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Fr. Dougal
23rd Oct 2009, 19:31
Does anyone know will an change in clearway change V1? I'm thinking an increase will give you a greater distance to get to 35ft and so increase V1.

hetfield
23rd Oct 2009, 19:43
Not necessarily, only if you are go limited.

pipertommy
23rd Oct 2009, 19:54
V1 is a relation to stopping distance versus a/c weight-temp-PA, a clearway could be a grass field at the end of the runway . I am a bit rusty with this interesting one:}

STBYRUD
23rd Oct 2009, 19:58
Youre right though, its something you can safely overfly / hover over but not brake to a safe stop on. But yeah, clearway doesn't necessarily increase V1, as hetfield correctly pointed out.

FE Hoppy
23rd Oct 2009, 21:14
if you were OEI runway limited then including clearway could increase MTOW and reduce V1.

Clearway is for the go not the stop. Having clearway allows you to continue from a lower speed hence lower V1.

Stop way allows you to stop from a higher speed and could therefore increase V1.


NOTE: This is very simplified.

Sir George Cayley
23rd Oct 2009, 21:14
In a Clearway there is No Stopping.

Just had to throw that in in for UK baby boomers into nostalgia.

Back to the thread........

Old Smokey
23rd Oct 2009, 23:20
V1 is for Stopping OR Going. In a Balanced Field scenario, Stop and Go would have the same V1, the same limiting weight, and the same Stop or Go performance.

If the Clearway is to be increased without a commensurate increase in Stopway, we are obviously no longer talking about Balanced Field, and the RTOW will be the lesser of the Accelerate-Stop, or the Accelerate-Go limit.

So, to answer the question, if the Takeoff WAS Accelerate-Stop limited, there would be no change in V1 or the RTOW. If, however, the Takeoff was NOT Accelerate-Stop limited, higher RTOW would be possible with a commensurately increased V1. (It would also allow for reduced V1s at the original weight provided that Vmcg etc. did not get in the way). In re-working the increased TODA case, care must be exercised to not 'bust' the Accelerate-Stop limit for the lesser ASDA at the higher weight/s.

FE Hoppy
23rd Oct 2009, 23:56
Question for OS.

How can a take off be accel stop limited in isolation? I can always stop below v1 so by reducing v1 i can declare a higher stop limited weight.

For the go case as weight increases for a fixed TODA v1min must increase.

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

adding clearway moves the go line such that i can have my failure at a lower speed and still go. Now where the go and stop lines cross is at a lower speed (v1) and a higher weight.

V1 is for stopping or going but we could well have a V1min defined by go and a V1max defined by stop. Clearway has no effect on V1 max but will reduce V1 min so long as it isn't limited by something else. (VMCG)

Airbus explain this from the V1 perspective in their getting to grips series as follows:

For a given takeoff weight, any increase in V1 leads to a reduction in both TODN-1 and TORN-1. The reason is that the all engine acceleration phase is longer with a higher V1 speed, and, consequently, in case of an engine failure occurring at VEF, the same V2 speed can be achieved at 35 feet at a shorter distance.

But it's saying the same thing. Increase V1 leads to reduction in TOD n-1 is the same as increase TOD n-1 leads to reduced V1.

But I'm open to another interpretation.

john_tullamarine
24th Oct 2009, 01:23
Points to note -

(a) there is a bunch of limits (two of which are ASD and TOD) EACH of which is looked at in isolation to determine a maximum takeoff weight. (The skill bit lies in the juggling process to end up with the best limiting weight).

(b) some of these limits are affected by V1 selection and optimising each may well require a range of V1 values

(c) with respect to (b) a range of V1 values is not much use for a given takeoff schedule so the usual operations engineering aim is to find that ONE V1 value which, when plugged into each separate calculation, gives a set of takeoff weights such that the lowest within the set (the "limiting" case, as we are wont to say) is as high as we can achieve.

It follows, for a given RTOW table, that we might well end up varying the V1/Vr ratios across the table. Although this usually becomes overly onerous if the process is being done manually .. with an AFM model on the PC .. no probs ...

One advantage of doing a lot of manual calculations is that one gets a VERY good feel for the underlying ebb and flow of the optimisation process.

(d) keep in mind that, as soon as we introduce clearway, we MUST consider TORA and TORR so that we don't end up with grass, dirt, lights and aerial structure in the tyre treads ...

(e) some folk probably get a tad confused when the AFM charts "appear" to be ignoring or somehow merging different limits. This is just an artefact of the graphical presentation techniques .. at the end of the day, EACH relevant limit is being considered and built into the overall calculation.

Simple questions such as "what does such and such varying do to V1 ?" necessarily can only be answered simply by stating (or inferring) various limitations on other boundary conditions. Generally I find such questions more confusing than illuminating for the educational process unless those other boundary assumptions are clearly stated. As they say, the devil is in the detail and I can present two apparently different stories for many questions if I am selective in the boundary conditions applying.

Having played with a number of aircraft for which clearway becomes very critical in optimising TOW the following may be useful to consider (but keep in mind that I can make a slight change in the assumptions and the story necessarily then may change to a greater or lesser extent)

(a) from whatever starting point where we have a given weight from the sums ..

(b) if the TO is ASD-limited, then there is no option to increase V1 without reducing weight .. which is not the aim of the game. (It is presumed that the TO has been optimised to the extent that the figures are near limiting for the TOD case - if not, why not ?).

However, if we get a bit more TODA by introducing or extending clearway, then we can look at reducing V1 a little which MAY allow us to juggle a resulting small increase in ASD-limited TOW provided that one of TORR and TODR doesn't then immediately become limiting.

(c) if the TO is TOD limiting, then the increase in TOD should give an increase in TOW due to the extra TOD or MAY allow us to increase V1 a little and get a bit more TOW providing that neither of ASD/TOR immediately becomes limiting.

(d) if the TO is TOR limiting, then we can only look at increasing V1 to improve TORR, providing that ASDR doesn't immediately become limiting.

However, one would expect that this option had been exploited fully first time around so, in reality, TOR-limited cases ought not to respond to an increase in clearway.

(e) if nothing were limiting first time around, then we could have increased/decreased V1 as appropriate until one case became limiting in such a way that we ended up with an optimised (ie maximised) TOW. If then we can get some more clearway, we could revert to (b) or (c) and maybe tweak a bit more weight out of the calculations.

For any runway analysis it is very much a case of juggling things a bit this way or that to end up with something becoming limiting but in the way which gives us the maximum RTOW for the chart. Sometimes one can spend quite a lot of time on this iteration process .. hence the reason we all loved the advent of microprocessors to do the slave labour numerically intensive bits of the exercise.

If the above is confusing, either OS or I can run up some pictures to help clarify the words.


How can a take off be accel stop limited in isolation?

Quite easily.

Find a runway with a given set of ASDA/TODA/TORA .. when you do the sums for a given set of ambient conditions, you will find that each results in a particular maximum TOW. Once ALL the various sums have been done, the lowest resulting TOW becomes the limiting TOW for that particular set of ambient conditions.

This set of weights will have a predictable relationship depending on the particular Type and that predictable relationship may well be very different for another Type.

If the ASDR ends up being the limiting factor (ie results in the lowest TOW) then the TO becomes ASD limited.

I can always stop below v1 so by reducing v1 i can declare a higher stop limited weight.

Very true .. but the down side is that the TODR/TORR increases and that might prove to be less than useful.

For the go case as weight increases for a fixed TODA v1min must increase.

True - due to increasing Vs unless I am missing your point ? Alternatively, once TODR (or, if you prefer, in this case, TODA) becomes limiting, we can only increase that limiting weight by increasing V1 with the caveat that ASDR may soon become the limiting problem.

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

If we are only considering ASD/TOD.

adding clearway moves the go line such that i can have my failure at a lower speed and still go. Now where the go and stop lines cross is at a lower speed (v1) and a higher weight.

However, as soon as you start going down this path we also need to be looking at ASDR, TORR as well as TODR - one will become the limiting case. Much easier to present this discussion with some graphs to illustrate the considerations.

Clearway has no effect on V1 max but will reduce V1 min so long as it isn't limited by something else. (VMCG)

But note that increasing clearway may permit an increasing V1 until either TODR or TORR becomes limiting. It can be rather confusing to folk if we ignore the TOR consideration ..

For a given takeoff weight, any increase in V1 leads to a reduction in both TODN-1 and TORN-1. The reason is that the all engine acceleration phase is longer with a higher V1 speed, and, consequently, in case of an engine failure occurring at VEF, the same V2 speed can be achieved at 35 feet at a shorter distance.

I think I would be emphasising things a little bit differently. Yes, the AEO distances up to the failure point will increase somewhat. However, the main consideration is that the OEI distances between a lower V1 and the nominated Vr will be SIGNIFICANTLY greater than between a higher V1 and the same nominated Vr. (Note, also, that Airbus are including both TOD and TOR in their discussion).

But it's saying the same thing. Increase V1 leads to reduction in TOD n-1 is the same as increase TOD n-1 leads to reduced V1.

.. but not a useful observation when considering what a varying clearway might be doing to the calculations .. except to the extent that the comment has relevance to juggling V1 in the presence of clearway to get a better ASDR/ASDA margin at the same weight. I think that you might be missing the underlying point that the aim is to increase TOW in general.

The driving point in the discussion needs to be that, in the presence of whatever I have in the way of runway etc. lengths, the aim is to juggle V1 to maximise TOW.

OverRun
24th Oct 2009, 02:19
(d) keep in mind that, as soon as we introduce clearway, we MUST consider TORA and TORR so that we don't end up with grass, dirt, lights and aerial structure in the tyre treads ...

Fish, JT, you forgot the fish in the tyre treads. For when the clearway is over the ocean (like IoM Ronaldsway).

hiphippyhippo
24th Oct 2009, 03:45
To simplify things, there are actually a couple of V1 options available - Balanced and Optimized V1.

Balanced V1 is when accelerate stop equals accelerate go. This is based on the assumption that the clearway and stopway are of equal usuable distances. In reality this may not always be the case. However, this is the speed that we will obtain from the AA charts or FMC.

So if a balanced V1 is used, the aircraft's take off performance cannot be optimized for the given day's condition. Therefore an Optimized V1 should be utilized if available.

Optimized V1 takes into account the the actual lengths of the clearway and stopway should they be of unequal distances. The V1 is thus 'unbalanced' in this sense. As FE Hoppy has pointed out if there is a stopway with no clearway, the stopping distance is greater hence V1 can be increased.
Vice versa if there is a clearway with no stopway, the V1 can be unbalanced to provide a lower V1.

On the 777, the V1 provided by the EFB is an Optimized V1. At least thats how the EFB was configured for the operator I'm working with.

john_tullamarine
24th Oct 2009, 10:14
Fish, JT, you forgot the fish in the tyre treads

That's what I get for publishing in a non-peer-reviewed journal ... missing such an obvious and critical consideration ...:}

FE Hoppy
24th Oct 2009, 17:07
(a) from whatever starting point where we have a given weight from the sums ..

if the TO is ASD-limited, then there is no option to increase V1 without reducing weight .. which is not the aim of the game. (It is presumed that the TO has been optimised to the extent that the figures are near limiting for the TOD case - if not, why not ?).

If the figures are not AT the limiting TOD case then it is possible to reduce V1 until the TOD limit is reached. This will make the ASD limited weight equal to the TOD limited weight will it not?

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


However, if we get a bit more TODA by introducing or extending clearway, then we can look at reducing V1 a little which MAY allow us to juggle a resulting small increase in ASD-limited TOW provided that one of TORR and TODR doesn't then immediately become limiting.

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

if the TO is TOD limiting, then the increase in TOD should give an increase in TOW due to the extra TOD or MAY allow us to increase V1 a little and get a bit more TOW providing that neither of ASD/TOR immediately becomes limiting.

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.

(d) if the TO is TOR limiting, then we can only look at increasing V1 to improve TORR, providing that ASDR doesn't immediately become limiting.

However, one would expect that this option had been exploited fully first time around so, in reality, TOR-limited cases ought not to respond to an increase in clearway.

Agree.

if nothing were limiting first time around, then we could have increased/decreased V1 as appropriate until one case became limiting in such a way that we ended up with an optimised (ie maximised) TOW. If then we can get some more clearway, we could revert to (b) or (c) and maybe tweak a bit more weight out of the calculations.

If nothing was limiting then an increase in TOD is irrelevant.

For any runway analysis it is very much a case of juggling things a bit this way or that to end up with something becoming limiting but in the way which gives us the maximum RTOW for the chart. Sometimes one can spend quite a lot of time on this iteration process .. hence the reason we all loved the advent of microprocessors to do the slave labour numerically intensive bits of the exercise.

If the above is confusing, either OS or I can run up some pictures to help clarify the words.


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.


How can a take off be accel stop limited in isolation?

Quite easily.

Find a runway with a given set of ASDA/TODA/TORA .. when you do the sums for a given set of ambient conditions, you will find that each results in a particular maximum TOW. Once ALL the various sums have been done, the lowest resulting TOW becomes the limiting TOW for that particular set of ambient conditions.

I dissagree that each will result in a particular MTOW. 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.

This set of weights will have a predictable relationship depending on the particular Type and that predictable relationship may well be very different for another Type.

If the ASDR ends up being the limiting factor (ie results in the lowest TOW) then the TO becomes ASD limited.

Or you use a lower V1 until VMCG or TOD limits equal ASD limit.

I can always stop below v1 so by reducing v1 i can declare a higher stop limited weight.

Very true .. but the down side is that the TODR/TORR increases and that might prove to be less than useful.

So ASD in isolation is not limiting.

For the go case as weight increases for a fixed TODA v1min must increase.

True - due to increasing Vs unless I am missing your point ? Alternatively, once TODR (or, if you prefer, in this case, TODA) becomes limiting, we can only increase that limiting weight by increasing V1 with the caveat that ASDR may soon become the limiting problem.

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

If we are only considering ASD/TOD.

adding clearway moves the go line such that i can have my failure at a lower speed and still go. Now where the go and stop lines cross is at a lower speed (v1) and a higher weight.

However, as soon as you start going down this path we also need to be looking at ASDR, TORR as well as TODR - one will become the limiting case. Much easier to present this discussion with some graphs to illustrate the considerations.

And why wouldn’t we?

Clearway has no effect on V1 max but will reduce V1 min so long as it isn't limited by something else. (VMCG)

But note that increasing clearway may permit an increasing V1 until either TODR or TORR becomes limiting. It can be rather confusing to folk if we ignore the TOR consideration ..

For a given takeoff weight, any increase in V1 leads to a reduction in both TODN-1 and TORN-1. The reason is that the all engine acceleration phase is longer with a higher V1 speed, and, consequently, in case of an engine failure occurring at VEF, the same V2 speed can be achieved at 35 feet at a shorter distance.

I think I would be emphasising things a little bit differently. Yes, the AEO distances up to the failure point will increase somewhat. However, the main consideration is that the OEI distances between a lower V1 and the nominated Vr will be SIGNIFICANTLY greater than between a higher V1 and the same nominated Vr. (Note, also, that Airbus are including both TOD and TOR in their discussion).

Airbus words not mine.

But it's saying the same thing. Increase V1 leads to reduction in TOD n-1 is the same as increase TOD n-1 leads to reduced V1.

.. but not a useful observation when considering what a varying clearway might be doing to the calculations .. except to the extent that the comment has relevance to juggling V1 in the presence of clearway to get a better ASDR/ASDA margin at the same weight. I think that you might be missing the underlying point that the aim is to increase TOW in general.

Point not missed at all. What I’m trying to say is exactly that. But the only way adding Clearway to an already runway limited TO can increase the MTOW is if it comes with a reduction in V1.

The driving point in the discussion needs to be that, in the presence of whatever I have in the way of runway etc. lengths, the aim is to juggle V1 to maximise TOW.

And having done that if someone gives me clearway I previously didn’t have I may be able to juggle my V1 LOWER to give a HIGHER TOW.


So what effect will stopway have on V1 and MTOW?

john_tullamarine
25th Oct 2009, 00:37
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 ?

Squealing Pig
1st Dec 2009, 22:01
I need to calculate ASDR in isolation in relation to assesing if the last 3rd of a runway is needed for an LVP take-off (EU Ops 1.430 Apendix 1) would I need to add public transport safty factor to this ?

john_tullamarine
18th Oct 2017, 00:30
Just saw this post while searching for something else.

What is a PT safety factor ?

Otherwise, ASDR is essentially raw, except for later aircraft where the A/L 42 2 seconds thing gets to be a pad.

galaxy flyer
18th Oct 2017, 01:21
Holy thread revival, John, only 8 years old!

Cheers

pattern_is_full
18th Oct 2017, 02:09
jt - you responded on this other thread, so you knew at some point ;)

http://www.pprune.org/questions/563187-public-transport-landing-factor.html

Same thing I expect - but applies (it appears) only to planning for a landing, so I don't see any connection to clearway and/or takeoffs.

john_tullamarine
18th Oct 2017, 04:06
Ah, right oh.

The normal certification factor for landing is 1.67 with some jurisdictions reducing this for alternates to 1.43. Others may well do other things. Certainly no relevance to ASDR. Other than the 2 seconds pad at V1 for recent certifications, best to treat ASDR as raw data and very critical, considering that the flight test and back room aerodynamicist work will discard anything not "quite up to scratch".

FE Hoppy
19th Oct 2017, 18:37
This thread is so old I started taking apart my own post before reading the author!

john_tullamarine
19th Oct 2017, 23:53
Guess I should go away, find a dark corner, and hide in disgrace ?

pattern_is_full
20th Oct 2017, 00:17
Never say that ;)