Taxi speeds - how fast is too fast

[ettamakwetta]

Re: Taxi speeds - how fast is too fast

Hi Guys
This is actually quite a complicated problem. I am currently finishing off some analysis regarding safe operating speeds, and have been curios to know what the highest speeds are that anybody might have seen. The problem with calculating these types of stability margins is the fact that you have so many non-linearities, especially from the tyres and the oleos. This means that you can get crazy situations that you could have two wildly different behaviours, even though you have the same thrust and steering angle settings.
Regards
Etta

[Boeing Freighter]

Re: Taxi speeds - how fast is too fast

Hi ,

I have no figures to give unfortunately but some airports have taxiways that are certified ( or something ) for controllers to give pilots " high-speed taxi " clearence.

Frankfurt-Mail is one such airport , but i am unsure of the requirements to have a high-speed taxiway , or just how fast you can go.

[kenparry]

There is no simple answer. One of the critical things on long taxi distances is tyre heating. About 10 years ago Boeing brought out a training package about the topic; one bit I remember is that if you taxi at MTOW (MTOM if you are using JAR) for 10 km at 60kt you will heat up new tyres to the temperature at which the safety plugs will blow. OK, I don't know an airport where you would taxi for 10km, but if you taxi that fast for say 6km (which is possible at a few places) and then start the take-off with no delay, are the tyres going to survive? If you reject the T/O, it's going to be worse. Boeing advice was, if I recall correctly, don't use sustained speeds above 20kt. But, as someone already said, it should be in the ops manual - and if so, that's what to do.

[Bungfai]

Depending on taxiway surface condition and most of the time thinking of passengers comfort, 30 kt and max 15 round the corner. Anyway if faster than 80kt, auto brake will activate!

[777fly]

I remember the Boeing training document, about taxi speeds and tyre heating, mentioned by Kenparry. It contained a graph which illustrated the amount of heat that was going into the tyres Vs taxi speed, due to tyre hysteresis. I was fairly linear up to about 20kts, then went semi-exponential through the 25-30kts range and upwards. The message, as I remember it, was that a MTOW 'heavy' which was taxiied at 25kts for 5Km in 25-30 degrees ( very possible at a number of Middle East airfields) was going to be near tyre fuse-plug failure nearing the end of 2Km+ takeoff run. The main issue was tyre carcase heating. 20kts, briefly 25kts was recommended for taxy. If tyres are designed to handle speeds of around 150kts and heavy braking on runways and 40kts on high speed turn-offs, taxiing at ANY speed, short of getting airborne, is possible for short periods. Safety and common sense dictate otherwise and most company flight manuals give recommended taxi speeds in the 20-25kt range ( maximum figures, good conditions, etc)