Coffin corner - is it better to descend on the fast side?
Thread Starter
Join Date: Nov 2002
Location: Heart of Europe
Posts: 198
Likes: 0
Received 0 Likes
on
0 Posts
Coffin corner - is it better to descend on the fast side?
A question or two for the pro's of flight testing.
If I ever would encounter a hairy situation when flying close to buffet onset and stall speed and the only way out is down and get a bit overspeed - what happens to an airliner when entering the zone above MMO/VMO?
Buffeting?
Flutter?
Nothing but increased drag?
Loss of e.g. aileron control?
Others?
The question arose out of a discussion where I said that it might be better to descend by reducing thrust and keeping attitude when hitting turbulence close to the margins (VS vs. MMO/VMO) even when risking that during the gusts I may hit an overspeed condition.
My argument was that MMO/VMO will not lead to a situation from which it may be difficult to recover as long as I would not dive the plane, whereas a stall definitively will.
Still we're out of tolerance and therefore the question: What really happens when going overspeed?
What happens with 10kts?
What happens with 25kts?
Any enlightenment very welcome.
If I ever would encounter a hairy situation when flying close to buffet onset and stall speed and the only way out is down and get a bit overspeed - what happens to an airliner when entering the zone above MMO/VMO?
Buffeting?
Flutter?
Nothing but increased drag?
Loss of e.g. aileron control?
Others?
The question arose out of a discussion where I said that it might be better to descend by reducing thrust and keeping attitude when hitting turbulence close to the margins (VS vs. MMO/VMO) even when risking that during the gusts I may hit an overspeed condition.
My argument was that MMO/VMO will not lead to a situation from which it may be difficult to recover as long as I would not dive the plane, whereas a stall definitively will.
Still we're out of tolerance and therefore the question: What really happens when going overspeed?
What happens with 10kts?
What happens with 25kts?
Any enlightenment very welcome.
Last edited by error_401; 25th May 2012 at 21:41.
Do a Hover - it avoids G
Join Date: Oct 1999
Location: Chichester West Sussex UK
Age: 91
Posts: 2,206
Likes: 0
Received 0 Likes
on
0 Posts
What happens with an overspeed will be type dependant.
I would expect no serious real problems untill 5 -10% above a V speed.
But I could be out of date on today's certification margins and don't have the time to look them up right now.
I would expect no serious real problems untill 5 -10% above a V speed.
But I could be out of date on today's certification margins and don't have the time to look them up right now.
Last edited by John Farley; 19th May 2012 at 17:32.
The demonstration of design dive speed essentially requires:
At the design cruise speed Vc/Mc and maximum cruise power, select a flight path 7.5 degrees lower and hold for 20 seconds. This is followed by a 1.5g pull out. At the commencement of the pull up a power reduction and use of speed brakes is permitted. The maximum speed reached is Vdf - demonstrated dive speed.
The speed increment would typically be 0.07M.
So your proposed method would be unlikely to exceed a speed which has been demonstrated during certification, and still has enough "fat" for a 1.5g pull. .
Of course you could take no action until the weight burn off reduced the stall speed sufficiently!!
At the design cruise speed Vc/Mc and maximum cruise power, select a flight path 7.5 degrees lower and hold for 20 seconds. This is followed by a 1.5g pull out. At the commencement of the pull up a power reduction and use of speed brakes is permitted. The maximum speed reached is Vdf - demonstrated dive speed.
The speed increment would typically be 0.07M.
So your proposed method would be unlikely to exceed a speed which has been demonstrated during certification, and still has enough "fat" for a 1.5g pull. .
Of course you could take no action until the weight burn off reduced the stall speed sufficiently!!
Join Date: Aug 2009
Location: UK
Posts: 18
Likes: 0
Received 0 Likes
on
0 Posts
The 20 second dive plus 1.5g pull-up may not have been flown before. The rules allow the design dive speed to be established from a ‘rational analysis’ of this manoeuvre. There is no requirement to demonstrate it in flight test.
See the excellent summary in this post:
http://www.pprune.org/tech-log/254943-demonstrated-design-dive-speed-vdf.html#post3002448
If you pull 1.5g at VD or MD, you might be the first to do so.
See the excellent summary in this post:
http://www.pprune.org/tech-log/254943-demonstrated-design-dive-speed-vdf.html#post3002448
If you pull 1.5g at VD or MD, you might be the first to do so.
Please excuse the poor wording of the first sentence of my previous post.
Vd/Md are design speeds which don't have to be demonstrated in flight. Vdf/Mdf are determined during flight test - high speed testing is done at speeds up to Vdf/Mdf (by definition these cannot exceed Vd/Md).
Typically the manufacturers that I have flown with use the -7.5 degree flight path for twenty seconds from Vmo/Mmo, with maximum cruise power, to simulate a type of upset required by FAR 25.253. The justification being that this is a method of establishing Vd/Md (by test or analysis), already written in the certification standards (FAR 25.335) so should be accepted by the regulatory authority during the determination of Vdf/Mdf.
Whatever method of upset simulation used (and accepted by the certification authority) the aircraft will be flown to, and recovered from, speeds significantly greater than Vmo/Mmo
Vd/Md are design speeds which don't have to be demonstrated in flight. Vdf/Mdf are determined during flight test - high speed testing is done at speeds up to Vdf/Mdf (by definition these cannot exceed Vd/Md).
Typically the manufacturers that I have flown with use the -7.5 degree flight path for twenty seconds from Vmo/Mmo, with maximum cruise power, to simulate a type of upset required by FAR 25.253. The justification being that this is a method of establishing Vd/Md (by test or analysis), already written in the certification standards (FAR 25.335) so should be accepted by the regulatory authority during the determination of Vdf/Mdf.
Whatever method of upset simulation used (and accepted by the certification authority) the aircraft will be flown to, and recovered from, speeds significantly greater than Vmo/Mmo
Last edited by zzuf; 22nd May 2012 at 14:38. Reason: typo
IMHO the use of the term ‘coffin corner’ w.r.t. commercial operations generates inaccurate impressions of the boundaries of normal operations and the safety margins provided by the certification process.
In some specialised military operations it may be necessary to all of the available performance; with this there is some risk.
In commercial operations the safety margins, as in posts above, and together with altitude, min selectable speed, and buffet boundary limitations, provide protection from hazardous situations (minimising risk) as currently envisaged by regulation (industry experience).
This and similar threads may fail to appreciate these aspects; they fail to define exactly how a hazardous situation might arise. ‘What Ifs’ are useful for understanding and planning ahead, but these have to be tempered with knowledge of the safety aspectsof the subject.
The apparent shortfall in this area suggests that training in commercial aviation is not teaching or explaining performance and aerodynamic aspects in sufficient detail; this might also be reflected by similar discussions on RTO’s and go-arounds, and loss of control accidents.
In some specialised military operations it may be necessary to all of the available performance; with this there is some risk.
In commercial operations the safety margins, as in posts above, and together with altitude, min selectable speed, and buffet boundary limitations, provide protection from hazardous situations (minimising risk) as currently envisaged by regulation (industry experience).
This and similar threads may fail to appreciate these aspects; they fail to define exactly how a hazardous situation might arise. ‘What Ifs’ are useful for understanding and planning ahead, but these have to be tempered with knowledge of the safety aspectsof the subject.
The apparent shortfall in this area suggests that training in commercial aviation is not teaching or explaining performance and aerodynamic aspects in sufficient detail; this might also be reflected by similar discussions on RTO’s and go-arounds, and loss of control accidents.
Last edited by safetypee; 22nd May 2012 at 13:36.
Join Date: Oct 2007
Location: fort sheridan, il
Posts: 1,656
Likes: 0
Received 0 Likes
on
0 Posts
safetype is quite right.
when I think of coffin corner, I think of the U2 spyplane and a very narrow gap between stall and overspeed.
most of us will not fly the U2.
Most of us will fly very docile transports and we shall try to keep them in the meaty part of the envelope.
sonic barrier
thermal thicket
coffin corner
are terms most of us will never really worry about. unless we don't know much.
when I think of coffin corner, I think of the U2 spyplane and a very narrow gap between stall and overspeed.
most of us will not fly the U2.
Most of us will fly very docile transports and we shall try to keep them in the meaty part of the envelope.
sonic barrier
thermal thicket
coffin corner
are terms most of us will never really worry about. unless we don't know much.
Thread Starter
Join Date: Nov 2002
Location: Heart of Europe
Posts: 198
Likes: 0
Received 0 Likes
on
0 Posts
@ zzuf
Thanks that answers my question. That means that my solution to an upset problem with "limited buffer" towards buffet and stall would be a viable way to get out of trouble. Maybe my next plane will take me up there.
@safetypee
I agree that the term may be misleading. Nevertheless used among "professionals" I think a good shortcut to describe the situation when margins are reduced. We know what is meant by it.
Thanks that answers my question. That means that my solution to an upset problem with "limited buffer" towards buffet and stall would be a viable way to get out of trouble. Maybe my next plane will take me up there.
@safetypee
I agree that the term may be misleading. Nevertheless used among "professionals" I think a good shortcut to describe the situation when margins are reduced. We know what is meant by it.
Last edited by error_401; 25th May 2012 at 21:39.
error_401. I disagree; IMHO many commercial aviation professionals misuse the term ‘buffet boundary’, and without good technical description few really understand what it is, or what it might mean when applied to a modern commercial aircraft.
Similarly, knowledge of the margins between normal / abnormal operations and a hazardous situation is weak, or again misunderstood to an extent that it may bias the choice of action in an upset / stall / overspeed encounter.
If an aircraft encounters a stall condition at high altitude then the protection devices will still work; the recovery procedure should be followed.
In the event of overspeed encountering buffet, the aircraft response will depend on type, but within the expected range of upset conditions the aircraft will still respond to the controls. The certification requirements and flight test evaluations should define the safety margins to an extent that commercial pilots should not fear such situations.
Of course this does not exclude the possibility of encountering something completely unexpected (by regulation or otherwise). My own unfortunate experience included encountering a gust at medium altitude which resulted in a control limited climb – full forward stick and still going up (overflying a lenticular cloud). Subsequent investigation showed that this was at the limit of certification expectation.
My lesson learnt was to maintain the appropriate control input and wait, the situation slowly corrected.
What pilots must consider, is that in these situations, although the encounters are sudden, they are dynamic, something will change; aircraft speed, control effectiveness, or the effect of the atmosphere (upset condition).
There is significant risk in assuming that the aircraft has entered some mythical no-fly area or that the controls have failed. In just a few seconds things will change, the important aspect is to identify the change, reassess the situation and ensure that the control input is correct.
Do not over react as this may induce a real upset. Dispel all thoughts of altitude bust, caution and warnings, structural failure, etc; control your thinking.
Rule 1, fly the aircraft.
Similarly, knowledge of the margins between normal / abnormal operations and a hazardous situation is weak, or again misunderstood to an extent that it may bias the choice of action in an upset / stall / overspeed encounter.
If an aircraft encounters a stall condition at high altitude then the protection devices will still work; the recovery procedure should be followed.
In the event of overspeed encountering buffet, the aircraft response will depend on type, but within the expected range of upset conditions the aircraft will still respond to the controls. The certification requirements and flight test evaluations should define the safety margins to an extent that commercial pilots should not fear such situations.
Of course this does not exclude the possibility of encountering something completely unexpected (by regulation or otherwise). My own unfortunate experience included encountering a gust at medium altitude which resulted in a control limited climb – full forward stick and still going up (overflying a lenticular cloud). Subsequent investigation showed that this was at the limit of certification expectation.
My lesson learnt was to maintain the appropriate control input and wait, the situation slowly corrected.
What pilots must consider, is that in these situations, although the encounters are sudden, they are dynamic, something will change; aircraft speed, control effectiveness, or the effect of the atmosphere (upset condition).
There is significant risk in assuming that the aircraft has entered some mythical no-fly area or that the controls have failed. In just a few seconds things will change, the important aspect is to identify the change, reassess the situation and ensure that the control input is correct.
Do not over react as this may induce a real upset. Dispel all thoughts of altitude bust, caution and warnings, structural failure, etc; control your thinking.
Rule 1, fly the aircraft.
Join Date: Sep 2006
Location: any town as retired.
Posts: 2,182
Likes: 0
Received 0 Likes
on
0 Posts
Challenger 601
many times when trying to cross the pond, we were on the edge of the buffet boundry, the performance charts, showed a very small margin of speed, between the MMM and Stall.....
in Glf5, similar when trying to outclimb CB in Asia.
The solution was to ease nose down, and accelerate, advising ATC of loss of altitude.
the reference to pond was when attempting to obtain level flight prior to entrance.
glf
in Glf5, similar when trying to outclimb CB in Asia.
The solution was to ease nose down, and accelerate, advising ATC of loss of altitude.
the reference to pond was when attempting to obtain level flight prior to entrance.
glf
Last edited by Gulfstreamaviator; 29th May 2012 at 15:59. Reason: not my reason....
From CS 25 AMC.
CS 25.251(e) states that probable inadvertent excursions beyond the buffet onset boundary may not result in unsafe conditions.
It also requires that the envelopes of load factor, speed, altitude and weight must provide a sufficient range of speeds and load factors for normal operations.
An acceptable means of compliance with the requirement is to establish the maximum altitude at which it is possible to achieve a positive normal acceleration increment of 0·3 g without exceeding the buffet onset boundary.
Does this mean that at the buffet boundary (1.3 g), there should not be a stall warning, thus this defines the margin between stall speed and buffet boundary?
Also, is there an operational maximum altitude based on ATC climb requirement – 300ft/min?
Thus in a civil aircraft there should not be a ‘corner’ point, but even if a limit was reached it should be safe to exceed the buffet boundary by a small amount.
CS 25.251(e) states that probable inadvertent excursions beyond the buffet onset boundary may not result in unsafe conditions.
It also requires that the envelopes of load factor, speed, altitude and weight must provide a sufficient range of speeds and load factors for normal operations.
An acceptable means of compliance with the requirement is to establish the maximum altitude at which it is possible to achieve a positive normal acceleration increment of 0·3 g without exceeding the buffet onset boundary.
Does this mean that at the buffet boundary (1.3 g), there should not be a stall warning, thus this defines the margin between stall speed and buffet boundary?
Also, is there an operational maximum altitude based on ATC climb requirement – 300ft/min?
Thus in a civil aircraft there should not be a ‘corner’ point, but even if a limit was reached it should be safe to exceed the buffet boundary by a small amount.
Thread Starter
Join Date: Nov 2002
Location: Heart of Europe
Posts: 198
Likes: 0
Received 0 Likes
on
0 Posts
@ safetypee
Thanks for the thoughts.
From talking to guys who actually do high altitude flying I've come to the conclusion that they are mostly scared about "ovespeed". That was the original reason for the question. Seen from a pilot's perspective I need something at hand that works when things start to go awry. As you put it - very nicely - fly the aircraft - but how when in thin air?
To do so in a high altitude setup I need a better way to get out than the A340 crew A340 incident ending up pulling the stick trying to stop an overspeed situation and ending up in far worse. (I leave aside the other things as improper task sharing, masked A/P disconnects etc.)
That was the simple aim of my question. If able to give up altitude, which widens the margins, the better. Else, can I simply nail the pitch, disconnect A/P and A/T and reduce thrust by an initial 15 % to get speed back to normal without loosing my wings if it overspeeds by let's say .03 Mach?
According to the other posts - obviously from a certification point of view, yes there is room, and as you put it - being gentle on the controls helps - but keep in control and don't panic. As you have put it - "situations are dynamic".
I fully agree with what you state and I try to tap a bit of the experience that can be found here
@ Gulfstreamaviator
I don't get the last part: "the reference to pond was when attempting to obtain level flight prior to entrance." Can you enlighten me a bit?
Thanks for the thoughts.
From talking to guys who actually do high altitude flying I've come to the conclusion that they are mostly scared about "ovespeed". That was the original reason for the question. Seen from a pilot's perspective I need something at hand that works when things start to go awry. As you put it - very nicely - fly the aircraft - but how when in thin air?
To do so in a high altitude setup I need a better way to get out than the A340 crew A340 incident ending up pulling the stick trying to stop an overspeed situation and ending up in far worse. (I leave aside the other things as improper task sharing, masked A/P disconnects etc.)
That was the simple aim of my question. If able to give up altitude, which widens the margins, the better. Else, can I simply nail the pitch, disconnect A/P and A/T and reduce thrust by an initial 15 % to get speed back to normal without loosing my wings if it overspeeds by let's say .03 Mach?
According to the other posts - obviously from a certification point of view, yes there is room, and as you put it - being gentle on the controls helps - but keep in control and don't panic. As you have put it - "situations are dynamic".
I fully agree with what you state and I try to tap a bit of the experience that can be found here
@ Gulfstreamaviator
I don't get the last part: "the reference to pond was when attempting to obtain level flight prior to entrance." Can you enlighten me a bit?
Per Ardua ad Astraeus
Join Date: Mar 2000
Location: UK
Posts: 18,579
Likes: 0
Received 0 Likes
on
0 Posts
error - the LAST thing you should do if you are 'at high level' and encounter an overspeed is 'zoom up' since you will worsen your performance margins by climbing..
I am confused by
- I assume you mean descending, which will further increase your speed.
As safetypee says, forget all about 'coffin corner' in commercial operations - you will not be near it.
You started by talking about 'turbulence' and needing to descend - yes you will if you need to retain sufficient 'spare' performance in mod/severe turbulence. In normal ops, however, there will always be sufficient speed margin for you to descend without an 'overspeed', and even if you do, as JF says, no 'biggie'. As sp says, be gentle!
I am confused by
If able to give up altitude, which widens the margins, the better.
As safetypee says, forget all about 'coffin corner' in commercial operations - you will not be near it.
You started by talking about 'turbulence' and needing to descend - yes you will if you need to retain sufficient 'spare' performance in mod/severe turbulence. In normal ops, however, there will always be sufficient speed margin for you to descend without an 'overspeed', and even if you do, as JF says, no 'biggie'. As sp says, be gentle!
Thread Starter
Join Date: Nov 2002
Location: Heart of Europe
Posts: 198
Likes: 0
Received 0 Likes
on
0 Posts
BOAC
Well - sorry for having been confusing, thought I'd put it nicely.
You have summarized what I think about that issue and will most probably do should it happen.
Well - sorry for having been confusing, thought I'd put it nicely.
You have summarized what I think about that issue and will most probably do should it happen.
error 401, fret not; you put your head into the Lion’s den and got some roaring.
The importance of this is that you asked the question; hopefully you achieved some satisfaction – a different viewpoint or new knowledge.
It is very important to ask questions, especially in aviation, and better to ask them on the ground before you encounter the situations for real.
The importance of this is that you asked the question; hopefully you achieved some satisfaction – a different viewpoint or new knowledge.
It is very important to ask questions, especially in aviation, and better to ask them on the ground before you encounter the situations for real.
Thread Starter
Join Date: Nov 2002
Location: Heart of Europe
Posts: 198
Likes: 0
Received 0 Likes
on
0 Posts
@ all contributing to this question
Thanks - this is what makes pprune a great forum. One gets an answer. And answers from different angles of aviation with some critical reasoning make for the best judgment one can get. No need to find out about everything by onself.
Cheers
Thanks - this is what makes pprune a great forum. One gets an answer. And answers from different angles of aviation with some critical reasoning make for the best judgment one can get. No need to find out about everything by onself.
Cheers
Join Date: Dec 2002
Location: Europa
Posts: 612
Likes: 0
Received 0 Likes
on
0 Posts
The safest thing is to do is know your weight and Va at that weight. Fly at Va and the worst that can happen is a stall which can be recovered by unloading the wings in the standard fashion. At Va you can move ONE control to full deflection without overstressing the airframe You have a 50% margin on top of the certified "g" limits. As John Farley says at the high speed end there is only 5 to 10% margin above Vne and you are almost certain to bend the airframe. The certification is based on aluminium deformation characteristics. Beyond that velocity is unknown flight path response to control input -probably not linear and likely flutter and loss of major components. I would rather carry out a stall recovery at Va than face an increasingly high speed dive well beyond MMo/Vmo.
Oh no, not the Va myth again! I thought that had well and truly been put to bed. Here is one attempt to do so:
Design Maneuvering Speed Limitation Statement
Clean stall in a jet transport mmm! Hope it is not one where the certification stall was defined by deterrent buffet levels.
I would choose high speed buffet any day.
Anyway, as Va min is approaching 1.6 Vso - if you can fly at Va you are nowhere near "coffin corner".
Design Maneuvering Speed Limitation Statement
Clean stall in a jet transport mmm! Hope it is not one where the certification stall was defined by deterrent buffet levels.
I would choose high speed buffet any day.
Anyway, as Va min is approaching 1.6 Vso - if you can fly at Va you are nowhere near "coffin corner".
Last edited by zzuf; 7th Jun 2012 at 00:38. Reason: add info
Join Date: Jun 2010
Location: Canada
Age: 37
Posts: 382
Likes: 0
Received 0 Likes
on
0 Posts
In regards to what John Farley said... under Transport Category airplanes, they must be free from any aeroelastic effects under all configurations and design conditions by an increase of 15 percent in equivalent airspeed at both constant Mach number and constant altitude... among a few other requirements.
Part V - Airworthiness Manual Chapter 525 - Transport Category Aeroplanes - Transport Canada
The Transport Canada Reg is essentially an exact copy of the FARs. Reference FAR 25.629.
Part V - Airworthiness Manual Chapter 525 - Transport Category Aeroplanes - Transport Canada
The Transport Canada Reg is essentially an exact copy of the FARs. Reference FAR 25.629.