Talk:Balanced field takeoff

I still find this BFT thingee difficult to understand. And it is not as though aviation terminology is a complete stranger to me. I think the article, as it stands, is (1) not correct, strictly speaking and (2) is not readily understandable by the average WP reader.

"Balanced field takeoff is the theoretical principle". No, it is not. It is a term used for something. That's a question of good English, not of aviation. The something for which BFT is a/the name is not "theoretical" - it is practical, used everyday. It is not a "principle", it is a practice.

No source is cited. There must be an official definition of this term.

The explanation uses jargon which is itself unexplained. And terms which have their own WP articles are not linked.

I write this as someone not best suited to make the improvements, but as a frustrated reader. Please help!

Paul Beardsell (talk) 08:27, 21 September 2009 (UTC)

But I've made some effort and I think I have improved and clarified the article. But work remains! Please review and fix. Paul Beardsell (talk) 08:47, 21 September 2009 (UTC)

This article has been neglected for a long time but I am pleased to see User:Psb777 and others have done something positive to improve it. I have made some refinements to the article. Following is my explanation of some of my edits:

(1) It was suggested that BFL had something to do with prolonging engine life and improving engine reliability. That is not correct. Reducing thrust is a reduced-thrust takoff (or reduced power takeoff). It is entirely possible to carry out balanced field takeoffs without any regard for reduced thrust or power.

(2) It was suggested that increasing mass reduces braking performance. This would be true if braking is reliant on aerodynamic braking such as reverse thrust or a tail parachute. However, on most runways the primary braking force is that due to wheel brakes. The braking coefficient of friction is approximately constant so the braking force is proportional to weight on wheels. More weight on wheels means a greater braking force, but also a greater mass and therefore the same deceleration. As a first approximation, braking performance is unaffected by the aircraft weight (or mass).

(3) It was suggested that balanced field takeoff is affected by density altitude and engine thrust. It is much more accurate to say takeoff performance is affected by engine thrust which, in turn, is affected by air pressure and air temperature. Air density determines the conversion of indicated airspeed to true airspeed but this effect is minor compared with the lapse of engine thrust due to increasing pressure altitude.

(4)I have deleted the paragraph about takeoffs on runways shorter than the balanced field length, especially the bit about such takeoffs being a bit less than legal.

Now that we have some interest and momentum towards raising the standard of this article, let’s keep it going. I will try to dig out some more references and in-line citations. Dolphin51 (talk) 12:08, 21 September 2009 (UTC)

---

I've numbered Dolphin51's points so I can reference them

(1) That A is a member of set B does not mean it is not also a member of set C. The definition you cite for BFT can be made to apply for a particular aircraft and airfield (taking into account a/c mass, flap deployment, density altitude, runway surface) by adjusting the engine thrust. And this calculation is done everyday, regularly, by airline pilots. They make a takeoff into a BFT deliberately, by calculation, in order to be kind to the engines. They *also* do the calculation to see if a legal takeoff is possible i.e. they have at least the min BFT distance available at max thrust. I think Dolphin51 misses the point of BFT entirely, and now the article does too.

(2) You're right.

(3) The article, as I left it, did not say what you say it said. The calculation of what the minimum BFT length is, on a particular day, depends upon the factors listed.

Later: And density altitude is *critically* important for the required take off speed, not just engine thrust. At a high airport on a hot day you'll need so much runway the only way to perform a BFT will be to offload cargo &/or pax. Paul Beardsell (talk) 19:56, 22 September 2009 (UTC)

(4) it *is* possible to take off with less than the minimum BFT length available. It is done every day, legally, by private pilots and by military pilots and by the pilots of cargo-only aircraft. It is *also* done, occasionally and illegally, sometimes deliberately and sometimes by mistake, by airline pilots. Is this not of interest?

Paul Beardsell (talk) 23:34, 21 September 2009 (UTC)

On-line references (which are not Wikipedia!) do not agree with how BFT is currently defined here. E.g. at : "The BFL is determined by the condition that the distance to continue a takeoff following failure of an engine at some critical speed (Vfail) be equal to the distance required to abort it. It represents the 'worst case' scenario, since failure at a lower speed requires less distance to abort, whilst failure at a higher speed requires less distance to continue the takeoff." See also . I think the FAR regs (I don't have a copy) used as reference must say something different to the definition currently in the article. Paul Beardsell (talk) 23:59, 21 September 2009 (UTC)

I am in agreement with the explanation of BFT given in . If you feel the current definition in the article is not compatible with this source please feel free to edit the article to make it compatible.

Notice that the explanation given in the above source says nothing about reducing thrust to preserve the engine. You have written that airline pilots on a daily basis carry out takeoffs that are both BFT and reduced thrust. I don't disagree with that, but I can say that many BFT takeoffs are also carried out at maximum takeoff thrust. It is reasonable to say that thrust setting is immaterial to a definition of balance field takeoff. My position is that BFT is achieved by the appropriate choice of V₁, not an appropriate choice of engine thrust. Boeing and some other manufacturers of transport category airplanes provide a web chart as a simple graphical means of selecting a single value of V1/VR ratio (or V₁), and this single value achieves the balance between accelerate-stop and accelerate-go distances.

You have written that private pilots takeoff legally on runways shorter than BFL for the aircraft weight. (The article presenlty contains no reference or in-line citation to support this matter so it should not be restored until a source is identified.) Can you explain how this is possible? In the case of a transport category airplane, the BFL is the shortest runway length at which takeoff can be made legally. If the runway is shorter than BFL the requirement for accelerate-stop distance can be satisfied, or the requirement for accelerate-go distance can be satisfied, but not both. Your statement is correct for single-engine airplanes, and normal category multi-engine airplanes, but only because accelerate-stop distance and one-engine-inoperative takeoff distance are usually not defined for such airplanes.

Balanced field is terminology not used in airworthiness standards such as Part 25 of the Federal Aviation Regulations. §25.109 talks about accelerate-stop distance; and §25.111 and §25.113 talk about takeoff distances, but balanced field is not included and is not necessary in the regulations. These regulations can be viewed at:

• §25.109 Accelerate-stop distance
• §25.111 Takeoff path
• §25.113 Takeoff distance and takeoff run
  

Dolphin51 (talk) 05:22, 22 September 2009 (UTC)

Look, I'm sorry, but plainly the article partly misrepresents the BFT concept and discussions here demonstrate several misunderstanding re aviation in general. Your questioning that it is possible for a private pilot to take off in under the minimum BFT length for his aircraft (mass, thrust, config) at the given airport (runway length, weather, wind etc) demonstrates this. It is both done (by me and countless others) as standard procedure at short strips and it is legal (otherwise I would not own up to it here). Also: A (good) pilot with surplus thrust will deliberately turn many a take-off into a BFT by reducing thrust so as to use more runway length - you continually seem to deny this point. A BFT is not a property of the airfield! It is a property of the given takeoff and that depends upon choices made by the pilot as well as other factors. If more thrust is used than is necessary then there is MORE takeoff length available than necessary to abort - and such a takeoff is *not* BALANCED (the full stop length and safe take off length are not equal!) and so is not a BFT. The margin of safety is more than is necessary. And doubtless the airlines ops manual tells the pilot in such circumstance to "calculate a reduced thrust to so as execute a BFT" - or words to that effect. Paul Beardsell (talk) 14:21, 22 September 2009 (UTC)

I'm pleased to see my hunch satisfied that it was wrong to cite the FAA citation to support BFT as it was then defined in the article - it was wrong and the FAA makes mistakes but usually not of that type. As you discovered: they do not define the term so the citation was improper. I'm glad to see it fixed. What the regs say is that the available distance must be MORE THAN OR EQUAL TO the distance required to abort having reached V1. A BFT obeys that rule by making the two distances EQUAL using the only reasonably available mechanism available to the pilot - the thrust is reduced. Paul Beardsell (talk) 14:21, 22 September 2009 (UTC)

Now, I regret if my frustration is showing BUT the point of this discussion is to make a better article. I'm going to improve it later and I ask that some thought is applied before they are rolled back, if they are. Paul Beardsell (talk) 14:21, 22 September 2009 (UTC)

But, importantly, we cannot complete our work here until we are clear as to what V₁ is and, I suggest, we are not. See Talk:V_speeds#V1_and_the_Balanced_Field_Takeoff Paul Beardsell (talk) 20:56, 22 September 2009 (UTC)

I continue to be puzzled by your comment that private pilots takeoff on runways shorter than the BFL. I have asked you to explain but, apart from strong assertions, you haven’t attempted to do so. Perhaps you are talking about private pilots flying normal category airplanes such as propeller-driven Cessnas and Pipers. These airplanes have neither accelerate-stop distance information nor one-engine-inoperative takeoff distance information so the concept of BFL is undefined and it isn’t reasonable to say they are taking off at less than BFL. If this is not an explanation for your comments please take a little time to explain what is meant by implying that a transport category airplane can legally takeoff on a runway shorter than the BFL appropriate to the airplane weight, altitude, temperature, runway wind component etc. For fifteen years I have been teaching pilots of transport category airplanes that it is never possible to takeoff legally on a runway shorter than the BFL appropriate to the airplane weight etc. If the all-engines-operating takeoff distance (the one with the 115% factor) is more limiting than the BFL, as it is sometimes in 3 and 4 engine airplanes, the runway must be even longer than the BFL appropriate to the weight.

Pipers and Cessnas. The accelerate stop distance can easily be determined (or closely estimated) by adding the take-off roll to the landing roll. These figures are published. But forget that! You make the same mistake again: The data doesn't have to be written down in some legal format for a BFT to be possible! You're all wrapped up in documentation and rules - legalities: Many experienced light aircraft pilots are quite capable of judging take off distance and determining decision points by counting fence posts, if necessary. They regularly will be making educated guesses at when best to reduce to max continuous power or raise the u/c. And the rule of thumb to be at half take-off speed 1/3rd down the strip has a similar purpose to BFT claculations. Anyway, a BFT calc or approximation or guess is always possible, if you know your a/c. Paul Beardsell (talk) 05:05, 23 September 2009 (UTC)

You make several mistakes here, to be blunt.

• You confuse "legal" with "possible". It is possible to drive at 100mph on the public roads, it isn't legal. Similarly a 747 pilot can take off on a runway so short that his wheels just manage to lift off the ground at the very end of the runway. It would be possible but would *NOT* be legal, with paying passengers.
• Now, if the pilot were a PPL flying his own 747 off his own private strip on a non-commercial op having to use all the strip would likely be legal. Exactly this is done every day on short strips around the world where BFTs would be impossible. And, as private ops in a light twin, they are legal.
• Read the cited material. BFT does not only apply to multi-engined aircraft, and does not necessarly involve the failure of an engine.
• The 115% etc is just obfuscation and is adding detail for no good reason - it effects the substance of the argument not at all.

Paul Beardsell (talk) 03:58, 23 September 2009 (UTC)

I agree that good pilots and operators will use reduced thrust for takeoffs on runways that are longer than necessary for the airplane’s weight. This is called a Reduced Thrust Takeoff. My position is that it is a good idea to takeoff using the V1 dictated by balanced field considerations. If using maximum takeoff thrust it is a good idea to use the V1 dictated by balanced field considerations. At reduced takeoff thrust it is also a good idea to use the V1 dictated by balanced field considerations.

As you well know, the definition of V1 is in dispute over at V speeds so we must wait for that until we can settle this. My guess is that the consensus definition of V1 will not allow it have such a high value that a light twin (or even a 737) can do a BFT on a 10km sea-level runway AT FULL THRUST. Becuase that is what you are saying. But I'm just-a-guessing. We need to get a definition of V1. Paul Beardsell (talk) 03:58, 23 September 2009 (UTC)

After deciding to carry out a Reduced Thrust Takeoff, the next question is what takeoff thrust settings should be used? I am familiar with the assumed temperature method. You are suggesting another method in which the pilot determines the takeoff thrust setting at which the BFL exactly equals the runway length available. Entirely sound reasoning.

That is what I read in the cited material. Paul Beardsell (talk) 03:58, 23 September 2009 (UTC)

You seem to be stating that when a pilot uses reduced thrust for takeoff and selects the V1 appropriate to BFL, the takeoff is called a Balanced Field Takeoff. I am stating that considerations of BFL give a single value of V1 (or V1/VR ratio) and this is true for all takeoff thrust settings, not only reduced thrust settings.

BF*L*? Assuming that's BFT. I am saying nothing new and I suggest we watch out for WP:OR here. The definition of BFT says that if the remaining distance to stop and the remining distance to get safely airborne are the same then that is a BFT. If the pilot connives for that to occur by whatever method (loading more cargo, taking off from the intersection, not using flaps, wiggling his feet on the rudder pedals during the roll, OR BY REDUCING THRUST) then that is a BFT by *definition*. Paul Beardsell (talk) 03:58, 23 September 2009 (UTC)

Please take a little time to explain your view about being able to takeoff on runways shorter than the BFL. Very best regards. Dolphin51 (talk) 03:08, 23 September 2009 (UTC)

I have already done so above, and am incredulous that I should be repetitively questioned on this by someone who lectures pilots on aircraft performance. Paul Beardsell (talk) 03:58, 23 September 2009 (UTC)

Thanks Paul. I now see your point of view on carrying out takeoffs with less runway than the BFLength. I intend to focus solely on BFT in transport category airplanes, and I will leave you to add material and citations about BFT in Pipers, Cessnas, military airplanes, cargo airplanes etc. Dolphin51 (talk) 05:19, 23 September 2009 (UTC)

The article continues to say: "To achieve a balanced field takeoff, V₁ is selected so that the one-engine-inoperative takeoff distance is equal to the accelerate-stop distance." I flagged that sentence with {{fact}} because I could not see any support for it in the cited material. And I do not know for sure that the statement is wrong or right. However, my tag has been removed and a citation has been supplied and here it the supplied text: "If we let A be the distance traveled by the airplane along the ground from the original starting point to the point where V1 is reached, and we let B be the additional distance traveled with an engine failure (the same distance to clear an obstacle or to brake to a stop), then the balanced field length is by definition the total distance A+B." Now, that strikes me as being correct, and the source may be authoritative. But, in what way does the cited material support the statement which references it? In particular, where does it say that "V₁ is selected"? That must be supported by citation, the inference, if true, would be WP:SYN. I note that the definition of V₁ is in dispute at V speeds. Resolving this question may help resolve that issue. In the interim back goes the tag. Paul Beardsell (talk) 13:17, 23 September 2009 (UTC)

Paul, don’t tell me you aren’t conversant with §25.107 Takeoff speeds. Para (a)(2) says V₁, in terms of calibrated airspeed, is selected by the applicant … …

If you agree this does in fact say V₁ is selected, feel free to add it to the article as an in-line citation. Dolphin51 (talk) 23:43, 23 September 2009 (UTC)

While we continue to commit the sin of WP:OR I think standard practice is not as described. V1 is looked up on a table for the relevant combination of takeoff factors [a/c weight etc, density altiude etc]. The pilot reads out of a table the necessary runway length to get to today's V1 and also the necessary braking distance from that speed, adds the operator's/regulators' fudge factors and then compares that to the total strip length available. If he does not have enough runway length something has to change to permit a legal takeoff. If he has excess runway length he DOES NOT (in my uninformed opinion) modify V1, he calculates (or looks up) a reduced thrust level (thus making engine failure less likely and also improving maintenance costs) so as to use the entire field length. THUS, BY DEFINITION CREATING A BFT. Paul Beardsell (talk) 13:32, 23 September 2009 (UTC)

What is true? Is V1 adjusted and full thrust used thus creating a high speed takeoff that potentially uses all the runway in the case of engine failure? Or is V1 left well alone and thrust reduced and a takeoff at standard speed performed? The latter, surely. Anyway that is my SPECULATION and it remains here. If something else goes in the article it must be supported by citation, Paul Beardsell (talk) 13:32, 23 September 2009 (UTC)

---

At Talk:V_speeds Dolphin51 has now stated that for a BFT there is only one possible V₁. So, V₁ is not a matter of choice, it is not selected. So, all agree: Matter closed and the article has been amended appropriately. Paul Beardsell (talk) 01:38, 24 September 2009 (UTC)

The following has been removed from the article as the assertions are unsupported by citation and, except for the last bit, are untrue, I think.

A higher V₁ leads to a shorter takeoff distance with one-engine-inoperative,^{[dubious – discuss]} and a higher accelerate-stop distance. Conversely, a lower V₁ leads to a longer takeoff distance with one-engine-inoperative,^{[dubious – discuss]} and a shorter accelerate-stop distance.

A higher V1 for the same aircraft (with the same mass etc etc) would require a LONGER takeoff distance and thus a LONGER accelerate-stop distance. Similarly: A lower V1 would requires a SHORTER takeoff distance.

Paul Beardsell (talk) 13:49, 23 September 2009 (UTC)

Imagine a mulit-engine airplane taking off at a weight which requires a rotation speed VR of 120 knots. Also imagine V1 is 100 knots. Assuming the critical engine fails at V1, the airplane must accelerate from 100 to 120 knots with one-engine-inoperative before being rotated for lift off. Now imagine V1 is higher - 110 knots. In this second case the acceleration from 100 to 110 knots is done with all engines operating, and it is only the remaining 10 knots from 110 to 120 that must be done with one-engine-inoperative. The distance required to accelerate from 100 to 110 with all engines operating is shorter than the distance to accelerate through this speed range with one-engine-inoperative. Newton's second law of motion. The end result is that the OEI takeoff distance is roughly inversely proportional to the choice of V1. The higher the value of V1, the shorter the OEI takeoff distance. The shortest OEI takeoff distance occurs for V1 equal to VR.

In the case of accelerate-stop distance, in the first case where the airplane must reach V1 of 100 knots and then decelerate to a halt, it requires less distance than the second case where the airplane must reach V1 of 110 knots and then decelerate to a halt. The distance required to accelerate to 100 knots with all engines operating is less than the distance required to accelerate to 110 knots with all engines operating. Similarly the distance required to stop from 100 knots is less than the distance required to stop from 110 knots. The end result is that accelerate-stop distance is roughly proportional to the choice of V1. The higher the value of V1, the longer the accelerate-stop distance. The shortest accelerate-stop distance occurs for V1 equal to Vmcg.

V1 is permitted to be anywhere between Vmcg and VR. The optimum value of V1 is the value that causes the accelerate-stop distance to equal OEI takeoff distance. This single, optimum, value of V1 delivers the shortest permissible distance required for takeoff. When these two distances are equal we say they are balanced and the chosen value of V1 delivers a balanced field takeoff.

You have written that the pilot determines V1 by looking in a book but how do you imagine that single value of V1 gets into the book in the first place? Dolphin51 (talk) 23:26, 23 September 2009 (UTC)

---

What Dolphin51 wrote is this:

A higher V₁ leads to a shorter takeoff distance with one-engine-inoperative, and a higher accelerate-stop distance. Conversely, a lower V₁ leads to a longer takeoff distance with one-engine-inoperative, and a shorter accelerate-stop distance.

It seems what he meant to write was this:

A higher V₁ leads to a shorter remaining takeoff distance with when one engine becomes inoperative at that speed, and a higherlonger required accelerate-stop distance at that speed. Conversely, a lower V₁ leads to a longer remaining takeoff distance with when one engine becomes inoperative at that speed and a shorter accelerate-stop distance.

But the point is moot now Dolphin51 has established there is only one possible V₁ for a BFT.

Paul Beardsell (talk) 01:47, 24 September 2009 (UTC)

Dolphin51 wholeheartedly agrees that for a BFT there is one, and only one, value of V₁. In the entire field of large airplane performance, considering all the different requirements, there is only one requirement that leads to a single, unambiguous, value of V₁ and that is the balanced field takeoff. All other requirements (such as climb gradient, obstacle clearance etc.) lead only to a range of permissible V₁ speeds. Dolphin51 (talk) 02:03, 24 September 2009 (UTC)

Consider it one of my many character flaws, but I have to point out that this is a complete about face from your earlier and recent position that a pilot selects various V1 speeds depending on field length so as to achieve a BFT. A whole lot of unnecessary energy has been expended to get here. Or are you going to say you have not resiled from that position? Paul Beardsell (talk) 02:32, 24 September 2009 (UTC)

You provided me with a website containing a fabulous diagram that has two lines, one representing ASD and the other TOD (OEI), showing the way the two distances behave for varying V₁, and showing that where the two lines intersect there is the BFL and the single value of V₁ that delivers that BFL. I have my own set of similar diagrams for the overhead projector, and I have been talking to them since 1994. Hopefully, all that I have written here has been consistent with that diagram. You seem to think not, so if that is so I apologise.

The diagram shows V₁ varying continuously from a low value to a high value. Perhaps I described this as the pilot selects various V₁ speeds. I could have written imagine V₁ varying continuously from Vmcg to VR, but that sounds a bit too much like the engineer talking.

As to exactly who selects V₁, it goes something like this. The FAA specifies that V₁ must be selected somewhere between Vmcg and VR. The airplane manufacturer's Performance Department selects a large bunch of V₁ values and runs them through the computer to produce ASD and TOD (OEI) information. That information ends up in the flight manual and performance manual. If the aircraft is operated by an airline, the airline's Performance Department or Jeppesen selects certain of the information out of the manuals and publishes it in convenient tables for the Captains to use on the flight deck. If it is a Learjet or Citation that isn't operated by an airline, the pilot doesn't have the luxury of neat tables of data so he must make his own selection of V1, VR and V2 using the performance charts. Dolphin51 (talk) 03:17, 24 September 2009 (UTC)

But the subject of this article is balanced field takeoff. Only one V₁ exists for any given BFT, it is not a question of selection in the sense of whimsy, which is how any reader of the article would very likely interprete what was written in the article. I know now we agree on this point, I know now you mean "selection of the one correct value out of the tables", but that is *not* how it read. I am simply trying to get an article which as unambiguously as possible tells the layman reader what a BFT is. Paul Beardsell (talk) 11:37, 24 September 2009 (UTC)