Balanced Field Length

[Flying Clog]

Could someone out there please explain balanced field length in idiot's terms!

I have been asked this question at interview, know it's a favourite question for some of the big airlines, and would like to be able to answer it with more confidence in the future!

I hope others will benefit from any responses as well.

Rgds,

Flying Clog

[mutt]

In very basic terms, its the amount of runway required to permit two takeoff scenarios.

1: Accelerate Stop = Accelerate with all engines to a point called Vef, (1 second prior to V1) at this point the engine fails,you initiate an aborted takeoff and stop at a certain point.

2: Acclerate Go = Accelerate with all engines to a point called V1, where the engine fails, continue the takeoff and achieve a screen height of 35 feet at a certain point.

When the distance required for 1 is the same as 2, then its a balanced field. In a lot of cases it will be all of the runway length available, but not always.

Please remember you asked for the simple version!



Mutt.

[Alex Whittingham]

There are two meanings. Mutt has given a definition which I think originates in America, the JAA definition is that TODA = ASDA.

P.S. I believe the certification rules have now changed to allow 2 seconds between VEF and V1.

[oldebloke]

As of amendment 42(1978)of FAR 25-127/129..the introduction of VEF(formerly used on the flight test card)was made known to the pilots.VEF is generally one second prior to V1,but is called by the Constructor(with the DC10/B727 it could be as high as three seconds prior to V1,due no yaw promt for 'Recognition'..
So it's generally one second prior to V1 for the pilot recognition,to react by V1...
And it's two seconds added to the failure at V1 thence the stopping actions for the Accelerate/Stop DISTANCE
check the reading of the the two seperate FAR's(a)V1 definition,and (b)accelerate stop distance....
Cheers....:ok:

Balanced field length(as described above)hardly applies these days due to improved brakes,and the availability of \'Clearway\'..
If,for a given weight,one can stop (after the failure)on the runway one doesn\'t have to cross the end of the runway at 35\'...
With \'clearway\'one need only cross the end of the clearway at 35\'....
The T/O distance stipulates that half of the distance ,from L\'off to 35\'
,need only be over runway.....

:p

[square leg]

TOD=ASD


This will be the case on certain A/C when you FLEX on T/O.

[Alex Whittingham]

Stone me, you're right!

[Interconti]

In addition to mutt's comments or to say it in other words;

If V1 is chosen that the 1-Engine Out Takeoff Distance equals the Accelerate-Stop Distance we speak of Balanced Field Length.


Cheers

[ftrplt]

In addition to Interconti's comment;

at that V1 you will also have your maximum takeoff weight for the same given conditions

[Old Smokey]

ASDR = TODR (1 Eng Inop at VEF)

The variation in 1 second and 2 seconds diff between Vef and V1 depends upon when the aircraft was certified. 2 seconds is now the requirement, BUT (and it's a big BUT) due to the "Grandfather Clause" aircraft certified before the regulatory change are not required to be re-certified......and that accounts for most of the aircraft currently flying.

Balanced Field is a nice theoretical concept for an ideal world, in practice variation between Stopway and Clearway length put it in the back corner and Accelerate-Stop and Continued Takeoff situations considered separately will give the optimum field performance. Still...a useful tool for creating general Takeoff Charts / Tables (Most FMCs use it).

[Kennytheking]

Flying Clog,

BFL by definition is where V1 is chosen so that ASD = AGD

In order to understand the concept, you need to look at the effect that V1 has on each of these aspects.

1. Increase V1 means increased ASD(higher abort speed = more runway)

2. Increase in V1 decreased AGD(this is where lots of people have a problem....what it boils down to is that it is easier to continue the take off with a higher V1)

Go and draw the relationships as a graph with distance on the vertical scale and V1 on the horizontal scale.

When you put the 2 graphs together on one graph, the point where they intersect is your balanced field length.

Note that ANY variation of V1 results in a higher distance required to do the take off. If you increase the V1, the ASD increases and if you reduce V1 the AGD increases.

The BFL represents the minimum runway length that you need for a given weight(or more importantly the greatest weight you can carry off a given field length)

This brings us the the principle of an unbalanced field........that is where clearway is factored into the calculation. To contradict myself.....you can carry more weight off an unbalanced field(than a balanced filed), by reducing the V1. Unfortunately this calculation is very complex. You need to factor in Take off run(all engines), Take off run(OEI), Take off Distance(All engines), Take off distance(OEI), brake energy limits and Accelerate stop.

The main benefit of using BFL in flight planning boils down simplicity at a small weight penalty.

Ok. Just my bit. Feel free to ask questions.

Cheers

KTK

[jetjockey737]

I may be creeping slightly away from the original question, for which I apologise.

On some FMCs in 737s, the FMC will calculate V speeds based on the info you have put in...ZFW, GW, Ambient and Assumed temps etc. Am I correct in thinking that these are balanced field speeds?..I am presuming this as then we overwrite them with our figures derived from the performance manual, which are figures that are runway specific.

[mutt]

Non adjusted FMC speeds are based on a "balanced field" for the weight and not the specific runway.

If its your policy to use optimized Vspeeds or account for the clearway/stopway you will have to adjust the FMC speeds accordingly. However, I would say that it negates the usefullness of the FMC if you need to overwrite it every time, might be a good time to assess your company policy.

In our case, we dont account for optimized Vspeeds, clearway/stopway. FMC speeds are only adjusted for MEL's and contaminated runways.


Mutt.

[Cap 56]

Mutt’s definition is crisp, clear and concise in the context of a simple explanation as was requested by the tread opener.

Really there is no complicated explanation either a definition is a definition.

The difference between JAR and FAR had traditionally to do with how you define the timeframe between Vef and V1, that’s were some discussion/differences may arise although I must say that I am no longer up to speed with the latest changes.

Of course, the real distance must be within the available ones (ASDA, TODA) but these are merely limitations, as is the rule on the maximum clearway that can be used but these are merely limitations and have nothing to do with the definition of the balanced take-off.

A clear differential analysis needs to be made between these concepts.

The fun during the interviews really starts when one applies the assumed temp concept to a balanced RTOW analysis.

1. Does the T/O remains balanced ?
2. Is the max trust one can reduce 75 % of the max ISA or 75 % of the maw thrust of the day ?

To make it perfectly clear, in theory, you can select a V1 to balance or unbalance the T/O even on a runway without clearway or stopway.

Stopway can be considered as clearway but clearway can not be considered as stopway. That’s why if we unbalance the Take Off we can use some space beyond the runway to increase the max weight that can be carried but this is not unlimited.

Not more that half of the airborne distance (between V lof and 35 ft) may be situated over the clearway.

One defines thus a take-off run, measured horizontally from brake release to half the air distance that must not exceed the runway length. A 15 % margin is applied to the all engine case.

Margines are also build in against over and underrotation.

[Alex Whittingham]

I beg to differ. The JAA consider, at least at ATPL exam level, that a balanced field exists where TODA = ASDA, quoting the distances available as opposed to distances required. This may well be a follow on from the UK military and the UK CAA exam system which used the same definition. I accept that it is at odds with the more widely accepted American definition.

[Cap 56]

Hello Alex,

Would your interpretation then imply that on a 4000 m runway a JAR 25/ FAR 25 light jet would not be able to take-off at all since never would ASDA=TODA simply because the runway is too long ?

Surely even if the calculations were to be make for a weight much higher than the structural MTOW as is always done to cater for operational reductions the ACTUAL TOD would be lower than the TODA.

So it may be the case that the UK CAA needs to polish up the ATPL questionnaires.

I accept that it is at odds with the more widely accepted American definition.

It is also at odds with other European CAA policy.

[Alex Whittingham]

No, it is not connected in any way with the definition you quote. It just means TODA=ASDA.

'Balanced Field' graphs were used by the RAF V bombers. They didn't have regulated take-off graphs for all the runways they used so they carried graphs for 'standard' airfields of, for instance 9,000ft TODA and 9,000ft ASDA at sea level, 1000ft and 2000ft reasoning that, if they found themselves at a SL airfield that had a TODA of 9200ft and an ASDA of 9300ft and they could get airborne using the 'balanced field' 9000ft graph they were erring on the side of caution.

What European CAA policy are you referring to?

[Cap 56]

Dutch, German, French and Belgian ATPL questions and courses, Delft Technical University Performance course, that dates from AFTER WW II.

It occurs to me that the RAF had a simplified system for easy decision making. In practice this means that they used the definition in another context then the one it was established in the first place.

Balanced means that TOD=ASD whatever amount of runway remains ahead of you does not change the fact that the take off is balanced.

\'Balanced Field\' graphs were used by the RAF V bombers. They didn\'t have regulated take-off graphs for all the runways they used so they carried graphs for \'standard\' airfields

It\'s RTOW charts we are talking about, there fore your arguments altough valid in practical terms to avoid a take off beyond the limits are not valid in the context of this discussion.

[Alex Whittingham]

I'm afraid you will find nowadays that the Dutch, Germans, French and Belgians all use the same questions as the British. We all work to a common syllabus.

The question asked for the definition of balanced field. Now you can tell me you have a different definition to mine, which is fine, but simply restating your opinion over and over again doesn't make you right, it's just the written equivalent of raising your voice.

Still waiting for a pointer on the European CAA policy you referred to.

[Mad (Flt) Scientist]

Sounds like Alex is talking about a characteristic of the airfield - i.e. there is a balanced field length available, due to the particular construction of the runway/overrun etc., while the usual way BFL is used is in the context of an aircraft's field performance, when we are really talking about a balanced field length requirement.

It's no different, philosophically, than talking about TODR and TODA, except that for whatever reason, no-one ever appends the "A" or "R" to BFL.

If Alex (and the RAF V bomber crews!) had called their "BFL" "BFLA" and the rest of the world called their version "BFLR" then it'd be much clearer. As it is, saying "balanced field length" can be as confusing as just saying "take off distance" - if it's not obvious from the context, there are two interpretations.

[Alex Whittingham]

Just so. Both definitions can be found on the net.

[Cap 56]

Mad (Flt) Scientist

I agree, but one does not choose the V 1 so the ASD to meet the ASDA.

V1 is selected so ASD=TOD and that's what balanced take off is all about.

To state that balanced take means that ASDA=TODA is the same as to state that IF a takeoff is balanced then you are limited by the runway lenght and that's not true at all.

This is clearly indicated on any RTOW chart.

[Mad (Flt) Scientist]

... one does not choose the V1 so the ASD to meet the ASDA. V1 is selected so ASD=TOD and that's what balanced take off is all about.

To state that balanced take means that ASDA=TODA is the same as to state that IF a takeoff is balanced then you are limited by the runway lenght and that's not true at all.

This is clearly indicated on any RTOW chart.

On the first point - you would choose V1 to get ASD(R)=ASDA if you were trying to get out of an airfield where you were being limited by the "stop" case, either due to a disparity in "go" and "stop" performance or a disparity in ASDA and TODA. V1 is selected to make ASD(R)=TOD(R) if you wish to have a BFL(R) - but that's unlikely to be the optimal case for a given airfield unless it happens to have a BFL(A).

In fact, a BFLR aircraft is more likely to be restricted by the runway than a non BFLR aircraft unless the field itself is balanced i.e. BFLA. So the two concepts do have a linkage.

BFL(R) is a simplification for ease of use, just as much as BFL(A) is. In both cases you are giving up some flexibility in scheduling speeds (and hence losing efficiency of operations) in order to obtain simplification.

[Cap 56]

Balanced take off has nothing to do with characteristics of an airfield but everything with the actual performance of the aircraft in terms of a chosen V1.

Once these data are set they are compared to the TODA and ASDA to make the take off legal but these are two different issues.

The calculation of V1 so TOD=ASD is the only process that balances the take off, verifying that the TOD and ASD are lower than TODA and ASDA in not a performance calculation at all, as such there is only one correct definition and it's important that this is clearly understood.

[Alex Whittingham]

Not true. You can't make what you believe into 'the truth' just by restating it.

[Cap 56]

If you take a B 737 at 50 ton its impossible to choose a V1 that would make the balanced TOD equal to a TODA in case of a 4000 m runway.

The error in your reasoning is that you mix the actual performance calculation with the runway characteristics.

Calculations are done for each temperature at max available thrust for the prevailing conditions until the highest weight is found with an associated V1 that balances the associated TOD and ASD.

After this calculation is finished it is compared to the TODA and ASDA, these distances may very well be much greater than the calculated TOD and ASD for the limiting temp and thrust. As a consequence you have a balanced take off were TOD < TODA and ASD < ASDA

Ref: Dr Prof JJ Ruijrok Delft University Section Design and Flight Mechanics research in Aircraft Performance, Propulsion and Noise.

[Alex Whittingham]

Look, I'm not saying your definition is wrong and I do understand how it works, thank you. All I'm asking you to accept is that a second definition exists. I can show it to you in regulatory documents if that would help.

[Cap 56]

Just by stating and repeating that there are WMD in I*** does not mean that you will find them;

British nonsense, same with your statement about ASDA and balanced take off.

And it is not my definition, it\'s the way things are done. BASTA.

[Alex Whittingham]

Good reasoned arguement, well done.

[john_tullamarine]

.. I'm not too fussed about who wins an argument when the argument relates to semantics.

However, if the tone deteriorates any further, either the relevant poster(s) will be constrained or the thread will be locked.

Please keep in mind that we play the ball here .. not the player.

[Kennytheking]

Girls, girls, girls,

Alex and CAP56, you are both right. You will see from my previous post that I tried to explain BFL in terms of selecting V1 to balance the ASDR & AGDR........this is the American way of doing things.

I had to refer back to my books(The Aircraft Performance Requirements Manual by RV Davies). According to this book the Brits do say that a Balanced Field exists when ASDA = AGDA.

This was was a revelation to me! I have to now sit down and rethink a subject I thought I understood. I am not sure why they did it this way, but I am sure we can get to the bottom of it.

KTK

[Alex Whittingham]

I think the concepts are the same. As the Mad (Flt) Scientist neatly summarised above the difference only lies in the definitions.

The B737 charts I have to hand input 'Field Length Available' (I'm assuming this is TORA=TODA=ASDA) to output FLL TOM. A second set of tables take density altitude and TOM to derive a V1. The whole arrangement seems designed for simplicity at the possible expense of accuracy. The British balanced field charts of the '50s and '60s, not just the V bombers - I used them only as an example, are identical in concept. The only issue is the definition, the Americans refer to a balanced field as occurring when TODR=ASDR, the Brits when TODA=ASDA.

[Cap 56]

john_tullamarine

It is not about semantics at all.

If you do not see that, you have not understood anything and I am not surprised.

Boeing publishes simplified tables, the real calculations are done by computer and if you know how the software is programmed it becomes all very clear.

Any performance engineer who would give Alex Whittingham's definition would fail his exam.

Alex Whittingham

By using the charts, the way you describe; you only calculate the balanced MTOW and associated V1for a particular runway under certain prevailing conditions and that was not what the original question referred too.

As such you do not answer the question hence you fail the question on the exam.

If the question would have been “calculate the max TOW for a particular runway” the Alex argument is correct. However balanced take off does NOT mean that at all times you are limited by the ASDA and the Brits are wrong if they claim this to be the case.

REf: Pierre Volosin, Boeing Performance engineer.

So finally there seems to be a British definition that is NOT accepted on the continent as is the case with many other issues that are not accepted on the continent.

I have had to deal with this attitude before and sometimes there is simply no way you can get trough their thick scull until an incident happens and all the **** comes to the surface.

I am sorry I have to put it like this but that’s the way it is.

(word deleted - site policy)

[bookworm]

However balanced take off does NOT mean that at all times you are limited by the ASDA and the Brits are wrong if they claim this to be the case.

You keep whining on about "balanced take off". The subject of this thread is the definition of balanced field length, not balanced take off. Are you surprised that they have different definitions?

[Cap 56]

The difference is that you can balance a take-off but you can not balance a field length.

That is, you can not balance a ASDA nor TODA it\'s just a distance.

To balance the take off means to take the action or imply the condition in choosing a V 1 so TOD=ASD thats all.

[Mad (Flt) Scientist]

Sorry, cap_56, but you can balance a field.

Either you do it in actuality, by means of construction of the airfield such that ASDA and TODA are, in fact, equal - which obviously you CANNOT do on a day-by-day basis - or you can assume the field to be balanced, by assuming that BFLA=min(ASDA,TODA), which, taken in conjunction with data relating field performance to (balanced) filed length requirements - the BFL you are using - enables a simplified assessment of the takeoff conditions to be undertaken.

In fact, if all that is available for take off performance data are BFL (required) then you have to assume a balanced field available because you have no way to take credit for the excess ASDA or TODA, as you have no ASDR or TODR data.

[Cap 56]

or you can assume the field to be balanced, by assuming that BFLA=min(ASDA,TODA), which, taken in conjunction with data relating field performance to (balanced) filed length requirements - the BFL you are using - enables a simplified assessment of the takeoff conditions to be undertaken.

If I assume a bull to be a cow then I could milk him.

Twisted reasoning to prove a point that has no relevance.

I realise I am involved with some Brits that can not accept a simple truth.

I just hope that the chap that raised the question does not get completely confused by your assumptions.

Cheers and good luck with your B-Levels

[benhurr]

Flying clog

In the land of the JAA...

And to answer the question at interview I would suggest the following:

"Both the TODR and the ASDR increase with increasing mass. The MTM at a given runway is obviously achieved by selecting the V1 value so that the TODA and the ASDA are of equal length."

"When both the required distances are of equal length the take off is said to be a balanced field length take-off. The balanced field length take off method is normally used because it gives the highest allowable take-off mass for the available runway."

I am quoting from the BAE notes (thanks to Graham Morris who is in the UAE somewhere I think and flew V bombers)

[mutt]

Theoretical world meets the real world. Alex your answer is correct as per the requirements of the written exams, however as soon as someone sits in a FMS equipped aircraft or uses QRH V-speed values, CAP56 is correct.

Now aren’t we all glad that Flying Clog asked for the simple explanation :):)

Benhurr, I most certainly wouldn’t give that answer for an interview!

Mutt.

[benhurr]

mutt.

I qualified it at the start with "In the land of the JAA"

I agree about real world/JAA world - I wonder if the interviewer would appreciate the difference?

[alatriste]

BFL again a very hot topic. I´m for CAP 56 point of view, but I disagree about that the stopway is always a clearway. The clearway must be at least 500 feet wide while stopway must be at least runway widht (tipically 120-150 feet).
Clearway covers airbone path with engine-out directional problems, while stopway just cover RTO .

[Mad (Flt) Scientist]

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??

[john_tullamarine]

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 ?

[Old Smokey]

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.

[Mad (Flt) Scientist]

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)

[alf5071h]

For the Airbus view of this subject, with diagrams; see the excellent document
Getting to grips with aircraft performance (http://www.nathangb.com/wingfiles.htm#Performance)

[john_tullamarine]

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 ...

[Old Smokey]

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 -

In our case, we dont account for optimized Vspeeds, clearway/stopway. FMC speeds are only adjusted for MEL's and contaminated runways.

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.

[mutt]

Old_smokey,

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°,

Or better yet, if you are back here in October, we can discuss it over a beer or two.

Cheers.

Mutt. http://www.stopstart.fsnet.co.uk/smilie/guin.gif

[Old Smokey]

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

[SR71]

Can I resurrect this thread?

Can we have your reply Mutt to Old Smokey's question?

Cheers.

:ok:

[mutt]

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.

[SR71]

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...

:ok:

[Idunno]

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.

[BizJetJock]

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

[SR71]

....then this is a penalty that many companies are happy to take in the pursuit of simplicity = less chance of silly cock-ups!

Not trying to be provocative or anything, but to me, if we consider a normal TO with a V1 of 135kts and we have a catastrophic failure/fire at 105kts ( << V1) but went flying anyway and subsequently wind up in the bushes, thats a silly cock-up isn't it?

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.

:uhoh:

[BizJetJock]

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"

[mutt]

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.

[SR71]

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.

Some airlines do actually give crews min/bal/max V-speeds and allow them to make their own decisions

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?

:ugh:

[BEagle]

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!

[FE Hoppy]

balanced field calculations do not take into account obstacles so how do you account for required clearance when using bf tables?

[Old Smokey]

Correct FE Hoppy, BFL does not consider obstacles. Operator's policy and techniques will vary, but for the Companies / Aircraft types that I'm responsible for as Performance Engineer, there are 2 approaches.

Primarily, I only use BFL for the production of 'General Takeoff Charts and Tables', this takes care of the 'Field Limits', i.e. Accelerate-Stop or Accelerate-Go to the screen height (nominally 35 feet). Now, from the end of the Runway, obstacles for the 1st and 2nd segment climbs are evaluated, and the 2 associated 1st / 2nd segment climb limits found. Now we have 3 limits, namely (1) The Field Limit, (2) the 1st segment limit, and (3) the 2nd segment limit. The MBRW is then the lesser of the 3 limits found. This may lead to some pretty horrendous limits, particularly when the 1st segment obstacles are 'close in'.

Alternatively, a technique is described in the Operations Manual/s for when the 1st/2nd climb limits are too restrictive. Steadily reduce the Field length in increments, thus making the distance to the obstacle greater, and the required gradient less. Thus, whilst the Field limit is reducing, the obstacle limit is increasing. Keep on reducing the Field length until the Field Limit and the Obstacle limit are as close as possible to each other, and Voila!, you've done a reasonable job of optimising the takeoff weight. The big 'sufferer' in all of this of course is the accelerate-stop limit (because you've been reducing the Field length), which need not have been so as the Accelerate-Stop is unaffected by obstacles.

The techniques described here are crude, but effective in achieving reasonable takeoff weights whenever the RTOW is invalidated. It's a brief and incomplete description, but it will keep you safe.

Take Care,

Old Smokey

[Mad (Flt) Scientist]

Alternatively, a technique is described in the Operations Manual/s for when the 1st/2nd climb limits are too restrictive. Steadily reduce the Field length in increments, thus making the distance to the obstacle greater, and the required gradient less. Thus, whilst the Field limit is reducing, the obstacle limit is increasing. Keep on reducing the Field length until the Field Limit and the Obstacle limit are as close as possible to each other, and Voila!, you've done a reasonable job of optimising the takeoff weight. The big 'sufferer' in all of this of course is the accelerate-stop limit (because you've been reducing the Field length), which need not have been so as the Accelerate-Stop is unaffected by obstacles.

My first question is "do all the crews realise exactly what you're doing".

I'd be concerned that someone who *thinks* they know (but doesn't) will note you've used a shorter BFL than the available runway, and think "aha, I can takeoff from the intersection, rather than taxiing all the way to the end of the runway...."

[Old Smokey]

Mad (Flt) Scientist,

To answer your second question first, the Operations Manuals that I've produced and alluded to PROHIBIT the use of intersection departures when General Charts / Tables are used.

To respond to your first question, "do all the crews realise exactly what you're doing", probably not. Whilst well written Operations Manuals are produced, and recurrent performance refresher courses etc. are carried out, there's still a significant number of pilots 'out there' who, having passed Performance 'A', promptly put it into the 'tick in the box' category and hope that they never need to use it. Methinks that the number of very basic performance related queries / responses on Pprune's pages are indicitave of this.

The best that we can do is produce the best possible performance material, training syllabi, and guidance to trainees, and hope that their professionalism is such that they would studiously apply it.

[FJJP]

Em, why was the question asked in the first place? Flying Clog claims to be an airline captain - is this subject not comprehensively covered as part of the ATPL study?

[Old Smokey]

FJJP.

To answer your question which I quote -
- is this subject not comprehensively covered as part of the ATPL study?
I respond with my own previous quote -
there's still a significant number of pilots 'out there' who, having passed Performance 'A', promptly put it into the 'tick in the box' category and hope that they never need to use it.

Old Smokey

[mutt]

Old_smokey,

What type of aircraft are you talking about?

Mutt.

[mutt]

Old_smokey,

If you were just talking about the basic BFL charts in an operations manual I would agree with you that they do not account for obstacles, as they are not airport/runway specific.

However the Boeing software methodology reduces the takeoff weight for the obstacle and then provides a balanced field solution. This data is then presented as a limiting takeoff weight and the speeds can be obtained directly from the FMS.

All companies don’t use this method; I have another electronic AFM that even though the input selection is "balanced", obstacles will “unbalance” the output.

It is therefore imperititive that crews KNOW exactly what their ops engineers are trying to do, this can only be achieved through adequate training and company procedures.

Mutt.

SR71,

Still interested in knowing how you actually brief the takeoff procedure. Using the lower V1 of 135kts, with the knowledge that you can safely abort at 165 kts. Do you brief your PNF that your maximum abort speed is 135 or 165?

Mutt.

[411A]

The most limiting sea level runway that I have personally operated from is FUK.
Takeoffs to the south (runway 16) are obsticle limited, whereas takeoffs to the north (runway 34) were field length limited...with TriStar -500 equipment.
Our routes were such that field length limited takeoffs were always performed on runway 34 (wind permiting of course) and therein was a problem.
The takeoff data was for a balanced field, yet I had no doubt whatsoever that trying to stop from anywhere near V1 would positively result in an overrun, something I personally was not about to tolerate.
So, I always had a short discussion with the other flight crew, and suggested that a reduction of V1 was appropriate, thereby avoiding an overrun (if required to stop) yet allowing the continued takeoff, with a lower screen height achieved, should an engine fail at the most unfortunate time.
Wet runway takeoff data (reduction of V1) was used, directly from the takeoff data supplied, and was derived from Air Canada manuals.
Seemed reasonable to me...and still does.

IMO, whatever it takes to keep you safe, and avoid that certainly unpleasant incident/accident.

[Alex Whittingham]

...but of course using the wet V1 left you about 10kt closer to a Vmcg that had been calculated without crosswind and might even have put you under your 'real' Vmcg on the day.

[BEagle]

Which would have caused the utterance of a singularly appropriate expletive considering the ICAO code of the aerodrome in question.....

In HMFC's VC10s, we used either aerodrome-specific regulated take-off graphs, or 'general' balanced field graphs if RTOGs weren't available. For normal conditions a mid-range V1 was used; this varied for contaminated ops or for wet RWs.

Reduced thrust ('factored power') take-offs were OK using most general balanced field graphs; however, on some RWs it was necessary to use max chat - and to treat the locals to the 'silent and serene' sound of the mighty Vickers Whisperjet!

[Old Smokey]

Mutt,

Agreed, on the one hand I was referring to basic Operations Manual BFL Charts where obstacles have to be considered separately.

On the other hand I was referring to those occasions when a published RTOW is invalidated (Usually by the appearance of a new obstacle), and in the interim period (often only a matter of hours) before the computer programme can generate temporary RTOWs, crews are required to use the "General Performance" data from the Operations manuals. This procedure is well spelled out to our crews, as I alluded to in a response to Mad (Flt) Scientist with the words "Whilst well written Operations Manuals are produced, and recurrent performance refresher courses etc. are carried out, there's still a significant number of pilots 'out there' who, having passed Performance 'A', promptly put it into the 'tick in the box' category and hope that they never need to use it. Every effort is made to convey to the pilot group the reasoning required to effect this, and it is gratifying that there is a growing awareness of performance appreciation and appropriate application in my company (This did not exist before, and I take some delight in having turned the tide).

The computer programmes that I've developed (from Manufacturer's AFM and Performance Manual data of course) rarely use the Balanced Field principal, as we regularly use Clearway and Stopway in MOST normal operations which, except in a few cases, are rarely equal. The major de-equalising (is that a word?) factor of course, is the presence of obstacles where the Optimised Continued Takeoff/Obstacle limit does not conflict with the separately computed Accelerate-Stop limit, the lesser of the 2 being the limit. This cannot be the case when forced to use "General Charts" when Balanced Field data is used in the instance of the RTOW being invalidated.

I mentioned elsewhere that I was primarily referring to the B777, but these same general principals, with their own idiosynchrasies, apply to the other aircraft that I do performance work for.

Sometimes I think we say the same things, but use different words.

Regards,

Old Smokey

[john_tullamarine]

Thread appears to get bogged down from time to time but is, nonetheless, a very important message to get across to the newchums periodically ..

Consider -

(a) the main aim is to come up with numbers to provide the operating crew with data unlikely to cause embarrassment - the bean counters may take a contrary view, of course

(b) the next aim is to be able to defend the numbers and processes in court

(c) the operating crew, however, must know enough about the basis of the numbers to operate in a manner compatible with the analysis.

(d) sometimes this results in pilots with not much more than a very elementary "do it this way" knowledge right through to folk who are competent to do the whole thing .. end user performance work is hardly rocket science .. one just needs to be methodical, disciplined, have good obstacle data and housekeeping.

(e) whether the analysis is balanced or unbalanced really is unimportant, provided that the numbers don't put the aircraft (on paper) off the side/end of the runway, into the hill, etc.

(f) balanced calcs are great for general charts to be used in association with climb gradient data by the pilot when the RTOW tables are invalid. Quick and dirty answer to the immediate problem without taking forever to do the sums ..

(g) except for folk who are used to using the AFM, it generally is not feasible for the line pilot to pull out the book on the ramp and do the full analysis. When one considers that the main problem is defining the actual obstacles, it would be pointless for the pilot to attempt an optimised calculation anyway.

(h) flight standards considerations are extremely important. While the bean counters have their part to play, often the difference between a balanced and unbalanced calculation is not significant so there may be the opportunity to use the simpler data to make it a bit easier on crew workload, etc. A similar argument can be made for facilitating the use of computer based cockpit assistance .. FMCs and the like.

(i) generally, unbalancing will provide extra payload so it is a good thing if the alternative is to leave a bunch of people behind at the terminal

(j) unbalancing to optimise the numbers may result in speed schedules which are not comfortable for the pilot .. considerations of high speed aborts become very relevant from the viewpoints of standardised practices and corporate risk control

.. and the list can be extended to further detail without too much effort ....

[Alex Whittingham]

Well....... its a neat summary but its an engineer's approach. The list is too long, you got down to (j) before you got bored, the pilots lost you at (c).

The problem isn't pilots who don't know what they're doing, they're likely to follow SOPs. The problem is not pilots who really know what they're doing, they will consider all the variables. The real problem, as always, is pilots who think they know what they are doing and don't, always the majority.

The answer can only be,

1. Operators, prepare the SOPs with the advice of specialists. Ask for what you want, the specialist will give it to you.
2. Pilots, always follow SOPs, do not deviate. You ain't paid to think.

Steps back and awaits incoming.

[SR71]

Mutt

Still interested in knowing how you actually brief the takeoff procedure. Using the lower V1 of 135kts, with the knowledge that you can safely abort at 165 kts. Do you brief your PNF that your maximum abort speed is 135 or 165?


I've never questioned the standard SOP...until now!

Alex,

2. Pilots, always follow SOPs, do not deviate. You ain't paid to think.


Hey, who are you calling a chimp?

;)

[Alex Whittingham]

Not you, unless you have a sense of humour. Have you noticed how little free thought pilots actually have?

[john_tullamarine]

Alex,

.. actually, I'm an engineer who was perverted by being a pilot .. and a pilot who, likewise, suffered at the hands of boring engineers ... come to think of it, at times I'm not quite sure what I am. Mind you, many others have no hesitation in suggesting various answers to that question .. but that's entirely another matter ..

In fact, the poor guy who drew the short straw and got me to train as an intake airline FO didn't know what had hit him .. fortunately he had a good sense of humour and perservered so here I am still ...

But I concur with your thoughts ... 100 percent.

Well, maybe 99.9 percent.

A lot of sim training work suggests to me that the "legend in his own mind" is a menace and that the better breed of pilot is one who

(a) follows SOPs unless there is a very pressing and sensible reason to do otherwise

(b) doesn't do non-standard things without a good deal of thought and consideration, being well aware that a lot of armchair philosophy went into developing most SOPs.

(c) always considers the need to be able to explain his/her action(s) at a later time

[Alex Whittingham]

No offence meant chaps! I was being slightly 'tongue in cheek' but there was a serious point behind it.

[john_tullamarine]

... but I'm improving .. this time I ran out of puff at (c) ...