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Old 24th Oct 2009, 23:37
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john_tullamarine
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Good to see some spirited discussion ramping up ...

Subject to the general caveat that these sorts of discussions are difficult to generalise as any general argument necessarily involves specifying boundary conditions ...

If the figures are not AT the limiting TOD case then it is possible to reduce V1 until the TOD limit is reached.

yes - subject to not going below the Vmcg limited V1min and watching ALL of TODR, TODA and TORR, TORA. Whichever of TOR and TOD becomes limiting first will be the runway distance consideration of note.

This will make the ASD limited weight equal to the TOD limited weight will it not?

it may, on occasion, if circumstances so conspire .. but generally not as the two are independently calculated

One Maximum TOW limited by the intersection of the go and stop lines. With the appropriate and singular V1.

if we constrain the boundary conditions for the calculations to suit that outcome

So as I said earlier clearway will result in a reduced V1 and increased MTOW.

it may do and, in general, will.

How can an increase in TOD lead to an increase V1 unless the V1 was not optimised before hand? If it was then an improvement in the accel go case will reduce V1 rather than increase it.

I think you are extrapolating the balanced case to support your argument. In general, we need to look at the parameters in isolation and take whichever happens to end up being the limiting case. In the general case only one of the limitations will be limiting so, if you are able to play around with that limit (in this case by postulating some extra TODA) then there is no necessary reason why another of the limits should constrain that activity. The balanced case is a specific special case which was of considerable use in the olden days .. but not to any real extent these days.

one graph with v1 along the x axis and TOW on the y with two lines one plotting accel go and the other accel stop for a fixed TOD and ASD is all you need.

the value of this sort of picture lies in looking at what happens when you find some extra distance.

I would say that if a predetermined V1 is used then this is true but if V1 is optimised then ASDR and TODR can be equalised.

Providing that I remain within the range of V1s and distances wherein I can influence the outcome, generally I should be able to find a V1 where I can balance the takeoff from the aircraft's viewpoint (which is what you are looking at). However, in general, the only advantage of doing this is that we can generate some easier to use generalised charts for manual calculation and the calculation can be done a little more quickly. To get the best RTOW, and for computer-generated numbers in general, usually there is no advantage in sticking with a balanced field scenario.

So ASD in isolation is not limiting.

..until the V1 and weight selected cause it to become so

If I plot the go line and the stop line where they cross must be the OEI runway limited weight.

if we are talking about limit cases with no involvement of hypothetical distance changes to the basic runway geometry

.. the only way adding Clearway to an already runway limited TO can increase the MTOW is if it comes with a reduction in V1.

... depends .. if the takeoff were previously

(a) limiting and balanced and the TOR were not limiting, then yes. Reducing V1 allows an increase in ASD-limited TOW while the additional TODA may allow a useful increase in TOW if you juggle the numbers suitably. One of ASD, TOR or TOD will become limiting as you play with the numbers.

(b) ASD-limited, then yes. Argument is similar to (a).

(c) TOD-limited but not TOR/ASD-limited, then, generally, no. Usually a small increase in V1 will give a better result than a small reduction.

(d) TOR-limited, then the clearway addition is, as you observed elsewhere, irrelevant.

I still think that your thinking is a tad contaminated by a defacto presumption of a balanced situation.

So what effect will stopway have on V1 and MTOW?

Perhaps a separate thread so we don't confuse the neophytes totally ?
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