Balanced Field Length

[BMM389EC]

Hi All

I was wondering what, if any, the operational significance of a Balanced Field was?

Regards

[Mikehotel152]

The V1 call becomes more crucial than where TORA is more than TODA?

Or do I need to drink more coffee before posting technical stuff!

[Fullblast]

No room for errors, in both cases of take off or reject.

[Onceapilot]

The significance here is that you need to do a Perf A course!

[aterpster]

Called "Introduction to Transport Aircraft Operations" in the U.S. :ok:

[A Squared]

Well, first you have to make sure that you're using the correct definition of balanced field.

I say this because when I first encountered the term, it was described as when the distance required to stop, beginning at the decision to reject, was the same as the distance to accelerate to flying speed. (in the context of a light twin) Didn't make any sense to me at the time, and no wonder, because it's complete gibberish. I have encountered this "definition" a number of times since, so it wasn't just me not understanding what was being described.

So, to make sure that we're on the same page, the actual definition of balanced field is when, for a transport category aircraft*, the runway distance required to accelerate to V1, lose an engine and continue the takeoff, is the same as the distance required to accelerate to v1 lose and engine and reject the takeoff, stopping in the remaining runway.

We on the same page as far as the definition? OK

The *significance* of this is the balanced field length for a given set of conditions (gross weight, field elevation, temperature, etc) is the minimum runway length for which there is, at all points during the takeoff run, a safe course of action in the event of an engine failure which will result in the airplane not crashing. IOW, with a runway length less than balanced field length, there is a window of time when if an engine fails, you don't have enough runway to stop, and you don't hae enough runway to continue the takeoff.


I think that "balanced field" is not a good term for that, because the words don't really imply the definition and lead people to try to invent alternate definitions that seem to match the definition of the words, but are meaningless. Perhaps a better title would be something like: "Minimum safe runway length"





* If someone is speaking of "balanced field" for a light twin which does not have the capability to continue a take off in the event of an engine failure, he hasn't a clue what he's talking about, and what he's saying may be disregarded.

[john_tullamarine]

A search of the archives on this topic will give you hours of entertaining reading.

operational significance of a Balanced Field

(a) provides the easiest and quickest procedure to figure runway limited take off weights as all the various limitations are built into (and hidden within) the AFM BFL takeoff chart(s)

(b) provides a useful quasi-Standard for comparisons between different aircraft .. in a manner similar to using a Standard Atmosphere for performance comparisons

(c) many runways are, or approximate, a BFL situation

Beyond that, there really isn't any great significance.

Note that the calculation for a non BFL takeoff involves consideration of ALL possible limiting cases, takes somewhat longer to do longhand, and will provide the best weight - generally in excess of the BFL weight which might be fitted to that runway.

Then again, next to no-one does this stuff by hand any more (ah, memories of mountains of cross plotted data to get to the end ..) as the preferred option is to stick the AFM data into a computer simulation which means that BFL, in the main, is of very little interest anymore to anyone.

Keep in mind that those with an airports engineering bent probably will talk of BFLs in terms of TODA = ASDA rather than what is of interest operationally, viz., TODR = ASDR

where TORA is more than TODA

Most unlikely ever to be the case. TODA, on the other hand, generally exceeds TORA

you need to do a Perf A course

Not really the case. A bit of reading and practice with some AFMs is more than adequate. Indeed, I know a number of pilots who are reasonably competent in performance engineering without being an operations engineer or having a ticket in Perf A.

in the context of a light twin

While some OEMs may provide data relating to their own definitions of BFL the term has no Design Standard relevance to light aircraft

the actual definition of balanced field is when

More specifically, TODR = ASDR

a safe course of action in the event of an engine failure

A generalisation. May be dependent on the actual aircraft performance data and very definitely is dependent on the inherent assumptions being matched by reality. It really is very important to keep in mind that the books give a presumed outcome in the case of this, that, and the other things being as specified .. if the real world situation differs sufficiently then all bets are off.

with a runway length less than balanced field length

Not the case - if the BFL required exceeds that available .. then the operation is outside the AFM requirements unless RTOW is reduced to suit the available distances. What you say is valid but ought never to occur.

[TheRobe]

JT - Could you make a simple subject MORE complicated?

Balanced field is just a calculated field lengh that at the point where you can depart safely with one engine inoperative, you can also have X amount of runaway remaining to safetly stop the aircraft. The purpose is to insure either a safe departure or safe rejection.

Now let's talk about assumed temp...lol....

[john_tullamarine]

JT - Could you make a simple subject MORE complicated?

Mea culpa .. apologies to all .. actually, if you really want me too, I probably could double the wordiology for not much gain :E

Balanced field is just a calculated field lengh that at the point where you can depart safely with one engine inoperative, you can also have X amount of runaway remaining to safetly stop the aircraft.

ie TODR = ASDR


The rest of my yarn reflects the amount of confusion amongst (especially) the junior piloting ranks when folks get to talking about BFL matters

[A Squared]

in the context of a light twin

While some OEMs may provide data relating to their own definitions of BFL the term has no Design Standard relevance to light aircraft

Ummmm yessss, that was kind my point, now, wasn't it.




a safe course of action in the event of an engine failure

A generalisation. May be dependent on the actual aircraft performance data and very definitely is dependent on the inherent assumptions being matched by reality. It really is very important to keep in mind that the books give a presumed outcome in the case of this, that, and the other things being as specified .. if the real world situation differs sufficiently then all bets are off.

Sigh........ yes, I suppose that I could have put in a disclaimer that this was assuming that all the assumptions were correct. I thought was obvious without stating it, and redundant if you did. Apparently I was mistaken.

with a runway length less than balanced field length

Not the case - if the BFL required is less than that available .. then the operation is outside the AFM requirements unless RTOW is reduced to suit the available distances. What you say is valid but ought never to occur.

Can you point to the precise words of mine which led you to believe that I was claiming that such an operation would be allowed by the AFM of a transport airplane? Or are you just in the habit of "correcting" things that clearly weren't said because it makes you feel important?

[David Horn]

I hated thinking of this in terms of TORA / ASDA / TODA etc. It simply balances the risk so that, generally, stopping at V1 - 1 knot isn't significantly more dangerous than continuing at V1 + 1 knot, or vice-versa.

Example 1:

“I think rejected take-offs are more dangerous so will set a low V1. This ensures I'll stop with loads of runway left.". Good so far, but consider the failure after V1. You now need the entire runway to get airborne and just meet the legal requirement at screen height. However, you might be digging a chimney out of the wheels and you'll definitely need new underwear.

Example 2:

“I think that EFATO is more dangerous, so I'll set the highest possible V1 to give the best performance." This time, though, when you reject before V1, you need the entire runway + any stop end to come to a halt, with the brakes on fire and the nose gear six feet away from falling into the shark filled pit at the end of the runway.

So the balanced field calculation takes both scenarios into account and tries to give the best shot at either case. This is a simplistic overview (and, possibly, completely wrong...), and when, for example, the calculation is performed by computer many, many, more factors are taken into consideration and the balance is skewed one way or the other.

[Centaurus]

you can also have X amount of runaway remaining to safetly stop the aircraft

Safely stop the aircraft? It may be legally safe but mess something up for a couple of seconds during the abort pocess and see if that is `Safe.` And don't even visit the probability of red hot brakes after the high energy stop:ugh:

[TheRobe]

OMG - Your not one of those guys that believe that you always go are you? Better to take a flying bomb up in the air vs blow a tire on the ground is it?

[Microburst2002]

Let me try to simplify:

When you increase the speed at which you consider that the engine failure will occur, which equates toincreasing V1, the ASD increases but the TOD decreases.

You need to meet both requirements, to have enough runway meters to able to stop or to continue after an engine failure, so the laongest of both distances, ASD or TOD, will be the limiting one.

When the ASD is equal to the TOD, you have the minimum runway distance required.

The balanced field v1 gives you an optimum. That is its operational significance.

Then you can take credit of stopways and clearways, wich will give unbalanced field v1, but optimum anyway.

I dont know how to put a picture, im trying copy paste to no avail...

[DaveReidUK]

I dont know how to put a picture, im trying copy paste to no avail...

http://www.lissys.demon.co.uk/pug/pics/p37.jpg

[Brian Abraham]

OMG - Your not one of those guys that believe that you always go are you? Better to take a flying bomb up in the air vs blow a tire on the ground is it?
Gals and Gents, should you be addressing posts by TheRobe you are wasting your time. He is a sciolist who pretends to be a pilot, and anything to do with this subject you can bet he is at the forefront telling the professionals they know zip. Some will recall some of his previous incarnations - SSG etc more recently SillyPeoples

Balanced Field Length. (as defined by the FAA) The runway length (or runway plus clearway and/or stopway) where, for the takeoff weight, the engine-out accelerate-go distance equals the accelerate-stop distance.

http://www.faa.gov/documentLibrary/media/Advisory_Circular/AC%20120-62.pdf

[TheRobe]

Brian, when I get to Australia we are having some words.

[mutt]

Are you going to teach him about helicopters?

[john_tullamarine]

.. quite clearly, the lad would have been an asset at dinner the other week ... he could have taken up the subject with Centaurus directly and have had the three of us as adjudicators, perhaps ?

Are you going to teach him about helicopters?

I suspect that that might have been an interesting exchange ...

[Microburst2002]

Thanks davereid! How do u do it

From this graph you can see that the v1 corresponding to the balanced field is the one requiring the shortest runway. Any other v1 will require a longer runway.

That's why it is operationally important.