Balanced field length
Balanced field length
The concept is fairly straightforward. For any aeroplane you have a take-off distance required, and a landing distance required (TODR and LDR).
Altering paramaters such as profile drag, residual thrust, weight, etc to alter these values is an inevitable practice in design. Generally (not universally) decreasing TODR tends to decrease LDR and vice-versa. What a designer tries to achieve is a balanced field length that is that TODR=LDR for the same aeroplane.
In effect, this means that the field length for all operations should have been optimised to give as short a distance as possible.
Having said that, this is really an Engineer's interpretation, and it may be that the term is used differently in ops.
G
Altering paramaters such as profile drag, residual thrust, weight, etc to alter these values is an inevitable practice in design. Generally (not universally) decreasing TODR tends to decrease LDR and vice-versa. What a designer tries to achieve is a balanced field length that is that TODR=LDR for the same aeroplane.
In effect, this means that the field length for all operations should have been optimised to give as short a distance as possible.
Having said that, this is really an Engineer's interpretation, and it may be that the term is used differently in ops.
G
The Reverend
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The FAR takeoff field length, often called the FAR balanced field length, contains certain inherent safety features to account for engine failure situations. This takeoff field length is defined in several slightly different ways. Briefly, if an engine should fail during the takeoff roll at a critical speed, called the decision speed V1, the pilot is offered the option of two safe courses of action. He may elect to continue the takeoff on the remaining engines, in which case, the takeoff distance is defined as the distance from the point at which the takeoff run is initiated to the point where the aircraft has reached an altitude of 35 feet. In the second alternative, the pilot may elect to abort and apply full braking. The decision speed V1 is chosen in such a way that the sum of the distance required to accelerate to V1 and then decelerate to a stop is the same as the total distance for the case in which the takeoff is continued following engine failure. If an engine should fail before V1 is reached, the aircraft is usually brought to a stop on the runway; whereas, if an engine fails at a speed greater than V1 the takeoff is continued. The distances are based on smooth, hard, dry runway surfaces.
This one in plain language, hope it helps.
This one in plain language, hope it helps.
still learning....
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BFL
Genghis-
?????......haven't heard that one before. In ops, it's when the accel-stop and accel-go distances are the same.
In the big jets, at least, the takeoff distance is much longer than the landing distance for any given set of conditions.
?????......haven't heard that one before. In ops, it's when the accel-stop and accel-go distances are the same.
In the big jets, at least, the takeoff distance is much longer than the landing distance for any given set of conditions.
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The term has (at least) two different meanings depending on whether you're working to FARs or JARs. It has been a source of confusion for years.
In the FAA system HotDog's reply is correct, in the JAA system its a definition of distances available, TODA = ASDA, as SF145 says. I confess I have never heard of Ghengis' third definition but there are more things in heaven and earth....
It is not always true that ASDR=TDR. Some graphs allow you to take account of long stopways, others don't.
In the FAA system HotDog's reply is correct, in the JAA system its a definition of distances available, TODA = ASDA, as SF145 says. I confess I have never heard of Ghengis' third definition but there are more things in heaven and earth....
It is not always true that ASDR=TDR. Some graphs allow you to take account of long stopways, others don't.
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Multi the engine failure is not at V1..It ocures ,for cert',at VEF(selected by the builder).whereas the engine at a point before V1 to enable the pilot to Recignize/react the stopping process.
It'd genarally about 1 second prior to V1,although in the old 3 engine aircraft as high as 3 seconds were allowed(no yaw centre eng' etc)
cheers
the engine'failed' at VEF
It'd genarally about 1 second prior to V1,although in the old 3 engine aircraft as high as 3 seconds were allowed(no yaw centre eng' etc)
cheers
the engine'failed' at VEF
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This issue has been discussed before and John Tullamarine gave a very good explanatiion...he seems to know what he's talking about. I saved it and here it is again (with your permission John).
(My emphasis)
Maxalt
BFL is a simplified approach to the sums, mainly of use for typically level-ish runways without any clearway declared or where a quick, manually-derived, answer is needed and we are prepared to forgo any benefit to be derived by considering the small margin of TODA over TORA.
Consider that many flight manuals provide a BFL subset of charts which is significantly easier and quicker to use than the detailed charts.
In this case the declared TODA should include the stopway (typically 60m or so) or remaining strip length and the declared TORA usually is not going to be limiting (ignoring the strange climb antics of helicopters).
There will be little advantage in playing with all the charts as the end result will be much the same weight.
There is a very important advantage for an operator with lots of runways to look after in that simplified rules can be generated for flight crew use (correction data and the like) ... in my view, this is a very valid consideration and the main reason that many people emphasise the BFL approach.
But it is important that pilots realise BFL is just a constraint on the more general takeoff weight analysis and, if you need the last kilo RTOW on the day, then the generalised analysis is the better procedural way to go. This can become quite an involved, pain in the neck, process when done manually. However, if it be simulated on the trusty PC, then it becomes a doddle ... working up the simulation is quite straightforward but takes a lot of time. When the work has been done, the production of optimised RTOW tables is quick and straightforward ...
Consider that many flight manuals provide a BFL subset of charts which is significantly easier and quicker to use than the detailed charts.
In this case the declared TODA should include the stopway (typically 60m or so) or remaining strip length and the declared TORA usually is not going to be limiting (ignoring the strange climb antics of helicopters).
There will be little advantage in playing with all the charts as the end result will be much the same weight.
There is a very important advantage for an operator with lots of runways to look after in that simplified rules can be generated for flight crew use (correction data and the like) ... in my view, this is a very valid consideration and the main reason that many people emphasise the BFL approach.
But it is important that pilots realise BFL is just a constraint on the more general takeoff weight analysis and, if you need the last kilo RTOW on the day, then the generalised analysis is the better procedural way to go. This can become quite an involved, pain in the neck, process when done manually. However, if it be simulated on the trusty PC, then it becomes a doddle ... working up the simulation is quite straightforward but takes a lot of time. When the work has been done, the production of optimised RTOW tables is quick and straightforward ...
Maxalt
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All of the above, but:
TODR = ASDR , Without SLOPE, without WIND
From V1 the distances to 35 ft and to the end of the paved RUNWAY surface are equal.
If any slope, wind, stopway, clearway are present then the Equivalent BFL must be found (so-called D-value).
BFL started life in the States, where generally NO stopways/clearways are declared.
TODR = ASDR , Without SLOPE, without WIND
From V1 the distances to 35 ft and to the end of the paved RUNWAY surface are equal.
If any slope, wind, stopway, clearway are present then the Equivalent BFL must be found (so-called D-value).
BFL started life in the States, where generally NO stopways/clearways are declared.
