the gear is subjected to more significant stresses which will be felt more acutely inside the aircraft.

JammedStab 12th Jun 2015 08:30


Originally Posted by Bergerie1 (Post 9008875)
A great aeroplane with no handing vices at all - as opposed to some earlier jet types I have flown. The only thing one had to take care over was not to taxi to fast round tight corners on wet surfaces. In these conditions the nose wheel adhesion was a little limited and would scrub. So long as one entered the turn really slowly there was no problem.

Especially on the paint at the runway threshold. The 747 does not seem to have a huge amount of weight on the nosegear depending on the CG.

Take a look at this video of a Pan Am landing with no body gear extended which is not that far aft of the wing gear(landing at 5:30, tips onto tail at 7:00). So only that body gear seems to be keeping the nose down in certain circumstances.

https://www.youtube.com/watch?v=tl_wXfSwRzM

stilton 13th Jun 2015 10:13

Have had issues with nosewheel scrubbing on the 75/ 767.


Discovered just a little inside brake cures it right away.

Feather #3 14th Jun 2015 07:38

Yes, Stilton, it did.

G'day ;)

Capt Quentin McHale 14th Jun 2015 12:03

JammedStab,


After consulting with some of our engineers I totally agree with your body gear/nose gear steering angles and ground speeds etc. However it was pointed out that your Captain from example 1 was more than likely incorrect, because if you are holding on the runway, your nose gear should be straight and therefore you body gear should also be straight and locked after nose gear angle is less than 20 degrees. If not, then you will get a "gear not centred" message on thrust lever advancement. Did you get that message on your takeoff roll? A continuous creaking noise does not make sense if you are holding/not moving but stranger things have happened in this game.

As for example 2, when performing a sharp 90 degree turn and feeling vibration through the steering cables and tiller, 2 things come to mind. 1... a dry nose gear steering collar as I previously posted or 2... nose wheels scrubbing in the turn due to entering the turn a bit too quickly. I have observed this in both wet and dry (nose wheels smoking in dry) conditions.

You are correct as far hydraulics (#1 system) are concerned. It was also pointed out to me by the engineers that you have a 7 degree limit both left and right for rudder pedal steering at low speeds (steering tiller inputs will override rudder pedal inputs) and if a nose gear steering cable breaks then a roller moves out of detent and allows the nose wheels to caster.

JammedStab 14th Jun 2015 15:14

Thanks for the info,

That noise that was heard while holding in position went away on its own after about a couple of minutes(held for about three minutes for traffic). It seemed to be quieter and coming from the back instead of through the tiller. No takeoff warning although I would think that rolling forward a bit would straighten out anything if it was not already straight.

Just to clarify, the noise I initially mentioned could very well be from the tire scrubbing(I was thinking that it was a hydraulic system noise). The sound only happens on occasion from a relatively sharp turn and reducing tiller input just slightly makes it go away. It does seem to be speed related(around 10 knots) as it does not happen in a slow sharp turn. Most people seem to do the turn at 10 knots although depending on the intersection, a tight turn is usually not required. That being said, I don't remember ever seeing any angled scrub marks on the nosewheel on the walkaround.

Castering nosegear in a 747? Have had the entertainment of taxiing with something like that in a light single only. It could be interesting on a bigger aircraft.

NSEU 15th Jun 2015 00:23


Castering nosegear in a 747?
Indeed. Look at any picture of the 747 nose gear side on. The axle is displaced to the rear (compared to the position of the strut).

The nose gear also has automatic (mechanical) centering on lift off. If you lose hydraulics on the nose gear, when the strut extends, centering cams engage. This stops the gear damaging the aircraft during retraction.

InSoMnIaC 15th Jun 2015 01:46

Never stop in a turn with full tiller still applied in a heavy 74. If the Body gear doesn't straighten up, you will have considerable trouble moving again

Capt Quentin McHale 16th Jun 2015 23:08

InSoMnIaC,

I understand what you are saying, but, the body gear will not "straighten up" until the nose gear is less then 20 degrees L or R of centre.

CCA 17th Jun 2015 03:28

While the tiller may be centered the aircraft also needs to have had a reasonable distance to roll to allow the body gear to centre as well.


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