Balanced field length, what to worry when departing from 10,000 ft. runway?

[superfreight208]

Request info??

 

[CUEBOAT]

question is too vauge, what kind of airplane? temp? pressure alt? obstacles? DP to comply with? etc..etc..

 

[O-Line]

It's very easy for jets to have required takeoff distances of over 10,000 on high, hot days. Even a lowly Citation V, which can get airborne in under 2000', needs 9651' (thanks Ultranav) when departing Denver at MTOW, on a 92' day with a wet runway.

Remember Takeoff Distance in Jets is the largest of 3 numbers (and you don't know which it is, because the checklist only gives Takeoff Distance).
1. Accelerate/Stop
2. Accelerate/Go
3. 115% of both engines to clear a 35' obstacle

 

[superfreight208]

Just looking for basic definition?

Just looking for basic definition of balanced field length, aircraft?...CRJ?
Were to find definition?

Thanks

 

[Guitar Guy]

Try this for an explaination though it's a little more about takeoff speeds: http://www.aerospaceweb.org/question/performance/q0088.shtml

I would also add that when calculating takeoff performance there are many other criteria to be considered. These include maximum brake energy, maximum tire speed, Vmc-limited considerations, anti-skid operative/inoperative, VR speed limits (VR can never be less than V1 for Part 25 aircraft), etc. And then there is obstacle clearance criteria.

In addition, it's important to remember that balanced field length refers to the case where the accel-stop distance equals the accel-go distance. This is not always the case. In fact, UNBALANCING can be very beneficial in many cases, including obstacle clearance, sloped runways, brake/tire speed concerns, contaminated runways, and instances where clearway or stopway is available.

 

[Sig]

I think I'm smelling what you're steeping in with the first post- "Why can't we just land immediately after popping one on rotation on a two-mile stretch of concrete?"

Thrust moments and/or inertia vectors aside (or, a steep-linear drop in thrust due to a lost engine versus mass at given 'v' to damp the steep drop- for the time being because of the lack of free lunches) , you will still most likely* be well below Vref at that configuration when the motor pops and when this thought experiment seems like a great idea. The plane is certified to climb (CLIMB) when it all goes horribly wrong at this point- because the other one is screaming away. It can't land with a comparatively clean wing whilst slow and getting slower (reducing thrust on fewer powerplants), period. You'll go BANG.

*This ain't no panacea, just a probable good fit for what I think the question sought. Who knows- maybe it was more about the above scholars' answers that are not only pertinent, but friggin' interesting. Dang fine reading regardless.

 

[TIS]

It's very easy for jets to have required takeoff distances of over 10,000 on high, hot days. Even a lowly Citation V, which can get airborne in under 2000', needs 9651' (thanks Ultranav) when departing Denver at MTOW, on a 92' day with a wet runway.

Remember Takeoff Distance in Jets is the largest of 3 numbers (and you don't know which it is, because the checklist only gives Takeoff Distance).
1. Accelerate/Stop
2. Accelerate/Go
3. 115% of both engines to clear a 35' obstacle

#3 - WHAT???? There's nothing in Part 91 that says you have to do that. That must be your company policy. I'm guessing you're at NetJets. That's a 135 thing.

 

[TIS]

Just looking for basic definition of balanced field length, aircraft?...CRJ?
Were to find definition?

Thanks

Well, there really isn't an FAA approved defintion because it's not an FAA term. It's really an artificial concept that operators and manufacturers use to mess with takeoff performance issues.

Simply put a balanced field is a runway that is exactly long enough to permit an acceleration to V1 and either continue or abort. if one continues one must cross the departure end of the runway at 35' AGL. If one aborts, one will stop on the last inch of pavement.

How is this possible? The answer is generally that you reduce V1 so that you would make a decision to continue earlier, thereby making the runway long enough to also abort prior to V1.

This isn't all there is to it but it's the general idea.

TIS

 

[Guitar Guy]

#3 - WHAT???? There's nothing in Part 91 that says you have to do that. That must be your company policy. I'm guessing you're at NetJets. That's a 135 thing.

I do work at NetJets and the 1.15 all-engine consideration is not a company policy, it's spelled out in certification rules under Part 25, specifically 25.113 (a) (2). Here's the rule:

§ 25.113 Takeoff distance and takeoff run.

(a) Takeoff distance on a dry runway is the greater of—

(1) The horizontal distance along the takeoff path from the start of the takeoff to the point at which the airplane is 35 feet above the takeoff surface, determined under §25.111 for a dry runway; or

(2) 115 percent of the horizontal distance along the takeoff path, with all engines operating, from the start of the takeoff to the point at which the airplane is 35 feet above the takeoff surface, as determined by a procedure consistent with §25.111.


Both Parts 121 and 135, under takeoff requirements for turbine-powered aircraft, reference the takeoff distance published in the Airplane Flight Manual. Thus, the 1.15 all-engine takeoff distance is a consideration for all Part 121 and 135 operations. As for the applicability of the 1.15 all-engine distance to Part 91 operations, see 91.605, "Transport category civil airplane weight limitations".

 

[Guitar Guy]

Well, there really isn't an FAA approved defintion because it's not an FAA term. It's really an artificial concept that operators and manufacturers use to mess with takeoff performance issues.

While there is no definition of "balanced field length" under 14 CFR Part 1, the term is used by the FAA. Perform a search on the FAA homepage using the term "balanced field length" and you'll see that several documents are found. One of those documents is: http://www.faa.gov/avr/arm/a1-48.doc (The document is titled as "Improved Standards for Determining Rejected Takeoff and Landing Performance".)

On page 3, the following statement is made:

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.


The term "balanced field length" is also found in FAA-H-8083-25, "Pilot's Handbook of Aeronautical Knowledge" on pages 9-32 and -33. However, that document provides a very scant discussion of balanced field length. If you are curious to see it, try this link: http://av-info.faa.gov/data/traininghandbook/faa-h-8083-25-2of4.pdf and scoll down to find the correct page.

Well, I hope all of that is either interesting or sleep-inducing, depending on your point of view.

 

[TIS]

I do work at NetJets and the 1.15 all-engine consideration is not a company policy, it's spelled out in certification rules under Part 25, specifically 25.113 (a) (2). Here's the rule:

§ 25.113 Takeoff distance and takeoff run.

(a) Takeoff distance on a dry runway is the greater of—

(1) The horizontal distance along the takeoff path from the start of the takeoff to the point at which the airplane is 35 feet above the takeoff surface, determined under §25.111 for a dry runway; or

(2) 115 percent of the horizontal distance along the takeoff path, with all engines operating, from the start of the takeoff to the point at which the airplane is 35 feet above the takeoff surface, as determined by a procedure consistent with §25.111.


Both Parts 121 and 135, under takeoff requirements for turbine-powered aircraft, reference the takeoff distance published in the Airplane Flight Manual. Thus, the 1.15 all-engine takeoff distance is a consideration for all Part 121 and 135 operations. As for the applicability of the 1.15 all-engine distance to Part 91 operations, see 91.605, "Transport category civil airplane weight limitations".

Okay, I see what you're trying to say but you're mixing apples and oranges here. Part 25 is a CERTIFICATION regulation and has NOTHING whatsoever to do with day-to-day operations under Part 91, 121, or 135. All you have to do as a pilot is adhere to applicable sections of the regulations under which you operate, including operating in compliance with the AFM's procedures and limitations.

That 115% clause you're pointing to is what the manufacturer has to publish as the takeoff distance in the AFM - not something you have to add if the runway is wet or for any other reason. The 115% figure is accounted for in the accelerate go distance you come up with in the takeoff performance calculation. The reason it exists is to add a degree of conservatism to the data the pilot will ultimately use to perform the takeoff calculation. In other words, from a practical standpoint, the takeoff distance is the greater of the accelerate stop or the accelerate go distance - for the pilot of a Part 25 certified airplane.

BTW, I'm curious why you mentioned a wet runway in your example. The FAA does not approve data related to wet runway takeoff performance - at least not on any large aircraft I've ever flown. Too many variables that cannot be accounted for in every case. (But then again, I haven't flown a Citation V.) That's why I suspected you were a NetJets guy. I know you guys have something you have to do when the runway's wet - seen it get more than a few of your guys in some hot water with owners when paired with your fuel tankering policy.

TIS

 

[Guitar Guy]

TIS,

For background, I worked as an aeronautical engineer for 11 years before I started flying as my vocation. During my time as an engineer, I worked for Boeing and also for a Part 121 carrier doing aircraft performance work.

The manufacturer-published AFM, as required by Part 25, does have direct bearing on large turbojet operations under Parts 91, 121 and 135. See the takeoff performance criteria listed in those Parts. Part 121.189 and Part 135.379 both state that takeoff weights may not exceed the weights (or "limits") shown in the Airplane Flight Manual for the given conditions. Part 25.1587 (Performance Information) under paragraph (b), states in part, "(e)ach Airplane Flight Manual must contain the performance information computed under the applicable provisions of this part (including §§25.115, 25.123, and 25.125 for the weights, altitudes, temperatures, wind components, and runway gradients, as applicable) within the operational limits of the airplane..." Note that Part 25.115 does specifically reference 25.113 which contains the 1.15 all-engine distance requirement. Therefore, to say that operations under Part 121 and 135 do not having any relation to Part 25 is incorrect. And again for Part 91, see 91.605.

By the way, I reviewed my posts on this thread and I did not see where I mentioned wet runways with regards to the 1.15 all-engine distance. However, Part 25 does discuss the 1.15 all-engine requirement on both wet and dry runways. The only difference is that for wet runways, the aircraft need only be at 15 feet at the end of the takeoff distance (this sometimes called the "screen height"), with the proviso that it reach V2 by 35 feet. See 25.113 (b) (2).

As for the FAA not requiring wet runway data in the AFM, that is true. Other operating authorities, however, do require this data in the AFM. When I worked in the AFM group at Boeing, the British CAA was the most famous of those entities requiring wet runway accountability. (I do believe you'll see a requirment for wet runway data in the AFM as the FAA works to harmonize with the JAR-OPS.) For the FAA, wet runway data was contained in a document called the "Performance Enginner's Manual" or PEM. In it, wet runway data was classified as "advisory" information. (This is also similar to how contaminated runway data was disseminated.) The Part 121 carrier I worked for published the wet runway data for crews and dispatchers to apply as the crew or dispatcher deemed necessary for a given departure. This is similar to how NetJets operates.

As for the accel-go and accel-stop distances outweighing the 1.15 all-engine distance, this is generally true for the twin-engine aircraft I worked on. However, at relatively extreme thrust-to-weight ratios, the 1.15 all-engine distance could be limiting for a twin-engine jet. I do recall that the 1.15 all-engine distance tended to be more of a limiting condition for tri-jets like the 727.

The bottom line to all of this is that FAR Part 25 testing and requirements drive the data (i.e. AFM) that must be adhered to in operations under Parts 121 and 135. The 1.15 all-engine distance is a consideration in all takeoff weight determinations though it is likely transparent to us as crews.

 

[TIS]

By the way, I reviewed my posts on this thread and I did not see where I mentioned wet runways with regards to the 1.15 all-engine distance.

You are absoltely correct - my apologies! It was O-Line who made mention of that and now I've gone and made a mess of two different ideas. Again, my apologies.

That said I do still have a bone to pick with you. I think you're still trying to mix apples and oranges. I do not dispute that one must pay attention to the AFM when one operates a large turbine powered aircraft certified under Part 25. I do NOT agree that compliance with the AFM implies adherence to Part 25.

As I said before, Part 25 relates to aircraft certification. it is not a flight operation section of the regulations. It must be adhered to by persons or entities who build aircraft. It is in no way intended that pilots adhere to it. Pilots need only adhere to the published results of the certification process found in the AFM. Which is why I say that the following is pretty much irrelevant from a pilot's point of view:

However, Part 25 does discuss the 1.15 all-engine requirement on both wet and dry runways. The only difference is that for wet runways, the aircraft need only be at 15 feet at the end of the takeoff distance (this sometimes called the "screen height"), with the proviso that it reach V2 by 35 feet. See 25.113 (b) (2).

Again, if it's not in the AFM it's not something a pilot can even consider. I've got loads of contaminated runway data for the Gulfstreams I fly but it's advisory only. It does not carry the force of law like the AFM does.

(I do believe you'll see a requirment for wet runway data in the AFM as the FAA works to harmonize with the JAR-OPS.)

I dunno. We'll see. The reason is that I don't think you'll ever see an aircraft manufacturer putting things in writing that they're responsible for from a liability standpoint in this country. Too many lawsuits. All it'd take is one accident on a slippery runway where the pilot relied upon manufacturer's contaminated runway data and someone's gonna be paying some big settlements.

As for the accel-go and accel-stop distances outweighing the 1.15 all-engine distance, this is generally true for the twin-engine aircraft I worked on. However, at relatively extreme thrust-to-weight ratios, the 1.15 all-engine distance could be limiting for a twin-engine jet. I do recall that the 1.15 all-engine distance tended to be more of a limiting condition for tri-jets like the 727.

Yeah but are you saying that the AFM would contain both the straight A/S vs A/G as well as the 115% figures for the pilot to decide which is the applicable limit? I'm afraid I can't agree with that assertion if that's what you mean. Again, as a certification regulation the manufacturer is obligated to make the determination of which is the longer distance - the A/S-A/G distance or the 115% number and then publish the final figure in the AFM. The pilot has nothing to do with it. The pilot uses what the AFM depicts as the limiting figure for that part of the calculation.

The bottom line to all of this is that FAR Part 25 testing and requirements drive the data (i.e. AFM) that must be adhered to in operations under Parts 121 and 135. The 1.15 all-engine distance is a consideration in all takeoff weight determinations though it is likely transparent to us as crews.

I think this shows that we're simply talking past each other. I just choose not to accept that I am regulated by Part 25 though I do agree that I am bound by the results of the certification process it engenders.

TIS

 

[gern_blanston]

SuperFreight, don't forget to put 'second segment climb' on your list of things to worry about. Doesn't have anything to do with balanced field length, but it's definitely something you have to consider on high-and-hot takeoffs.

 

[superfreight208]

Great Info!!

Thanks all that replied, very good information.


superfreight

 

[superfreight208]

Thanks everybody for the great information!!!!

Very informative, I will definitely lookup the websites provided.