St Elmo's fire and Coffin Corner

pilot602

If specified, this will replace the title that
Re: St Elmo\'s fire and Coffin Corner

St. Elmo's Fire is a movie about Alien Abduction and Coffin Corner is what they used to call a stretch of highway I used to live near ....


Seriously, St. Elmo's Fire is a discharge of Static Electricity on the surface of an aircraft which if bad enough can cause severe interference with radio communication/navigation. Static wicks are used to dissipate/alleviate the problem.

Coffin Corner can mean several things. In GA/Civil aviation I believe it's the place on the power chart where an aircraft "gets behind the curve" and is then operating in the region of revers command. In helicopters it's something to do with takeoff speed, I think.
 

MikeD

Administrator
Staff member
Re: St Elmo\'s fire and Coffin Corner

St Elmos fire:

A) An '80s movie

B) Static electrical discharges normally encountered in storms, etc that can appear in cockpit areas, on wings, on windscreens, etc.

Coffin Corner: Many terms. Two common.

A) Behind the power curve

B) The crosswind to downwind portion of the bombing pattern on a range where aircraft can potentially midair if one extends the pattern while the following aircraft cuts it off.

C) The local coffin dealership.
 

Alchemy

Well-Known Member
Re: St Elmo\'s fire and Coffin Corner

I always thought coffin corner was the altitude where the critical mach number and stalling speed of an aircraft become almost equal. This is mainly an issue for high altitude aircraft like the U-2, if I'm not mistaken. There is a very narrow airspeed window in which the airplane can be operated safely at those altitudes...too fast and you risk structural damage, too slow and you stall.
 

Derg

New Arizona, Il Duce/Warlord
Staff member
Re: St Elmo\'s fire and Coffin Corner

I've seen LOADS of St. Elmos fire in the cockpit.

It's makes the front windscreen look like a Tesla device and is freaky. Then we start building up a lot of static electricity and sometimes go momentarily "NORDO" (No Radio) because of the static. Fortunately, COM2 (at least on the -88/90) is a little more audibly legible than COM1 in those conditions so we're not completely radio deaf.
 

Mr_Creepy

Well-Known Member
Re: St Elmo\'s fire and Coffin Corner

Yeah Alchemy that's what I heard too. Lear drivers always say that up above the 40s they approach the coffin corner if they bank too steep.

You have to really set it up on the CRJ (and we only did it in the sim of course), by getting up to FL 410 and yanking over in to a 45+ degree bank. The two rattlesnakes on the Airspeed tape start heading towards each other and you get stall warning and overspeed at the same time.
 

MikeD

Administrator
Staff member
Re: St Elmo\'s fire and Coffin Corner

[ QUOTE ]
I've seen LOADS of St. Elmos fire in the cockpit.

It's makes the front windscreen look like a Tesla device and is freaky. Then we start building up a lot of static electricity and sometimes go momentarily "NORDO" (No Radio) because of the static. Fortunately, COM2 (at least on the -88/90) is a little more audibly legible than COM1 in those conditions so we're not completely radio deaf.

[/ QUOTE ]

Want fun? Try having St Elmos fire multiple lightening-tesla-like strikes on the windscreen while air refueling, when the AR boom on our jet is right in front of the windscreen and it's leaking fuel (worn seal) aft onto the windscreen where the tesla is going off.

Loads of fun.
 

pilot602

If specified, this will replace the title that
Re: St Elmo\'s fire and Coffin Corner

[ QUOTE ]
[ QUOTE ]
I've seen LOADS of St. Elmos fire in the cockpit.

It's makes the front windscreen look like a Tesla device and is freaky. Then we start building up a lot of static electricity and sometimes go momentarily "NORDO" (No Radio) because of the static. Fortunately, COM2 (at least on the -88/90) is a little more audibly legible than COM1 in those conditions so we're not completely radio deaf.

[/ QUOTE ]

Want fun? Try having St Elmos fire multiple lightening-tesla-like strikes on the windscreen while air refueling, when the AR boom on our jet is right in front of the windscreen and it's leaking fuel (worn seal) aft onto the windscreen where the tesla is going off.

Loads of fun.

[/ QUOTE ]

It's not worn ... it's engineered that way. You know. To keep you alert during the operation.
 

Derg

New Arizona, Il Duce/Warlord
Staff member
Re: St Elmo\'s fire and Coffin Corner

Yeah Mike! Inflight entertainment for ya!


Besides, I hear that drives you hog drivers nuts, something interesting but you can't shoot at it!
 

CAVOK

New Member
Re: St Elmo\'s fire and Coffin Corner

[ QUOTE ]
I always thought coffin corner was the altitude where the critical mach number and stalling speed of an aircraft become almost equal. This is mainly an issue for high altitude aircraft like the U-2, if I'm not mistaken. There is a very narrow airspeed window in which the airplane can be operated safely at those altitudes...too fast and you risk structural damage, too slow and you stall.

[/ QUOTE ]

That is what I have always thought also. I have heard that the U-2 has about a 3-5 knot window to operate at between stall and overspeed when they are at altitude. Not too sure if that is correct (never flown one
), but I think I heard that somewhere.
 

ananoman

New Member
Re: St Elmo\'s fire and Coffin Corner

[ QUOTE ]
I always thought coffin corner was the altitude where the critical mach number and stalling speed of an aircraft become almost equal. This is mainly an issue for high altitude aircraft like the U-2, if I'm not mistaken. There is a very narrow airspeed window in which the airplane can be operated safely at those altitudes...too fast and you risk structural damage, too slow and you stall.

[/ QUOTE ]

This is correct. The speed of sound changes due to temperature. It ranges from about 660 kts at sea level decreasing to 574 kts in the stratosphere were the atmosphere is a constant -56.5 celcius.

Most aircraft will encounter mach effects before this due to the acceleration of the air over the tops of the wings. Designers can delay mach effect somewhat by using thinner wings without much camber or sweeping the wings, but all subsonic aircraft should encounter mach buffet at or before 574 kts at altitude.

As altitude increases and the speed of sound is decreasing somewhat, the stall speed is increasing. It is true that the indicated airspeed that results in a stall remains constant, but the resulting true airspeed increases dramatically from sea level to 50,000'. This is not usually a huge problem with normal aircraft, but it can result in limitations mandating autopilot use at very high altitudes. In an extreme example like the U2, pilots are restricted to a very narrow band of available airspeeds.
 

IrishSheepdog

Sitting in the median
Re: St Elmo\'s fire and Coffin Corner

Coffin corner is found in various jet aircraft. It is the area in flight performance where the max operating mach/speed (decreasing with altitude) meets the stall speed (increasing with altitude). Simply put, go faster, you'll overspeed and possibly "mach tuck". Go slower, you will stall.

Mach tuck is when the air above the wing separates, therefore causing a strong pitch downward. Normally uncontrolable, with negative consequences. You can experience mach tuck at speeds below mach 1.0. Normally happens above mach .95. As the air flows over the airfoil, it speeds up, and can exceed mach 1.0. If it does, it separates.

Most jet transport aircraft have a large "coffin corner". You'll usually find this a concern in aircraft such as the Citation X, Learjet series, or other high performance aircraft that fly in the upper Flight Levels (above FL450) at high mach numbers.
 
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