I believe its something you figure out from a performance chart. Something an airline would have publications under different conditions (ie Wx). I believe the actual def. is the total length of the runway it takes to accelerate to V1, abort, and come to a complete stop. Kind of like an accelerate-stop distance for light twins.
Coulb be wrong, or not complete with that def. though.
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.
This can be manually calculated using the combined takeoff and accelerate stop data, but is most often presented as singular information on cheat charts or in aircraft flight manual data. Look in the published data for your specific aircraft.
The answers you're getting are correct but they don't seem to directly address your question. I'm not an expert but here are some ideas that might help your understanding. This is part technial and part practical (for simplicity), so don't nit pick the technical. Please read to the end before you pick it apart.
In Transport Category aircraft the pilot is required to compute takeoff performance prior to each departure (FAR 121). When the aircraft is certified it must be able to meet the pre-established performance criteria for every departure. The criteria considers takeoff weight, temperature, wind, runway contamination, runway available (length) and obstruction clearance in the takeoff path. Pucker-factor is also a consideration, but the FAA doesn't mandate its calculation.
1. It must have sufficient runway available to accelerate to V1, abort the takeoff and come to a complete stop in the remaining runway + clear way (if any). The takeoff weight must be adjusted as necessary to guarantee that this can happen on each takeoff.
2. It must be operated at a maximum takeoff weight that will allow the aircraft to continue the takeoff, after the failure of a critical powerplant at V1, accelerate through Vr to V2, clear the departure end of the runway at + 35 ft., and continue the climb while clearing all obstructions in the takeoff path, until it reaches an "enroute climb" altitude (which is usually 1500 ft). During the climb, the aircraft must be able to accelerate to a "clean" configuration as the departure progresses.
Note1. This is different from the performance required of non-transport category aircraft. The basic difference is that the FAA allows aircraft that are not T-category to fly through obstructions (when takeoff performance is calculated). T-category aircraft must fly over the obstructions. (yeah it sounds stupid, but that's the FAA).
Note 2. The criteria are slightly different for recips, turboprops and jets. This is because these different types would not be able to meet the criteria without very long runways or severe weight penalties if we did not "adjust" the flight path. (It's a little more complex, but that will do for now)
3. When, at a given takeoff weight & temp., the distance to accelerate to V1, abort, and stop within the clearway, is the same as the distance to continue (after failure of engine) accelerate, lift off, and cross the end of the runway at 35 ft, (gear down, flaps at TO setting, [propeller (if you have one) windmilling,], the field is said to be "balanced".
Essentially as the airport, runway, obstructions, wind and temperature change, we adjust the maximum takeoff weight for that runway to whatever weight will allow the aircraft to comply with the performance criteria and clear the obstructions, on that departure.
A. It is possible for the pilot to calculate the allowable weight from a/c performance charts in the AFM and determine accelerate/stop criteria can be met (at a given weight). However, it is not practical (much to time consuming and inaccurate) to do this.
B. It is impossible for the individual pilot to calculate the climb for the remainder of the takeoff path and ensure that the aircraft will clear all obstructions. The reason the pilot cannot do this is because the pilot has NO WAY to know the physical location of, elevation of or distance to, any existing obstruction.
Because of "B" above, runway analysis tables (charts) have been developed for each aircraft. The information is all pre-calculated and the pilot is presented with a simple and practical means of accurately determining the maximum takeoff weight for each departure.
Most airlines, and that includes the majors, do not have the inhouse capability to create runway analysis charts. So, they are purchased from companies that specialize in this work. In addition to takeoff performance, the charts also include pre-calculated approach and landing performance.
BTW, I'm talking about how airlines do it. I have no idea how corporate operators do this and suspect that in most cases they are guessing or not doing it.
All jets, that I know of, can meet the criteria while accelerating and climbing at the same time. Turboprops don't have enough power to do this, so turboprops climb to 400 ft, level off to accelerate to flap retract speeds, and then climb again.
It ain't over yet and now it starts to get complicated.
In a commercial (airline) operation we never want to leave revenue behind, so we always want to be able to operate at the highest permissible takeoff weight. Enter the "unbalanced" field length.
4. Whenever we have excess runway available (more than required for a balanced field) it presents opportunity. At certain airports/runways accelerate stop distance is not a problem, but accelerat go distance is. In other words, we have plenty of runway in which to abort and stop if we have a failure at V1. More than we need (so in theory, V1 could be a much higher value -- but we never do that).
On this runway however, we have a problem with accelerate go distances. So in order to improve our accelerate go performance (make the distance greater) we play a little game. We artificially lower the V1 speed which, in turn, increases the accelerate go distance. By doing this, we can now carry MORE weight than we could if the field was "balanced". When these calculations are peformed, we have an "unbalanced" field.
Can a pilot "unbalance" the field in the cockpit? For practical purposes, the answer is NO. If ANY of these calculations had to be made in the cockpit, some we could do, some we could not do accurately and, the time it would take would bring everything to a screeching hault. This is why we have Runway Analysis Data. It's all done for us in advance.
If an "unbalanced" field is advantageous (allows higher TO weight) on a particular airport/runway, the runway analysis manual will include that data. The pilot can decide if he wishes to use the balanced field data or the unbalanced data for the particular departure.
Be careful and CYA. (Particularly in smaller T-category aircraft that are weight/payload critical - especially on HOT days)
A. If you plan the weight for a particular runway and you are close to maximum, and your runway is changed after you taxi, be sure you remember to recalculate (go into the RA Data) for the new runway, before you depart. Why? The runway length is probably different and (more importantly) the obstructions may be entirely different.
Chances are no one will ever know if you did this correctly or not, until the day you have to abort, you blow a tire or (God forbid) you have a wreck (that you survive). That's when folks will start checking and second guessing everything you did. It is definitely NOT in your best interest to be discovered operating above max takeoff weight.
A word to the wise: From the day you get your first airline job, until the day you retire, there will always be at least 3 people continuously trying to take your license away. 1) Your doctor; 2)the FAA; 3) the company. If you want to retire collect the big pension and be able to tell war stories to your grand kids, fly by the book.
I hope that helps and I hope too that I didn't make too many mistakes.