Operational Meaning of Balanced Field

[KnowledgeSeeker]

Ok, Ok everywere you look there is the definition of balanced field: acc-go equals acc-stop. Mabey I have forgoten some fundamentals here, but nowhere do i find the operational meaning of having those two equal.

 

[A Squared]

Balanced field length; It's the shortest runway you can use and have at all times a course of action which will (theoretically) result in you not crashing if an engine fails. On a balanced field, if the engine fails before V1, you can abort and have enough runway to stop safely. If the engine fails at or above V1, you have enough room to continue the takeoff on one engine.

If your runway is shorter than balanced field length, there is a period of time during the takeoff run when you no longer have enough runway to stop, but you haven't gained enough speed to takeoff on one engine in the remaining runway.

If you runway is longer than balanced field length, it just gives you a wider safety margin.

The term "balanced field length" leads people astray, because the fact that the acc-go and acc-stop distance is equal is not of primary importance. It is true, but that is not the real goal. The issue is; do you have at all times during the takeoff, an option which will keep you safe, according to the performance charts? The balanced field is the shortest field where you do.

*edited to remove confusing typo

 

[KnowledgeSeeker]

A squared,

Thank you very much for the post. Sounds good.

My only question though, is that you said:

On a balanced field, if the engine fails before V1, you can abort and have enough runway to stop safely. If the engine fails at or above V2, you have enough room to continue the takeoff on one engine

didn't you mean something different...by definition of V1 "...the pilot can continue the takeoff and achieve the required height above the takeoff surface within the takeoff distance."

 

[Snoopy58]

K.S.,

You're right; what most people mean when they say "successfully continue the takeoff" includes a bunch of things, including achieving the required altitude by the departure end, required climb gradients, etc. Depending on the airplane, there can be lots of assumed things... "#1 windmilling on NTS, other 3 engines at takeoff power, gear down, flaps in takeoff position, 5 degrees of bank away from the dead engine, gear retraction initiated within 3 seconds, feathering the dead engine initiated within 6 seconds, 35' by departure end of runway, etc etc etc ad infinitum." A takeoff in a flat area might be considered "successful" even if you didn't quite make 35' above the departure end, but the engineering data uses that as the starting point, since departure procedures use that as the basis for obstacle clearance.

There is one instance which can occur where "balanced field" actually is a player: "how much weight can you take out of here?" Under certain circumstances, the answer will put you at exactly balanced field -- you can stop the airplane no faster than your V1, and you can successfully fly the airplane (minus an engine, with all the "assumed" things above) no sooner than V1. You have exactly balanced field, and they load up the airplane to exactly that weight. Any more weight & you'd have the dead zone of no option if you lose an engine between X and Y airspeed.

There are plenty of other times, though, when you aren't runway limited. Perhaps you are climb limited -- you have plenty of runway to stop or to go, but above some particular weight you can't clear the obstacles minus an engine. In that case, balanced field won't be much of a player.

Or perhaps the conditions at your landing field limit your max takeoff weight to something well below what would give you a balanced field, and so you don't have to perfectly reproduce what the test pilots did in order to survive an engine failure at V1. That's frequently the case if your max landing weight (for normal ops) is significantly below your max T/O weight.

Cheers,

Snoopy

 

[A Squared]

Knowledge seeker,

Yes, I meant V1, not V2. Good catch, sorry for the confusion.

 

[80/20]

TYPICAL OPERATIONAL USE OF BALANCED/UNBALANCED TAKEOFF:

BALANCED: Quick and easy way to:
(a) Check that you can take-off on a given runway
(b) find max weight for a runway
(c) find the highest assumed temperature (make a reduced thrust take-off)

UNBALANCED: Adjust V1 to improve performance under special circumstances by for example:
(a) Increasing V1 to improve climb (obstacle limited takeoff)
(b) reducing V1 to have better stopping margins on a slippery runway

If conditions permit, a balanced takeoff will result in the shortest possible runway, but the actual runway available is usually longer than the minimum balanced field length required

 

[CLCAP]

When a manufacturer designs an airplane (at least a jet) they always strive to publish only balanced field length numbers. (even on contaminated runways)

The logic works like this -

If acc stop would be shorter than acc go - the manufacturer would have to publish the higher rwy length - that is acc go.

In order to minimize the runway required it wuld make perfect sense to increase v1 (that would lenghten acc stop but shorten acc go) until they balance.

If acc go is shorter - the reverse applies.

When those two meet wou have the shortest possible distance for a safe takeoff - and you can market the airplane as such.

 

[80/20]

REDUCED V1 TECHNIQUES & UNBALANCED TAKE-OFF?

When the actual airplane weight is less than the Field Length Limit Weight, there is more runway available than is required by the regulations to perform the take-off. V1 can be chosen from a range of permissible speeds between the minimum V1 and the maximum V1. The minimum V1 speed still satisfies the continued take-off criteria, the maximum V1 speed meets the rejected take-off requirements, and any value of V1 chosen between these two limit speeds would actually provide performance in excess of that specified by the continued or rejected take-off criteria. An example would be if the V1 speed is determined in the usual manner from simplified presentations in the airplane operating manual, Quick Reference Handbook (QRH), or most onboard computer systems. This speed is typically a balanced V1 which means the actual engine-out accelerate-stop and accelerate-go distances will be equal to each other but less than the actual runway available.

If V1 were reduced to a speed below the standard QRH value, an additional surplus of accelerate-stop distance is available. However, the lower the V1 speed, the greater the spread between V1 and V2 and the greater the distance required to accelerate (with one-engine out) to the take-off safety speed, V2. This added engine-out acceleration requirement increases the accelerate-go distance. In fact, it maybe possible to reduce V1 to the minimum V1, so that the accelerate-go distance exactly matches the runway available. The resulting lower V1 must be checked to insure that it conforms to the Vmcg limit criteria for that aircraft.

If the V1 speed were chosen to be less than the balanced V1 but greater than the minimum V1, additional distance margins would exist for both the continued and rejected take-off conditions. Any V1 speed that meets this criteria is referred to as a “reduced V1 speed”. A reduced V1 speed less than the balanced V1 will result in a "un-balanced take-off"

New transport category jet operating manuals, Quick Reference Handbook (QRH), and most onboard computer systems have (and are in some cases required) to include this information. Most onboard computer systems will automatically reduce V1 on a slippery runway. Many manufacturer are now recommending to reduce V1 on slippery runways. The most common example is the B-737 Slippery Runway Take-off V1 adjustment table in the QRH.