Originally Posted by
rickair7777
Assuming fixed GW, CG, and AOA...
At higher density altitude:
1) The true airspeed necessary to generate sufficient lift increases. This means that you have to fly FASTER to maintain your stall margin.
2) The reduced air density causes the AS indicator to read lower than true.
As the pilot flys faster (to keep indicated airspeed at normal values), the TAS will then be higher than at sea level.
Convienently, the amount of reduction in IAS at higher density altitudes approximately matches the required increase in TAS to maintain normal stall margins...so we just fly the same numbers regardless.
Note: your groundspeed will be higher on landing, so the distance remaining markers will go by more quickly.
Originally Posted by
patton33
No, indicated stall speed should remain the same.
If you have a multi engine rating refer to your Vmc vs Stall speed diagram, should answer the question in graphical terms.
Originally Posted by
Lalo37
Good info! Thanks Rick
Folks, I hate to disagree with all you guys, but this issue is pretty important so I think we need to correct this before MORE people get incorrect information.
Stall speed
INCREASES with altitude. Whether you look at IAS, CAS, EAS or TAS, they
ALL increase with altitude.
Since PFDs display CAS (not IAS), and TAS has no importance whatsoever in the stall margin, we should only really talk about CAS and EAS.
EAS is really the airspeed that we need to be concerned about when relating airspeed to stall margin, and the stall speed in EAS increases with altitude.
A good rule of thumb is the 2kts/5000ft rule, but at cruise altitudes the increase is even greater than this approximation and varies depending on aircraft type.
The CAS stall speed will increase by an even greater degree due to compressibility effects... and since this is what we see on the PFD, we should keep and even greater "buffer".
I urge everyone to review some
good aerodynamics or performance books because IMHO there should be no confusion whosoever over such a basic and fundamental aerodynamic principle. High altitude aerodynamics must be one of the most poorly understood areas of flight training and unfortunately it is not taught enough (FAA requires it for a commercial but applicants are seldom quizzed on it....and my experience is that regional airlines do a very poor job at teaching it).
Unfortunately, there are WAY to many books that contain incorrect or in some cases outdated information, so be careful about what you trust.
Long story short -> if you ever find yourself having to deviate from company profile at high altitudes, remember stall speed will be higher than what you would otherwise encounter at sea level. 