Question: How does a commercial jetliner's highest possible groundspeed vary with altitude (assuming wind=0) ?
My understanding is that drag decreases with altitude, therefore maxspeed increases with altitude.
This is also what Cpt. Simon Hardy, a B777 instructor, says:
« Aircraft [referring to airliners] go faster at altitude than at low level. At sea level the aircraft [referring to a B777] can only do 330 kn. And at altitude, 30.000 ft, it can do Mach 0.86. So it's a sort of sliding scale – as you descend, you get slower. » (source)
But today I read exactly the opposite on an aviation blog, which I found curious:
« When a plane gets so high, you have to fly slower, since the speed of sound (and hence true airspeed for a given Mach number) decreases with altitude. » (paraphrasing, source)
« Real basic point: You’re flying higher, you’re flying slower. You want to go fast […] you just fly lower. » (source)
... so which is it ?
The perfect answer for me would ...
• include a graph (Y: altitude, X: maxspeed)
• include citable references
EDIT (in response to the comments below):
I specified maxspeed as the highest possible groundspeed (assuming wind=0), although I'd rather prefer not to make that distinction, because from my experience this complicates the issue more than necessary: Of course the altitude will affect the different types of speed more or less, but isn't it nonetheless possible to make an approximate statement like "higher=faster, lower=slower" and explain why that is?
For a concrete example, let's assume:
- aircraft = B777
- wind = 0 km/h (for argument's sake)
- weight = typical weight of a B777 ~45min into a 9 hour long haul flight
- aircraft has already reached cruising altitude
If at that point the pilot wanted to "get away" as fast as possible (let's say from waypoint IGARI), what altitude should be chosen?
Is the quoted blog post correct in saying that at an usually high altitude, the plane would fly slower, therefore it would have to go lower to fly faster?
it's certainly possible to make a vague/general statement like "higher/faster, lower/slower" and explain why that is.I would say that this is NOT possible without defining a specific speed type. IAS = higher/slower, but TAS = higher/faster (everything else being equal). So, which do you want? (each individual type of speed is discussed elsewhere on this site) $\endgroup$