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fieldlanding
28th Apr 2002, 00:08
Could someone explain why at a given airfield as A/C weight increases V1 increases.

As I understood it with a heavier aircraft it takes longer to accelerate (uses more runway) and then to stop (uses more runway) therefore a lower V1. A light aircraft accelerates quickly (uses less runway) and brakes quicker (uses less runway) Thus a light aircraft could reach a higher speed and still stop on the runway than a heavy one.

Sorry for being thick!!!

mutt
28th Apr 2002, 00:37
It really depends on what type of V1 you are talking about, your concept will work for optimized V-speeds, but in this FMS driven world of ours where we use balanced field V-speeds you can expect V1 to increase with weight.

Mutt.

mustafagander
28th Apr 2002, 04:10
The simplest field analysis for the performance engineer is the balanced field case where acc/stop = acc/go. Ignoring a few complicating factors, there is only one set of conditions under which balanced field = actual field. All other flyable scenarios are conservative. So providing data for balanced fields is (for the pilot user) simple and conservative. Also the flying techniques are unchanging.
Hence, it is often possible, at light weight/cool day/good headwind combinations, to actually be able to find a theoretical V1 at or even above Vr.

fieldlanding
28th Apr 2002, 08:44
By always using a balanced field for pref. factors I assume then that in all but the worst case ie an aircraft at MTOW , the V1 speed is artificaly reduced for the ease of calculation. (Temp assumed constant for the same day just different TOW)

Does this not present a hazard then in that in all cases except for the most limiting (an aircraft at MTOW ) a failure at V1 (or even slightly above the lighter the aircraft) would still be stoppable on the ground.

Where does this then leave Wet V1 reductions?

If according to the perf detail an a/c at ISA at airfield "X" and TOW 60T has a V1 dry of 159 and a Wet V1 reduction of -5 giving a Wet V1 of 154. If the aircraft was at TOW of 40T the V1 dry is 126 the wet reduction is -12 giving a Wet V1 of 114.

Surely if the heavy aircraft can stop on a wet runway up to 154 why do we reduce the light V1 even further to 114 when its orginal dry figures were less than the heavy wet figures?

SuperRanger
28th Apr 2002, 11:05
fieldlanding,

i believe most of us tend to think of each TOW equates a single set of V speeds as most of us 'calculates' our TO perf from a Specific Takeoff Chart (at least we do i.e. a tabulated chart with perf figures corresponding to each TOW at a certain ambient conditions) which is to save us time. I doubt many of us would calculate a takeoff perf using Generalised Method which would probably result in not many flights departing on time :)

if we really look at it carefully, for a given set of ambient conditions, there is a MTOW. This MTOW will correspond to a set of MAX V speeds (i call it MAX for lack of a better term). many a times, we are taking off much less than the MTOW, therefore we can actually use ANY speeds right up to the MAX V speeds.

Let not consider WET conditions yet. In you example, assuming 60T and 159kts is the MTOW for this particular runway under this conditions. therefore, if you are taking off at 40T, you can use a min. V1 of 126 right up to a max V1 of 159kts. just like you say, there is no reason why the acft cannot stop at 159kts at 40T when it can stop at 159kts at 60T, right?

Now, if we move on to WET speeds calculations. for FAA certified acft i.e. Boeing, there is actually NO requirement for WET V speeds calculations. it is only in recent times where Boeing included "Advisory Informations" which gives Slippery Runway performance figures. But for JAA certified acft i.e. Airbus, they do infact have Wet V speeds.

In this discussion I will stick to Boeing (as I am not Airbus qualified). To calculate Slippery Runway degradation, the correct method is to enter the table at the MTOW and MAX V1 and apply the corrections. If your ACTUAL V Speed is LESS than MAX WET V1, then there is NO requirement to apply corrections to your ACTUAL V1.

Again from your example, since the MAX WET V1 is 154kts and your ACTUAL V1 is only 126kts, you should not apply the -12kts to the V1. The -12kts correction only applies IF your MAX V1 is 126kts (which happens if you are taking off from a very short runway).

I hope my explanation makes this very ambiguious topic clearer for you and not the other way around :)

Happy Landings...

SR

mustafagander
28th Apr 2002, 11:12
fieldlanding,
K.I.S.S. is the word.
Fast accurate and conservative results are what pilots and regulators want.
Think about it - a low V1 on a field that's "too long" means bags of room to stop or fly on if the worst happens near V1.
Conservative and allows for a bit of mishandling, although none of us would need it!!!
Being committed to fly is usually not too bad a thing. The performance engineers have lots of margins built into the calculations. I'm a true believer after spending some time as a technical assistant to our Chief Performance Engineer. A great education in real world performance optimising when we did all the sums for long haul ops in B707 ex MEX and ACA.

PETIGRAM
28th Apr 2002, 23:02
SuperRanger:

"for FAA certified acft i.e. Boeing, there is actually NO requirement for WET V speeds calculations."

It's my understanding that the NG Boeings (at least the B737-700) do have and require the use of published WET V Speed data. I could be mistaken here - but I distinctly heard that these aircraft are required to use Wet Rwy info - perhaps someone with more positive info can clarify it.

Petigram

fieldlanding
29th Apr 2002, 09:13
Thanks for all your points they are of great help esp. SR.

MustafaG. what you are saying about a "long runway" and a low V1 on a light aircraft is exactly my point. V1 is there to in effect take any decission away from the pilot and commit you to getting in the air. There are cases in the RTOW where V1 is only just over VMCG and the airfield is huge. It seems daft to me that you may well be commited and yet have ample room to abort safely.

Dont get me wrong I do it by the book V1 I go, no doubt about it just wondering about the in's and outs of it.

Thanks Guys& Gals

john_tullamarine
29th Apr 2002, 11:52
Then ... why not push for your ops eng section to schedule ASD-limited data for each runway for crew information ... then you have an acceptable V1 range for pre takeoff risk assessment processes ....