Balanced Field Length
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Cap56, I'm not too sure why you feel this is such an emotional issue.
As far as I can tell, re-reading the thread, no-one has claimed that the "V1 such that ASDA=TODA=BFL" definition is "wrong". It's simply not "unique".
It's hardly the only TLA in aviation that has multiple meanings.
oh, and just out of curiousity, what's a "B-level"? is that something a "Brit" would know about??
As far as I can tell, re-reading the thread, no-one has claimed that the "V1 such that ASDA=TODA=BFL" definition is "wrong". It's simply not "unique".
It's hardly the only TLA in aviation that has multiple meanings.
oh, and just out of curiousity, what's a "B-level"? is that something a "Brit" would know about??
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Good folk ..
These sorts of threads are wonderful to see as they encourage people to be forthright in challenging each other's not always absolutely correct understanding of this and that. At the end of the day, it is our hope that people challenge their own ideas and, occasionally, change them. It is a truism that, if you ask a question of 20 flying folk ... often you will get 30 slightly (and sometimes wildly) varying answers.
A couple of points ..
(a) several of the posters in this thread are known to be very experienced, practising performance and/or flight test engineers. Mutt, for example, looks after the numbers for a LOT of big aeroplanes flown by a well known airline ...
(b) BFL calculations, generally, do NOT give maximum RTOW (unless we are talking about aircraft whose AFM provides ONLY BFL data - some do, most don't). Depends on the runway and the aircraft. However, what BFL calculations DO do is to give the easiest and quickest calculation.
BFL calculations generally give a good weight quickly.
... and, as has been observed earlier .. there are no sheep stations riding on these sorts of questions .. so there ought not to be any need for rising temperatures ?
These sorts of threads are wonderful to see as they encourage people to be forthright in challenging each other's not always absolutely correct understanding of this and that. At the end of the day, it is our hope that people challenge their own ideas and, occasionally, change them. It is a truism that, if you ask a question of 20 flying folk ... often you will get 30 slightly (and sometimes wildly) varying answers.
A couple of points ..
(a) several of the posters in this thread are known to be very experienced, practising performance and/or flight test engineers. Mutt, for example, looks after the numbers for a LOT of big aeroplanes flown by a well known airline ...
(b) BFL calculations, generally, do NOT give maximum RTOW (unless we are talking about aircraft whose AFM provides ONLY BFL data - some do, most don't). Depends on the runway and the aircraft. However, what BFL calculations DO do is to give the easiest and quickest calculation.
BFL calculations generally give a good weight quickly.
... and, as has been observed earlier .. there are no sheep stations riding on these sorts of questions .. so there ought not to be any need for rising temperatures ?
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Methinks that the paranoia and emotion arising here is not so much as it is a performance issue, but a VERY commonly asked interview question.
If there is any residual curiosity out there from a purely performance perspective, refer to Mutt's last post.....I concur.
If there is any residual curiosity out there from a purely performance perspective, refer to Mutt's last post.....I concur.
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If it is the interview thing as O_S suggests, then it's a shame that interviewers would apparently rather have a stock, learned-by-rote, answer than a response indicating that the interviewee may actually have done some thinking for themselves and, in fact, be better informed.
There is no single right answer in engineering, and even when the rules appear to mandate one it is a foolhardy man who closes his eyes to the real world behind the regs. I, for one, am very wary of people who know the regs, procedures, etc., by heart, but appear to have no understanding of the topic itself.
(Not intended at anyone in this thread, before someone takes offence; I have seen other cases, though, where the regs are quoted blindly out of context)
There is no single right answer in engineering, and even when the rules appear to mandate one it is a foolhardy man who closes his eyes to the real world behind the regs. I, for one, am very wary of people who know the regs, procedures, etc., by heart, but appear to have no understanding of the topic itself.
(Not intended at anyone in this thread, before someone takes offence; I have seen other cases, though, where the regs are quoted blindly out of context)
For the Airbus view of this subject, with diagrams; see the excellent document
Getting to grips with aircraft performance
Getting to grips with aircraft performance
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MFS' comments on interviewing unfortunately are right on the mark. Many interviewers know not much more than the interviewee and, on occasion, far less... which makes things hard for both parties... particularly as we get older and suffer fools less politely than we did as youngsters ....
On the one hand, an imperative (often seen with pilot interview processes) is to weed out those who ought not to have got to the interview stage (except for the misplaced value of their innovative CVs)... hence the use of canned question and answer ...
On the other hand I recall an interview for an engineering job many years ago where the principal interviewer (who became my next boss and from whom I learnt a lot ...) asked not one straight forward question ... all the questions were thought provoking.
As he confided some months later over a coffee .. he hadn't been terribly interested in the specific answers so much as the thought processes which the to and fro discussion on each question revealed.
Mind you, I did find one question .. along the lines of "tell me a bit about supercharged gas turbine engines" and the subsequent discussion on engine design and operating envelopes interesting ...
On the one hand, an imperative (often seen with pilot interview processes) is to weed out those who ought not to have got to the interview stage (except for the misplaced value of their innovative CVs)... hence the use of canned question and answer ...
On the other hand I recall an interview for an engineering job many years ago where the principal interviewer (who became my next boss and from whom I learnt a lot ...) asked not one straight forward question ... all the questions were thought provoking.
As he confided some months later over a coffee .. he hadn't been terribly interested in the specific answers so much as the thought processes which the to and fro discussion on each question revealed.
Mind you, I did find one question .. along the lines of "tell me a bit about supercharged gas turbine engines" and the subsequent discussion on engine design and operating envelopes interesting ...
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Mutt,
You consistantly make inordinately good sense, but I'm a little puzzled by the rationale of your company policy spelled out in your recent posting -
Is it true that you don't take any credit for Clearway / Stopway ? Perhaps your Company's operations are on good long runways and CWY/SWY credit is not needed. Where possible we do not use them if unwarranted, but will immediately do so on shorter runways where an operational advantage exists. (This requires a policy statement to pilots re the use of FMC speeds. Certainly, we adjust FMC speeds for MEL / Contaminated runways). I'm interested to know how you can justify this penalty (although the good long runway explanation might be all that's needed).
When you state that you don't account for optimized Vspeeds, are you referring to their 'fine tuning' due to CWY/SWY, or to Vspeeds optimized for V2 overspeed / Improved Climb which is not considered by the FMC ? In the case of the former, OK, I'm with you, you don't use CWY/SWY, but if it is your policy to not use the latter, I cannot see the justification. On a recent departure from JED at 43°, we squeezed on another 2600 Kg of payload in using 'Improved Climb' (I hate that Boeingism). These higher speeds most certainly require over-writing of the FMC balanced field speeds.
Genuinely interested to see your reply, we're always looking at Company policy improvements for maximising commercial advantage, or similarly towards operational simplification if there is no commercial penalty involved.
You consistantly make inordinately good sense, but I'm a little puzzled by the rationale of your company policy spelled out in your recent posting -
In our case, we dont account for optimized Vspeeds, clearway/stopway. FMC speeds are only adjusted for MEL's and contaminated runways.
When you state that you don't account for optimized Vspeeds, are you referring to their 'fine tuning' due to CWY/SWY, or to Vspeeds optimized for V2 overspeed / Improved Climb which is not considered by the FMC ? In the case of the former, OK, I'm with you, you don't use CWY/SWY, but if it is your policy to not use the latter, I cannot see the justification. On a recent departure from JED at 43°, we squeezed on another 2600 Kg of payload in using 'Improved Climb' (I hate that Boeingism). These higher speeds most certainly require over-writing of the FMC balanced field speeds.
Genuinely interested to see your reply, we're always looking at Company policy improvements for maximising commercial advantage, or similarly towards operational simplification if there is no commercial penalty involved.
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Old_smokey,
Will have to answer this at a later date, the Emerald Isle and a keg of Guinness beckons!
Or better yet, if you are back here in October, we can discuss it over a beer or two.
Cheers.
Mutt.
Will have to answer this at a later date, the Emerald Isle and a keg of Guinness beckons!
On a recent departure from JED at 43°,
Cheers.
Mutt.
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Mutt,
Thanks. I look forward to both, the answer and the beer. The October part is very much in the hands of the forces of darkness that control my roster.........Will try to twist their tails a bit.
Regards,
Smokey
Thanks. I look forward to both, the answer and the beer. The October part is very much in the hands of the forces of darkness that control my roster.........Will try to twist their tails a bit.
Regards,
Smokey
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Yikes, thanks for dragging this up SR71, I had forgotten about it!
The easiest way is to deal with each item individually, I will add that we operate an extremely diverse fleet so my answers will refer to our aircraft in general and not particularly every aircraft type.
Stopway Not accounted for. Goes back to B707 days so I don’t know the rational behind it.
Clearway (a) Generally speaking, a large percentage of our operations use a generic takeoff chart for all runways in a particular airport, takeoff weight given for 4 temperatures based on the worst runway at that airport. Using this, there is no point in using clearways. (b) Some takeoff chart programs don’t allow clearway’s. (c) Airports in home country are long.
Increased V-speeds. Big NO NO, crews have no desire to use higher than required V-speeds. Initial aircraft/home base takeoffs were field length limited, therefore improved climb wouldn’t work!
The goal is to keep takeoff calculations as simple as possible, basic takeoff charts are used with the QRH/FMC V-speeds with minimum corrections. The desire to achieve this is usually greater than the desire to get the highest amount of “thrust reduction”, or additional payload!
Mutt.
The easiest way is to deal with each item individually, I will add that we operate an extremely diverse fleet so my answers will refer to our aircraft in general and not particularly every aircraft type.
Stopway Not accounted for. Goes back to B707 days so I don’t know the rational behind it.
Clearway (a) Generally speaking, a large percentage of our operations use a generic takeoff chart for all runways in a particular airport, takeoff weight given for 4 temperatures based on the worst runway at that airport. Using this, there is no point in using clearways. (b) Some takeoff chart programs don’t allow clearway’s. (c) Airports in home country are long.
Increased V-speeds. Big NO NO, crews have no desire to use higher than required V-speeds. Initial aircraft/home base takeoffs were field length limited, therefore improved climb wouldn’t work!
The goal is to keep takeoff calculations as simple as possible, basic takeoff charts are used with the QRH/FMC V-speeds with minimum corrections. The desire to achieve this is usually greater than the desire to get the highest amount of “thrust reduction”, or additional payload!
Mutt.
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Cheers Mutt,
One thing (amongst others!) bothers me though with analyses that don't exploit the maximum available distances for "go-ing" or "stopping".
As Old Smokey says, there will be a performance implication, but there will also be a reject implication.
In any situation where a spread of V1 speeds is applicable, if I am working with normal speeds as opposed to improved speeds, by forcing me to go flying for a Vef > V1 in this particular case, rather than reject, I am being denied the opportunity to take advantage of the fact that ASDA(V1_normal) < ASDA(V1_improved) whereupon there may be ample distance available to reject even after V1?
The inherent conservatism (i.e. the ability to stop within the remaining distance available) in ASDA that exists in the case of a rejection at normal V1 speeds exists up to V1_max doesn't it?
Isn't it the case that when departing from a runway using normal V-speeds but that affords you the option of increased V-speeds, technically it is the increased V-speed V1 that is the latest safe point for a reject or am I missing something?
FWIW, a quick peek at RW 24 @ PRG in our FCOM shows a ~30kt spread of V1 speeds for normal versus increased V-speeds T/O. At an average of 150kts, thats probably worth a good 500m...
One thing (amongst others!) bothers me though with analyses that don't exploit the maximum available distances for "go-ing" or "stopping".
As Old Smokey says, there will be a performance implication, but there will also be a reject implication.
In any situation where a spread of V1 speeds is applicable, if I am working with normal speeds as opposed to improved speeds, by forcing me to go flying for a Vef > V1 in this particular case, rather than reject, I am being denied the opportunity to take advantage of the fact that ASDA(V1_normal) < ASDA(V1_improved) whereupon there may be ample distance available to reject even after V1?
The inherent conservatism (i.e. the ability to stop within the remaining distance available) in ASDA that exists in the case of a rejection at normal V1 speeds exists up to V1_max doesn't it?
Isn't it the case that when departing from a runway using normal V-speeds but that affords you the option of increased V-speeds, technically it is the increased V-speed V1 that is the latest safe point for a reject or am I missing something?
FWIW, a quick peek at RW 24 @ PRG in our FCOM shows a ~30kt spread of V1 speeds for normal versus increased V-speeds T/O. At an average of 150kts, thats probably worth a good 500m...
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I've always believed that the reason we use Balanced Field Length figures is that it simplifies calculation (as mentioned earler by A.Whittington) and also presentation.
In an airline with multiple destinations to airports with multiple runways - doing a detailed runway analysis for each and every combination of runways/temps/pressures etc would obviously be impractical.
Using a Balanced Field Length takes a 'Standard Runway' template and moves it from runway to runway where possible, thus avoiding multiple individual analyses.
This standard template is one which gives MAX performance (Max RTOW) from the aircraft at ISA conditions, and allows for corrections where necessary (temp/amb press/wind etc).
In the event a runway cannot give max RTOW using BFL then a specific analysis would have to be done.
As Mutt says, the catch (IMHO) arises when you put the BFL template for a small jet onto a 14,000' RWY. You may in fact then only be 'lookig at' the first few thousand foot of the runway (the BFL template) and you are potentially ignoring thousands of feet which are available beyond that.
This means that you could be led to believe you must make the V1 decision far earlier than actually possible in reality.
In current methods of presentation you often don't know if you are using BFL or not. Perhaps it would be a good idea if all Perf Charts had this annotated on the page?
By the way...all of the above could be utter bullcrap. I am simply stating my own (self deduced) understanding of the situation. Please correct me if I'm wrong.
In an airline with multiple destinations to airports with multiple runways - doing a detailed runway analysis for each and every combination of runways/temps/pressures etc would obviously be impractical.
Using a Balanced Field Length takes a 'Standard Runway' template and moves it from runway to runway where possible, thus avoiding multiple individual analyses.
This standard template is one which gives MAX performance (Max RTOW) from the aircraft at ISA conditions, and allows for corrections where necessary (temp/amb press/wind etc).
In the event a runway cannot give max RTOW using BFL then a specific analysis would have to be done.
As Mutt says, the catch (IMHO) arises when you put the BFL template for a small jet onto a 14,000' RWY. You may in fact then only be 'lookig at' the first few thousand foot of the runway (the BFL template) and you are potentially ignoring thousands of feet which are available beyond that.
This means that you could be led to believe you must make the V1 decision far earlier than actually possible in reality.
In current methods of presentation you often don't know if you are using BFL or not. Perhaps it would be a good idea if all Perf Charts had this annotated on the page?
By the way...all of the above could be utter bullcrap. I am simply stating my own (self deduced) understanding of the situation. Please correct me if I'm wrong.
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Idunno
A very clear explanation of the practical (as opposed to exam) use of BFL, with one small inaccuracy - the BFL figures in the manual do not give the MAX performance, just a simplified one that is fairly close to max. If you apply your template to a particular runway and it is not long enough, you may be able to "make it fit" by doing a full performance analysis taking into account stopway, clearway etc. Obviously if the runway is a Balanced Field (exam definition), then there will be no benefit.
The penalty for this simplification is exactly as has been described - you ignore all the runway beyond the "template" which could be used for stopping after an engine failure or accelerating to an increased V2 for obstacle clearance. But if your operation is one where you are rarely "performancearily challenged"
, then this is a penalty that many companies are happy to take in the pursuit of simplicity = less chance of silly cock-ups!
Happy Flying
BJJ
A very clear explanation of the practical (as opposed to exam) use of BFL, with one small inaccuracy - the BFL figures in the manual do not give the MAX performance, just a simplified one that is fairly close to max. If you apply your template to a particular runway and it is not long enough, you may be able to "make it fit" by doing a full performance analysis taking into account stopway, clearway etc. Obviously if the runway is a Balanced Field (exam definition), then there will be no benefit.
The penalty for this simplification is exactly as has been described - you ignore all the runway beyond the "template" which could be used for stopping after an engine failure or accelerating to an increased V2 for obstacle clearance. But if your operation is one where you are rarely "performancearily challenged"
, then this is a penalty that many companies are happy to take in the pursuit of simplicity = less chance of silly cock-ups!Happy Flying
BJJ
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....then this is a penalty that many companies are happy to take in the pursuit of simplicity = less chance of silly cock-ups!
The situation with PRG above is the same isn't it? We have our catastrophic failure/fire at V1 = 135kts but could reject anywhere up to 165kts.
In this case, if we wind up in the bushes, we say the guys did the right thing?
Incongruous.
My last performance course was woefully inadequate.
IMHO, we live in a day and age where we shouldn't have to rely on a BFL analysis template for the various fields we operate to. The number-crunching ability of your average desktop is phenomenal. The only reason more don't complain about the implications of the simplifications is probably to do with the fact that not many of us (myself included) fully appreciate how our PERF manuals are compiled.
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SR71
In your example, if there is enough runway top stop from 165 instead of 135, then there is also plenty of margin to continue from 135 as well. The type of "silly cock-up" I was referring to was someone trying to do the full calculations from the graphs in the AFM and getting it wrong. A friend of mine had a very interesting departure in a 707 after getting Fahrenheit and Celcius mixed up!!
However, you are absolutely right about modern PC's. One of the companies I fly for regularly have just purchased a system that has a worldwide runway and obstacle database and all the performance for all the different aircraft they operate, which is being loaded onto each aircraft's laptop! This is definitely the way forward.
"Aim for the stars and you may just miss the trees"
In your example, if there is enough runway top stop from 165 instead of 135, then there is also plenty of margin to continue from 135 as well. The type of "silly cock-up" I was referring to was someone trying to do the full calculations from the graphs in the AFM and getting it wrong. A friend of mine had a very interesting departure in a 707 after getting Fahrenheit and Celcius mixed up!!
However, you are absolutely right about modern PC's. One of the companies I fly for regularly have just purchased a system that has a worldwide runway and obstacle database and all the performance for all the different aircraft they operate, which is being loaded onto each aircraft's laptop! This is definitely the way forward.
"Aim for the stars and you may just miss the trees"
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SR71,
There are many different ways to accomplish a safe takeoff calculation; your goal seems to be to use the Maximum V1 so that you only take to the air at the last possible moment. This is a perfectly valid reason but it doesn’t appeal to everyone.
Recently observed on a B777 flight, mid weight where the choice of Flaps 05 or 15 were available, Captain chose Flaps 15, when asked why; he replied that he was concerned with brake fires with higher V-speeds. He therefore wanted to abort from the lowest possible V-speed; his preferred choice would be Minimum V1.
We have operated the A300 for about 20 years with optimized V-speeds, crews hated the idea that for a 1000 increase in takeoff weight, there could be a 30 knot increase in V-speeds, they campaigned for years to have the speed rational changed to Balanced.
So basically there isn’t one easy answer to satisfy everyone. Some airlines do actually give crews min/bal/max V-speeds and allow them to make their own decisions, however around here the predominant mindset that was developed for the Classic B747’s and carried over to all fleets is for lower speeds to aid the accelerate stop.
Mutt.
There are many different ways to accomplish a safe takeoff calculation; your goal seems to be to use the Maximum V1 so that you only take to the air at the last possible moment. This is a perfectly valid reason but it doesn’t appeal to everyone.
Recently observed on a B777 flight, mid weight where the choice of Flaps 05 or 15 were available, Captain chose Flaps 15, when asked why; he replied that he was concerned with brake fires with higher V-speeds. He therefore wanted to abort from the lowest possible V-speed; his preferred choice would be Minimum V1.
We have operated the A300 for about 20 years with optimized V-speeds, crews hated the idea that for a 1000 increase in takeoff weight, there could be a 30 knot increase in V-speeds, they campaigned for years to have the speed rational changed to Balanced.
So basically there isn’t one easy answer to satisfy everyone. Some airlines do actually give crews min/bal/max V-speeds and allow them to make their own decisions, however around here the predominant mindset that was developed for the Classic B747’s and carried over to all fleets is for lower speeds to aid the accelerate stop.
Mutt.
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Mutt,
I suppose I haven't explained myself that clearly.
Your aforementioned 777 skipper...I concur. I'd always seek to use the TO configuration that kept the V-speeds to a minimum depending on the TOW for exactly the reason he suggests.
However, whatever type of TO I am accomplishing, I'd like to know the max V1 at which an abort is possible bearing in mind the actual ASDA.
Presumably your skipper above isn't aware of whether or not it is the AGD or ASD that is limiting the BFL to a particular value, whereupon once V1 arrives, his decision is determined for him.
But isn't forcing me to go flying at a V1 determined from a balanced field analysis where it isn't the ASDA that is limiting imposing an artificial constraint on me unnecessarily? And with potentially the worst of consequences...
Sure, we have loads of margin in the case of an abort, but we've forced the decision to commit to getting airborne before we needed to.
Returning to my scenario above concerning PRG where we are considering a normal TO with V1=135kts and where an increased V2 speed TO for the same TOW gives V1=165kts, as BizJetJock says, if I have a catastrophic failure at 135kts, from a performance point of view, theoretically getting airborne doesn't present me with a problem, but why take the risk in this scenario?
In this instance, I know that I could quite safely abort from anything up to 165kts.
I think this sensible. I would still always use the lowest speeds which allowed me to get airborne for the TOW on the day, but am I missing something when I suggest that if a particular TO is possible at a particular TOW via a range of V-speeds, technically the option to abort always exists up to the max V1 possible?
I suppose I haven't explained myself that clearly.
Your aforementioned 777 skipper...I concur. I'd always seek to use the TO configuration that kept the V-speeds to a minimum depending on the TOW for exactly the reason he suggests.
However, whatever type of TO I am accomplishing, I'd like to know the max V1 at which an abort is possible bearing in mind the actual ASDA.
Presumably your skipper above isn't aware of whether or not it is the AGD or ASD that is limiting the BFL to a particular value, whereupon once V1 arrives, his decision is determined for him.
But isn't forcing me to go flying at a V1 determined from a balanced field analysis where it isn't the ASDA that is limiting imposing an artificial constraint on me unnecessarily? And with potentially the worst of consequences...
Sure, we have loads of margin in the case of an abort, but we've forced the decision to commit to getting airborne before we needed to.
Returning to my scenario above concerning PRG where we are considering a normal TO with V1=135kts and where an increased V2 speed TO for the same TOW gives V1=165kts, as BizJetJock says, if I have a catastrophic failure at 135kts, from a performance point of view, theoretically getting airborne doesn't present me with a problem, but why take the risk in this scenario?
In this instance, I know that I could quite safely abort from anything up to 165kts.
Some airlines do actually give crews min/bal/max V-speeds and allow them to make their own decisions
Surely all the airline interviewer wanted to hear was something along the lines of:
"An airport has a runway at the end of which is a clearway and a/c must reach their screen height at the end of the clearway. Some of the clearway includes a stopway, which is purely there to support an a/c after a rejected take-off.
The maximum distance from the far end of the runway to the end of the clearway is never to be more than 150% of the actual runway length; however, if only the stopway is considered rather than all the clearway, the distance available to meet screen height is equal to the distance available from starting the take-off to stopping if the take-off has to be rejected. Such a case, where ASDA=TODA, is referred to as a 'balanced field'."
KISS!
"An airport has a runway at the end of which is a clearway and a/c must reach their screen height at the end of the clearway. Some of the clearway includes a stopway, which is purely there to support an a/c after a rejected take-off.
The maximum distance from the far end of the runway to the end of the clearway is never to be more than 150% of the actual runway length; however, if only the stopway is considered rather than all the clearway, the distance available to meet screen height is equal to the distance available from starting the take-off to stopping if the take-off has to be rejected. Such a case, where ASDA=TODA, is referred to as a 'balanced field'."
KISS!
