This graph nicely depicts how Vs (stall speed) increases with height and Mmo (maximum speed or maximum Mach number) decreases with height. Where they meet is the coffin corner.
This is a blurry copy of the Airbus PFD (Primary Flight Display). On the left is the speed tape. On top is Mmo (red) "barber pole" and the bottom (yellow) is speeds nearing the stall. In the middle is the "coffin corner." F.Y.I the term "barber pole" (think the helical shape of a barber's pole) means Mmo. When you enter the red zone the airplane starts yelling at you.
So now, you are at maximum altitude and you encounter some wake or bumps from a jet stream. This is when the “pucker factor” increases. For the Airbus we leave the autopilot on and for the most part let the auto-thrust do it’s thing. If the speed wavers too much then it’s off with the auto-thrust. “Jacques from Airbus” says best maneuvering speed above FL 200 is .76. But what happens if this is too slow? Most pilots keep the speed in the middle of the coffin and start thinking about plan B. Lower altitude.
I had better not go any further with this. I would hate to enthrall fear to my edgy seat of the pants fliers. I for one am experiencing sweaty palms as I type this.
I’ll be teaching some high level meteorology next week at the Brampton Flight Center. The facilitator wanted me to include such topics as high-level turbulence and avoidance from an airline pilot's perspective, jet streams and the dreaded coffin corner.
I do not proclaim to be a NASA aerodynamic PhD but here is my take.
One aerodynamic term is the “coffin corner” in other words when an aircraft’s stall speed Vso (minimum speed) nears the aircraft maximum speed Mmo. Mmo (critical Mach number) is the maximum speed at which air can travel over the wings without losing lift due to flow separation and shock waves. This of course is predicated on weight (the heavier the worst) and altitude (the higher the worst). Remember stall speed increases with altitude and maximum allowable speed decreases. It’s when VSO and MMO begin to join. Mmo is different for each aircraft. The little bus is Mach .82, the big bus (A330) is Mach .86 and the manly B777 is Mach .89.
So what does it mean to airline jockey? Well, when ATC asks you if you are able a higher altitude and you oblige knowing full well you are at the limits for that weight, you might want to keep an eye on things.. Airbus does the calculations and predicts a recommended maximum altitude (REC MAX FL), predicated on a buffet of .3g. One could go higher but you better be certain it’s as smooth as a “baby’s bum.”
I do not proclaim to be a NASA aerodynamic PhD but here is my take.
One aerodynamic term is the “coffin corner” in other words when an aircraft’s stall speed Vso (minimum speed) nears the aircraft maximum speed Mmo. Mmo (critical Mach number) is the maximum speed at which air can travel over the wings without losing lift due to flow separation and shock waves. This of course is predicated on weight (the heavier the worst) and altitude (the higher the worst). Remember stall speed increases with altitude and maximum allowable speed decreases. It’s when VSO and MMO begin to join. Mmo is different for each aircraft. The little bus is Mach .82, the big bus (A330) is Mach .86 and the manly B777 is Mach .89.
So what does it mean to airline jockey? Well, when ATC asks you if you are able a higher altitude and you oblige knowing full well you are at the limits for that weight, you might want to keep an eye on things.. Airbus does the calculations and predicts a recommended maximum altitude (REC MAX FL), predicated on a buffet of .3g. One could go higher but you better be certain it’s as smooth as a “baby’s bum.”
So now, you are at maximum altitude and you encounter some wake or bumps from a jet stream. This is when the “pucker factor” increases. For the Airbus we leave the autopilot on and for the most part let the auto-thrust do it’s thing. If the speed wavers too much then it’s off with the auto-thrust. “Jacques from Airbus” says best maneuvering speed above FL 200 is .76. But what happens if this is too slow? Most pilots keep the speed in the middle of the coffin and start thinking about plan B. Lower altitude.
But what a minute! Everyone knows “Jacques from Airbus” built the Airbus such that it does not stall. Remember, you are in the threshold of the operating window and things happen much in line with the unsinkable Titanic.
Another scenario is when dodging thunderstorms. The higher the better for allowing visual separation, however, sometimes it’s difficult to avoid overhangs or convective clouds forming below and you may get yourself in a situation when you encounter some solid turbulence and your airspeed begins to fluctuate. Remember the margin is now small. The flight controls are less effective, the engines are “sucking wind” and the aircraft beings to wallow in the sky. Not a nice feeling. Yes, I’ve been there and it’s uncomfortable.
Many think Air France 447 encountered the coffin corner. It’s pretty well confirmed they flew into a MCC (Mesoscale Convective Complex) i.e. a group of ‘bad ass’ thunderstorms. They were heavy and at a fairly high altitude and then BAM! Turbulence! A recipe for the grim reaper to visit.
Many think Air France 447 encountered the coffin corner. It’s pretty well confirmed they flew into a MCC (Mesoscale Convective Complex) i.e. a group of ‘bad ass’ thunderstorms. They were heavy and at a fairly high altitude and then BAM! Turbulence! A recipe for the grim reaper to visit.
I had better not go any further with this. I would hate to enthrall fear to my edgy seat of the pants fliers. I for one am experiencing sweaty palms as I type this.
22 comments:
from YVR. Met? love it. u never get tired studying the weather.
Have fun at ur class.
Thanks Anon in YVR. Unfortunately it's small class- only six this year. Doug
Once Mmo and Vso join, I take it the airplane has been converted into a nice rock.
If Jacques is not in control and this occurs, will the stall be recoverable or will the airplane bust into small bits before one can have hope of recovery?
Although you hesitated to say the worst for worry that it would cause angst, inquiring minds want to know.
Either way the airplane is no longer flying...
I'm not sure how "Jacques from Airbus" will handle this. I bet I'll get a post clarifying this.
Stay tuned Mark.
Doug
Wow, that was an extremely interesting read, Doug. I didn't realize that there are such tight margins at that FL. Also, I thought that the higher altitude a plane flies, the more efficiently the engines work. But I guess the coffin has to do with airflow across the wings? I can't say I totally understand the idea, maybe it's worth another read. Regardless, do you do any public lectures, or any lectures that some of us can catch?
Lavi
Hello- Although the scenario re coffin corner is potentially grim I for one am glad for your sweaty palms- that tells me that your on-going situational awareness is operating at peak performance! LS-P
Lavi. You're right. Generally, the higher the better as far as engine efficiency, but the higher one goes wing and flight control efficiency decreases. This is where the designers/engineers/test pilots come into play - challenging the envelope. For me, I'll stick to 'straight and level flight', thank you.
As far as lectures/talks, yesterday I had a request to do a talk in front of another Probus (Pro business) group in April. I guess I wooed the other group.
But on the technical side, I teach at the Brampton Flight Center and do a couple of classes at Seneca college. (Both are in Toronto and it's mostly weather). I've also done two presentations for safety groups.
Thanks for the post.
My next post won't be as scary.
Capt. Doug
Flying Kites Mom. That's a good way of putting it- situational awareness coupled with peak performance. I guess the older I get, the smoother I like my flights. Less paper work.
Thanks for the post. How are things in France?
Capt. Doug
I'm probably showing that they were right to take me off my flying course, when I hadn't gone solo on the Chipmunk in 13 hours, but are "Full throtle height" and "Coffin Corner" at the same place, or are they completely different?
Richard. Your question is deep. But no, they pertain to two different things yet both factor in density.
Here's an interesting tidbit I found while researching your question: As altitude increases the thrust reduction caused by reduced air density becomes more significant. At 40 000 ft a jet engine only develops 31% of the thrust it would at mean sea level for the same thrust lever setting at the same gross mass.
Here's one definition of "full throttle height" I found from the same source. Looks like it's more of a piston engine performance thing.
‘Full throttle height’ is defined as that altitude above which the supercharger is unable to compensate
for the reduced air density and both the BHP (brake Horsepower) and airflow decrease in direct proportion to any further
increased altitude. The precise altitude at which this occurs varies according to the combination of manifold absolute pressure (MAP -Manifold Absolute Power) and RPM. The full throttle height will be quoted in the Aeroplane Flight Manual for the maximum rated and maximum cruise power conditions.
Here's my little postscript on this very interesting posting...
-Indicated stallspeed remains the same, while true stall speed increases the higher the altitude.
-Vmo stays the same, in respect to percent of speed of sound, but as Mach is dependent on solely temperature, gaining height gives Vmo as a lower TAS. Don't forget the tropospause will exert influence on the TAS. Going higher will always give lower indicated airspeed, due to the less dense air.
Here's an an inaccurate example - IAS not in the equation:
Cruising FL340, Mach 0.8, and indicated airspeed 260kts. Additional Mach 0.02 up to the mach-buffet, and 40kts down to the stall speed buffet (220kts). This is a pretty wide margin.
FL 430 speed up a touch at Mach 0.815. Since the air is a lot less dense, indicated airspeed will be a lot lower, c. 225kts. That means you have 5kts down to stallspeed and Mach 0.05 to machbuffet.
With this, you are more or less placed in a corner where a minor variance will make ALL the difference.
Just my well-worn example, which I hope helped.
Ian
Ian, thanks for your "well-worn" example. It's good to depict such a complicated topic from a different perspective.
I like your take dealing with specific numbers. You guys know your stuff. No wonder the British exams are the toughest exams to pass. :)
Know anything about Sir Frank Whittle? I may have a little skit to do for the Discovery Channel (Inventions that Changed the World)
I thought you were flying today?
Doug
What about supersonic planes like the old Concorde?
Or is that a completely different set of cicumstances?
Whywhyzed. Probably applied. But I'm not sure what transpires with mach buffet for an SST.
Hi Doug. Thursday takes me to Kenya - much warmer than here!
Whittle: mmm.
I understand that in many circles
Dr. Hans von Ohain and Sir Frank Whittle are both recognizsed as being the co-inventors of the jet engine. Each worked separately and knew nothing of the other's work.
Technically, Frank Whittle was the first to register a patent for the turbojet engine in 1930. Hans von Ohain was granted a patent for his turbojet engine in 1936. The German got his machine into the air first though, I think in 1939, whereas Whittle didn't get his airborne till 1941 with the Gloster E28...Not much help though...
Ian
Ian. You know your aviation history as well! The Discovery Channel will be putting Frank Whittle on the pedestal as the sole inventor. Like you said, he registered the patent first.
Doug
The absolute best (or perhaps worst) coffin corner is the one experience in the Lockheed U2 spy plane at 70,000 feet. The aircraft's stall speed at that altitude is only 10 knots less than its maximum speed.
Douglasr. Thanks for the info. I heard the U2 needed lots of computers to stay out of the coffin. Capt. Doug
I don't know if you are still reading comments from this post. Have you seen the report regarding the AF447 accident? Go to the Flightglobal website if not. You nailed the explanation. I found this post extremely helpful in understanding the comments about what happened in the final minutes of the flight. This is why I love this blog!
Hi Michael.
I did see your comment and I was going to respond later tonight. Really! :)
You have to remember (I have yet to read any reports and this is only my take), but they were in violent storms. There were warnings going off in the flight deck causing major concern, plus they were being tossed about so abruptly they probably could not read the instruments or they were not functioning properly even if they could. Plus...they were scared! Not a nice thing for a pilot to admit, but I bet they
knew their fate especially if they entered a flat spin.
When I saw the weather charts and their track going right through the stuff, it made my skin crawl!
I'll start reading the reports.
Thanks for the kind words about my blog.
Doug
Fockewulf190
Those were two great links you provided. They are both very thorough, but I just scanned them.
Why? I've had my theory about this accident all along. And I prefer not to second guess dead pilots.
I will tell you one thing, we have a new directive out for stall recovery...in a nutshell..."reduce the AOA (Angle of Attack).
This whole episode makes my skin crawl. I'll leave you to it. It's too disheartening for me.
Captain Doug
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