Wet Runway and balanced field....
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Wet Runway and balanced field....
Hello!
I was hoping someone could help with this...
What effect does a wet runway have on a balanced field? Does reducing V1 "unbalance" the field?
I am under the impression that just because you reduce v1 for a wet runway, your ASDA should still be the same as your TODR, although you will achieve v2 at 15' instead of 35'.
Some other similar questions are...
Does the TODR change when the runway is wet?
Does the ASDR change when the runway is wet?
Many thanks in advance for all who can help me get my head around this stuff!
Cheers,
sunbird
I was hoping someone could help with this...
What effect does a wet runway have on a balanced field? Does reducing V1 "unbalance" the field?
I am under the impression that just because you reduce v1 for a wet runway, your ASDA should still be the same as your TODR, although you will achieve v2 at 15' instead of 35'.
Some other similar questions are...
Does the TODR change when the runway is wet?
Does the ASDR change when the runway is wet?
Many thanks in advance for all who can help me get my head around this stuff!
Cheers,
sunbird
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Well, there are a few caveats.
For one, not all performance data is predicated upon ASDR=TODR i.e. a balanced field. It may not be possible for some types, for one thing (if you are limited by the single engined "go" case, then reducing V1 obviously won't help, and if V1=VR it can't go higher either ...)
Secondly, not everyone wants a balanced field calc - if you want to exploit a difference between ADSA and TODA, say, then unbalancing may be the way to go.
But, to answer the questions:
Does the TODR change when the runway is wet?
yes, though not much - there's a potential for some additional drag on a wet (though more on contam) so you may need SLIGHTLY more runway for the accel phases. It's pretty small, of course.
Does the ASDR change when the runway is wet?
Hell, YES. The "S" part of ASDR is a damned sight harder on a wet runway. It's the increase in ASDR (especially relative to TODR) which makes reducing V1 a good idea most of the time; reduced V1 reduces ASDR and increases TODR, bringing both back into relative equilibrium.
For one, not all performance data is predicated upon ASDR=TODR i.e. a balanced field. It may not be possible for some types, for one thing (if you are limited by the single engined "go" case, then reducing V1 obviously won't help, and if V1=VR it can't go higher either ...)
Secondly, not everyone wants a balanced field calc - if you want to exploit a difference between ADSA and TODA, say, then unbalancing may be the way to go.
But, to answer the questions:
Does the TODR change when the runway is wet?
yes, though not much - there's a potential for some additional drag on a wet (though more on contam) so you may need SLIGHTLY more runway for the accel phases. It's pretty small, of course.
Does the ASDR change when the runway is wet?
Hell, YES. The "S" part of ASDR is a damned sight harder on a wet runway. It's the increase in ASDR (especially relative to TODR) which makes reducing V1 a good idea most of the time; reduced V1 reduces ASDR and increases TODR, bringing both back into relative equilibrium.
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V1 is reduced by 10kts to cater for stopping,and off the top of my head its TODR X 1.15 for a wet runway. ASDR is explained here http://www.caa.co.uk/docs/33/CAP698.pdf 
Last edited by pipertommy; 7th Nov 2007 at 15:23. Reason: adding link
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Guys,
Thanks for the info.
One last, but very important question....
What effect does a wet runway have on a balanced field length?
Does it unbalance the field?
Or, Is the field still balanced.
I cannot seem to get this idea straight.
I think it's mostly semantics, so if someone can explain it in a very simple way, I'd really appreciate it.
Cheers,
sunbird
Thanks for the info.
One last, but very important question....
What effect does a wet runway have on a balanced field length?
Does it unbalance the field?
Or, Is the field still balanced.
I cannot seem to get this idea straight.
I think it's mostly semantics, so if someone can explain it in a very simple way, I'd really appreciate it.
Cheers,
sunbird
Join Date: Dec 2006
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Sunbird !
In the all engine scenarios both the wet and the dry rwy cases have the same safety factor (15% for JARs) TODR,TORR. In the engine out cases: there is no safety factor and there's always some kind of reduction: TODR: screen from 35 to 15 feet (DRY to WET), TORR: distance equidistant from lift-off point to screen(DRY) to just lift-off point on the WET rwy. These all take into account the slower acceleration etc. on the wet rwy. ASDA: only gross distances, but again there is allowance for the WET rwy scenario.
Balanced field: TODA=ASDA, or any Clearway= Stopway, TODC computers ignore some of the clearway available, just to make the calculations easier(and of course your clearway cannot exceed 50% of the rwy length that it is associated with). The TODC might unbalance the field for a wet rwy, but if you play around with it a little bit you will see, that 90% of the time it just plain reduces your V1 by 10 knots (737 JARs,allows for the degraded braking performance during an RTO on a WET rwy), and leaves all other aspects the same (reduced thrust, VR, V2...). If you have contamination (wet snow , slush...)that is a different story...
Hope this helps
In the all engine scenarios both the wet and the dry rwy cases have the same safety factor (15% for JARs) TODR,TORR. In the engine out cases: there is no safety factor and there's always some kind of reduction: TODR: screen from 35 to 15 feet (DRY to WET), TORR: distance equidistant from lift-off point to screen(DRY) to just lift-off point on the WET rwy. These all take into account the slower acceleration etc. on the wet rwy. ASDA: only gross distances, but again there is allowance for the WET rwy scenario.
Balanced field: TODA=ASDA, or any Clearway= Stopway, TODC computers ignore some of the clearway available, just to make the calculations easier(and of course your clearway cannot exceed 50% of the rwy length that it is associated with). The TODC might unbalance the field for a wet rwy, but if you play around with it a little bit you will see, that 90% of the time it just plain reduces your V1 by 10 knots (737 JARs,allows for the degraded braking performance during an RTO on a WET rwy), and leaves all other aspects the same (reduced thrust, VR, V2...). If you have contamination (wet snow , slush...)that is a different story...
Hope this helps
