IAS vs Groundspeed and tyre limits on takeoff.

[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