If VMCG is limiting, how would you increase aircraft weight?
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What do you mean Vmcg is limiting. V1 is lower than Vmcg? and therefore the first should be increased to equals Vmcg.
This scenario might arose when ATOW is very low and DA is low as well, it is usually no problem to increase V1 cause of in these conditions runway is not limiting.
Another flap setting for take off could solve the problem in certain aircrafts.
I donīt Know if this script helps you. Maybe I didnot catch the question.
This scenario might arose when ATOW is very low and DA is low as well, it is usually no problem to increase V1 cause of in these conditions runway is not limiting.
Another flap setting for take off could solve the problem in certain aircrafts.
I donīt Know if this script helps you. Maybe I didnot catch the question.
Dancing with the devil, going with the flow... it's all a game to me.
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V1 is not lower than Vmcg! If V1 were lower than Vmcg the aircraft could yaw off into the terminal with the asymetric thrust following an engine failure...... Vef being lower than V1 and all that. Vmbe can be limiting too. Vr must be greater than 1.05mca and not less than V1. Vmcg and Vmbe are the minimum values of V1 and Vr is the max value as far as I understand. Hope this is right - sitting the exam on Monday. Eek! 
Don't think you could increase the weight.
VFE.
Don't think you could increase the weight.
VFE.
Moderator
.. I think that we need a bit more of the question before we can come up with a convincing answer ...
Possibly The Don is thinking about a situation where, on a shortish runway for the Type, the weight is down anyway due to the declared lengths and the V1 becomes limiting due to rated thrust considerations (ie Vmcg) while trying to fit the ASDR within the ASDA. In such a case it is typical to see a lower than optimum weight due to ASDA considerations while there is plenty of spare distance for TORA/TODA at the weight in question. The trick is to find a way to reduce the V1 to reduce the ASDR which may provide a few more kilos ...
Especially if the takeoff may be unbalanced then use of a derate (as opposed to a flex or reduced) thrust may provide a reduction in Vmcg and, in turn, may permit a lower V1 resulting in a better ASDR-limited weight provided that the TORA/TODA considerations are OK at the higher weight .... all a case of juggling the oranges and lemons to get the best weight on the day.
Possibly The Don is thinking about a situation where, on a shortish runway for the Type, the weight is down anyway due to the declared lengths and the V1 becomes limiting due to rated thrust considerations (ie Vmcg) while trying to fit the ASDR within the ASDA. In such a case it is typical to see a lower than optimum weight due to ASDA considerations while there is plenty of spare distance for TORA/TODA at the weight in question. The trick is to find a way to reduce the V1 to reduce the ASDR which may provide a few more kilos ...
Especially if the takeoff may be unbalanced then use of a derate (as opposed to a flex or reduced) thrust may provide a reduction in Vmcg and, in turn, may permit a lower V1 resulting in a better ASDR-limited weight provided that the TORA/TODA considerations are OK at the higher weight .... all a case of juggling the oranges and lemons to get the best weight on the day.
Last edited by john_tullamarine; 3rd Aug 2003 at 08:51.
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To john_tullamarine
Hi!
This is a tricky one.
Your comment is quite fair, but I think d way d quest is put, is too theoretical, prob from some b--s--t test, most probably from around the Pacific rim.
What do you think if we throw in contaminated rw?
Any way, the guy, who started it must provide more info.
Cheers
This is a tricky one.
Your comment is quite fair, but I think d way d quest is put, is too theoretical, prob from some b--s--t test, most probably from around the Pacific rim.
What do you think if we throw in contaminated rw?
Any way, the guy, who started it must provide more info.
Cheers
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Just reading through Performance A and i was curious of how it is possible to answer the question with this statement:
''By reducing take-of thrust. This would allow a lower value of VMCG and so the V1 could be reduced, resulting in a greater take-off weight for the same take off distance.''
My confusion is this'' a greater T/o weight for the same distance''.
As i understand, a higher weight would result in an increased T/O distance required, because of a slower acceleration.
Appreciate your efforts guys, many thanks.
The Don!
''By reducing take-of thrust. This would allow a lower value of VMCG and so the V1 could be reduced, resulting in a greater take-off weight for the same take off distance.''
My confusion is this'' a greater T/o weight for the same distance''.
As i understand, a higher weight would result in an increased T/O distance required, because of a slower acceleration.
Appreciate your efforts guys, many thanks.
The Don!
Different factors require different amounts of runway length. Or another way, different factors allow different weights for a given runway length. One of those factors will be more limiting than the others. This critical factor will be the one that determines the length needed (or weight available for the length). That means that there is some 'spare' weight capacity available & waiting to be used in all the other factors provided the critical factor can be adjusted so that it is not so limiting.
If Vmcg is the most critical - limiting - factor (by setting the min V1/Vr ) then reducing it reduces the amount of runway required. That lets one add additional weight until it or some other factor becomes limiting again.
In the example above the thrust to get airborne doesn't seem to be the problem. Instead Vmcg causes a limit so altering that factor gives a bit more margin to use for more weight.
That's my understanding...
If Vmcg is the most critical - limiting - factor (by setting the min V1/Vr ) then reducing it reduces the amount of runway required. That lets one add additional weight until it or some other factor becomes limiting again.
In the example above the thrust to get airborne doesn't seem to be the problem. Instead Vmcg causes a limit so altering that factor gives a bit more margin to use for more weight.
That's my understanding...
Moderator
Sir Donald ..
My previous post serendipitously was the answer to your question. ... the reference in your subsequent post to reducing thrust MUST be read to mean using a derate(d) thrust (equivalent to a lower rated thrust engine) and NOT reduced or flexible thrust (where the rated thrust, which determines Vmcg, remains the same regardless of the flex values) ... one can, of course, use both simultaneously .. but only the derate situation is relevant to your question.
I'm guessing from your profile that you don't have a heavy aircraft background yet so, with a presumed small aeroplane outlook, your confusion is quite understandable.
For the typical bugsmasher operation, one is only concerned with TODR and TODA .... for that sort of operation mindset, your observation is quite valid, especially if we are looking at the combination of a reduced V1 (which is not relevant to small aircraft) and reduced rated thrust. The secret to resolving the confusion lies in the fact that heavies require consideration of a bunch of different calculations for determining the (most limiting) RTOW..... ie not just a case of looking at the TOD consideration ..
Going on from Tinny's previous post, the main concerns in the low weight/short runway regime are
ASDA and ASDR
TODA and TODR
TORA and TORR
Making up some numbers to illustrate his point .. and ignoring things like contamination and lineup (which just modify the numbers) ... unfortunately, I don't have any suitable AFMs with this sort of problem fix to hand so make-believe numbers will have to do .... doesn't affect the philosophy at all ...
ASDA = 2000 m
TODA = 2300 m
TORA = 1900 m
For the higher rated thrust case ...
RTOW = 80,000 kg
V1min = 100 kt
ASDR = 2000 m
TODR = 1600 m
TORR = 1200 m
Clearly, V1min is causing a problem by keeping the V1 higher than we need for the runway distances in question
For a lower rated thrust case ....
RTOW = 85,000 kg
V1min = 95 kt
ASDR = 2000 m
TODR = 2200 m
TORR = 1700 m
By reducing the rated thrust, which reduces Vmcg and V1min (or minV1 .. whichever you prefer) we can push up the ASDA-limited RTOW. Provided that the increased TODR/TORR don't cause either of them to become more limiting than the ASDA case, then the end result is a modest increase in RTOW. All a matter of juggling the oranges and lemons ...
If Mutt sees this thread he can provide all sorts of real world examples as his big birds routinely have this problem ....
My previous post serendipitously was the answer to your question. ... the reference in your subsequent post to reducing thrust MUST be read to mean using a derate(d) thrust (equivalent to a lower rated thrust engine) and NOT reduced or flexible thrust (where the rated thrust, which determines Vmcg, remains the same regardless of the flex values) ... one can, of course, use both simultaneously .. but only the derate situation is relevant to your question.
I'm guessing from your profile that you don't have a heavy aircraft background yet so, with a presumed small aeroplane outlook, your confusion is quite understandable.
For the typical bugsmasher operation, one is only concerned with TODR and TODA .... for that sort of operation mindset, your observation is quite valid, especially if we are looking at the combination of a reduced V1 (which is not relevant to small aircraft) and reduced rated thrust. The secret to resolving the confusion lies in the fact that heavies require consideration of a bunch of different calculations for determining the (most limiting) RTOW..... ie not just a case of looking at the TOD consideration ..
Going on from Tinny's previous post, the main concerns in the low weight/short runway regime are
ASDA and ASDR
TODA and TODR
TORA and TORR
Making up some numbers to illustrate his point .. and ignoring things like contamination and lineup (which just modify the numbers) ... unfortunately, I don't have any suitable AFMs with this sort of problem fix to hand so make-believe numbers will have to do .... doesn't affect the philosophy at all ...
ASDA = 2000 m
TODA = 2300 m
TORA = 1900 m
For the higher rated thrust case ...
RTOW = 80,000 kg
V1min = 100 kt
ASDR = 2000 m
TODR = 1600 m
TORR = 1200 m
Clearly, V1min is causing a problem by keeping the V1 higher than we need for the runway distances in question
For a lower rated thrust case ....
RTOW = 85,000 kg
V1min = 95 kt
ASDR = 2000 m
TODR = 2200 m
TORR = 1700 m
By reducing the rated thrust, which reduces Vmcg and V1min (or minV1 .. whichever you prefer) we can push up the ASDA-limited RTOW. Provided that the increased TODR/TORR don't cause either of them to become more limiting than the ASDA case, then the end result is a modest increase in RTOW. All a matter of juggling the oranges and lemons ...
If Mutt sees this thread he can provide all sorts of real world examples as his big birds routinely have this problem ....
Last edited by john_tullamarine; 4th Aug 2003 at 08:54.
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BA used to use this technique on the TriStar. Early models had RB211-22B engines, later ones had the more powerful RB211-524B4s. The more powerful engines created a VMCG limit on short runways, effectively the aircraft had to accelerate to a higher V1 than it would otherwise use to get over VMCG.
By reducing the EPR to match the 22B full thrust condition they were able to use the lower VMCG figures that matched the less powerful engines and get off in a shorter distance for the weight. The only extra consideration was that you had to swear a great oath in the take-off brief that, if an engine failed after V1, you would not increase the thrust and invalidate the calculation.
By reducing the EPR to match the 22B full thrust condition they were able to use the lower VMCG figures that matched the less powerful engines and get off in a shorter distance for the weight. The only extra consideration was that you had to swear a great oath in the take-off brief that, if an engine failed after V1, you would not increase the thrust and invalidate the calculation.
