Balanced/Unbalanced field length

[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]

Redused V1 or an ubalanced field T.O

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]

Originally Posted by 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.

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]

Optimisation principle

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) ?

[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.