V1(vmcg) and cross wind B757-200
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V1(vmcg) and cross wind B757-200
Hi,
I need some expert advice.
I am supposed to go to a place which is 30 meter (98 feet) wide and 5280 feet long with a B757-200. Our V1 are at or above Vmcg (under FAR 25.149 rules). the company limitation is 25 Kt cross wind for departure.
1/ My understanding is that any crosswind will act as a critical engine (adverse yaw), Vmcg is only valide with NO cross wind. With 25 Kt cross wind I will not have enough rudder to keep the aircraft on the runway if I have an engine failure at or above V1 Am I correct?
2/ The B752 landing gear is 24 feet wide. on a 98 feet wide runway, 30 feet of drift (at Vmcg) will only give 7 feet of runway on the side (assuming no cross wind and immediat rudder correction) am I right?
thanks for your help
stardust
I need some expert advice.
I am supposed to go to a place which is 30 meter (98 feet) wide and 5280 feet long with a B757-200. Our V1 are at or above Vmcg (under FAR 25.149 rules). the company limitation is 25 Kt cross wind for departure.
1/ My understanding is that any crosswind will act as a critical engine (adverse yaw), Vmcg is only valide with NO cross wind. With 25 Kt cross wind I will not have enough rudder to keep the aircraft on the runway if I have an engine failure at or above V1 Am I correct?
2/ The B752 landing gear is 24 feet wide. on a 98 feet wide runway, 30 feet of drift (at Vmcg) will only give 7 feet of runway on the side (assuming no cross wind and immediat rudder correction) am I right?
thanks for your help
stardust
Moderator
Stardust,
Can't speak specifically to the 757 ... perhaps Mutt can when he sees your post ... however, the following general principles apply
(a) crosswind will, if the failure is on the windward side, add to the undesired yawing moment and increase the aircraft's lateral deviation from centreline .. not good. Conversely, if the failure is on the leeward side, things improve ... problem is you can't bet on the situation.
(b) FAR requirements adopt a nil wind Vmcg.
(c) as the V1 reduces toward, and below, Vmcg in nil wind conditions at critical thrust levels (normally near SL, max rated with a high thrust tolerance motor, aft cg, etc ..) the observed centreline deviations increase rapidly and significantly until a point where the aircraft track is uncontrollable and runway departure occurs. This is a REAL, not imagined, scenario and the onset of problems is surprisingly rapid .. ie occurs in a small speed spread.
In a crosswind situation, the notional effect is for the real Vmcg to increase. Boeing would have this data and it may appear in the relevant Boeing Performance Engineers' Manual. Such data as I am aware of (but not specifically for the 757) suggests that this increase is in the typical range of 50% to something in excess of 100% of the crosswind, depending on the particular aircraft Type.
I wouldn't like to be on the aircraft in the circumstances you describe with a decent crosswind and a Vmcg failure .... interesting to watch from the sidelines, though ...
Can't speak specifically to the 757 ... perhaps Mutt can when he sees your post ... however, the following general principles apply
(a) crosswind will, if the failure is on the windward side, add to the undesired yawing moment and increase the aircraft's lateral deviation from centreline .. not good. Conversely, if the failure is on the leeward side, things improve ... problem is you can't bet on the situation.
(b) FAR requirements adopt a nil wind Vmcg.
(c) as the V1 reduces toward, and below, Vmcg in nil wind conditions at critical thrust levels (normally near SL, max rated with a high thrust tolerance motor, aft cg, etc ..) the observed centreline deviations increase rapidly and significantly until a point where the aircraft track is uncontrollable and runway departure occurs. This is a REAL, not imagined, scenario and the onset of problems is surprisingly rapid .. ie occurs in a small speed spread.
In a crosswind situation, the notional effect is for the real Vmcg to increase. Boeing would have this data and it may appear in the relevant Boeing Performance Engineers' Manual. Such data as I am aware of (but not specifically for the 757) suggests that this increase is in the typical range of 50% to something in excess of 100% of the crosswind, depending on the particular aircraft Type.
I wouldn't like to be on the aircraft in the circumstances you describe with a decent crosswind and a Vmcg failure .... interesting to watch from the sidelines, though ...
Last edited by john_tullamarine; 9th Jul 2002 at 21:33.
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1/ My understanding is that any crosswind will act as a critical engine (adverse yaw), Vmcg is only valide with NO cross wind. With 25 Kt cross wind I will not have enough rudder to keep the aircraft on the runway if I have an engine failure at or above V1 Am I correct?
Not really, the point where you will have a problem is if the V1=V1MCG, if the engine failure is any faster than V1MCG the aerodynamic forces on the rudder will have increased so it is “hoped” that you wont go off the side of the runway…… no guarantees though!
2/ The B752 landing gear is 24 feet wide. on a 98 feet wide runway, 30 feet of drift (at Vmcg) will only give 7 feet of runway on the side (assuming no cross wind and immediat rudder correction) am I right?
Remember that the 30 feet is the maximum as demonstrated by an “average” pilot, you might not need all of it.
I would suggest that you get your company to supply you with the V1min / V1Balance / V1 Max for this runway, this would then give you the option on windy days of increasing the V1 beyond V1MCG.
Good Luck.
Mutt.
Not really, the point where you will have a problem is if the V1=V1MCG, if the engine failure is any faster than V1MCG the aerodynamic forces on the rudder will have increased so it is “hoped” that you wont go off the side of the runway…… no guarantees though!
2/ The B752 landing gear is 24 feet wide. on a 98 feet wide runway, 30 feet of drift (at Vmcg) will only give 7 feet of runway on the side (assuming no cross wind and immediat rudder correction) am I right?
Remember that the 30 feet is the maximum as demonstrated by an “average” pilot, you might not need all of it.
I would suggest that you get your company to supply you with the V1min / V1Balance / V1 Max for this runway, this would then give you the option on windy days of increasing the V1 beyond V1MCG.
Good Luck.
Mutt.
Last edited by mutt; 10th Jul 2002 at 07:45.
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Mutt,
In regards to the Vmcg thread in the pprune flight test forum, I thought I would mention a couple of additional conservative considerations that are included in the determination of Vmcg:
1. Vmcg testing is conducted at aft cg, and with nosewheel either free to castor or lifted off the runway to minimize the stabilizing effect of the nose gear. This has been shown to have a greater effect on Vmcg than a wet runway.
2. Vmcg is determined with sudden failure of the critical engine.
3. The thrust level on the operative engine(s) is the maximum net thrust expected for a production engine (in contrast to the use of minimum thrust for the AFM performance data).
4. Only rudder can be used for control of the airplane (the use of other controls is limited to maintaining a wings level attitude), and the maximum force that can be applied to the rudder pedals is 150 lb.
Mearse.
In regards to the Vmcg thread in the pprune flight test forum, I thought I would mention a couple of additional conservative considerations that are included in the determination of Vmcg:
1. Vmcg testing is conducted at aft cg, and with nosewheel either free to castor or lifted off the runway to minimize the stabilizing effect of the nose gear. This has been shown to have a greater effect on Vmcg than a wet runway.
2. Vmcg is determined with sudden failure of the critical engine.
3. The thrust level on the operative engine(s) is the maximum net thrust expected for a production engine (in contrast to the use of minimum thrust for the AFM performance data).
4. Only rudder can be used for control of the airplane (the use of other controls is limited to maintaining a wings level attitude), and the maximum force that can be applied to the rudder pedals is 150 lb.
Mearse.
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Mearse,
The conditions that you posted are straight from FAR25, but for the sake of discussions sake, lets say that Stardust encounters all of these on a particular flight, there is a strong crosswind. The takeoff speed is set to V1=VMCG.
Do you think that he will have enough rudder authority to stay on the runway? Or even on a runway of normal width?
Stardust,
Are you going to let us know what you decided to do?
Cheers
Mutt.
The conditions that you posted are straight from FAR25, but for the sake of discussions sake, lets say that Stardust encounters all of these on a particular flight, there is a strong crosswind. The takeoff speed is set to V1=VMCG.
Do you think that he will have enough rudder authority to stay on the runway? Or even on a runway of normal width?
Stardust,
Are you going to let us know what you decided to do?
Cheers
Mutt.
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Like most of the performance and handling qualities regulations, the
conditions under which Vmcg is determined is a scenario intended to provide
a high level of safety for a wide range of conditions. An alternative
could be to require manufacturers to analyze each particular
condition/event that may occur, assess conditional probabilities, etc. and
determine Vmcg from such a detailed analysis based on a specific safety
level criterion. Instead, the basis of the current requirement is that
by determining Vmcg to be the speed at which, if the critical engine suddenly
fails, and limited the resulting runway tracking deviation to 30 feet
using only the rudder to control the deviation, and including other
conservatisms, a wide range of conditions/events are covered.
For the 757 on a standard 150 ft width runway, the 30 foot deviation requirement
leaves an additional 33 feet to the runway edge. In the operational situation,
the nose wheel will take side loads as well as provide some steering
capability, which adds quite a bit more directional control authority.
If not runway length limited, differential braking can also be used.
But on that day when you are at aft cg on a narrow, short, slippery runway
with strong crosswinds, and you have a sudden engine failure just before
V1 on one of your spanking new engines that are putting out more than the
specification thrust, and your nose wheel steering linkage fails, well you are outside of the certification requirements.
More directly to your question of just considering a strong crosswind, I agree that it would be prudent to use higher than the minimum V1 if that option is available.
Mearse
conditions under which Vmcg is determined is a scenario intended to provide
a high level of safety for a wide range of conditions. An alternative
could be to require manufacturers to analyze each particular
condition/event that may occur, assess conditional probabilities, etc. and
determine Vmcg from such a detailed analysis based on a specific safety
level criterion. Instead, the basis of the current requirement is that
by determining Vmcg to be the speed at which, if the critical engine suddenly
fails, and limited the resulting runway tracking deviation to 30 feet
using only the rudder to control the deviation, and including other
conservatisms, a wide range of conditions/events are covered.
For the 757 on a standard 150 ft width runway, the 30 foot deviation requirement
leaves an additional 33 feet to the runway edge. In the operational situation,
the nose wheel will take side loads as well as provide some steering
capability, which adds quite a bit more directional control authority.
If not runway length limited, differential braking can also be used.
But on that day when you are at aft cg on a narrow, short, slippery runway
with strong crosswinds, and you have a sudden engine failure just before
V1 on one of your spanking new engines that are putting out more than the
specification thrust, and your nose wheel steering linkage fails, well you are outside of the certification requirements.
More directly to your question of just considering a strong crosswind, I agree that it would be prudent to use higher than the minimum V1 if that option is available.
Mearse
Moderator
Mearse,
First, I note that you are new to PPRuNe posting, if not the site ... welcome and, as you appear to have an appropriate background, your input will be awaited in topics to come ... with any luck, we might all learn from your considerable experience ... and isn't that the main value of PPRuNe to us all ? Hopefully you are not constrained too much in your ability to enter into some of the discussions ....
Agreed .. Vmcg is a very conservative approach to a critical handling consideration and is intended to put a generally reasonable fence around that problem.
Interestingly, there are those certification flight test people about who would far prefer to see your alternative scenario adopted in the interests of better matching aircraft to role and providing more appropriate data and guidance to the operator/pilot. It is, of course, the pilot who is in the interesting position of being able to detect that he/she has a major problem evolving rapidly ... and the operator who has to wear the legal and financial cost of the ensuing investigation.
The sort of approach which Mutt and I adopt can be extended to many of the more hazardous activities with which pilots might be faced ..
(a) increasing takeoff speeds within the available range for a particular runway to provide better Vmca protection during a very low weight takeoff. (This can be be quite significant for swept wing jets)
(b) techniques for OEI training in turboprops
(c) OEI missed approach minimum heights
are just three f'instances immediately springing to mind.
The real point of discussions such as this is a philosophical one ... whether, with a little bit more knowledge of what goes into determining certification matters generally, (and Vmcg, specifically, in this case), Pilot Bloggs' employer ought to be providing sensible operational guidance for specific circumstances in which the already conservative limits might not be sufficiently so and for which there may be simple protecting remedies. This is not just for Pilot Bloggs' protection but is the stuff of sound corporate governance.
It has been my observation over many years that there is a surfeit of highly imaginative ignorance and folklore in this industry.
As one who has been involved with the educational side of the industry off and on over my entire career, it is painfully obvious that one of the critical problems is that ignorance begets ignorance and, in process, that which began as ignorance eventually becomes folklore and, unfortunately, often enshrined as gospel.
When I hear some of the technical nonsense which is endemic in areas of the aviation marketplace, for example, I can only despair as to the confusion caused in the minds of younger pilots coming up through the ranks.
One example I can bring to mind from the dim past involved a local proficiency check session on my then aircraft Type. The check pilot asked me a question along the lines of "under such and such conditions and with two engines out, what sort of climb performance might I expect ?" .. to which I answered "not much" (or words to that effect, your Honour) .. whereupon the said check pilot, in his usual paternalistic fashion, proceeded to talk me through the relevant charts, thereby arriving at a more accurate answer of several feet per minute climb rate. He, quite obviously, had not the slightest idea of how the data was derived and what significance it represented.
The folklore associated with the subjects suggested above could fill novels ....
Back to Vmcg, the very great majority of pilots has absolutely no comprehension of how the typical aircraft behaves in this corner of the envelope.
Some years back, Australia unilaterally implemented an ICAO suggestion that such things ought to be addressed for operations from narrower than "standard" runways. Some of the subsequent test programs (simulator and runway) showed very clearly that this is a problem well worthy of sombre reflection and consideration.
I have videos from some of the test programs which show aircraft quite dancing about on the runway during the intial gyrations following a contrived critical failure.
All Mutt and I are on about .. is that knowledge can be applied usefully ... and might just be the thing which saves the aircraft on a particular occasion .....
Just a thought ...
First, I note that you are new to PPRuNe posting, if not the site ... welcome and, as you appear to have an appropriate background, your input will be awaited in topics to come ... with any luck, we might all learn from your considerable experience ... and isn't that the main value of PPRuNe to us all ? Hopefully you are not constrained too much in your ability to enter into some of the discussions ....
Agreed .. Vmcg is a very conservative approach to a critical handling consideration and is intended to put a generally reasonable fence around that problem.
Interestingly, there are those certification flight test people about who would far prefer to see your alternative scenario adopted in the interests of better matching aircraft to role and providing more appropriate data and guidance to the operator/pilot. It is, of course, the pilot who is in the interesting position of being able to detect that he/she has a major problem evolving rapidly ... and the operator who has to wear the legal and financial cost of the ensuing investigation.
The sort of approach which Mutt and I adopt can be extended to many of the more hazardous activities with which pilots might be faced ..
(a) increasing takeoff speeds within the available range for a particular runway to provide better Vmca protection during a very low weight takeoff. (This can be be quite significant for swept wing jets)
(b) techniques for OEI training in turboprops
(c) OEI missed approach minimum heights
are just three f'instances immediately springing to mind.
The real point of discussions such as this is a philosophical one ... whether, with a little bit more knowledge of what goes into determining certification matters generally, (and Vmcg, specifically, in this case), Pilot Bloggs' employer ought to be providing sensible operational guidance for specific circumstances in which the already conservative limits might not be sufficiently so and for which there may be simple protecting remedies. This is not just for Pilot Bloggs' protection but is the stuff of sound corporate governance.
It has been my observation over many years that there is a surfeit of highly imaginative ignorance and folklore in this industry.
As one who has been involved with the educational side of the industry off and on over my entire career, it is painfully obvious that one of the critical problems is that ignorance begets ignorance and, in process, that which began as ignorance eventually becomes folklore and, unfortunately, often enshrined as gospel.
When I hear some of the technical nonsense which is endemic in areas of the aviation marketplace, for example, I can only despair as to the confusion caused in the minds of younger pilots coming up through the ranks.
One example I can bring to mind from the dim past involved a local proficiency check session on my then aircraft Type. The check pilot asked me a question along the lines of "under such and such conditions and with two engines out, what sort of climb performance might I expect ?" .. to which I answered "not much" (or words to that effect, your Honour) .. whereupon the said check pilot, in his usual paternalistic fashion, proceeded to talk me through the relevant charts, thereby arriving at a more accurate answer of several feet per minute climb rate. He, quite obviously, had not the slightest idea of how the data was derived and what significance it represented.
The folklore associated with the subjects suggested above could fill novels ....
Back to Vmcg, the very great majority of pilots has absolutely no comprehension of how the typical aircraft behaves in this corner of the envelope.
Some years back, Australia unilaterally implemented an ICAO suggestion that such things ought to be addressed for operations from narrower than "standard" runways. Some of the subsequent test programs (simulator and runway) showed very clearly that this is a problem well worthy of sombre reflection and consideration.
I have videos from some of the test programs which show aircraft quite dancing about on the runway during the intial gyrations following a contrived critical failure.
All Mutt and I are on about .. is that knowledge can be applied usefully ... and might just be the thing which saves the aircraft on a particular occasion .....
Just a thought ...
Last edited by john_tullamarine; 14th Jul 2002 at 01:56.
V1(MCG)
Stardust, I think everyone has given you lots of good information on how ,why ,etc.Are you able to get a simulator slot to run through a few scenarios before you go to this airfield?If so it will probably ease your mind,or say ,not my pay-scale ,chief ,over to you chief pilot;unless you are the chief pilot!As in most testing there is a built-in delay following a failure to allow for an average reaction;I think in this case ,if you have done your homework in a sim.,with your crew,you will be ready,and well-sharp.I hope it would be a non-event;even if it was an event ,you are going to be prepared,and not out to verify the published data.Always expect the unexpected!! Hope you get it done;we`ll all want to know. 
To those who feel this thread should be moved to "Tech-log", I disagree, this is core test and certification stuff. However, I've added a link to this thread on tech-log, so we might get some different views on the subject.
In the meantime, my complements to all participating in a very constructive discussion.
Genghis
Flight-test forum moderator.
In the meantime, my complements to all participating in a very constructive discussion.
Genghis
Flight-test forum moderator.
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AS J.T. says the fallacies of 'Certification'..
The european countries(northern)flew the DC9 off 'wet' runways for years with wet data in the manuals--DC never did any 'wet' testing,not required by FAA,results were 'analysis' ..Although the DC9's are now gone Canada never did have wet info.....
JAR stipultes the Height at the end of the take off distance on a wet runway to be 15'...As the rules say only 50% of the distance need be over runway enroute to the screen-what height does one really cross the lights????remember the screen is at the end of the clearway....
The european countries(northern)flew the DC9 off 'wet' runways for years with wet data in the manuals--DC never did any 'wet' testing,not required by FAA,results were 'analysis' ..Although the DC9's are now gone Canada never did have wet info.....
JAR stipultes the Height at the end of the take off distance on a wet runway to be 15'...As the rules say only 50% of the distance need be over runway enroute to the screen-what height does one really cross the lights????remember the screen is at the end of the clearway....
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V1(vmcg) and cross wind B757-200
In days of yore the BCAR regulations required Vmcg to be demonstrated in a 7 knot adverse crosswind. This figure was derived on a statistical basis - probability of having more than 7 knots adverse crosswind multiplied by probability of having an engine failure was less than probability of sky falling in and hence wasn't worth worrying about. The CAA was forced very reluctantly to accept zero crosswind, a reduction in safety, in the interests of harmonisation with the regulations of other countries. Interestingly, US Mil Specs require Vmcg for transports to be scheduled with crosswind, although they allow the manufacturer to take credit for nosewheel steering which JARs/FARs do not.
The effect of adverse crosswind is pretty dramatic, but then so is the effect of any reaction time and that is likely to be minimal if the edge of the runway is extra close. I don't know what the rudder forces are like on the 757, but the 150lb limit is also susceptible to adrenaline.
The fact is that the laws of probability have worked to date on this one, and the probability is that enough things won't go wrong simultaneously.
The effect of adverse crosswind is pretty dramatic, but then so is the effect of any reaction time and that is likely to be minimal if the edge of the runway is extra close. I don't know what the rudder forces are like on the 757, but the 150lb limit is also susceptible to adrenaline.
The fact is that the laws of probability have worked to date on this one, and the probability is that enough things won't go wrong simultaneously.
Moderator
From a certification viewpoint, I don't think that we have any quibbles ... whether it is 7kt, zero wind, or whatever small wind value is not a real concern (the manufacturers, naturally, will always push for nil wind as this gives the nicest field length figures for marginal runways) ... provided that crews are AWARE of the problems which can arise in a cross wind failure situation with a low speed schedule ....
The point which irritates me immensely is to see crews head off down the runway with minimum speed schedules in a strong crosswind when there is no need to expose the operation to the attendent increased risk ... fine to do so when there is no option and accept a risk .. but why do so when there is absolutely no need ? ... this is quite beyond my risk management attitude to life, death, and the universe ....
The point which irritates me immensely is to see crews head off down the runway with minimum speed schedules in a strong crosswind when there is no need to expose the operation to the attendent increased risk ... fine to do so when there is no option and accept a risk .. but why do so when there is absolutely no need ? ... this is quite beyond my risk management attitude to life, death, and the universe ....
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After a while,doing the same thing seems to be the easiest(pilots are basically lazy anyway!!
),other items that don't get considered:Using 'flex'thrust,and minimum flap off rough/contaminated runways. Using flex and normal speeds with strong winds and miles of concrete. Not looking into turns after takeoff in Terminal(busy)areas(not all traffic is on the screen)..
Always hitting the binders-with acres of concrete to spare.
Not using all the runway for high temp landings....
),other items that don't get considered:Using 'flex'thrust,and minimum flap off rough/contaminated runways. Using flex and normal speeds with strong winds and miles of concrete. Not looking into turns after takeoff in Terminal(busy)areas(not all traffic is on the screen)..Always hitting the binders-with acres of concrete to spare.
Not using all the runway for high temp landings....
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“this is quite beyond my risk management attitude to life, death, and the universe ‘
Needless to say, John, attitudes to risk management vary considerably from pilot to pilot. The heavy jet freight animals, for example, operate frequently at either maximum takeoff and landing weights, or are completely empty ferrying the aircraft to the next pickup. They are, by necessity, usually in corners of the envelope that carry the highest degree of risk, yet as far as I’m aware, have accident/incident rates no greater than their passenger-carrying brethren and sistren(?) operating the same aircraft type. Despite your concerns, there is much to be said for a strict, standardized ‘by the numbers’ operation, even if those numbers could occasionally be improved under certain conditions. We wouldn’t want a little finger trouble or momentary inattention to cause inadvertent correction of performance figures in the wrong direction. That could adversely affect one’s future earning potential.
Stardust, I would act on mutt’s advice before accepting the trip; insist on getting the data for that runway from your company.
Needless to say, John, attitudes to risk management vary considerably from pilot to pilot. The heavy jet freight animals, for example, operate frequently at either maximum takeoff and landing weights, or are completely empty ferrying the aircraft to the next pickup. They are, by necessity, usually in corners of the envelope that carry the highest degree of risk, yet as far as I’m aware, have accident/incident rates no greater than their passenger-carrying brethren and sistren(?) operating the same aircraft type. Despite your concerns, there is much to be said for a strict, standardized ‘by the numbers’ operation, even if those numbers could occasionally be improved under certain conditions. We wouldn’t want a little finger trouble or momentary inattention to cause inadvertent correction of performance figures in the wrong direction. That could adversely affect one’s future earning potential.
Stardust, I would act on mutt’s advice before accepting the trip; insist on getting the data for that runway from your company.
Moderator
I concur that standardisation is extremely important ... but do keep in mind that, sometimes, the operator's flight standards experts get it wrong .... not so long since I did some work for a flag carrier which, for its own reasons, did no training at this end of the envelope while encouraging pilots, in certain circumstances, to operate just there ... until some of the problems were pointed out .... in the sim as the aircraft rolled through the vertical ...
All I am trying to suggest is that the soft material between the ears was put there for more important reasons than just keeping the ears apart .... if you see something which seems to be strange or not very sensible ... it might be an idea to bring it up with management for a review .... ?
All I am trying to suggest is that the soft material between the ears was put there for more important reasons than just keeping the ears apart .... if you see something which seems to be strange or not very sensible ... it might be an idea to bring it up with management for a review .... ?
