Balanced Field Length question

[slim75]

Ladies and Gentlemen,


A question being asked on a particular airline FO entry exam is as follows: does the balance field length ALWAYS have to be equal? a brief explanation to your answer to help me understand would be greatly appreciated. thank you all in advance for your time in answering this inquiry.


kind regards,


slim

[Greedy]

Slim,
In considering the definition of Balanced Field Length I imagine the performance of a hypothetical aeroplane configured like a Cessna 337.(A push/pull twin-google it if you are not familiar). Except my imaginary aeroplane has powerful engines and is capable of taking off on one engine alone. There is no difficulty in keeping straight after engine failure at any speed.
Consider the following scenarios:
Accelerate Stop Distance- What if we accelerated all the way to a safe take off speed then failed an engine and stopped. The distance used on a runway would be relatively large. If we accelerated to 5 knots and stopped the distance would be a few meters. We could graph the distance for all intermediate speeds between 5 kts and safe take off speed.
Accelerate Go Distance- similarly if we take off from a standing start with a failed engine the distance required would be relatively long compared with normal take off.It follows that the lower the speed the engine is failed the greater the Accelerate Go Distance. Graph the result for intermediate failure speeds between 5kts and safe take off speed on the same graph.

The two lines on your graph will cross. This will be the speed and distance at which Accelerate Go and Accelerate Stop will be equal.

Hope this helps with the concept.
Greedy

[john_tullamarine]

does the balanced field length ALWAYS have to be equal ?

By definition, the answer must be "yes", providing we modify the question to specify ASDR=TODR. Note that the concept of BFL doesn't relate to ASDA and TODA as that wouldn't be terribly meaningful.

I think what they probably are getting at is more along the lines of "do you always need to have a BFL calculation ?" to which the answer is, subject to the AFM's scheduling non-BFL data, "no".

By way of explanation, generally BFL will give you the quickest and, possibly, best weight for a typical runway. However, if there is a noticeable difference in ASDA and TODA then, depending on the ASD versus TOD characteristics of the particular aircraft, unbalancing the takeoff may provide a weight better than for a BFL calculation.

[Goldenrivett]

As an additional observation, when we calculate the maximum Flex temperature (assumed temperature method) then the assumed stop margin (even on long runways) is often only a few meters.

We have effectively calculated the required thrust for the take off mass to produce a BFL for that runway.

Apparently the savings in engine maintenance > extra fuel cost incurred.

[john_tullamarine]

We have effectively calculated the required thrust for the take off mass to produce a BFL for that runway


That doesn't follow at all. The result may/may not produce a BFL.


(.. however, you definitely have the right idea sailing ...)

[Goldenrivett]

.. however, you definitely have the right idea sailing ...
Dohh!
I'll go back to sailing then John.

[LeadSled]

Slim 75,
By definition the answer (and it is a poor question, if it is asked in an interview, and probably reveals more about the knowledge (or lack thereof) of the interviewer than the interviewee) "Yes".
It a bit like asking to define "2", and asking is "2" always equal to "2".
Having said that, there are often considerable advantages, in doing performance calculations, to "unbalance" the field length, ie: the accelerate/stop distance and the distance to clear the "screen height" are not the same, ie: not "balanced".
Tootle pip!!

[mutt]

J_T, in theory the assumed temp / Flex temp will produce thrust and speeds figures for a BFL on that runway. However in reality due to the difference in actual temperature and the assumed/flex temp, it wont be balanced.

To answer the original question, YES. If you are using a FMS for performance calculations it will always be balanced, however as soon as you apply any corrections it will no longer be balanced.

[alldaysushi]

J T, is correct by definition, ASDR=TODR except in assumed temp scenarios.

BFL, I tend to think of as favoring a go/no go, mentality for any group of conditions.

Safe journeys...sushi

[Alex Whittingham]

To add a bit from an ATPL theory perspective, which part of the interviewer's distant past may be where this question originated, the balanced field length calculation starts by assuming ASDA=TODA and then produces a solution for V1 and OEI FLL TOM where ASDR=TODR. Possibly relevant is that the EASA definition of balanced field in the theory exams is that ASDA=TODA. I'm struggling slightly with JT's statement that additional stopway or clearway leading to an unbalanced solution may produce a greater OEI FLL MTOM, by my understanding it will always produce a greater OEI mass. Could you expand, please JT?

Concerning the original question it only needs slight rewording to make it 'Does the field length for take-off always have to be balanced?', which improves it no end and can lead to an interesting discussion about where V1 goes and where the FLL TOM goes with, for instance, increased stopway. These questions feature in the ATPL exams, Maybe it was one of those.

[john_tullamarine]

My position with the ops eng hat on has always been that TODA=ASDA is irrelevant .. further, depending on the runway numbers and the AFM data, especially the V1/Vr range available, it may not be possible to adjust to the declared distances. The concern is more with what the aircraft can produce rather than what the runway offers.

it will always produce a greater OEI mass

Some thoughts here -

(a) I would expect much the same outcome. Just my conservative engineer nature to avoid "must" where "may" can be used reasonably .. one occasionally finds something in the way of the exception coming out of the woodwork to bite one on the tail should one be excessively pedantic in one's opinions.

(b) the calculation cannot be done solely for the OEI case .. the AEO case often is limiting depending on the aircraft and the runway dimensions.


(c) some aircraft don't offer the option of non-BFL. As I recall the DC9 was in this arena ?

[OK465]

it will always produce a greater OEI mass.

one occasionally finds something in the way of the exception coming out of the woodwork to bite one on the tail should one be excessively pedantic in one's opinions.

Is it not true that if you're already at the structural limit with the appropriate BFL V1, any extra weight allowance available as a result of the existing ambient conditions can only be used to satisfy performance penalty reductions (i.e. runway contamination, antiskid inop, etc.) down to, or even possibly below this limit? An available range of V1s is somewhat moot.

[john_tullamarine]

.. and I missed that point .. thanks.

[FE Hoppy]

How about a VMC limited Balanced field limited TOW.

Is there a case where a bit of extra stopway could allow a slightly higher V1 and therefore give a few extra KGs of TOW?

[slim75]

All,


thank you for taking the time to respond to this inquiry. I appreciate the insights. your points add more clarity to what's written in Gary Bristow's "Ace the Technical Pilot Interview."

[LeadSled]

FE Hoppy,
I have seen some famous errors made by performance engineers who did not take into account Vmcg limited V1 in calculating available assumed temperature thrust reductions.
A "same only different" problem used to arise with the RR powered B747SP on very short runways, you could only use Rating 1, not full rated thrust, because the full rated thrust Vmcg limited V1 could not be accomodated in the balanced field length, but the reduced Vmcg for Rating 1 was just enough that a solution could be calculated, and there was no way of using an unbalanced filed length, that only comes in to play with very long runways.
Tootle pip!!

[john_tullamarine]

LS, I thought Wal S would have kept a tight rein on the ops engineering ship during your tenure .. ?

[LeadSled]

John_T,
Of course, but this was after his time, not only did we discover this wrinkle with the SP, but even the usual Boeing performance guidelines for assumed temperature de-rate calculations could, at best, be described as "ambiguous". At least Wal's graphical presentation of performance made it possible for a pilot to see the problem, the tabulated data preferred by many operators would have made it quite hard for a pilot (even with an engineering background) to see the problem.
It took me a while to work out the error,(resulting in excessive de-rate) but finally, when the penny dropped, it earned me a couple of free beers from the then incumbents of the performance department.
I took a very personal interest, seeing as I was flying B767-238 regularly out of the critical runways, at the time.
Tootle pip!!

PS: Wal's "Performance of the TurboJet Aircraft" is still one of the best primers on SFAR422B/FAR 25 performance, after all these years, a pity somebody has not updated it for recent amendments to FAR 25, and re-issued it.

[john_tullamarine]

still one of the best primers


Indeed .. have a couple of copies filed away.


He was a superb lecturer .. we had him for some Industry coursework lectures when I was an undergrad ..


.. not to mention very dry and quite entertaining.

[galaxy flyer]

How might some one interested in Perf A find a copy?

GF

[john_tullamarine]

You're on the list for a scanned copy when I locate the document .. they are archived in a bunch of boxes at present pending unpacking into a new work room .. which is taking a lot longer to finish than SWMBO would prefer .. I am presuming that copyright won't be a problem. If so, I may have to check with QF first.