Originally Posted by
Cralis
(Virtual/PC pilot)
I've noticed that as you ascend to your cruise altitude of 360, your 'speed' drops in the primary display. Additionally, I notice that max speed (Vne?) reduces as well. My reading leads me to believe that the speed reduction on the primary display reduces because the air pressure on the pitot decreases, or less air is being forced in. But why does the Vne reduce as you climb? Is it a calculation of Vne based on altitude?
Hi Cralis, what you quote as Vne in a jet airliner is actually Vmo (max operating speed). This is typically a fixed value up to about FL280 when Mach number becomes the dominant factor and it switches to Mmo (max operating mach no). Again, this is generally a fixed value for the aircraft type. e.g. for the B777-200, Vmo is 330kt & Mmo is M0.87. As you continue to climb to cruise level, for a fixed mach no, your IAS will decrease as will Mmo in IAS terms
Then I see we have Ground Speed. So that would be using GPS over the ground, I'm guessing. Is that what ATC would see, when the aircraft reports speed?
Yes and no. Ground speed is the GPS speed over the ground but when reporting speed to ATC, it's either IAS or mach no. e.g. "280 knots' or "mach decimal 84"
Then we have IAS... So indicated airspeed would match what we see on the primary display, I think.
Yes
And TAS would be the real speed the aircraft is moving through the air, regardless of pitch (i.e. If we were climbing vertically, speed would not be 0 - it would be relative to the airflow).
Is TAS based on the altitude (air pressure).
IAS (indicated air speed) is an indication of the dynamic air pressure the aircraft 'sees' as it flys through the air, and is what the wing needs to produce lift. TAS (true air speed) is the speed the aircraft is actually flying though the air. At Sea Level, they are pretty much identical. As the aircraft climbs and the air density reduces, to get the same IAS the aircraft has to fly faster to 'see' the same pressure, so TAS increases as you climb at a fixed IAS. Groundspeed is essentially TAS ± headwind component.
If all of these are correct - why wouldn't the primary display just display TAS, which seems to be the most important speed for the pilot? With Ground Speed being the most important to ATC. IAS seems to be the least useful.
Most of you assumptions were incorrect. IAS is by far the most important for a pilot as that is what provides the lift.
Hope that explains it all.