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# Contaminated runway Vs clearway

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# Contaminated runway Vs clearway

3rd Nov 2008, 17:18

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Contaminated runway Vs clearway

Here is the following problem... had a few days ago:

Runway is 1800m long and raining (quite a lot). If applying the take off corrections through AFM (not using your airport analisys) you`ll find 4 tables (an A320 AFM) for:

Wet runway weight and speed decrements

1- Bleeds OFF no clearway
2- Bleeds OFF with clearway
3- Bleeds ON no clearway
4- Bleeds ON with clearway

Well the runway had a clearway, not long however, only 180 metres.

But the greatest weight decrement was using "with clearway" option. That`s the opposite of what I tought...!

So that`s my question... if you have a clearway you`re supposed to get more weight off the runway... right? Because you can achieve V2 at 35ft beyond runway end, on the clearway.

An other question... If option "no clearway" gives you best weight (more weight for takeoff). Can you actually use it, instead of "with clearway" option? Remember runway had a 180m clearway...

Thanks!
3rd Nov 2008, 20:32

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Quote:
So that`s my question... if you have a clearway you`re supposed to get more weight off the runway... right? Because you can achieve V2 at 35ft beyond runway end, on the clearway.
[Unquote]

Yes: it will certainly never reduce it, and would probably increase it in the case you describe. But I think you may be misunderstanding the idea behind decrements. Correct me if I'm wrong, but the weight decrement you are talking about is meant to be applied to the RTOW that you have already calculated separately for a dry runway on the day?

TORA = 1800m
TODA = 1980m

On the dry runway, the 1980m TODA will give a noticeably higher RTOW than if there was no clearway (TODA 1800m)

The clearway is also useful for the contaminated runway case, but not as useful as it is in the dry runway case. The contamination has a big effect on runway performance (usually the stopping), but none on the airborne performance (theoretically). In other words, the length of the runway is the more critical thing in the contaminated case. So when applying the weight decrement (CORRECTION) for the contaminated case there has to be a greater penalty than if the TORA was as much as the TODA.

Once again: if the TODA and TORA were 1800m, the dry RTOW would obviously be lower than with your TODA of 1980m. Despite applying a smaller decrement to get the contaminated RTOW, the resulting contaminated RTOW would still be lower than the one you would obtain in your case, where you are applying a slightly bigger decrement to a higher RTOW.

Quote:
If option "no clearway" gives you best weight (more weight for takeoff). Can you actually use it, instead of "with clearway" option?
[Unquote]

NO!! That would be like keeping your fat, and eating it as well.

Hope this helps...

Last edited by Chris Scott; 4th Nov 2008 at 00:35. Reason: Minor changes to reflect the fact that the extra clearway MIGHT not improve the theoretical RTOW in the contaminated case.
4th Nov 2008, 00:42

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I've been thinking more about this one. Decrements are a blunt instrument, because they try to cover a range of situations in one simple figure. So they can only be approximate, and have to cover a worst-case situation. In the exact situation you had the other day, the availability of that short (10%) bonus of clearway would probably improve the contaminated RTOW very slightly, if full and accurate calculations were available for with-versus-without. But because of the limitations of the decrement system, it's possible that you might end up with a slightly lower RTOW, because of the higher decrement you have described.

PS: You described the runway as simply wet. You may notice that I have talked of the "contaminated" case. Perhaps I should explain that under UK performance rules unless they've changed recently there is normally no weight decrement for a wet runway, provided there is no measurable depth of water. What we normally do is to reduce the V1 by a variable figure usually about 10kts and accept a lower screen height at the end of the clearway (15ft). We may also assume the use of one thrust reverser in the wet runway case, but I'm not sure and it's getting late...
4th Nov 2008, 05:19

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Because you can achieve V2 at 35ft beyond runway end, on the clearway.
May be because you need 15ft clearance only on contaminated ?
4th Nov 2008, 08:44

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Chris Scott,

Thanks a lot, great explanation! But still can`t get it why after all calculations "with clearway" option gave me a lower TO weight than "without clearway". It should be the opposite. I understand that with clearway speed decrement is greater than without clearway, but the weight decrement shouldn`t.
Strange... Ohh well, performance has so many "grey" issues!

Most airlines do not account for clearway on takeoff for wet or contaminated runways... obviouslly 15ft at the threshold is already quite low, using a clearway would reduce that margin for almost nothing increasing the risk of not being airborne at all at the end of the runway.
But the airline where I`m working for has no policy regarding that and regulations have nothing about it either (at least that I know).
But I remember when I flew the 737 some years ago I belive the FCTM recommended not using the clearway for wet runways.

Oh and I learned something new... I didn`t know that CARs allowed no weight reduction for wet runways... First time I hear that, I wonder if many more countries allow that.

Henry, you`re right, its 15ft not 35.

Thank you guys
4th Nov 2008, 12:18

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Quote from TO MEMO:
But still can`t get it why after all calculations "with clearway" option gave me a lower TO weight than "without clearway". It should be the opposite.
[Unquote]

I'm not sure either. Is there any chance that you could post all the actual figures you are using for the two cases?

There's no doubt that the DECREMENT for the "with clearway" case should be greater than the one for the "no clearway" case. But that does not in itself mean that the resulting figure for contaminated RTOW will be lower than the contaminated RTOW if the clearway is closed down. But, as I said when editing my first post, each decrement has to allow for the "worst" case, and your 10% clearway is comparatively short.

If you run the actual figures past us, there's a good chance that I or someone else will be able to sort this out from the unhurried comfort of our armchairs... [I well remember what it's like trying to make sense of decrements in the left seat of an A320, or whatever, as schedule approaches.]

Chris
4th Nov 2008, 12:58

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But still can`t get it why after all calculations "with clearway" option gave me a lower TO weight than "without clearway". It should be the opposite.
Its right, IIRC it has to do with the takeoff flare distance. I will try to dig out the appropriate Airbus blurb on it.

Mutt
5th Nov 2008, 19:34

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CS,

Here it is. Runway 1800 m long. No wind
On the airport analisys 69,1T is the MTOW for dry runway.
You can find on the following link the FCOM page for wet runway decrements.

HotShare - Visualizando MTOW.JPG
6th Nov 2008, 00:18

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Hi again, TO MEMO,

Thanks, I see that the decrement for "fluid contaminated" runway case is much greater (by about 2T) if the dry RTOW allows for the clearway, just as you said.

Does your dry RTOW of 69.1T allow for the clearway or not, and what flap is it based on? I do not fly A320s any more, so I don't have access to any A320 performance manuals, even if I knew which A320 you are flying.

So I am going to need 2 dry RTOWs to complete my explanation, and they must both be based on the flap you intend to use in the "fluid contaminated" case:
(1) Dry RTOW for TORA=1800m and TODA=1800m (i.e., no clearway);
(2) Dry RTOW for TORA=1800m and TODA=1980m (clearway=180m).

I realise that if you are using airport-specific tables you may not be able to give me a figure for (1). But if you can, I suspect that the dry RTOW (2) may be in the order of a couple of tonnes greater than (1). By the way, if you are not sure whether to use Flaps 2 or Flaps 3, let's look at the dry figures for both.

Chris
6th Nov 2008, 10:17

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CS,

I`m starting to see your point here.
Indeed the dry RTOW of 69,1T is for Flaps 3 (chosen flap setting for TO)and it accounts for clearway.
If I can still remember Flaps 2 RTOW was around 66T
Actually, all tables in the airport analisys allowed for clearway if I`m not mistaken.

As you said, indeed if I had a table with "no clearway" it would be probably a couple of tonns lower.

Thanks
6th Nov 2008, 14:36

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Good!

In the contaminated case, the clearway is of much less actual value than in the dry case in fact it may not help at all. The runway position at the wet V1 has to be earlier than for the dry V1, to ensure stop-capability despite the reduced stopping performance. So, in the "go" case, you may be able to reach the screen height before the end of the runway, in which case the clearway is merely an unnecessary luxury. AND, if the rules permit screen height to be lowered from the normal 35ft to 15ft for the wet case, that is even more likely to happen. That's why the decrement has to be higher for the clearway case.

I wish we had some real figures to illustrate the argument, though. Such is life...

Chris
7th Nov 2008, 10:36

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CS,

9th Nov 2008, 08:35

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The first 2 charts will show you that the weights increase with a clearway.

These 2 charts show you that the contaminated runway takeoff weight DECREASES with the addition of a clearway.

The reasons are:
1: Requirement to have half of the takeoff flare distance over the runway...

Hope it makes sense..

Mutt

Last edited by mutt; 10th Nov 2008 at 12:16.
9th Nov 2008, 23:50

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Hi mutt,

Am most impressed that you have your own personal airfield!

Will have to put my thinking cap on re "take-off flare distance". Must admit it's a new concept to me, unless the name has changed in this millennium. At the moment, I'm guessing it may be the distance taken in the first segment (rotation and gear retraction)? Are you saying that the first segment has to be completed over the runway in the contaminated case, whereas the clearway can be used for it in the dry case? And is the screen height reduced from 35ft to 15ft in the contaminated case?

In the meantime, is there any chance you could improve the sharpness of the tables, please? I've no problem on my browser in enlarging the smaller figures on my screen, but they remain a bit too blurred to be readable. Thanks for all the trouble in producing them.

Chris
10th Nov 2008, 03:44
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At this time of the day, Mutt will be hard at work in his day job ... so I'll jump in ...

"take-off flare distance"

read "distance from lift off to screen height" .... engineer-speak (especially if one comes from an aerodynamics or FT background) ... please tolerate our occasional use of in house mumbo-jumbo jargon.

first segment (rotation and gear retraction)?

first segment is the distance from the screen height to the completion of gear retraction .. and may, or may not, exist .. depending on how fast the retraction sequence is.

Are you saying that the first segment has to be completed over the runway

not at all .. Mutt's earlier comment -

Requirement to have half of the takeoff flare distance over the runway

refers to the take off run required (TORR) part of the takeoff calculation which (for most jurisdictions) requires at least half of the distance to screen to be over the declared TORA. Simply put, this tries to avoid too many instances of dust, dirt, and grass blowing around the aircraft at the far end of the runway ...
10th Nov 2008, 08:24

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Thanks very much, John, and I'll now wait in the hope mutt can help with the image problem.

Should have realised that first segment starts AFTER screen height.

Chris
10th Nov 2008, 11:31

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I think you guys are rather over complicating the situation, the reason is actually quite simple and logical.

Most runway specific take-off data used by airlines uses an "unbalanced" field, ie. TODA is greater than TORA because the clearway can be included in the distance required to reach screen height giving a higher RTOW. JAR-OPS (now EU-OPS for most) forbids the use of clearway when the runway is contaminated so if you enter the tables with a RTOW that used it you must make a larger decrement than if you had used balanced field take off data.

You need to check the runway page as you do the initial calculation, if TORA is greater than TODA you are using unbalanced performance and you must use the larger decrement. If TODA = TORA then the performance is balanced and you can use the smaller decrement.

If, like the original poster, you are non specific performance you may well find that the best weight is achieved by not accounting for the clearway at the initial stage and using the smaller decrement which is why the specific pages that some airlines produce for contaminated runways normally produce a better weight, they have discounted clearway at the beginning and use a smaller decrement.
10th Nov 2008, 22:15
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if TORA is greater than TODA you are using unbalanced performance

not quite... "balanced" relates TOD to ASD, rather than TOR... but, we know what you mean here.
11th Nov 2008, 16:55

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Yup, fair point.

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