Originally Posted by
RVSM Certified
But, here is what I don't understand: In high density altitude conditions, why does the delta between indicated airspeed and true airspeed begin to widen or increase?
Example: Normal departure airport field elevation is 3,000 ft msl. Atmospheric conditions are such that the HDA is 7,300 ft msl. So, the aircraft will now need more runway for the ground-roll. But,
why does the aircraft
also require a higher true airspeed/ground speed? I don't yet understand why.
I've read stories where pilots have flown into higher field elevation airports, spent some time on the ground, came back to the aircraft and loaded it up with full fuel, cargo and possibly a passenger or two,
after a significant increase in the density altitude and then has very difficult trouble getting the nose off the ground during the ground-roll, having to either abort the landing, or ditching off the end of the runway.
This blows my mind and I'm trying to make sure that I don't do the same thing. Are these cases simple matters of just needing more airspeed? Or, are these matters a case where the HDA factor had grown well beyond the takeoff performance capability of these aircraft and no matter what the pilot did, the takeoff was doomed to end in an abort, regardless?
Thanks for the help in advance, guys.
Indicated airspeed is generated by air molecules impacting the pitot tube. The faster the true airspeed, the more molecules impact the pitot.
However as altitude increases, the air becomes less dense so there are fewer molecules impacting the pitot, so indicated airspeed drops even though true airspeed remains the same.
A high density altitude does the same thing as a high MSL altitude...less dense air.
Operationally, you normally land and takeoff at the same IAS regardless of density altitude. The IAS measures molecules, and molecules make the wing fly so a given number of molecules impacting the pitot corresponds to a certain number flowing over the wing.
At high density altitude, a higher TAS is required to achieve the needed IAS. This results i higher groundspeed, which means a longer runway is required (for both TO and LDG).
As for humidity...air is a mixture of molecules, at a given pressure (altitude) a certain volume of air contains a fixed number of molecules. These molecules can be any of several gasses, including water vapor. Air is mostly made up of oxygen and nitrogen molecules (O2 and N2)...these molecules are actually heavier than a water molecule because H2O has two hydrogen atoms which are the lightest atom on the periodic table. Since the presence of water molecules means less N2 and O2 molecules, that volume of air is going to be less dense. This jeans it will have less effect on the pitot tube and the wing.
Density altitude also effects propulsion...less O2 for the engine to burn, and the prop is just a spinning wing, so it suffers too.