Balanced Field Length
 

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

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.

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.

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

TOD=ASD


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

Stone me, you're right!

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

In addition to Interconti's comment;

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

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).

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

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.

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.

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.

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.

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.

It is also at odds with other European CAA policy.

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?

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.

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.

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.

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.

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

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.

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.

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.

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

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.

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.

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.

Good reasoned arguement, well done.

.. 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.

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

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.

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)

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?

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.

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.

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

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)

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.

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?

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 .