Pretty interesting indeed!

Was engaged with a long conversation of a very long cup of coffee post office hours with a colleague, discussing the difference between aerodynamic stall, and the stall defined as “lift less than weight”. Its kinda funny. How did we come to this point?

Well, the lower stall/buffet margin (that is seen on the PFD as the red and black barber pole) comes up only when the aircraft weight is entered in the FMS. Change the weights and the stall limits change! Now, an aerodynamic stall is independent of the weight, and solely dependent on the angle of attack, and the airspeed. Hmmm!

So the weight entry in the FMS which generates the lower barber pole on the PFD signifies the “stall” defined as lift being less than weight. This happens when, for the same speed and the same AOA, the aircraft banks, rendering the vertical component of the generated lift less than the weight of the aircraft which always acts vertically downward.


We were in a nice discussion about Vmax and Vmo as well. Apparently, Vmo is a hard coded number, but the Vmax is dependent on variables.

Then came a very interesting fact: supposing the ISA deviation goes crazy in the positive direction, and you are at cruise, there can be an ISA deviation at which the available thrust si less than drag! Common sense, yes, but this common sense may not be in your FMS! So, your FMS may actually want you to continue, based on some “assumptions”. Ah ha! Something for you, David Learmount!

And we discussed a lot more on cockpit displays. Wonderful Day. Wonderful talk. Good night folks.