B747 Taxi & Turning Operations
 

Some details and guidance from those who know, please...

Given: Most low speed 747 (1-2-3-4-8?) directional control is provided by tiller orders to the nose gear. Limited nose wheel control is available via the rudder pedals, but I understand that this is limited to about 6-7 degrees and is really intended for brief periods of increasing rudder control while accelerating for TO.
I've just learned that the 747, at least in some editions, provides some degree of BLG steering control, assumed to be similar to the counter steering often seen on extremely long urban fire apparatus, to enable negotiating tight turns in confined spaces. My questions about the 747's BLG steering include:
1. Is this automatic, or under manual control of the taxiing pilot?
2. Do I have it right, in that this is actually a counter-steering function, either holding the wheels straight or turning them 'counter' to the intended direction of the turn?
3. How many degrees of deflection are available?
4. --- In actual practice, how does it really work? Do pilots like it, dislike it, or simply never think about it?
5. Is this feature common to all B747 editions or perhaps an (extra cost) option? Perhaps it is a feature that has changed its stripes over the years?
6. Whatever you know and can share is greatly appreciated. Thank you!


Ah yes... Of course, once more question! Can the 747's BLG offset be used during extreme cross-wind landings(*), or is it limited to ground taxi functions only?
(*) In past generations, a few VLA have had a MLG offset function available for cross-wind landings. The C-5 and B-52 instantly come to mind, but there may be others. As I recall, the feature was quickly disabled on the C-5 and then eliminated from the design of future generations. For the apparently everlasting B-52, now flown by the grand and great-grand children of its designers, I have no clue. (It sort of sounds like a crazy idea, but for an airframe with floppy wing spread of the B-52, perhaps it is necessary. Again, I have no clue. If you know, beyond the B747, do any other Very Large civil aircraft offer this feature?
Thanks, boys and girls. Any details, leads/links or insight are greatly appreciated.:ok:

I believe it's all automatics on the 747, it was different on the C-5. We had to select CASTER at each turn, switch on the pedestal, and it took the hydraulic lock off the aft gear allowing it to free caster when the nose wheel steering was cranked in. The pilot then selected CENTER coming out of the turn to hydraulically center the aft gear. The crosswind gear was always available on the A models, but was converted to the B model gear in the early '90s. The gear was different in the details and much simpler and more reliable.

Some form of aft gear steering is necessary on any long bodied plane, the 777 has it on the aft most gear.

The B-52 needs the crosswind gear because of its tandem body gear works, four gear, two forward, two aft.

1/ The 747’s Body Gear Steering system is automatic and starts to operate when the nose gear is turned more than 21 degrees. The 747-100/200/300 had an OFF – ARM switch. The 747-400 was entirely automatic (no switch).
2/ Yes it counter steered to reduce tire scrubbing and improve the turning radius.
3 / It could steer up to 13 degrees.
4/ I am not a pilot so can’t offer any operating experience but I think it was a system that worked in the background and no-one thought about it until it malfunctioned which was very rare.
5/ It was standard on all 747 models (not 100% sure about the SP).
6/ It could not be used for xwinds. It had to turned off for takeoff and only turned on after landing (automatic on the 747-400). It was connected to the TO Config warning system so the horn would sound if both body gears were not centered and locked when applying TO thrust.
9/ As for other aircraft the 777 steers the rear axle. I think the A380 steers the rear axle on the body gear. If you go back in time the DC8 allowed the rear axle on the inside of the turn to unlock and caster to improve turning radius. This was deleted on the DC8-62/63. I think some of the large Russian aircraft (AN124?) can steer main landing gears but know nothing about them.

Remember, only the body gear has steering, not the wing gear so you could never set it up in the same way as the B-52 even if there was a way to activate it from the cockpit for landing(which there is not).

http://cdn-www.airliners.net/uf/1137...phpJa7h1s.jpeg

Going from memory, as the tiller moves beyond 20 degrees of nosewheel steering, the body gear for the -400 starts to turn in the opposite direction to a maximum of 13 degrees presumably at max nosewheel steering angle(although that is just a guess). But, this is only at slower speeds. It is disabled once accelerating through 20 knots and enabled when decelerating through 15 knots so it is for low speed operation only.

I'd say in general that you never really think about it. Until it is MEL'd as inoperative that is. That being said I did see one pilot work his way through a 120 degree turn onto the parallel taxiway from the high speed exit in this condition so normal ops really are not affected but turning radius for close-in ops should be considered.

Non-centered body gear will cause damage if retracted. Therefore, there is a takeoff warning if it is not centered when power is advanced for takeoff. So prior to adding a bunch of thrust for takeoff it is best to make sure the tiller is centered to avoid a false warning. I believe the gear lever will not move to the Up position if the body gear is not centered(or if it is not tilted) and therefore the override function should not be used until it is verified why the handle will not move to Up(such as an EICAS message).

Mega Thanks!
 

@Galaxy, CV880, Jammed et al possibly pending: Hot damn and thank you. Between the three of you, I understand what I wanted to understand. As a relatively minor sub-system, rarely given much thought, I can just picture some of the head-scratching that may occur if/when the BLG steering is MEL'd- (if that is a MEL item) and when landing/taxiing on some less than perfect airfields. (For some, I'm thinking this may be a good opportunity to pull off then sit and wait for a two to the gate/stand.) Thanks guys. Much appreciated.
Addendum/Edit: Special thanks to @Jammed for the picture. It illustrates the subject perfectly and clarifies my meaning, even if my words were imperfect. Thank you.

The airline I flew classic B747,s for had a policy not to use the body gear steering while taxing on very slippery taxiways { ice covered, snow etc}. When the -400 was introduced the policy was dropped because the system had become automated.

One thing I have noticed when taxiing the B747 with the Body Gear Steering inoperative is that you have to "overshoot" your intended line of direction during turns far more (ie go much deeper into turns); it feels like taxiing a much longer aircraft all of a sudden.

Another piece of outstanding engineering on the most beautifully designed aircraft I have had the fortune to fly.

The MD-11 and the 777, especially the 300 require a fair amount of over-steering( as do many other others like A-340), so when I had a chance to fly the 747 for a short time it was unnatural being in such a big airplane that basically needed little or no over steering .....worked beautifully!

On the 747-400, the speed limits are based on wheelspeed information from the Brake System Control Unit (with a backup of IRU groundspeed at 40kts).

The system will still work with the IRUs switched off.

Thanks NSEU. Any explanation for the reason why you hear and feel a creaking or vibrating noise when the tiller has been moved to a significant angle while taxiing. As soon as you back off the tiller, it seems to go away. You can hear it in the flight deck and the lower deck. Sometimes as a passenger I have heard a cable movement kind of noise as well on the lower deck.

The other day we were lined up on the runway for a while and there was a similar noise. One guy said that it was possibly the body gear trying to straighten out. We were at max weight.

JammedStab,


From my experience that noise you're hearing in the flightdeck and on the maindeck below could be a dried out nose gear steering collar "groaning" during turns. Engineers tell me they usually give the collar a good lube and problem noise disappears. Also, one would think that sitting in the flightdeck with engines running, you would be too far away from the body gears to hear any noise emanating from them unless maybe a severe structural failure that I would guess you would feel through the airframe as well.

My first taxi experience was in the 742 after a few years of maintaining the body gear steering system an alot of time pushing back and towing , it was an absolute pleasure. A little thrust on the O/B engine, that nose will go 90 deg in a tight turn with the body gear to follow up.

The MD11 takes a bit more planning, the nose wheel only turns somewhere around 80 somewhat deg, speed is more critical as the opposing wheel comes off the ground due to the raked gear, I scrubbed many tires. First time getting used to the "ass in the Grass" is a bit of a challenge. None the less like anything else, it was easy after a bit of practice.

Body Gear Steering on the 747
 

The Body Gear Steering is automatic on the 747-400, it gets enabled/ disabled around 20kts. And yes, it steers in the opposite direction being on the opposite side of the center point of the turn as compared to the nose wheel. Imagine if you steered right in a car and your rear wheels turned (swiveled) right, the front of your car would probably turn left!

The -200/300 had a guarded toggle switch which the pilot flying would typically reach out and enable/disable before the RWY exit/before takeoff.

I have been observer on a 747-200 when a rolling take off went awry because the Captain rushed the take off and flipped the Body Gear Steering toggle switch while the body gear was not yet neutral/centered. We lost traction on a just about wet RWY and thanks to providence, rotated to safety before reaching the RWY shoulder!

I feel distinctly uncomfortable if the Body Gear Steering is not available. It is probably one of the major reasons why the 747 is a great airplane to taxi. The 777 and 787 do not compare in the feelgood, feel comfortable sweepstakes, and I have extensive experience on all 3 types.

SO, to pique your interest in things similar, @No_Fly_Zone, the 777-300ER have something called "Semi-Levered-Gear" - go figure ... ;)

Thanks Plus
 

@capinsan, thanks for the FU response. Much like I've already heard, but 'more better.' Of course I will look into the triple-7's steering the the other two or three that are considered Very Long Airplanes. Got it, but of course I'm wondering about the others. From the A380 down to the C150, if the pilots (alone) cannot taxi it, what good is it?:D Thanks.

It aint always the steering
 

dear @No_Fly_Zone

It ain't always the steering . . .

I've spent a fair amount of my Boeing career working various models of the 747, but being an engine guy I didn't even know the 747 had body gear steering until I read this thread.
A little over a week ago I was out at the flight line for some 747-8 testing, so during some down time I decided to check out the gear steering. HOLY COW :mad:, those are some big actuators!:eek:
Good to hear that the system works well :ok:.

Actually, I am not so sure about that based on two examples I experienced. In the first one we were at max takeoff weight holding in position on the runway for a few minutes. I was just in the observers seat. While in position there was a continuous creaking noise in the cockpit although not as loud as is frequently heard in the turns. An experienced captain said that it was likely the body gear steering being not quite centered but trying to become centered. But that could be incorrect.

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.

Nose and body gear both on the same hydraulic system.

I have no choice but to agree 100% with you Sir:

15 years of my life was spent on the classic 747 flying for 4 different firms and I enjoyed it all.
I feel fortunate as well.

Very interesting, anyone know if the SP did have this feature ?

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.

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.

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

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


Discovered just a little inside brake cures it right away.

Yes, Stilton, it did.

G'day ;)

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.

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.

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.

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

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.

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.