Amazing content brother!! Its funny how experienced pilots still believe low speed stall is just a regular stall. The truth is, shock waves start to appear due to acceleration of air over the wing surface when AoA is increased. Keep up the good work!

As always brilliant ....

Very fleshing idea about low speed buffet, instructors in my airline always talk about an airplane will always stall at a constant IAS, so as altitude increases the stalling speed will be the same IAS, but TAS will increase because higher altitude. Hence the low speed buffet. But I am inclined to your explanation because that makes more sense to me.

Awesome

Hello. Stumbled across this video while I'm studying high altitude aero for my ATP. This video helped me to understand coffin corner a little better because it actually made me think. However, there is an inadequacy in your explanation. The PFD/Attitude indicator does not show direct AOA of the wings. It is only telling the pilot the angle between longitudinal axis and the horizon (nose of the airplane vs horizon to put it simply). This is something that I have drilled into my student's heads over and over, the PFD/attitude is not an AOA indicator. Angle of attack is angle between chord line and relative wind. Pitch angle from the PFD/attitude indicator does not tell you that. Most planes do not have AOA indicators, although the attitude indicator can give you a general idea of where your AOA is at. But you are correct when you say a stall has nothing to do with speed, it does not. Only critical AOA. What I did get from this video is that at the low speed buffet, your AOA is high enough (although perhaps not at the conventional critical AOA) to now induce a supersonic flow over the wings due to the airplane's TAS through the air. This will cause a shockwave and move the center of pressure back, but not as much as with the high speed buffet. This shockwave will induce a low speed mach buffet. It is not necessarily a stalled condition as you defined it due to the angle of attack not being at critical AOA, but it is almost like a stall as there is separation of airflow from the wings surface and the boundary layer is disrupted. Which will result in buffeting and reduced control effectiveness as would happen in a stall at a lower altitude and at 17-18° AOA. So to conclude, with a low speed mach buffet, you perhaps have not exceeded the defined critical AOA, but your AOA is excessive enough to induce shockwaves from airflow acceleration and basically "stall" your airplane. However, as we don't really have AOA indicators in many airplanes, we use speeds as known guidelines to avoid "stalls" at higher altitudes (the low speed buffet), although an airplane can stall at any given airspeed. The air data computer in the airplane will calculate a corresponding IAS and put that on the screen as the speed the pilot should avoid to prevent the low speed (or high speed as well) buffeting. I think we could also compare low speed buffet and what is happening to the AOA to icing on wings: ice will start to disrupt the boundary layer and smooth airflow, and eventually separate the airflow at an AOA MUCH LOWER than that of the critical angle if the pilot tries to increase the AOA. This is stated in the Pilot's Handbook of Aeronautical Knowledge Chapter 5, implying that you can stall or have a severe reduction in lift even at an AOA much less than critical AOA if there is a reason for it to happen. This is basically also what is happening with a low speed mach buffet, except the reason for the early airflow separation is the shockwave rather than icing. Now is the low speed mach buffet a stall? Depends on how you define it and how you're looking at the whole critical AOA situation, but you definitely have severe reduction of lift. There's also fluid dynamics magic happening in all this I'm sure, but I'm not an aerodynamic engineer and I'm not even going to begin trying to solve this at that level. I just fly the things safely and have fun. This has been my two yen on the topic.

Don’t really fully understand why aoa wouldn’t decrease with an increase in altitude considering the air density is significantly reduced, I get airspeed is increased but certainly not as significantly as the air density is reduced at a cruising altitude. Maybe I just don’t understand the math of it. But in conclusion both low and high speed Mach buffet are essentially caused by air over the wing going super sonic causing shockwaves? I’d assume then low speed buffet causing the cop to move rear wards just like a high speed buffet? Also causing a Mach tuck? Im a pilot in training for my atp already took the atp written

thank you for your explanation! but why it called coffin btw?

I am not sure about your low speed buffet explanation. In the lift equation there is air density; which is pretty low at high altitudes. Decrease in speed will lead to reduction in lift. However you are already be at the critical value of air density that barely enough for required lift and I think thats why aircraft will stall.??

As you climb, the air becomes less dense, and your wings need more airflow to generate the same amount of lift. The true stall speed increases as you climb, regardless of the AoA.