IAS vs Groundspeed and tyre limits on takeoff.

[Arkwright]

Hi all,

I'm about to conduct a flight departing from a very high elevation aerodrome and was concerned about tyre limiting speeds.

I don't have access to the performance manuals at the moment, and wondered if there was a "rule of thumb" or table converting pressure elevation and temperature to IAS?

For some background data, the airport elevation is 9200', assume ISA pressure, average day temperatures between 20-25C and a rotate speed of approx 140 kias.

edit: Oh, and a tyre limiting speed of 195kts gs

Thanks

[MU3001A]

9,200' elevation, altimeter 29.92, IAS 140kts

TAS = 168kts @ 20C, 169kts @ 25C and 171 kts @ 30C

Good to go.

[FE Hoppy]

and the limit as written on the tyre side wall is 225 mph. It's converted to kts in your book as that's the only indication you have.

[Genghis the Engineer]

Originally Posted by Arkwright

Hi all,

I'm about to conduct a flight departing from a very high elevation aerodrome and was concerned about tyre limiting speeds.

I don't have access to the performance manuals at the moment, and wondered if there was a "rule of thumb" or table converting pressure elevation and temperature to IAS?

IAS doesn't change, you need to calculate TAS. Exactly the same as in-flight.

I'd also have words with whatever pillock removed the performance data from the aircraft.

For some background data, the airport elevation is 9200', assume ISA pressure, average day temperatures between 20-25C and a rotate speed of approx 140 kias.

edit: Oh, and a tyre limiting speed of 195kts gs

Thanks

Tyre limiting speed is in TAS.

So, look up the IAS:CAS correction for the aeroplane, calculate TAS for your rotate speed, subtract headwind - that's the number you want.

All do-able on your flight computer.

Assuming QNH=1013, my Aviat gives a density altitude of 12,400ft for 25deg.C, which would turn 140kn CAS (assuming no significant correction) into 169kn TAS - so unless you have a 26+kn tailwind component, looks fine to me.

Now, as to the idiot who removed your performance manual from the aircraft....

G

[Torque2]

(Tyre limiting speed is in TAS.)


Are you being type specific Genghis? The airbus tyre limiting speed is quoted as groundspeed, reference is FCOM 3 General limitations.

[Rainboe]

When I was on turboprops, we used to convert IAS>TAS roughly by multiply 3 times the tens of thousands of feet altitude. So at 10,000', the addition is 3 x 10, at 15,000', 3 x15=45kts increase for TAS. It used to work pretty well (I think, without decent navigation systems, you never really knew where you were, so the speed was pretty immaterial). It used to make dodging the Russians up the Berlin Corridor 'interesting'.

[renard]

I think the time you are most likely to come up against the tyre limiting speed is on landing in a non-standard configuration e.g reduced or no flaps.

I fly an RJ, and at 45t TOW, max rotate is 142kt.

Tyre limitng speeds are 190kts nosewheel and 210kts(?) for main wheels.

These are way in excess of any take off speed, but tie in nicely with the speeds that you will be reaching if you have to do a max landing wt flapless approach.


In a Canberra jet I believe that rotate was below Vmca and so for a short while after take off it would be disasterous to have an engine failure, especially if you were the observer without an ejector seat.

I met someone who had flown one in the RAF and asked why they just didn't take off faster. "Tyre limiting speeds" was the answer

[Arkwright]

Many thanks for all the replies.

MU3001A, perfect, just the answer I needed! I won't have easy access to the performance manuals until two hours before the flight, and being new to operating at these altitudes, this issue was causing me some sleeplessness!

Genghis, in the manuals that I do have easy access to, the manufacturers state tyre limiting speed as groundspeed. After all, TAS only equals GS in zero wind conditions?? This a/c allows a maximum tailwind for take off of 10 kts, so that would have to be added to TAS to confirm GS was under max V tire.
The perf manual is still on the aircraft, its just that I'm nowhere near the aircraft!

Rainboe, sounds like a useful rule of thumb, and comparing it to MU3001A's calculations, fairly accurate!

[FE Hoppy]

on your walkround why not look at what's written on the tyre. If you're not sure which bit to look at find the bit that looks like this: MAX SPEED 225 MPH. But don't take my word for it.

[stilton]

Well if your touchdown speed is faster than your tyre speed you have no option.

You cannot land

[Genghis the Engineer]

Originally Posted by Arkwright

Many thanks for all the replies.

MU3001A, perfect, just the answer I needed! I won't have easy access to the performance manuals until two hours before the flight, and being new to operating at these altitudes, this issue was causing me some sleeplessness!

Genghis, in the manuals that I do have easy access to, the manufacturers state tyre limiting speed as groundspeed. After all, TAS only equals GS in zero wind conditions?? This a/c allows a maximum tailwind for take off of 10 kts, so that would have to be added to TAS to confirm GS was under max V tire.
The perf manual is still on the aircraft, its just that I'm nowhere near the aircraft!

Rainboe, sounds like a useful rule of thumb, and comparing it to MU3001A's calculations, fairly accurate!

Tyre limiting speed is in groundspeed of-course, which is TAS-headwind.

My point was that you need TAS, not IAS/CAS/EAS to determine safety.


As per both my, and MU3001A's calcs which, re-assuringly, come up with the same answer.

In passing, from my own very limited experience of high density altitude runways, the big issue is generally that the high TAS pushed both take-off and landing distances up substantially so if you aren't looking at a very large increase in TODR over standard conditions, then re-check your calcs, they're probably wrong!

G