Balanced Field Length & V1

[uwochris]

Hey guys,

It's my understanding that BFL is the point where your accelerate-go and accelerate-stop distance are equal (i.e. the distance required to experience an engine failure at Vef and come to a complete stop= the distance required to experience an engine failure at Vef, continue the takeoff, and be 35 ft above the departure end of the runway). First of all, is this correct? I'm also under the asumption that it is very good to have runway length in excess of your BFL, as this increases your safety-margin; what happens, however, when you must takeoff from an airport where the TODA is less than your BFL?

Secondly, how exactly is BFL related to V1? The definitions seem similar, except that V1 is defined in terms of airspeed, while BFL is the distance.

Finally, can anyone give examples of how you calculate BFL and what factors affect it?

Thanks in advance,

Chris.

 

[guido411]

Replace your term Vref with V1 and you may have answered your question. Vref is an approach speed and not relevant, V1 is the speed you should have been using. BFL is the length where aborting or going AT V1 are the same distance. V1 is the decision speed. BFL garuntees if you abort prior to V1 you have enough room to stop (usually limited by brake energy) or after V1 continue continue the TO to be 35' above threshold when you cross it.

Calculating BFL (sometimes called TO Field Length) is usually done by looking at tabulated numbers (tab data) that are in the airplane. It's simple. In our plane you go to the section that has your flap setting and altitude for the day. Then you go down the first column to your weight go across the chart to your temp and the book gives you the numbers (V1, Vr, V2, Vref, TOFL). It looks sort of like, and works like, the multiplication tables you studied in grade school

Then there are graphs to correct these numbers for non standard factors like runway slope, anti-ice on, wind, etc because the tab data is calculated on dry, no slope, anti-ice off, no wind, etc.

Also important to remember that the tab data only garuntees meeting certification climb gradient 1.6% which is only good for VFR. If the wether is IFR you may have to lower weight or temps even more to meet the 3.3% required by Terps. Or even MORE for mountainous terrain.

If you find your BFL or TOFL is more than the available runway you can not legally take off. Part 25 certified aircraft HAVE to be able to make it under any circumstance. So you either change flap settings, lower weight, wait for cooler temps, or wait for lower pressure altiude.

Yes, a safety margin is always nice so longer runways are great. But during certification the FAA takes into account that the airplane is not new and you're not a test pilot. So they actually buffer it for you. If your BFL is shown to be 4000' the test pilots may have actually proved it to be 3750'.

Hope that helps.

 

[gern_blanston]

Here's a good overview of part 25 takeoff performance.
http://www.airplanedriver.net/study/part25.htm
V1 is the speed at which you can abort and stop before you run off the end of the runway, or continue the departure on one engine and make the 35-foot screen height at the end.
If you don't have balanced field, and you're in a part 25 airplane, you wait for it to cool off, leave some folks (or golf clubs) behind, or offload fuel.
Every part 25 airplane will have perfomance charts for BFL and second segment climb, and yes, we do the calcs for every takeoff.

 

[Yank McCobb]

Replace your term Vref with V1 and you may have answered your question.

Hope that helps.

The term he used was "Vef" (speed @ engine failure) and not "Vref" (approach speed). He is using the term correctly. The definition he quoted is exactly word for word from the official FAA definition. The same definition apprears word for word in our company manuals.

 

[guido411]

The term he used was "Vef" (speed @ engine failure) and not "Vref" (approach speed). He is using the term correctly. The definition he quoted is exactly word for word from the official FAA definition. The same definition apprears word for word in our company manuals.

Whoops,

Thanks Yank. Little dylexic this morning I guess.

 

[Yank McCobb]

Hey, not a problem. The first time I saw that in the book I had to do a double-take, as well.

Then again, when you get to be my age, schmalzheimer's disease is a daily battle.

 

[typhoonpilot]

It's my understanding that BFL is the point where your accelerate-go and accelerate-stop distance are equal (i.e. the distance required to experience an engine failure at Vef and come to a complete stop= the distance required to experience an engine failure at Vef, continue the takeoff, and be 35 ft above the departure end of the runway). First of all, is this correct?

Pretty much the standard definition, yes. Going a little bit further though and to help answer some of the other questions you have posed. The present day definition acknowledges that if there is an area beyond the runway over which an aircraft could safely fly after becoming airborne, ( the clearway ), then it would become unnecessarily restrictive to stipulate that the airborne distance to the screen height should be accomlished within the runway length. Also, if there should be a suitable area beyond the runway, that the aircraft could use for stopping in the event of an aborted take off ( a stopway ) then it again becomes restrictive to stipulate that the aircraft should be able to stop within the actual runway length.

A Balanced Field Length, then, is the distance within which an aircraft can;

a. Accelerate to V1 and then execute a stop within the Accelerate Stop Distance Available ( ASDA ), where ASDA = TORA + Stopway and;
b. With an engine failure at V1 continue the take off and cross the screen within the Take Off Distance Available ( TODA ), where TODA = TORA+Clearway.

In other words, Balanced Field Length exists whenever ASDA = TODA

When ASDA is greater than TODA ( i.e. with a Stopway only ); or when ASDA is less than TODA ( i.e. with a clearway only) then you have an Unbalanced condition that is still legal.

That brings us to:

Secondly, how exactly is BFL related to V1? The definitions seem similar, except that V1 is defined in terms of airspeed, while BFL is the distance.

Take a situation where you have a clearway. With the clearway you have more distance in which to reach your 35 foot screen height. It would make sense then that you could take off with a heavier weight. But, wait a second, if we are heavier it would take longer to stop on the available pavement, right ?. In this case, you could reduce the V1 by using a higher flap setting. The higher flap setting allows you to reduce the V1 and get airborne a little earlier, but it also increases your distance to reach 35 feet since the climb rate is slower.

Does that help ?


TP


p.s. some of this was plagiarized from a book on performance.

 

[uwochris]

Thanks for all the responses guys!

TP- just one comment here: I the definition I have seen for TODA is TORA+Clearway (i.e. no stopway). I'm not sure if this makes much of a difference or not, but I have never seen TODA defined to include the stopway.

 

[gern_blanston]

p.s. some of this was plagiarized from a book on performance.

Hey, Typhoon, what's the name of the book? I'm looking for something like that.

 

[typhoonpilot]

TP- just one comment here: I the definition I have seen for TODA is TORA+Clearway (i.e. no stopway). I'm not sure if this makes much of a difference or not, but I have never seen TODA defined to include the stopway.

Oops, right you are. I was referencing an Australian diagram and it was written on there so I just wrote it down without thinking. Perhaps what they were thinking is that stopway may be part of the area designated as clearway.


Found this in some further research, you might find it handy:

http://www.wingfiles.com/files/GTG_perfo.pdf

Hey, Typhoon, what's the name of the book? I'm looking for something like that.

Understanding Performance by A.J. Walters


TP