Uwochris
Balanced field length is more applicable to transport category aircraft type certificated under Part 25. Looking at your data by your name, most of your expeirence has been and probably will be for the next while in Part 23 aircraft. Most part 23 airplanes don't have any balanced field data published, and for the single engine aircraft, most don't have accelerate-stop data published either (and certainly not accelerate go...
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Balanced field length occurs by manipulating V1, or your decision speed. This speed represents the point at which you either stop or go, in a nutshell, or more specifically, the point at which you have decided to stop or go. My manipulating this number, the distances for accelerating and going, or accelerating and stopping can be adjusted. That's the theoretical world.
Reality is that the aircraft needs a certain amount of speed or steam to get off the ground, period. Manipulating the decision speed doesn't change that...it's still going to require that amount of room to get off the ground and go. One could merely call this rotation speed.
Prior to any departure in a turbine airplane, Takeoff and Landing Data is calculated. This includes the distances discussed in this thread, as well as several different airspeeds. V1 will be the decision speed, Vr will be the rotation speed, and V2 will be the speed that we first seek right after coming off the ground to ensure our initial climb performance.
If we reduce V1 to a lesser value (manipulate it) to come up with a "balanced field length," we are saying that by aborting sooner, we will use less distance to get stopped, because we have less energy to stop. We are also saying that we will need to accelerate longer on one engine, because we still need to reach that rotation speed and that initial climb speed, V2.
The numbers tell us that we can do it within the balanced field distance given, and that we'll reach a height of 35' at the end of that distance if we continue. In a part 25 airplane, the numbers have to work...sometimes folks like to say the performance is gauranteed. While that's not entirely true, the numbers in the Part 25 airplane have been demonstrated and verified in testing. In a part 23 airplane, this is not the case. Most of the numbers in your performance charts are theoretical, interpolated from some testing and some calculations. You are not gauranteed any particular performance after takeoff, or in a light twin, even in acceleration-go data. What you have before you is theory.
Manipulating down the decision speeds on takeoff may make the airplane legal, but may not do a darn thing but lull you into putting the airplane into an undesirable condition in a place where you don't want it.
Particularly in a Part 23 airplane, you should always plan on book performance plus your own margin of safety, at a minimum. I like to call that your own PFR's, or personal flight rules. The book says you can get off the ground with an accelerate go performance of 3,500', but you won't accept anything less than five thousand feet. Nothing wrong with that.
Once you've decided to set and stick by your own personal minimums, you need to consider the other factors that affect your performance. Once you do accelerate and go, how is the climb performance today, based on density altitude, and what kind of terrain do you have around you, and how comfortable are you climbing out on one engine...or executing a forced landing off the end of the runway? If you're not comfortable with that idea, if the density altitude won't allow you the climb performance, or if terrain and obstacles are too great to allow you safe passage out of the area or back around for another landing, what difference does it make if you have enough runway to takeoff? What you really have is adequate runway to crash off field. No performance chart tells you about that...it's something you have to consider under the circumstances.
Tabular data is great, but it's seldom if ever available for part 23 airplanes. You need to look at multiple charts and combine the results to decide if you can take off, and what you can do after you take off.
Switching back to the runway again, consider your accelerate-stop. The book states that you can get stopped within a certain distance, balanced field or not. So what? Can you do it, and would you want to? One of the most dangerous things you can do is accelerate and then do a high speed abort. The fact that the book says you can do it within a certain distance may be misleading.
If you're in a propeller airplane, turbine or piston, the numbers you get will be based on a feathered climb to get away from that runway if you decide to go. Some aircraft, turbine airplanes, utilize autofeather systems that are supposed to be checked before each takeoff, be operational, and turned on, each time. but what if one fails, or if you are unable to feather? I've certainly experienced that...and once in a four engine airplane, was unable to feather and do much more than stay at tree top level while getting the airplane turned around and landed. It caused enough consternation that once we disappeared from site, the base called out crash rescue because it was presumed we had crashed.
With all four turning for all they could and one doing it's darndest to hold us back, we could hardly maintain altitude. We couldn't feather because we'd lost our oil, which was covering the engine and wing, and had nothing left in that system to drive the prop to feather. You're probably taught in whatever you're flying that loss of oil pressure will cause the prop to feather, but don't count on it, and not all aircraft will do that.
The point is that there's more to the equation that merely playing with V1 to get numbers you want. Artificially moving the decision numbers down a little doesn't change the fact that the aircraft still requires a certain amount of airspeed and energy to go flying, and to get around and over whatever might be out there once it does. One can crunch the numbers all day to make the paying public feel good, but those numbers mean very little where the rubber stops meeting the road and the wing meets the air, especially in a part 23 airplane.
