Balanced Field Concept

[FlyLawyer]

With respect to multi-engine flying, how do the FAR's define "Balanced Field"? I am assuming that it is "accelerate stop" being equivalent to "accelerate-go". Thanks for the help.

FL

 

[501261]

"The choice of V1 speed affects the accelerate-go and accelerate-stop distances. A lower V1 speed, corresponding to an engine failure early in the takeoff roll, increases the accelerate-go distance and decreases the accelerate-stop distance. Conversely, a higher V1 speed decreases the accelerate-go distance and increases the accelerate-stop distance. When V1 is selected such that the accelerate-stop distance is equal to the accelerate-go distance, this distance is known as the balanced field length. In general, the balanced field length represents the minimum runway length that can be used for takeoff."

Balanced field length is used primarily by the manufacture’s marketing department to sell airplanes. As pilots (of jet aircraft) we generally don't care if the field length is balanced (or would we know) all we care about is that the runway available is greater than takeoff distance required.

 

[The FNG]

501261,

What is the reference for your quote?

In addition to V1, takeoff weight and takeoff flaps also affect balanced field. (Plus contamination, V1mbe, slope, critical tailwind, etc...)

I fly jets and care a great deal about balanced field; as well as obstruction clearance and minimum climb gradient both of which are related topics when discussing balanced field.

 

[avbug]

I care very much about balanced field length. If you don't care about it, you're not considering the possibility of an abort, and that's a dangerous concept. Know before you go, as they say.

Balanced field length isn't about selling airplanes at all. It's about safety. Of necessity, each takeoff calculation also needs to include the distance necessary to get stopped.

Why would you bother calculating V1, otherwise?

 

[501261]

I care very much about balanced field length.

Are you sure???

I think you are confusing Takeoff Field Length and Balanced Field Length. The two are very different and are often misused.

Balanced Field Length is the Takeoff Field Length IF AND ONLY If Accelerate Stop and Accelerate Go are equal.

In Pt. 25 airplanes operating manual look for Accelerate Stop distance or Accelerate go, guess what? You can't find it!!!! Why, because by regulation, the only thing that shows is Take off Field Distance, which includes the longest of 3 factors (Acc-stop, Acc-go, or 115% of all engine takeoff distance to 35').

Do we as pilots care if the reason your airplane need 6280' of runway because that is Acc-go or because it is 115% of all engine performance? Of course not, the only thing that matters to us is takeoff distance required. So why would we as pilots care if the Acc-stop and Acc-go are equal (and they almost never are)?

Balanced Field Length is can be useful if your flying Pt. 23 airplane where you can find Acc-stop or Acc-Go figures, but again what does it matter? If you're smart you use the largest of these figures as your minimum runway requirement, even though you are not legally required to use them.

 

[The FNG]

Here we go...

I agree with the definition of Takeoff Field Length. But that's different than "takeoff distance required".

"all we care about is that the runway available is greater than takeoff distance required"

That's as wrong as 2 boys F***ING.

I've always considered the true definition of Balanced Field as mearly theoretical. I'd even go so far as to say that Accel-Stop and Accel-Go are NEVER exactly equal. And exactly what is equal? To the foot, the inch, the millimeter?

As a practical matter, I use tabular data. So as long as I'm on the right page I guess I don't need to care about any of those silly distance thingies...

 

[avbug]

501261,

Are YOU sure? (see ending comments)

You identified takeoff distance as being the greater of three choices: accelerate-stop, accelerate-go, and all engines to 35' agl. Your reference is Part 25 for this requirement. However, Part 25 doesn't address anything but takeoff to 35'. If you calculate takeoff distance based on Part 25 takeoff performance information, you know how much runway you need to get off the ground, but not how much you need to get stopped if you have a problem.

You're describing critical field length, which is the minimum runway length (or runway plus clearway and/or stopway) required for a specific takeoff weight. This distance may be the longer of the balanced field length, 115% of the all engine takeoff distance, or established by other limitations such as maintaining V(1) to be less than or equal to V(R)

See 14 CFR 25.113, which defines takeoff distance. It doesn't address stopping at all. You may be thinking of 25.109 instead. This sets the requirements for establishing takeoff stop distance. However it centers about accelerating to Vef before aborting: it's all about getting stopped. That still doesn't solve your problem of determining what's needed for takeoff.

One needs to know both how much runway is available to abort the takeoff safely and still get stopped, how much runway is available to make a complete takeoff on all engines, and how much runway is available and required to continue the takeoff after an engine failure, and get airborne.

A balanced field length is the runway length in which the takeoff distance is the same as the number of feet required to accelerate an airplane to a go/no-go speed, decide to abort the takeoff, and come to a complete stop.

While technically balanced field length isn't the critical deciding element in determining suitability of a runway, one should consider the ability to get stopped, continue with a powerplant out, or go with all engines. Simply computing takeoff distance won't cut it. More information is required.

Balanced field length should be used unless a parameter exceeds it, such as the 115% all-engines concept. In this case, it becomes the longer of these distances (again, which is different than the Part 25 requirements or definition of takeoff distance). It becomes an unbalanced field length in that the accelerate-go/accelerate-stop distances are considered but are usually less than that required to accelerate on all engines plus an additional 15%. The 115% all-engines takeoff distance is usually the controlling distance for Part 25 airplanes. A V1 selected to achieve a balanced field length usually exceeds VR, a condition which is not allowed by the regulations. For these airplanes, V1 is normally selected as identical to VR and the balanced field length concept is not applicable.

14 CFR 121.189 sets operational takeoff limitations. These are not the same as certification takeoff requirements, and make allowance for stopway and clearway up to 1/2 of the runway distance. A generous margin is allowed for calculating the stopway or clearway into the takeoff performance calculations, in the interest of using shorter airfields. Takeoff run may not exceed runway length, but takeoff distance may. Likewise, accelerate-stop distance includes the stopway; the aircraft is allowed to leave the runway or exceed it. Is this a good idea? Not necessarily. Again some things are legal, but not necessarily safe or wise.

14 CFR 135.379 sets forth similiar operational rules as 121.189.

I will generally look for balanced field length. If more than that is required, I want to know about it, but I want to be able to do everything on the runway. I don't want to be aborting and taking advantage of a clearway off the runway. I want to be able to calculate my performance such that I can do everything I need to do on the runway.

Planning a takeoff simply such that one has adequate distance to get off the ground doesn't take into account aborting or other problems (deployed T/R, etc) that can arise during the takeoff. It doesn't take into account delays or kinks that may arise. It doesn't leave much room for safety, if any at all.

The point is simple. While I may or may not be concerned with the minimum specific BFL for a departure, I am very concerned with all my options. Balanced field length is important because it tells me that my safety options are met. I can get stopped if I need to, and I can lose an engine and still accelerate and go if I need to.

I used to fly airplanes that included two and three engine takeoff distance charts (four engine airplane). Generally I'd be even more conservative, and determine takeoff distance based on three engines from a standing start. This distance was always longer than accelerate stop, accelerate go, or 115% of all engines, and left me with a comfortable margin for making a decision.

The use of balanced field lengh is advantageous only in determining minimum distances that meet the ability to stop or go. As stated before, generally the all-engine distance plus 15% exceeds this distance. In this case, this is the distance to use.

Aicraft flight manuals provide a variety of performance data. Some do include balanced field information, some don't. It may be presented in different ways.

I don't know that there is a disagreement here: 501261 appears to be stating that one needs to consider the takeoff distance required, be it exact BFL, or a longer distance if the 115% rule makes it so (usually the case). If so, this is quite correct. I'd only add that this should be calculated on actual runway length, rather than stopways or clearways, if one intends to complete the operation on the runway and not in the sticks or a frangible overrun.

I think FNG is stating that he wants to see adequate runway to complete the operation he's intending to make. If so, he's right in saying it. FNG and 501261 are saying the same thing (as am I), just in different ways. The disagreement is semantics, which isn't really important here.

The confusion that exists is in individual definitions of takeoff distance required. That needs to be qualified. If we agree that takeoff distance required includes the above mentioned criteria, and that the available runway takeoff distance exceeds that, then all our requirements have been met at once. I think 501261 was simply saying that so long as we can do everything we need to do and still have runway left over, it's a good day. I think he's right, as is FNG.

 

[501261]

Avbug,

Nice summation and yes I agree with your conclusion.

The reason that I stated and believe that Balanced Field Length is simply a marketing tool on Pt. 25 Jet* airplanes is simple, in order to find the balanced field length, you have to find the Acc-stop, and Acc-go distances. Well, in Pt. 25 Jets you will NOT be able to find Acc-stop, or Acc-go in the Operating Manual! Avbug, you've flown Lear 25's, go into the manual and show me Acc-stop for an SL, ISA day at 15,000lb! You CAN'T find it, the only "runway"** numbers which you will find Takeoff Field Length. And Takeoff Field Length in a Pt. 25 Jet* will already incorporate Acc-stop, Acc-go, or 115% of all engine performance.

*The reason I say Pt. 25 jet, instead of just Pt. 25 is that I can't remember if the King Air 350 (or any other Pt 25 turboprop) has Acc-stop and Acc-go distances like the King Air 200 does. The King Air 200 has Acc-stop, and Acc-go distances in the manual because it is a Pt 23 airplane.

**FNG, thank you for correcting me. Takeoff Distance Required should also include climb performance, etc. What I meant to say is Takeoff Field Length.

*Rereading FNG's and Avbug's posts I think, the reason for the disagreement MIGHT be that Pt. 25 turboprops have Acc-stop and Acc-go numbers. I don't know if they do or not. Obviously, IF SB340's, L188, DC6's have acc-stop and acc-go numbers in the manual then balanced field becomes something to think about. However I am certain that Pt. 25 jets do NOT have Acc-stop, or Acc-go distances in the manual. Again the only runway numbers that Pt 25 jets show are Takeoff Field Length.

I have to go to work!

 

[The FNG]

Avbug and 501261,

I expect you to slap me around when I'm wrong. I'm always striving to learn which is why I asked for the reference.

The original post was actually correct. The rest of the thread was a semantics debate. But that's important too.

Have you guys actually ever seen an exact balanced field scenario. I never have. I've loosely used the term to mean that a given runway met both Accel-Stop and Accel-Go. But they were never actually equal.

I use tabular data that doesn't reference any performance related distance. Only weights. But I must understand what that weight assures me and what the limiting factor is. All too often I see people simply say, "the max weight is the max weight". Or, "I'll do all my takeoffs with max flaps instead of min flaps and I'll never have to worry about it", and never once consider second segment climb rate (or rather the lack thereof).

I think this dog has been kicked to death.

Later.

 

[LJDRVR]

Excellent debate guys!

This has NOT been kicked to death and here's why: If there was one single issue that professional pilots are "knowledge deficient" in, it would be A/C performance. I've been studying this stuff for years now in regards to climb gradient/VFR vs. IFR trivia, and I still do not have a 100% grasp. Several months ago I actually looked up Wally Roberts, generally acknowledged to be more knowlegable about TERPS than anyone out there. When I posed my question about non-diverse departure minutia to Capt. Roberts, he replied: "Well, that's about as clear as mud." I'm not making fun of Captain Roberts here, he was VERY gracious to give a total stranger five minutes of his time on the phone. My point is this: We as a group are very weak on this stuff.

For example, there's an MD-11 crew that possibly would not have destabilized their A/C during the flare in an attempt to get the jet on the ground in what they thought was a runway with less than 800' of margin, if they had understood the real definition of landing distance. (They actually had almost 1700' of margin, 2700' with max autobrakes) I don't mean to impugn the crew here, I'm glad they escaped the jet unhurt. My point is that if it can happen to a seasoned, professional crew such as this, then the rest of us should be debating it often in an attempt to educate one another. Part 121 types have nice runway analysis and approved escape routes. Those of us in part 91 and 135 have to know what we are doing. With a few notable exceptions, both FSI and SF historically do not do a very good job here.

Let's keep the debate going. I'm with 501261 here. I could care less about BFL. Avbug and FNG make some good points also. One bit of advice to the TAB DATA fans. Remember the only thing certified in your A/C are the AFM TOL numbers. TD may be approved, but it's not certified. I love the nice folks at Simuflite, and have no problem using TD at sea level on a nice long runway, but the only numbers I'm gonna trust to extracate myself from someplace like EGE, come from the nice test pilots at Learjet.

Warmest Regards to ALL!