Reduction in flight envelope at altitude.

[funfly]

As a GA pilot I have few concerns about the effect of height on stalling speed.
However following the AF447 thread I understand that on commercial aircraft at altitudes abouve FL 350 the margin between stalling speed and maximum speed can be extremely small - perhaps down to less than 10 kts in some cases. The implication being that these margins leave very little for variations in air properties and the phrase 'coffin corner' applies to a situation where the altitude and air temperature allows the maximum speed to fall below the stalling speed.
I understand the theory but I find that my inclinatioin to fly commercially has virtually been removed by this 'little' information. Can anyone who knows enlighten me as to how close commercial flight comes to flying at the top limit and what the margins would be on a conventional flight at, say, FL350 otherwise I don't think that I will fly commercially again.

[misd-agin]

The higher you go the smaller the operating enevlope is. That's the difference between minimum and maximum airspeed.

The following numbers aren't perfect and are just for an example -

Stall might be 180 KIAS, buffet might be 210 KIAS, minimum recommended speed about 230 KIAS, cruise would be around 250 KIAS, and max airspeed around 270 KIAS.

As you go higher you're dealing with decreasing KIAS to fly the same MACH. IE, Mach .80 is 320 KIAS at FL 300(guess), 270 KIAS(guess) at FL350, 250 KIAS at FL390, and 240 KIAS at FL 410.

So if you're cruising at FL 300 you're 90 KIAS above minimum recommended airspeed. At FL 350 you're 40 KIAS above, at FL 410 you're 10 KIAS above.

This is high altitude jet flying 101, especially when you're heavy, above OPT ALT, or near your MAX ALT. It's dealt with tens of thousands of times a day, day in, day out, for decades, by professional pilots.

Book a flight, ask for a beer, sitback, and relax. :ok:

[Mad (Flt) Scientist]

As a GA pilot I have few concerns about the effect of height on stalling speed.
However following the AF447 thread I understand that on commercial aircraft at altitudes abouve FL 350 the margin between stalling speed and maximum speed can be extremely small - perhaps down to less than 10 kts in some cases. The implication being that these margins leave very little for variations in air properties and the phrase 'coffin corner' applies to a situation where the altitude and air temperature allows the maximum speed to fall below the stalling speed.
I understand the theory but I find that my inclinatioin to fly commercially has virtually been removed by this 'little' information. Can anyone who knows enlighten me as to how close commercial flight comes to flying at the top limit and what the margins would be on a conventional flight at, say, FL350 otherwise I don't think that I will fly commercially again.

The kinds of narrow margins people have mentioned in the context of AF447 only occur AFTEr a variety of things go wrong.

Under normal circumstances a cruise speed is selected which provides a margin at the lower speed end equivalent to the ability to pull 1.3'g' before encountering buffet. If you consider that stall warning (or buffet) is usually required to give 5% warning of stall, then you could consider that the typical minimum margin to stall for a selected cruise speed is 1.05 * sqrt (1.3) = 1.197 - basically, 20% speed margin to stall at the lower end. That's comparable to the minimums typically applied for takeoff (1.13 to 1.20) and landing (1.23 to 1.30) so high altitude flight, properly planned and without extreme events, is no more at risk of stalling than any other phase of flight.

At the high speed end, you cannot cruise at higher than Mmo (max operating Mach number) - its an absolute flight manual limitation. But for certification the aircraft must be flown safely to a speed called Mdf (demonstrated flutter Mach number), must be shown to have acceptable handling to Mfc (which lies between Mmo and Mdf), must be shown to be structurally sound at the design Mach number, MD, and must be shown to be able to start at mmo and be subject to various "upsets" - including severe gusts - without infringing the various high speed limits or causing a hazardous condition. So there's a considerable margin at the "top end" also - typically of the order of 0.05 Mach. So an aircraft with a cruise "speed limit" of Mmo=0.85 will have been shown to be safe out to 0.90. Oh, and the flutter calculations at MD have to have a 15% speed margin on top, and the structural calculations have a 40-50% safety margin too.

It takes HELL of a lot to go wrong before any normal civil airliner starts to worry about "coffin corner" - the problem is fairly well understood and catered for these days. The aircraft it really was an issue for were military types pushing the envelope (such as the early U-2, which did cruise precariously close to "coffin corner"). No-one does that in a civil transport.

[ClippedCub]

The Curtis Jenny has a coffin corner too, stall speed is close to cruise speed.

[hawk37]

I'd like to nominate Funfly and MFS for their Q and A to be submitted to USA Today. This is just what the public needs to hear.
A 20 knot TAS wind shift seems much safer to handle at altitude than on take off or landing.

MFS, these mach speeds must be done for max aircraft weight I presume?

And so with a GW increase for an aircraft, the test pilots would have to revisit them all, and if necessary reduce VMO?

[Grunf]

Funfly,

Please take a look into this FAA AC (http://tinyurl.com/5vjvoz), in addition to MFS's excellent explanation.

Cheers

[wonwinlow]

Most of what you are saying is correct but...a new phenomenom related to the ITCZ is coming in to play, and it's not very comon. The sudden increase of temperature due to convective currents (and some sources are saying from -49°C to -19°C at FL 370)... now do the numbers and where that leaves your coffin corner?? suddenly you are flying outside the envelope! I remember a B747 in that same route lost 4000' a few years ago (2001 if I'm correct), and that temperature change range was a factor. Now remember it's not official, but this rumour is begining to make sense, the problem is that it ocurrs very rarely. Temperature is a key factor on your "coffin corner".

[FE Hoppy]

wonwinlow Most of what you are saying is correct but...a new phenomenom related to the ITCZ is coming in to play, and it's not very comon. The sudden increase of temperature due to convective currents (and some sources are saying from -49°C to -19°C at FL 370)... now do the numbers and where that leaves your coffin corner?? suddenly you are flying outside the envelope! I remember a B747 in that same route lost 4000' a few years ago (2001 if I'm correct), and that temperature change range was a factor. Now remember it's not official, but this rumour is begining to make sense, the problem is that it ocurrs very rarely. Temperature is a key factor on your "coffin corner".

I'm sorry but I think you are jumping to the wrong conclusions here. "Some sources" are talking crap. There is a huge difference between Indication and actual. The Indication may vary for many reasons. If the Actual temerature changed by 30C at high altitudes those of us who have spent thousands of hours up there would know about it by now.

Please don't post unsubstatiated rumour in the Tech Log Forum.

[wonwinlow]

Ok FE Hoppy, maybe the changes in OAT are a bit too much and it's OAT that I'm talking about, remember the lines on the altitude capability paper charts? ISA + 20, ISA + 15, etc??, ... Well they do influence your envelope, and a change in temperature when you are pushing your aircraft to secure a flight level or whatever other reason can have an impact on your performance, the point I'm trying to make if this change becomes large enough along with even moderate turbulence it will get you in some very tight situations.

Weather in that part of the world can be very violent and nasty, perhaps the worst next to the north american midwest. Now large changes in temperature did ocurr and happened to the B747 flight, and the satellite picture picture was similar showing a very active ITCZ.