Balanced/Unbalanced field length

[Kristian17]

I have searched the forums but cannot find any direct answer to my question. When you have worked out the V-speeds for a runway e.g. Vmcg 110, V1 130 Vr 135 V2 140, can you change the V1 speed up to 130 and down to 110 to make the field length unbalanced? If this is correct, is it left down to the pilots to alter the V1 speed, as I believe all speeds calculated from charts and FMC are balanced anyway? How far do you usually change the V speeds?
In the example I have used, if the rwy is very long and not ASD limited can you choose to increase the V1 up to 130 to lengthen the time on the ground incase of an emergency. Am I right in believing increasing the V1 you shorten the TOD, so this helps you clear any obstacles after approach in case of an engine failure.
Thanks for all your help.
Kristian

[foxmoth]

If you reduce the V1 to Vmcg at high weights you will probably not get off the ground! A balanced field means (if I have it right) that at V1 you can just stop or go and you will only have one V1. At a field that is longer (ie not balanced - I think the term "unbalanced" is incorrect) you may have a range of V1s but this will be shown as Vmcg 110, V1 125 130, Vr 137, V2 140, which means that your V1 range is between 125 and 130. Vmcg just tells you the min speed for keeping directional control.

[Mad (Flt) Scientist]

I have searched the forums but cannot find any direct answer to my question. When you have worked out the V-speeds for a runway e.g. Vmcg 110, V1 130 Vr 135 V2 140, can you change the V1 speed up to 130 and down to 110 to make the field length inbalanced? If this is correct, is it left down to the pilots to alter the V1 speed, as I believe all speeds calculated from charts and FMC are balanced anyway? How far do you usually change the V speeds?

Unless your charts/procedures allow you to select a V1 for yourself, don't. You are making assumptions about the basis for those numbers, and if your assumptions are incorrect then you could find yourself in trouble - both in terms of having inadequate performance and in legal trouble if you have a 'near thing' and are found to have made it up 'off the cuff'.

Not all performance chart data are 'balanced', for example. So using that as a starting assumption is not wise.

Additionally, there are margins between e.g. Vmcg and V1 in the regs - Vef not less than Vmcg, V1 not less than Vef plus margin - so you can't just set V1 to Vmcg anyway.

[kenparry]

Two different things are being confused here.

A "balanced field" is a runway which had the "stop" distance equal to the "go" distance. The stop distance is the runway length plus any paved stopway; the go distance is the runway length plus any clearway (inb essence a distance from the departure end of the runway guaranteed free of obstacles higher than the runway).

If the take-off mass is the maximum that the runway and weather conditions allow, then there is only one value of V1. If the mass is less, then a range of V1 speeds is possible, and for each type of aircraft there is a way of calculating that range of speeds.

So, you don't unbalance the runway by changing the speeds.

[MrBernoulli]

Kristian17

No offence, but that should be UNbalanced.:ok:

[oldebloke]

K,KenP hit it on the head.In your original question I think you meant can you increase your V1 to'135'as opposed to '130'as stipulated-YES if the braking will stop you on the runway upon a reject at 135..
One normally takes the V speeds for the actual weight-then go into the charts and see the gross weight V1(to provide the Acc/stop)..You can now see the variance between the speeds..BUT the discipline:ok: is to reject at YOUR vef prior to VI..
When we first acquired the Airbus 320 we had rediculous V1's (about 112knots-light weights)..we raised the issue with the company that they expected one to reject at failure below V1 on an 11000 foot runway-the firm got back to Airbus who stipulated that due to the braking efficiency the V1 speeds could be just below VR,and still stop in the 'required/weight distance'.
This simplied all the calculations and that is why the 2 V speeds are close tegether except in the gross weight case.
Reducing the V1 only applies in the WET runway case in conjunction with the reduced screen height(15')..whereas one can stop a little(sometimes 10knots)sooner on the wet surface,BUT one also has to accept the fact one will cross the screen lower(than dry) in the continued takeoff case..:)

[oldebloke]

That should be 'reject after the Vef prior to V1';)

[foxmoth]

I need to reread this then, my undestanding was that a balanced field had nothing to do with the stop distance and "go" distance being the same but to do with operating at max wt for that field (or at max derate), thus giving you just a single V1

[kenparry]

Balanced field means stop and go distances are the same, as I said in my previous post. And, again, if the mass is the max for the runway and the conditions, there is only one V1, because if you go from there (in the engine-out case)you will clear the screen by the requisite amount (just) and if you stop, you will stop at the end(just) if everything has been calculated correctly.

In the real world, balanced fields (runways) are rare - usually the Go distance is greater than the Stop distance, as most runways have some declared clearway.

You will find much more detail in textbooks for Group "A" performance - but it's a complex subject and easy to get yourself bogged down. Even with lectures from experts it's easy to get confused!

(Edited to clarify that I'm talking about the engine-out case: when all engines behave as advertised there is not a problem!)

[Kristian17]

I would like to thankyou all (kenparry, mad (flt) scientist, foxmoth and oldebloke) for your kind help in this complex subject and quick replies. Your answers have made things much clearer.

Kristian

P.S. Mr Bernoulli, thanks for spotting my error, I was not thinking correctly

[RYR-738-JOCKEY]

Kenparry, I agree with most of what you have written, but I am of the impression that you may indeed "unbalance" the runway by changing speeds.
You have a balanced T/O when TOD=ASD. If your GW is not runway limited you may alter this TOD/ASD relationship by changing the speeds (of course staying within the "band" of safe speeds). Increasing V1 will decrease the TOD because you will have a higher speed when the engine fails, but ASD will increase because you need more runway to decellarate from the higher V1.
As I see it, one will rarely have a balanced T/O, and thus one will have a range/band of speeds applicable for the current weight and conditions, and the RTOW's or the computer gives us the speeds for the optimum relationship between TOD/ASD by "balancing" the T/O as much as possible.

[kenparry]

No, 738 jockey, you don't unbalance the field. Being a balanced field is simply a characteristic of the runway, the ASDA(accelerate stop distance available) being equal to the TODA(take off distance available). Which is another way of saying that stopway and clearway are equal.

If you are not performance limited, you can, as you say, use a range of V1 speeds, but that does not change ASDA or TODA. It just means you don't need all the available length - which always is a comfort. When you are performance limited, there is no room for error - so that decision at V1 is really critical on those occasions.

[john_tullamarine]

"(Edited to clarify that I'm talking about the engine-out case: when all engines behave as advertised there is not a problem!)"

Not always the case.

Keep in mind that the AEO case has a fudge factor included and may become limiting especially at lighter weights, lower Hp, lower OAT etc.

[RYR-738-JOCKEY]

Kenparry: I think we are not on the same page here..I did not imply that I want to unbalance my T/O, my point being; it is possible to alter your speeds. Balanced field is not very relevant, because then we are talking about the TOD available versus ASD available. What the term balanced T/O refers to is of much higher relevance. Here we compare our actual TOD versus our actual ASD with the current GW and conditions. And it is here we can get the advantage of having a balanced T/O, which in essence means that we have the optimum relationship between TOD/ASD and therefore giving us equally "excess" distance in both the GO case, and the STOP case.
What we are after here is to be allowed to have the highest possible GW and at the same time being safe.

[oldebloke]

Try a copy of PJ.Swatton's book "aircraft performance theory for pilots(2000)'.
It covers the Performance Area quite well(JAA rules)but is pretty well aligned with the FAR's(US)now..
'Flying the wing' doesn't cover the 'wet' ops as well as the JAA rules(reduced V1/lower screen height)as the FAR's only cover 'new' equipement(B777),or
Performance 'B'(JAA rules)customers..Canadair have Certified to the JAA rules.
cheers:D

[john_tullamarine]

Very easy to get bogged down in semantics here.

Generally, BFL refers to the aircraft AFM takeoff calculations, ie if TODR=ASDR then the operation is defined as BFL and, in this case, TODR refers to whichever case is limiting in the calculation.

I guess you can talk about the term in relation to the physical runway environs but it doesn't really make much sense to do so.

For whatever reason, BFL sometimes acquires some mystical significance .. which is not really justified. BFL calculations usually give the quickest RTOW calculation answer. However, in general, the BFL answer won't be the maximum RTOW for the situation. Therefore, if the operator so chooses (and the AFM provides the data - and not all do) the extra effort involved in running an unbalanced calc can be done.

It is NOT acceptable to juggle the speeds arbitrarily for longer runways unless sufficient calculations have been done to ensure that such variation is justified by formal calculation. In EACH AND EVERY case, the set of AFM calculations MUST be run to ensure that ALL the required certification things are addressed .. to do otherwise is to invite considerable embarrassment at the Enquiry and potential liability .. big time bad news.

[Flyboy136]

Dear Kristian17 & others interested in this thread,

I have just done some research for the subject and can perhaps contribute the following:

The most useful definition of "Balanced Field Length"(BFL) which I have found this far is:

the distance obtained by determining the decision speed (V1) at which the take-off distance and the accelerate-stop distance are equal. In other words the V1 is moved around (between 1.05 VMCG and VR) until those 2 distances are equal. No factors such as wet, etc. are taken into account.

In other words, the aircraft manufacturer will simply determine, for a given weight, which decision speed will result in the same distance to reject and stop the plane as to achieve the 35' screen height at the required point. This is a general performance calculation which does not care on which runway, or at which temperature your take-off is taking place. (The calculation also assumes dry, sea-level conditions).

This is how the good old "speed booklet" for each weight is produced. Obviously, the speedbooklet does not "know" from which runway you happen to be taking-off. The advantage of the BFL calculation, as you stated quite rightly in your thread, is twofold:

Firstly, it allows a decision speed as close as possible to flying speed (Vr). This leaves you with the "comfortable" option of rejecting take-off right up until you are practically airborne. (Being forced to wait from an early (unbalanced) V1 until Vr while accelerating with a broken airplane must be a rather unpleasant experience indeed...)

Secondly, as you again stated quite rightly, the later V1 will ensure maximum performance in the first and second segments, as no potential energy (distance) will be lost accelerating with an engine out from V1 to Vr. Either you have a failure right up to Vr and you stop safely, or your engine fails practically at Vr and you are ready to go with your "optimal" one engine inoperative flight profile.

UNbalancing the take-off (ie adjusting V1 within the allowed limits of 1.05 VMCG and Vr) is, in any case, a runway-specific procedure which must take into account all relevant factors. (Also referred to as "scheduled" distance or "factored" distances). Multiple factors must be included in the calculation. They include:

runway condition: wet, contaminated, head/tailwind, temperature (to be determined by the pilot)

and

1st, 2nd segment obstacle clearance requirements (predetermined by the operator, in rare cases by NOTAM (eg. crane, etc.))

ONLY runway, temperature, obstacle, etc.-specific information will allow an adjusted (earlier) V1. As correctly pointed out in several related threads, it is not foreseen that the pilot be more or less arbitrarily shifting around the V1! There will be a very specific V1 for each particular runway and each particular set of weather conditions. Now all this begs the question: Why an UNbalanced field length, or take-off?

The answer is simple: You can play around with the V1 as much as you like, the aircraft Vr for a specific weight(or to be more precise Vmu) will not change. In other words, the aircraft will not be ready to actually fly any earlier because you need it to on that day! So if your 35' screen height (dry runway) is limiting, bad luck. However, if the Accelerate-stop distance (ASDA) is limiting (which it often will be in less than ideal runway conditions), then there is some room to play:

let' say that, on a particularly wet day, you decide (incorrectly) to go with a BFL calculation. An engine fails just before V1. You apply full brakes (no reverse is assumed). You may be lucky and stop before the physical end of the runway. But the ASD you just needed would be greater than the continued one-engine out take off distance to a screen height of 15' (wet) ie the take-off is unbalanced. So to right the wrong, we would have to reduce the aircraft RTOW until both distances match again. Bummer. Lots of angry offloaded people. Provided ASD is your limit today, we could also move the V1 earlier in order to achieve the same distance in case of a rejected take-off, as that required for a continued, one engine inoperative take-off to a 15' screen height. And now we see the advantage of an unbalanced take-off (which, by the way, the BCAR (British Civil Aviation Requirements) first allowed for in 1965).

Long-winded indeed, but hopefully helpful.

Cheers.

[john_tullamarine]

Flyboy 136,

Welcome to the PPRuNe Tech Log sandpit .. just a few comments on your post ..

the take-off distance and the accelerate-stop distance are equal.

Might be better to read "limiting takeoff distance ...[/i]"


No factors such as wet, etc. are taken into account.

It is appropriate to take into consideration whatever factors are relevant to the particular takeoff .. ie, the intent is to balance the particular takeoff .. it is not a generic thing.


This is a general performance calculation which does not care on which runway, or at which temperature your take-off is taking place. (The calculation also assumes dry, sea-level conditions).

I think a quick look at the typical AFM will indicate a dependence on Hp and OAT. Certainly not limited to SL conditions and not a general (ie generic) calculation. I can only presume that you have misread some guidance material along the way somewhere ?


UNbalancing the take-off (ie adjusting V1 within the allowed limits of 1.05 VMCG and Vr) is, in any case, a runway-specific procedure which must take into account all relevant factors. (Also referred to as "scheduled" distance or "factored" distances).

Distance and other factors apply equally whether we are looking at a balanced or unbalanced calculation .. both calculations are made using the same AFM data (although, often, there will be a simplified cut-down chart specifically for BFL calculations .. still based on the main AFM data, though.


1st, 2nd segment obstacle clearance requirements (predetermined by the operator, in rare cases by NOTAM (eg. crane, etc.))

The minimum clearance requirements are specified by regulation. The operator may impose a more conservative clearance profile although most would not for commercial reasons. NOTAM regularly advise changed or temporary obstacle data which must be plugged into the calculations ...


ONLY runway, temperature, obstacle, etc.-specific information will allow an adjusted (earlier) V1

For a specific takeoff on any given runway, there will be a range of usable V1. The lower end will be TOD/Vmcg limited, the higher either ASD/BE limited. You pick which suits your operating philosophy and needs on the day. One of these V1 values will result in a BFL situation if that is important to you. Depending on the runway, the day, and the takeoff weight, the calculated BFL data may or may not be limiting for the particular runway.


I suggest that BFL is only of much value for simplification of data .. general TO charts, typical on board computers etc .. For maximum RTOW, one normally needs to go unbalanced. Some AFMs, of course, don't give you the option, only scheduling balanced data, ... but that's another story.

[Alex Whittingham]

It might be worth pointing out that there are two definitions of balanced field. One, essentially American, has TODR = ASDR the other, of British origin, is that ASDA = TODA. It comes to almost the same thing in the end but the approach and semantics are different.

[john_tullamarine]

.. not wanting to be difficult but, as a practitioner (both in the sharp end and as an ops engineer), TODA=ASDA is, in effect, a useless definition.

TODA=ASDA has some significance to the civil engineer designing a runway .. I will refer this to Overrun to get his view ... but is of little interest to the pilot or ops engineer

TODR=ASDR is very pertinent to the operation.

As to whether the two definitions relate to each other will depend on the relative criticality of the runway to the aircraft. So, for instance, a 737 out of a 15000ft strip is not going to be interested .. whereas the 744 might be ..

[OverRun]

TODA=ASDA has some significance to the civil engineer designing a runway .. I will refer this to Overrun to get his view ... but is of little interest to the pilot or ops engineer
The detail of the discussion in this topic goes beyond the ambit of the airport civil engineer. Their planning of runway length is necessarily more prosaic. The myriad combinations of aircraft type, engine rating, load, and fuel are almost limitless. The aircraft type is often dependent on a particular airline, which over time changes equipment, routes and could even close down.

Since the structural life of the runway is typically 20 years, and the geometric life (its alignment) could be 50 years, it would therefore be foolhardy to precisely tailor the runway to an operation. What is done is a more generic approach to the design of the runway by the airport civil engineer. This leaves the task of fitting an aircraft/route to the runway (and its obstacles) to the aircraft performance engineer. And the task of day-to-day fine-tuning of load, wind, temperature, runway surface conditions, aircraft empty weight, and engine rating to the pilot.
There are several airport civil engineer approaches to designing runway length. The simplest is the FAA airport design program V3.2 which gives some generalised runway lengths:

Large Airplanes of 60,000 lbs or more
Stage length of 1000mi 5950 feet
Stage length of 2000mi 7600 feet
Stage length of 3000mi 8950 feet
Stage length of 6000mi 11,200 feet

FAA also has performance charts for the (older) aircraft. ICAO has their reference length approach. Because of the high cost of a runway, it is more common to use the manufacturer's Airplane Characteristics for Airport Planning manuals for a detailed design. With these, the design aircraft is forecast, the takeoff weight is determined (usually MTOW), and the aerodrome elevation and design temperature are used to find takeoff runway length required. Nil wind is usually assumed. There is a mechanism for adjusting for the runway slope. The runway length for landing is similarly checked. The design length is the longer of the two – almost always the takeoff length. This gives the basic runway length so that detailed civil engineering design can proceed. The airline is usually asked to do a performance analysis to refine the basic length, and they would incorporate their operational limitations, runway alignment length (i.e. line-up distance), and obstacles into the performance calculations. Out of this study would come any refinement of clearway and/or stopway.

As kenparry said, the balanced field length concept is a runway where ASDA (EMDA) = TODA. To me, as PJ Swatton says, it is the equality of the stopway length and the clearway length which is now referred to as 'balanced'. If the stopway does not equal the clearway, the field lengths are considered unbalanced. I looked at the declared distances for a dozen or so Australian airports, and they are generally unbalanced with the ASDA being < TODA. Very occasionally (Pearce 05) the ASDA is > TODA. However the frequency of "balanced" runways would possibly vary by country.

I'm not seeing any trend for providing stopways and clearways on new runways. The cost of the runway pavement, relative to the cost of getting the runway strip filled and levelled and the cost of the aerodrome overall, isn't excessive; an extra hundred metres or two of pavement is within most budgets. The incremental difference between building stopway pavement and building runway pavement isn't much. This thread has made me wonder whether long clearways have become redundant. The RESA rules, where RESA has to be provided and starts from the end of the TODA, means one now has to build some sort of pavement structure beyond the TODA anyway, plus a pavement structure to connect the runway and the RESA (i.e. along the clearway). Of course, where the runway IS space limited, then "squeezing a quart into a pint pot" may well see the necessity of providing either a clearway or stopway or both.

I have a feeling that it is now economically attractive to make the clearway shortish, so as to limit to overall length of facility provided. The whole thing can get rather long these days, you'll be pleased to read. I am looking at a 747 runway at the moment (medium haul), and we have approximately 3000m of runway (TORA). Add 60m for the runway strip at both ends. Add 240m of RESA at each end. That is a total of 3600m of length needed within the aerodrome boundary, of which the airline can only "use" 3060m for TODA.

So coming all the way round to JT's question – for the airport civil engineer, TORA, TODA, LDA and RESA are the direct design concerns; ASDA and achieving a balanced field length is less so. Having said that, this thread has made me think long and hard about what distances are declared and built. The debate over RESAs is hotting up amongst airport engineers as the deadlines loom closer for retro-fitting them, and this thread adds to our discussion.

[Alex Whittingham]

There was a use of ASDA=TODA. Back in the '50s and '60s (and continuing into the '80s on V Bombers), when runway specific Regulated Take-Off Tables or Graphs (RTOTs or RTOGs) were not easily available for all the possible airfields at which you might end up and when MTOM calculations from scratch were tortuous it was common practice to carry generic RTOGs for fields at, for instance, zero, 1000ft PA and 2000ft PA. These were calculated on a balanced field TODA = ASDA for, perhaps, 8000ft, 9000ft and 10000ft.

If you found yourself in the back of beyond at 600ft PA with an ASDA of 9020ft and a TODA of 9300ft you could use the 9000ft balanced field graph for 1000ft PA and get some quick numbers that erred on the side of safety.

This is the historical reason why this definition is so common on this side of the pond but, as JT says, it is of little use to a modern pilot.

[john_tullamarine]

mmm ... wonder if I should wind up Milt to stir the pot a tad more ... ?

Alex is quite correct .. BFL charts for general TO use are about as simplified as one can get .. and still very useful for when Capt I M A Pilot finds himself caught out in the scrub ... although most operators would opt for separating the data into general ASDR and TODR (with TORR inbuilt one way or another to avoid confusing folk). This latter gives a tad better result in most cases ... and operators are always interested in a tad better payload ....

[Norwegian]

I am still struggeling here, but trying to understand.

An balanced field Lenght it a term that an airport designer is using? for designing a good RWY for operators.

An unbalanced T.O is used by pilots in determing best use of A/C perfomance to have a larger TOM using a redused V1 (above 1.05 Vmcg) to comply with ASDR , in cases where TODA is not equal to ASDA.
What then with the distance in speed difference between V1 and Vr, if you get an OEI here you willnot be able to fly, or stop.

Still requirements for an airplane regarding Takeoff requirement is to accelerate to V1 and then either stop or go and then reach screenheight of 35feet.

Balanced field takeoff is where you have no problems with either stopping or going and be within T.O req. regarding regulations.

[Mad (Flt) Scientist]

I am still struggeling here, but trying to understand.
An balanced field Lenght it a term that an airport designer is using? for designing a good RWY for operators.
An unbalanced T.O is used by pilots in determing best use of A/C perfomance to have a larger TOM using a redused V1 (above 1.05 Vmcg) to comply with ASDR , in cases where TODA is not equal to ASDA.
What then with the distance in speed difference between V1 and Vr, if you get an OEI here you willnot be able to fly, or stop.
No. You have to choose V1 to meet all the requirements, and/or reduce TOM.
Still requirements for an airplane regarding Takeoff requirement is to accelerate to V1 and then either stop or go and then reach screenheight of 35feet.
Balanced field takeoff is where you have no problems with either stopping or going and be within T.O req. regarding regulations.

Think of it like this.

Any given runway has an ASDA and a TODA. Because of the difference in rules for calculating ASDA and TODA, they need not be equal. If the airport either by design or accident ended up with TORA=ASDA one could say that the RUNWAY was 'balanced' in that case. That's the runway design side, and much less common usage.

Any given aircraft (including consideration of weight and all other relevant factors) has a TODR and a ASDR. Again, these need not be equal.

However, because both ASDR and TORA are dependent upon the choice of V1, and have opposite dependency on it, one can typically choose a single V1 where TODR=ASDR. In that case one can speak of the aircraft performance being 'balanced' and its typical to describe TODR=ASDA=BFL and quote that distance as the 'balanced field length'.

If ASDA=TORA, then the balanced performance is the best optimisation of V1 possible. If ASDA isn't equal to TORA, then logically the optimum performance is obtained when one makes use of as much of both the ASDA and TORA as possible. This is achieved by 'unbalancing' the calculation and selecting a different V1 to the 'balanced' case.

Note, however, that at all times you must have TODR consistent with TORA and ASDR consistent with ASDA; you will still be able to stop or go as required. You're just taking credit for clearway or stopway that the balanced calculations were forced to ignore.

[Bigmosquito]

I think it would be better talking about V1/VR and V2/VS ratio for takeoff optimisation.
Depending on restriction(s). You may have a triple limitation like a second segment, TOD n-1, brake energy limitation and have as a result 2 V1/VR ratios.
One being a min. V1/VR and one being a max. V1/VR ratio. It is up to the operator to choose one.
Companies will use a performance provider (if not inhouse) for takeoff optimisation data.
This BFL is very theoritical and talks only about a specific distance, while operators are more interested in MTOW for a specific runway and conditions of the day.
The optimisation objective is to obtain the highest possible performance limited takeoff weight, while fulfilling all airworthiness requirements.
There are fixed and free parameters. Free parameters are for example, takeoff configuration, air conditioning, V1, V2.
By changing ratios V1/VR and V2/VS, operators will get the MTOW according to a specific runway, conditions of the day and aircraft configuration.
The V1/VR ratio as a minimum and is given by the aircraft manufacturer. The max. being 1 (regulatory).
Where did you get this definition 'V1 not less than 1.05 VMCG'???.
Rgds.

[john_tullamarine]

Norwegian,

If I may add a comment and paraphrase MFS and Bigmosquito ..

An balanced field Lenght it a term that an airport designer is using? for designing a good RWY for operators.

The term "BFL" can be used by the

(a) runway designer (and OverRun certainly has a lot of runs on the board in that discipline ..) talking about lengths at a particular airport, or by the

(b) ops engineer (performance engineer .. whatever term you prefer) and flight crew looking at how we can get the aircraft out of a particular runway on a particular day.

For your purposes, I think that you are concerned with (b) ?

There are two main goals, either of which is valid and reasonable in different circumstances ..

(a) simplicity of calculation (BFL is always going to give you the simplest calculation .. now wait for someone to cite an example where that might not apply ..) or standardisation (for example to make it easier to use FMC/FMS calculations) .. generally we will go with a BFL approach every time

(b) achieving maximum RTOW (RTOM, whatever term etc..) or some similar criterion. Generally, this will NOT be achieved for BFL and we usually have to do a few more calculations (associated with not using BFL) to get to the desired goal.

Now, if we are doing the non-BFL calculation, there will be a whole lot of calculations to do to ensure that ALL the various requirements are met .. the requirement which gives the LOWEST TOW then becomes limiting and defines the permissible maximum TOW for the conditions.

These can be presented in different ways according to the preference of the engineers (generally from the aerodynamics group at the OEM) and whatever standard approach a particular OEM might have. Some OEM, for some aircraft, don't even give you a choice .. they only present the AFM data for BFL conditions.

The OEM will set out to keep his AFM reasonably thin and not have it extend to numerous volumes so there are various tricks to the trade in developing the different typical styles of presentation .. and the previous post highlights some of these.

Main thing to keep in mind is that, doing it by hand (which would apply to any calculation done in the field by the pilot without a fancy computer in his nav bag), the BFL calculation (simplified for obstacle assessment) might take 10 minutes or so while the non-BFL optimisation for a runway with a few obstacles can take an hour or two to finish ... I used to figure on a half day to a full day for a typical manually derived runway RTOW chart.

Thank heavens we now have computers .. either one uses the OEM provided program or develops one oneself to emulate manual techniques and the PC can figure the answer in a matter of seconds/minutes according to the complexity and how far one wants to push the calculation "accuracy" ...


Balanced field takeoff is where you have no problems with either stopping or going and be within T.O req. regarding regulations.

Not quite.

You can have plenty of problems stopping or going whether the takeoff is BFL or not BFL. With the unbalanced T/O, either ASDR and/or TODR/TORR (generally for one of the TOD or TOR cases) will be limiting .. The only time that there is a reduced anxiety load for the pilot is when the required distances are very much less than the available distances and there are no obstacles of any significance for the calculation .. otherwise, every takeoff should still involve a bit of mental sweatiness for the pilot .. both seats, not just the PIC.

[Bigmosquito]

Thanks for the well detail inputs. Hard subject I guess.
Yes, I was concerned on your point b.
Understand the whole concept about BFL data and yes I know about poor AFM data sometimes. Guess it depends how much you pay for the plane...
For the long calculation, that's why my company is using an external performance provider. It relieves the pain that you just mentioned.
Still one thing to ask. Where did you get this V1 related to 1.05 VMCG?.
I thought V1 was established in relation to VEF (VEF not less than VMCG) with a time delay of 1 or 2 seconds depending on cetification prior to or after FAA amendment 25-42.
Rgds.

[john_tullamarine]

.. possibly thinking of Vr - refer 25.107(e)(1)(ii) (http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=9c39107da7522fe06bd56efba7f6cdb5&rgn=div8&view=text&node=14:1.0.1.3.11.2.155.11&idno=14) ?

[john_tullamarine]

aeroncaman,

A tad more complex than what you are suggesting but you certainly have the basic idea down pat ..

I am not sure that BFL means that the physical distance to accelerate to V1 is equivalent to the distance to stop from a reject just prior to V1.

Indeed, not ... especially if you have nosewheel brakes in the equation as well as main. If one of the posts suggested this, then I must be more attentive in future as I completely missed it ..

In general, stop distance will be considerably less than accel distance ...

[mutt]

We had to local CAA conducting certification testing on a B744 simulator, they wanted to conduct the BFL test and requested the sim people to mark a point 50% along the runway so that they could start stopping at the point!

They looked slightly red when we pointed out to them that BFL didnt mean 50% acceleration, 50% deceleration! :):)

The scary part is that they were certified and operating Captains on that aircraft!

Mutt.