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V1Pup
14th Apr 2002, 03:30
Hi all. What's the concensus on this question? I'm having an argument with a colleague re the brakes and undercarriage fitted to a 47-400. His view is that the oft recommended max taxi speed of 20 knots straight and 10 knots cornering is old-fashioned, and applied only to the previous generation of brakes but not to the modern carbon-fibre whatevers now fitted. So what's the gouge? What effect does fast taxi speeds have on the potential brake performance in an RTO, and does fast cornering, ie in excess of 20 knots, break down the sidewall strength of the tyres? Would be grateful for any tech insight on this...

G.Khan
14th Apr 2002, 05:41
Fast taxying in any aircraft is, I think, foolish but in a 747-400 it is a definite no-no, quite reckless in fact. 20kts on the straight and 10kts cornering would be MAXIMUM speeds for most operators.

Overheated brakes will have an adverse effect on an RTO and fast cornering will scuff the tyres as well as putting a considerable side load on the u/c. The aircraft weighs almost 400 tonnes when full and has massive inertia which must be taken into consideration when selecting any taxy speed.

Your friend that wants to taxy faster than the above quoted speeds is verging on the realm of Cowboy, IMHO.

:rolleyes:

Hugh De Payen
14th Apr 2002, 05:55
Quite right Mr Khan, in fact it is bordering on the realms of downright stupidity, IMHO.

Yeeehaaaa.

18-Wheeler
14th Apr 2002, 08:11
The typical maximum taxy speed on the Classic is 30kts, below 320 tonnes, and 20kts above 320 tonnes.
As stated above, the max speed in a turn is 10kts, but in wet conditions you often want to get down to less than that, say, 6kts.
In icy conditions, a few knots less than that, asymmetric power, and some people also turn off the body gear steering as well.
(Not too sure about the last one, non-standard procedure)
Being up three stories, you can trundle along at 30kts and hardly even notice it, so an eagle eye on the groundspeed readout is needed.

Quick edit - The weight of the plane doesn't really affect it's ability to stop at taxy speed, it'll pull up very darn quickly indeed! However, you have to allow for the extra weight as the wheel assembly's heat up a lot faster with the plane heavy. IT also depends on turn-around times, etc.
The main reason for the speed limit in turns is because if you go much faster than that, even on a very sticky taxyway, the nose-wheel will just scrub away and the plane will not turn. It gets worse when the plane is very light, as there is not much weight on the nose-wheel, and also again when it very heavy, purely from inertia.

V1Pup
14th Apr 2002, 20:55
GS on a 747-400 is read via the IRS units which project the GS onto the EICAS display...

Denzil
14th Apr 2002, 21:33
Regardless of taxi speed the carbon brakes are more efficient if you use heavier single applications in preference to constant dabbing on the brakes!!

Semaphore Sam
15th Apr 2002, 13:51
Those advocating 20 knots or slower (straight) must realize that, at light weights, almost constant brake application can be required. I believe it's better to let the speed to get to a max, then 1 constant application to a very slow speed, to limit the number of applications, and repeat. As for cornering, I agree, the slower the better.

18-Wheeler
15th Apr 2002, 14:38
Yep, when you're light you have to brake as you get up around 30kts, down to about 10kts, then let it pick up speed again.
After landing when you're light, you can shut down #3 engine, which helps a fair bit.

Checkboard
16th Apr 2002, 06:37
Tyre heat build up is due to a large part to the flexing of the tyre side walls, which in turn relates to taxi speed and weight.

You will find that exceeding the maximum taxi speeds will over heat the tyres before you experience a problem with the brakes.

Sopwith Pup
16th Apr 2002, 07:44
Salako......747 Classics are either equiped with INS or IRS, both give a ground speed readout displayed on the instrument panel.

Flight Detent
16th Apr 2002, 11:14
Hi 18-Wheeler & Checkboard,
Yes, the Classic has an indication of ground speed on both the selected page of the INS and on the HSI.
Also, don't forget that carbon brakes are significantly more efficient when they are operating at higher temperatures, but, of course, the tyres are not!

When's your next move 18-Wheeler?
Cheers

Dan Winterland
16th Apr 2002, 20:59
Body gear steering cannot be turned off on the -400 unlike the classic. However, it disarms above 20 knots and re arms once below 15.

If you land with the body gear retracted, you musn't use nose wheel steering under any circumstances when under 15 knots after landing, as the body gear turns in the wells causing lots of damage.

18-Wheeler
16th Apr 2002, 23:36
Dunno, FD, waiting like the rest of us ... They sent me an email saying that they'll be cranking up again around the middle of next month though.

Dan - Can't you pop a circuit breaker to turn the body gear steering off, somehow?

Flight Detent
17th Apr 2002, 14:00
Hi Dan,
Yes, you can disable the body gear steering, at least on a Classic, without turning the arming switch off, but I'm not sure on the -400, in fact, I'm not sure of anything on the -400, but that's another story!
The FE will know about that!
Cheers

Checkboard
18th Apr 2002, 06:57
Ahh: found it.

This (http://www.desser.com/tech/heatgen.html) is a technical site explaining heat build up in aircraft tyres.

http://www.desser.com/aircraft/img/heatgen_c1.gif

The vertical dotted line at 35 mph (30 knots) indicates the recommended maximum taxi speed. On the above chart, the curves constantly slope upward with higher taxi speeds. In other words, the faster an aircraft travels over a given distance, the hotter the tires will become.

Many people would expect the shoulder area to generate the most heat. In reality, the bead and lower sidewall area are the hottest. There are two major reasons for this:

1. All forces, in or acting on a tire, ultimately terminate at the bead. This is an area of high heat generation.


2. Rubber is a good insulator; or said another way, it dissipates heat slowly. The bead area, being the thickest part of the tire, retains the heat longer than any other part of the tire.