Balanced Field Length Q

[Leave 5 on the glide]

Ladies/Gents,

Having performed a search re balanced field length, and not found what I was looking for, I'd just like to clear something up on the subject.

Is balanced field length defined as when:

a) the accelerate stop distance (ASD) = accelerate go distance (AGD),
b) or when the accelerate stop distance available (ASDA) = take-off distance available (TODA)?

Thank you in anticipation,

leave 5 on the glide

[Robini]

Im almost sure it is ASD=AGD .

[Gary Lager]

Balanced field length is when no account is taken for any clearway, but you can account for stopway.

This means that TODA can't be any longer than ASDA.

So ASDA=TODA. This is the defintion of a 'balanced field'.

Your 'AGD' will be equal to your TODA if you are field length limited, or using a reduced thrust takeoff to achieve the same effect. So in some circumstances your two options are the same thing.

[Mad (Flt) Scientist]

This has come up before, I'm sure, but there are in fact two different usages of the term "balanced field".

The more general usage seems to be that which refers to the takeoff performance characteristics of the aircraft, in which case the term "balanced field length" is used when referring to a single takeoff field length, which is (either by design or by assumption) equal to both ASDR and TODR. It will be "by design" if the OEM or the operator is scheduling V1 such that the two distances "balance". It is "by assumption" where they are in fact different (and perhaps cannot in fact be equal) but regardless the assumption is made that "declared distance"=BFL=TODR=ASDR. In this latter case one is leaving some performance "on the table" for the simplicity of a single distance.

The more specialised usage seems to be mainly a British thing, and refers to the characteristics of the runway. In this case, the runway is said to be a "balanced field" if the ASDA=TODA.

The two terms actually have their uses, and in fact can be profitably used together provided one is careful and precise.

for example....

Aircraft X has a range of possible V1s, giving a range of ASDRs and TODRs which overlap. I can make my life simple and publish a single V1 for a given condition, such that TODR=ASDR for that condition. I have published the "balanced field length" for the aircraft.

I wish to know if I have optimal performance for airport Y. How can I find out? Well, ASDR=TODR is only optimum where TODA=ASDA. In other words, balanced field length data for the aircraft is only the best solution where the AIRPORT is a balanced field. If the field is not balanced I have to unbalance the aircraft if I am to optimise my takeoff performance.

[Leave 5 on the glide]

Thanks very much guys for your helpful input. While we're on the subject, can anyone recommend a text on this subject?

Thanks once again

[Mad (Flt) Scientist]

If you're after info on "Performance" there's a few links in the "Useful website and document reference" thread (http://www.pprune.org/forums/showpost.php?p=658373&postcount=5) here on Pprune.

The link to Airbus's "Getting to Grips with Aircraft Performance" is unfortunately dead, but it's available elsewhere (http://www.smartcockpit.com/pdf/flightops/aerodynamics/0018) and is quite useful.