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.