Coffin Corner?

[Lurk R]

Heard the term on another thread and just wanted to query the actual meaning. Have seen it used before and think it's along the lines of optimum flight levels, speeds and weights - can anyone please elaborate?

Thanks folks...

[m&v]

With the advent of the "jets" 707/dc8 etc,there was a marked absence of high Altitude flight experience.This led some pilots to consider climbing to early(too heavy,not condidered the temp'diff from ISA,or to avoid turb').With climb thrust the aircraft struggled to alt'-then it became evident that the "cruise" speed was very close to the Stall" speeds(slow speed/high speed ,critical mach)-with no power to spare.If cond'worsened with turb',there existed the possabilities of Stalling at alt,or exceeding to VMO and High speed stalling the wing as one met the Mach crit.After a couple of Incidents/accidents the Industry learned to respect the "optimum"weight for given alts.

[Captain Cessna]

I understand that coffin corner refers to the range of available airspeeds, which decreases as an aircraft climbs..

An aircraft can fly only in the speed range between the point of stall, and the point at which high speed buffet occurs (over control surfaces, airframe etc). As an aircraft climbs, the stall speed will increase and the IAS at which buffet occurs will drop. Therefore at some point, the two meet, meaning therefore that the point of stall, is also the point at which high speed buffet sets in. I believe this point is called coffin corner.

This problem is made worse by the fact that jet engines are more efficient the higher they are operated.

Hope i have got it right, after all, I am only a student :). Maybe a real engineer can confirm my mumblings.

(Looks like someone already has http://www.pprune.org/ubb/NonCGI/tongue.gif)


[This message has been edited by Captain Cessna (edited 21 November 2000).]

[Jim lovell]

As the aircraft climbs higher and higher the stalling IAS increases- but the VMO/MMO decreases. I beleive "coffin corner" refers to the situation where the difference between MMO and stalling speed is very small- e.g 5 kts or less in some cases. In this situation the aircraft cannot accelerate due to MMO and it can not decelerate due to VS. Remember the barbers pole drops after u pass changeover level because it has to conform to the MMO limit. I believe high flying business jets- Lears, Citation etc are most susceptible to this phenomenon?

[BOAC]

As I remember it JL is correct. I believe it was 'born' when a straight-wing twin jet US bomber came into service (anorak alert!-cannot remember the number, but it was before the B47 I think - could have been B45, and could have been a 'Martin')
and it could climb to this point. When crews tried to descend having realised their mistake the speed naturally increased as soon as they lowered the nose putting them at MCrit (shock stall, as it was known), which with a critical straight wing caused hideous wing drop and loss of control so I guess they just had to stay up there all day!

[Check 6]

It is not that the stall speed increases with altitude, the IAS decreases, thus approaches the stall speed. As altitude increases, the airspeed can approach Mmo, AND approach stall speed. The margin between Mmo and stall speed is the so-called "coffin corner." In a U-2, this margin is less than 10 KIAS at altitude. This does not give much margin for turbulence, or error. Turbulence is what is believed to have caused the loss of a U-2 between Japan and Korean approximately 5 years ago.

It is not true that Learjets have a "coffin corner" problem. In a Lear 35, the IAS at FL410 for example will be in the area of 210 KIAS, depending on the OAT. At a typical 15,000 pound weight, the Vref (clean) is 167 KIAS for banks up to 30 degrees, and 157 KIAS for banks up to 15 degrees. As you can see, the margin is approximately 43-53 KIAS. So, this is rarely a problem. One exception is where the AFCS (autopilot) is engaged in "altitude" mode, and severe turbulence including a strong downdraft is encountered. The AFCS will try to hold altitude by increasing the angle of attack, which at the same time will bring about an ever decreasing IAS. Now, this margin could possibly decrease to the point of concern. The remedy in this scenario is to immediately disengage the altitude hold, and request a "block altitude" from ATC. I have an excellent article on "coffin corner" written by Pete Reynolds, Vice President of Flight Test for Learjet (Bombardier), but it must be in my office, as I can not find it at home. I will post it if I can find it.

Check 6

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Kick the tires, light the fires, first off is lead, brief on guard.

[Lurk R]

Thanks heaps folks - just what I was after!!!

[Dick Whittingham]

IMHO - I've just learnt that - all aircraft have a coffin corner, it's just that some can't get there for lack of power or equally lack of a limiting Mach. Absolute coffin corner is where the stick shaker is on and you are at MNE. Practical coffin corner is where you are on the low speed buffet boundary, about 1.2Vs -at that height - and on the high speed buffet boundary -usually MMO. Many transport aircraft can't reach coffin corner fully loaded because no one is going to put in excessive engine power above that needed for economic cruise. Old square overpowered military jets did reach coffin corner.

Stall IAS does increase with height. The constant speed, weight for weight in level flight, is EAS, and for the same EAS IAS gets higher and higher as compressibility comes in.

Likewise, MMO gets lower as alpha is going up at the lower EAS at height, and with increasing alpha the shocks come earlier. The best picture I have seen of this is the 737 set of buffetboundary charts.

[Checkboard]

Usually if they "can't get there" it is because the two limits meet above the aircraft's certified ceiling (which is usually determined by the pressurisation limit in jet transports).

[Capt Claret]

Another consideration for coffin corner is manoeuvring, say a normally executed turn could cause a stall if the aircraft is too close to coffin corner. (stall IAS v angle of bank).

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bottums up !

[jtr]

Check6, I believe the stall speed will increase with altitude (above about FL200). Relates to reynolds number, and the compressibility of the air. (lots of "I think"'s in previous statement)
Most large modern jets have thrust limits that are similar (in alt) to buffet limits. Result being, if you do drag your butt up to max CRZ level (at 300'/min or less), flogging the engines all the way, then you have to sit there for the next 2 hours hopeing for a smooth ride, as Vs, and MMO, are looking pretty friendly with each other.

[wondering]

I might add U-2s have a problem with coffin corner at high altitude with their straight wings. Must be extremely difficult to fly them at optimum altitude.