Does Indicated Stall speed increase with altitude (Coffin Corner)?

Question

It is my understanding that Indicated Stall speed is mostly constant with altitude, the one that increases is True Stall Speed. Given that PFD show speed as IAS, how come the yellow speed range increase with altitude?

Answer 1

I think part of your confusion comes from mis-interpretation of the amber bands on the PFD. The picture you show looks like it is from a Boeing 777, which has the red barber pole and the amber band towards the top of the speed band defined like this:

5 Maximum Speed

Indicates maximum permissible airspeed as limited by the lowest of the following:

• Vmo/Mmo
• landing gear placard speed, or
• flap placard speed.

6 Maximum Maneuvering Speed

When displayed, indicates maneuver margin to high speed buffet. Bottom of the bar is the airspeed that provides 1.3 g maneuver capability (or an alternative approved maneuver capability as preset by maintenance). Bar may be displayed when operating at high altitude at relatively high gross weights.

Note: 1.3 g maneuver capability occurs at 40 degrees of bank in level flight.

^{(Boeing 777 FCOMv2 - 10.10.3 - Flight Instruments, Displays - Controls and Indicators)}

It also has a red barber pole and amber band towards the bottom of the speed band defined like this:

4 Minimum Maneuvering Speed

Indicates maneuver speed margin to stick shaker or low speed buffet. Top of the bar is the airspeed that provides:

• 1.3 g maneuver capability to stick shaker with flaps down.
• 1.3 g maneuver capability to stick shaker or VREF + 80, whichever is less, with flaps up below 20,200 FT.
• 1.3 g maneuver capability to low speed buffet (or an alternative approved maneuver capability as preset by maintenance), with flaps up above 20,200 FT.

Displayed with first flap retraction after takeoff.

Note: 1.3 g maneuver capability occurs at 40 degrees of bank in level flight.

5 Minimum Speed

Indicates the airspeed where stick shaker activates.

^{(Boeing 777 FCOMv2 - 10.10.5 - Flight Instruments, Displays - Controls and Indicators)}

Note that the top of the amber band above 20,200 ft is related to buffeting and not stall speed (near stick shaker activation) or something "preset by maintenance". Therefore, this does not indicate a change in stall speed. Why buffeting becomes the limiting factor at higher Mach number is already explained in this answer.

Having said this, it is also true that indicated stall speed increases with altitude. This is not really significant for smaller GA planes, where to first approximation stall speed will be at a constant IAS. At higher altitudes and at higher Mach numbers, the stall speed will increase for various reasons, which are well explained in this answer.