Aviation Education: Coffin Corner explained correctly

This is an aviation education video explaining in detail what coffin corner is.
0:39 General
1:00 PFD AOA
1:39 High Speed Mach Buffet
2:18 Low Speed Mach Buffet
3:24 STALL
4:41 PFD AOA
4:50 Summary

[@guillermocuevas1688]

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.

[@Linlds21]

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.

[@flightlevelview2115]

THank you for commenting. Thank you for watching. Which airline if i may ask.

[@yohanjojo4020]

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!

[@flightlevelview2115]

Thank you for watching. Jojo. Could you give a quick intro to yourself? Are you a pilot? Dispatcher? or enthusiast?

[@user-ey3rg3nc5s]

As always brilliant ....

[@flightlevelview2115]

Thank you for watching

[@kadirsarac2203]

Awesome

[@flightlevelview2115]

Thanks man

[@bcrow8163]

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

[@flightlevelview2115]

Thank you for commenting. Not sure what you were asking. Do you not understand why the speeds encroach towards each other? Could you ask again in a different way?

[@flightlevelview2115]

A stall “exceeding the Stalling AoA” does not depend on altitude (air density). Please write back i would love to chat, and perhaps you could give me another idea for a video

[@zaildarbains]

@@flightlevelview2115 i think bccrow thinks that Level flight AoA should be lower than 3 degrees as seen in your PFD. Above FL310 the aircraft is going fast , but the AOA is still high because IAS is low at HIGH ALT due to pressure and density . IAS determines AoA , not Ground speed. in the mid altitudes flying fast can result in 0 degree level flight because the you can attain IAS greater than 320 knots.

[@alexkurnia3127]

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

[@flightlevelview2115]

Coffin Corner is just a name, I "think" it got the name because because you're flying in such a tight speed margin.

[@Romeojulietless]

You get it wrong , you end up in a coffin 😉

[@mosty85]

​​@@flightlevelview2115I don't think that's correct. I think its more sinister than that. I believe the 'corner' part of it refers to the tight margins, but the 'coffin' part refers to the potential human consequences of losing control of the aircraft due to the small margin for error.

[@flightlevelview2115]

@@mosty85 thank you for commenting and watching. Are you a subscriber?

[@flightlevelview2115]

@@mosty85 thank you for the comment

[@angiecibis]

"Hello, gentlemen..." ?

[@orhananildemir4022]

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.??

[@flightlevelview2115]

Thank you for responding. I really appreciate it. Not too detailed but are you an airman by any chance? I just wana know for my channels sake who’s watching my content. What videos you prefer go pro or educational

[@orhananildemir4022]

@@flightlevelview2115 I am a Pilot, CPL holder. However the matter is, there is only one correct answer about the low speed stall at high altitudes. If what I know is incorrect I would appreciate to be corrected, if you have a source that proves your claim.

[@flightlevelview2115]

@@orhananildemir4022 my sources come from my aerodynamic knowledge. I think I explained in the video why the low spd mach buffet isn't due to a stall. A stall is defined as the CL not increasing with a further increase in AOA. (The CL is decreasing) A stall doesn't depend on altitude or spd. At 1 G level flight at high altitude, the stalling AOA is no different, and assuming the stalling AOA is around 15~17 degrees That means the airplane would have to be at about 15 to 17 degrees pitch up in order to "STALL". The airplanes do not arrive at the low spd mach buffet at 15 ~ 17 degrees. That's why I am ruling out stall from the reasons why the nose drops once you hit the low spd mach buffet. The low spd mach buffet involves shockwaves. If you would like to explain why you think the low spd mach buffet is due to stalling. Please feel free to comment. Let's have a discussion. Once again, thank you for taking the time to comment.

[@MikeSierra8282]

just check what is the stall and what is caused by. Stall is exceeding critical angle of attack which is as "flight level view" said constant with altitude and it should be around 16-17 degrees. Low speed mach buffet is exactly what he showed in this content. EVen if you think that way, why is this called low speed mach buffet? if there would be any stall they would probably call it stall buffet or just the stall:-) BTW. you can decrease critical angle of attack by lowering the flaps for example. Efective AoA increases and because of that AoA crit decreases

[@flightlevelview2115]

@@MikeSierra8282 thank you for commenting... are you a sub or a just a visitor?

[@fightingfulcrum8108]

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.

[@flightlevelview2115]

What? That is incorrect.

[@flightlevelview2115]

That is not correct at all.

[@graysonmalitsky5019]

@@flightlevelview2115 Could you explain why that’s incorrect? Not a pilot, but I would have thought true stall speed increased with altitude because of the air density. Would love to hear your explanation as an expert, and thanks for your content!

[@flightlevelview2115]

@@graysonmalitsky5019 thank you for commenting. Too many pilots focus on “speed” when discussing stalls. Yet all pilots know stalls are dependent on AOA. Rhetorical Question: Why do pilots focus on speed when they should be focusing on AOA? The stalling AOA doesn’t change with density altitude. At 1G level flight at 5000ft the stalling AOA should result in a corresponding pitch attitude. If the airplane is at 1 G flight at a higher altitude the stalling pitch attitude should be the same as the lower altitude. If the airplane reaches “low spd mach buffet” or (low speed shock stall) --some people find it easier to understand the term shock stall, its not quite accurate though-- at a different pitch attitude from where it should have normally STALLED i can conclude that the low speed shock stall is NOT airflow separation due to an exceedance of the stalling AOA. I hope that made sense. You said you were not a pilot. Are you a dispatcher or mechanic by any chance?

[@zaildarbains]

@@flightlevelview2115 i see . Your pointing out that Coffin corner low speed loss of lift is not the traditional stall caused by boundary layer separation. rather its airflow experiencing disruption due to Mach effect. the AOA is low enough for air to pass freely and create a low pressure on top of a wing but Mach effects are ruining our streamline. either ways the actions are the same. lower nose , increasing speed .