Balanced Field

[trippple7]

hey,
i am currently studying atpl performance and i have a few texts. the one thing they dont agree on is what constitutes a balanced field.

some say ASDA=TODR, other one says ASDR = TODR

i have the exam on tues, can anyone please tell me?

cheers

[CommandBars]

A balanced field is ?

• is when the TODA is EQUAL to the ASDA

What effect does a balanced field have on take-off performance ?

• A balanced field means that it is critical that an abort is carried out immediately on EF recognition. Also that the t/o on one engine could be marginal if continued as a balanced field is a take off right on the performance limits for the given aircraft weight – not a favourable scenario

Good luck !!

CB

[itchybum]

The FAR take-off field length is the longest of the following:

-The distance required to accelerate with all engines, experience an engine failure 1second prior to V1, continue the take-off and reach a point 35 feet above the runway at V2 speed (Accelerate-Go Distance).

-The distance required to accelerate with all engines, experience an engine failure 1second prior to V1, recognise the event, initiate the stopping maneouvre and stop within the confines of the runway (Accelerate-Stop Distance).

-1.15 times the all engine takeoff distance required to reach a point 35 feet above the runway.

Calculating a V1 speed that equates Accelerate-Go and Accelerate-Stop distances allows the maximum takeoff weight for dispatch from a given runway. This is known as a "Balanced Field Length"

In the Australian CAOs it is clearly stated that a factor of 1.15 is also used however they make no mention of the "1 second prior to V1" or "the confines of the runway".

From that I would say that Balanced Field Length is when the Takeoff Distance Required equals the Accelerate Stop Distance Required.

IE: TODR = ASDR.

[Wizofoz]

Itchybum,

Perfect explanation except that I would add that if it is a Balanced Field Length TO, TODR=ASDR AND ASDR=ASDA, as it isa runway-limited performance situation.

Command bars,

I'm afraid I disagree. TORA does not have to equal ASDA in a balanced feild. Any stopway can still be used in the calculation, and is included in ASDA, not TODA.

As to your second statment, I also disagree. TO performance will still be at least minimum required. An RTO near V1 will still be theoretically possible, but will require all the available stopping distance.

In any case, it makes no practical difference. Recognition below V1=RTO. Recognition at or above V1= Continue, same as in any Takeoff.

[john_tullamarine]

.. a couple of considerations ...

(a) BFL is a takeoff scheduling technique, generally used to simplify the calculations and/or to simply data presentation for crews on the line.

A given aircraft can be operating to BFL conditions where, say,

ASDR=TODR (whichever of AEO/OEI is critical)=6000 ft

from a 15000 ft runway ....

Provided that ASDR and TODR are not greater than the respective available distances, the available distances have naught to do with BFL.

ie BFL applies if AFM calculated ASDR=TODR

Some aircraft AFMs only provide data for BFL calculations.

In general, an unbalanced takeoff calculation will yield a slightly better RTOW than the balanced calculation ... but at the expense of a bit more legwork in the calculations.

(b) be careful quoting from the current regs .. aircraft are certificated to a particular frozen set of design regs and, in general, later changes to the rules are not retrospectively applied.

[Wizofoz]

John,

Agreed BUT Max TOW will be achieved when ASDA=ASDR=TODR, (assuming the takeoff is runway limited).

[john_tullamarine]

.. afraid not, Wiz ... unless we restrict our interest to the specific case where TODA=ASDA, then you should get the best weight for a balanced V1... providing that other cases are not more limiting than ASD and TOD.

Usually, TODA > ASDA, due to clearway extending beyond the end of the seal. Best RTOW will occur for whatever unbalancing V1 case gives the highest limiting weight. This need not necessarily occur for TODR=TODA and ASDR=ASDA.

"Runway limited", in usual parlance, only means that, of all the various cases to be considered, the limiting case (ie that giving the lowest TOW) is one of the runway cases.... and this limiting case need not necessarily be either TOD or ASD.

[itchybum]

Wizofiz I agree. I just didn't have the energy to go into that side of things and I hoped no one would notice... :ok:

[esreverlluf]

OK - now I understand Balanced Field theory - who can explain Unified Field Theory? Surely it can't be any more complicated.

[Two_dogs]

Get in, Sit down, Shut up and Hang on!

Two Dogs

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