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gjhome
22nd Jan 2008, 10:02
Hello,

I have a question regarding wet runway dispatch requirements. I was reading Airbus getting to grip with aircraft performance to brush up my knowledge a little bit.

On dry runways you may not use more than 60% of the LDA.
But do I need to use the 60% margin on wet runway calculations? Or
is it enough to use dry distance * 1,15??

The case for my airplane is that we have special performance tables for wet runways, how do I use this to comply with jar-ops requirements? The reason I ask is because if I use the 115% * dry runway, I end up with for example 1250M. But if i use the table for wet runways, it says 1850M.

In this case I dont find it safe to use the Jar-ops 115% factor, so I want to use what the book say 1850M, but which factors do I have to use on this value? Do I need to apply the 60% rule? Or the 115% rule? Or no factor at all?

I feel that if I use the 60% rule, the result is unreasonable long runway requirements.

Of course you should use your good judgement and always be on the safe side, but I would like to know what the correct jar-ops way is.

Thanks!

FlightDetent
22nd Jan 2008, 10:44
On dry runways you may not use more than 60% of the LDA. But do I need to use the 60% margin on wet runway calculations? Or
is it enough to use dry distance * 1,15??

I am only partly trained in this, but straight out of JAR OPS 1:

1.515 Landing - Dry runways allows a full stop landing from 50 ft above the threshold:
(1) For turbo-jet powered aeroplanes, within 60% of the landing distance available;
As a side note, 60% of LDA consumed for landing is definitely not a 60% margin, but that is not the point.

Landing – Wet and contaminated runways (a)... the landing distance available is at least 115% of the required landing distance, determined in accordance with JAR–OPS 1.515.

So, A/C needs 1200 m to stop, with max braking, manufacturers data factored for slope and wind effects.

For dry runway, this 1200 must not represent more than 60% percent of LDA, so the minimum LDA is indeed 2000m - I will call this Required_LD(dry).

If the runway is wet, this RLD(dry) must be increased by 15%, so the minimum LDA(wet) is 2300 m. Agree?

Adding 15% on top of wet makes no sense, as this serves to recalculate RLD(dry) to RDL(wet) and you already start with wet data.

You must use dry table and double factor them to obtain RLD(wet), that is the requirement.

If manufacturer provides you with WET actual LD data, and this ALD(wet) *1/0,6 (with slope and wind) will be shorter than the required RDL(wet) as calculated above, you are allowed to use this shorter minumum LDA.

FD (the un-real)

I am sorry for several editing versions, but that is what really OPS says.

gjhome
22nd Jan 2008, 11:12

I ended up digging in jar-ops and calculating examples.
I came up with this.

For a wet runway you should use, dry distance(with 60% margin) * 1,15.

For a contaminated runway, you should use the longest value of two:

Either the one above dry * 1,15 (wich includes the 60% margin)

Or

If your AFM contains values for contaminated runways,
The value from the book for the selected contaminanr * 1,15
In this case, there is notes in jar-ops saying that you should NOT include
the 60% margin.

The only doubt I have now, is that the later pharagraph say contaminated runways. A wet runway is not a contaminated runway, but my AFM still contains data for wet runways in this section. Can I use this, even thou a wet runway is not considered contaminated accordning to jar-ops

Fella
22nd Jan 2008, 11:51
The old 1.67 x landing distance required for dry and 1.92 x landing distance required for wet seems to work wonders...check it out, easier math:ok:

FlightDetent
22nd Jan 2008, 13:09
For a wet runway you should use, dry distance(with 60% margin) * 1,15.

For a contaminated runway, you should use the longest value of two:

Either the one above dry * 1,15 (wich includes the 60% margin)

If your AFM contains values for contaminated runways,
The value from the book for the selected contaminanr * 1,15
In this case, there is notes in jar-ops saying that you should NOT include
the 60% margin.

Now, I apologize for being a pain in the neck. The rules are probably relatively simple, but the wording is quite complicated. In order for both of us to get through the woodwork, we need to keep a high standard of precision.

1) It is not a 60% margin. It is 1/0,6 margin (=1,666`). ;) Let's call it 60% rule for simplicity.
2) It cannot find that you need to use the larger value of two, as I read it, you must use method number 1, and if appropriate calculation with contaminated data yields a shorter RLD, you may use it. I cannot see the "longest of" requirement.
3) 15% addition with AFM contaminated data is correct I you ask me: sub-pragaraph (b), also that no 60% rule should be applied to this,
4) However the wet data is described in sub-p (c), which offers possibility to use wet AFM data (no 15%+ here), but rules for dry runways must be applied first (60% rule).

?

FD.

PEI_3721
22nd Jan 2008, 16:44
As per Fella.
Dry landing distance is 1.67 of manufacturers’ value (equivalent to landing within 60% of available distance for the weight / prevailing conditions for jet aircraft. See JAR-OPS 1.515).

Wet landing distance is 1.93 of manufacturers’ value, i.e. 1.67 * 1.15 (115% of dry, JAR-OPS 1.520 “The landing distance available is at least 115% of the required landing distance, determined in accordance with JAR–OPS 1.515” ).

See slides 26-27 and speakers notes in Managing Threats and Errors During Approach and Landing. (http://www.flightsafety.org/ppt/managing_threat.ppt)
Also, remember that in practical terms the wet landing distance factor only provides a safety margin in specific conditions and thus may not be as large as that for landing on dry runways. In addition, wet runway conditions can vary much more than dry, the depth of water (up to the contaminated definition) and the runway texture (grooved or good / poor friction surface) can significantly reduce the safety margin. See reports from Transport Canada (http://www.tc.gc.ca/tdc/summary/14200/14273e.htm) which indicate that in extreme conditions safety margins of 2.2-2.4 may be required to provide the equivalent safety margin as for dry runways.

FlightDetent
22nd Jan 2008, 16:58
Gentelmen, please. No one disputes that your numbers are correct for dry performance AFM data and far mor practicable to use for planning than JAR definitions. However that is not gjhome's question.

We're looking for correct application of paragraphs 1,520 (b), (c), (d), and (e). If we can find one.

FD.

PS: Thanks for the links, great info!

Sir George Cayley
22nd Jan 2008, 17:24
Just one point to always bear in mind when landing is that the amount of runway behind you at the point of touchdown is of no use.

Airmanship still plays its part once the numbers are crunched.

Also, grooving makes little or no difference to grip on a wet runway. Its purpose is to facilitate drainage and adds only a small amount to the texture of the surface depending on width, depth and number of grooves per metre.

Any doubts? Go to CDG where the runways are not grooved.

Sir George Cayley

OverRun
23rd Jan 2008, 02:42
Sir George Cayley,

Not quite ad idem with you on the grooving and grip issue.

Surface texture (which some call macrotexture) for normal asphalt varies but an average asphalt sand patch macrotexture is around 0.65 mm. Grooving adds to the texture of the surface, although how much it adds does indeed depend on width, depth and number of grooves per metre as you suggested.

The groove style varies by airport/authority, as shown in the following table, and the resultant net effect on the texture of the grooved asphalt can be seen.

http://www.geocities.com/profemery/grooves.jpg

I think that it adds more than a small amount to the texture on airports which use the larger grooves.

Grooving has its limits – it won't totally cope with standing water due to ruts and birdbaths in the runway (common in worn-out runways). Grooving won't totally cope with deep standing water due to heavy rainstorms and it won't cope with standing water due to all the grooves and texture being filled up with rubber.

However grooving does make a difference to the grip on a wet runway if as the water gets deeper on the runway. The work of Benedetto shows how when the macrotexture increases, the decrease of skid resistance during heavy rainstorms is greatly reduced. His conclusion is important because it underlines the ICAO requirement for both friction and texture depth:

The numerical analysis on the case study of Milan Airport shows some interesting outcomes:
(a) the macrotexture plays a fundamental role especially during the heavy rainstorms: in fact, if the skid resistance in good weather conditions depends substantially on the microtexture, in poor weather conditions the skid resistance depends on the macrotexture, because of only the greater pavement irregularities can penetrate the water film and can assure of the tire-pavement contact;

His Figure illustrates how skid resistance (shown on the Y-axis) reduces as speed (shown on the X-axis) increases. The depth of water is shown as n. For a dry surfacing with no water on (water depth or n = 0mm), there is minimal change. But for a surfacing with water on (the Figure shows depths from 0.1mm to 1.5mm), the skid resistance drops off rapidly with increasing speed. The deeper the water, the more pronounced the drop in skid resistance. This effect continues beyond the depth of 1.5mm. What grooving does is work to offset the effect of water on skid resistance. Even if the depth of water is more than the groove depth, Benedetto found that the grooving still acted to reduce the loss of wet friction at speed.

http://www.geocities.com/profemery/benedetto.jpg

If the runways at CDG are not grooved, then the French must be providing macrotexture in other ways. The French do pay attention to macrotexture for runways. They get their macrotexture from the choice of surfacing, its grading, and rugosity. While their standard BBA and BBME asphalts have a macrotexture around the 0.6-0.8 mm mark, their BBM, BBTM 0/10, ECF asphalt mixes have macrotexture over 1mm.

And in their airport asphalt guide:
STBA 2003: Guide d’application des norms. Enrobés hydrocarbonés et enduits superficiels pour chaussées aéronautiques. Direction Générale de l'Aviation civile. Bonneuil-sur-Marne, France.
the French say Le chapitre 7 de l’ITAC indique qu’une macrotexture exprimée en hauteur au sable (NF P 98 216 1) inférieure à 0,5 mm risque de conduire à une glissance excessive; lorsque la valeur dépasse 1 mm, l’adhérence est satisfaisante, pour peu que la microtexture soit conforme which I poorly translate as being Chapter 7 of the Instruction Technique sur les Aérodromes Civils (ITAC) indicates that a macrotexture (sand patch method) less than 0.5 mm is likely to lead to excessive slipperiness; when the value exceeds 1 mm, skid resistance is satisfactory if the microtexture also conforms.

Their airport asphalt guide goes on with a lot of detail about macrotexture. I don’t know enough about the exact asphalt wearing course mix on any of the runways at CDG. However the French are more advanced than most with their asphalt surfacings (IMHO including Australia, USA and the UK), and I think that they could well be providing their macrotexture through innovative asphalt engineering rather than grooving.

Sir George Cayley
23rd Jan 2008, 14:35
Overun

Apologies if I misled, but thanks for picking me up on it.

What I was trying to say applies only to the UK and referred to the current perceptions of the increase in grip grooving adds to UK asphalts.

In fact your table of effects quite neatly highlights this when comparing the UK in general to the others. There are plenty of runways that have not been resurfaced recently so the half worn figures are more relevant. Stansted was done last year I recall.

I think taking your post in total it just about answers all the questions and as such should be required reading for all.

Would it be fair to say that aircraft operators seeking performance credits should in fact be looking at micro/macro texture values as opposed to whether the surface is grooved?

Sir George Cayley

OverRun
24th Jan 2008, 07:38
Sir George Cayley,

No problems. In a purist engineering sense, linking performance credits with micro/macro texture values as opposed to whether the surface is grooved is technically more correct. But I think the grooving approach is better. When dealing with people, a purist engineering approach may not always be best. Macrotexture is a subtle concept and is not easy to see, not often measured, and (as I know from experience) capable of being distorted, disputed, mis-interpreted and generally twisted. I’ve seen this from all sorts of technical people, both on airports as well as high speed motorways/freeways.

I like grooving because it is simple (and the same is true for porous asphalt). The pilots can see it is there. The airport can see it is there. The authorities can see it is there. No arguments. And it works. Grooving worked at BRS. As soon as the new runway overlay was being immediately grooved, the complaints and incidents stopped. And there hasn’t been one since. Imagine if someone was trying to sort out the BRS problem if grooving was not available as a strong tool. Imagine trying to argue the subtleties of macrotexture with counter arguments about high friction results and so on. Not something as easily done.

I think that grooving is one slice of the James Reason model of Swiss Cheese. Take away that linkage between grooving and performance, and pretty soon you’ll have airports taking away grooving, and taking away that slice of Swiss Cheese.

I know that airports do not like friction treatments such as grooving or porous asphalt. They are expensive, they have a comparatively short life, and they require expensive routine maintenance. They do not add to the capacity of the airport, they don’t make the runway longer or wider, they don’t add to the structural capacity of the pavement, and they won’t let more planes take off. Airports can’t charge extra for them, and can’t rent them out. The airports provide friction treatment now because they have to.

I don’t think macrotexture is a strong enough argument to keep all of the airports grooving. So airports would start dispensing with grooving / friction treatment given a small gap to do so. I know they would based on many years experience of arguing the cost of runway overlays. Take the grooving slice off the Swiss Cheese, and it becomes that much easier to fit the line all the way through the Swiss Cheese to the accident.

deing
31st Jan 2008, 14:38
Another question but now for take-off. In the manual for a performance class A aircraft I only have dry runway take-off field lengths. What corrections need to be applied for a wet runway, jar-ops 1.490 doesn't mention anything about wet corrections.

Cov HEMS
28th Mar 2008, 09:56
Anybody got an answer to this wet runway take off question???

tribo
28th Mar 2008, 15:01
Current Certification Requirements Amendment 25-92

• Amendment 25-92 of the FARs required inclusion of wet runway takeoff performance in the AFM

• Provided a method to account for wet runway wheel braking capability that was based on ESDU 71026 ─ Both smooth and skid-resistant surfaces are addressed

• Method documented in the FAR’s and AC 25-7

Have a look at slide 24 at this link:

http://www.smartcockpit.com/data/pdfs/flightops/aerodynamics/Wet_Runway_(Physics-Certfication_and_Application).pdf

http://www.easa.eu.int/ws_prod/g/doc/Agency_Mesures/Agency_Decisions/CS-25_Amdt4.pdf

Info can also be found at these links:

http://www.tc.gc.ca/CivilAviation/certification/CPR/FARAmend.htm

clevlandHD
31st Mar 2008, 15:12
How do concrete rwys compare to alsphalt in terms of friction?
Cost wise, I guess they are much more expensive to built? What about life span?
Is grooving efficiency on concrete as time limited as on bitumen?
Which material is best, and why?

OverRun
1st Apr 2008, 04:58
Concrete is more expensive and much slower to build, but lasts longer than asphalt. Concrete is terrible to repair or rebuild (unless you overlay it with asphalt). Concrete is better in very hot climates under slow moving heavy aircraft because it doesn’t rut like asphalt. Concrete is better for very busy airports. IMHO where you see concrete, it is because the engineers have done all the sums and it came out best. And vice versa where you see asphalt.

While some type of textural finish can be constructed in a concrete pavement surface prior to grooving [i.e. brush or broom finish, heavy burlap drag finish, wire combed or wire tined construction], these texturing techniques cannot be substituted for saw cut or plastic cut grooves because they do not prevent aircraft from aquaplaning.

Grooves last longer in concrete than asphalt. They are much more stable. They don’t close-up like in asphalt, and rarely get damaged by the rubber removal methods. If you cut the grooves deeper at the start (say 8mm instead of 6mm), they can be very long lasting indeed.

I still prefer asphalt runways with maybe concrete runway ends. When things go wrong with asphalt, it generally can be fixed with overnight closures while the runway stays open 16-18 hours a day. But for a brand new mega-airport with say 6 runways and lots of redundancy and lots of money, concrete is often the better choice.

clevlandHD
1st Apr 2008, 08:58
Thanks OverRun.

safetypee
1st Apr 2008, 18:33
As a general observation (IMHO), concrete runways have reduced friction characteristics in comparison to asphalt. Perhaps this is because of deteriorating surface treatments, or lack of timely repair – as above.
See slide 26 in the presentation Landing Threats, (www.flightsafety.org/ppt/managing_threat.ppt) where the ‘wet smooth’ curve represents friction on concrete and the ‘wet grooved’ that of asphalt. There are of course differences in each category with or without grooving, surface texturing, porous friction layers, etc.
Also see Runway Research, (http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19890068908_1989068908.pdf) and Factors influencing aircraft ground handling performance. (http://hdl.handle.net/2060/19830019708)

alf5071h
2nd Apr 2008, 02:35
gjhome, re contaminated data (#3). If there is still some confusion, remember that contaminated performance is obtained from a different process than dry/wet.
Wet operations are based on factored dry data (1.67 factor); basic dry data comes from manufacturers tests.
In wet conditions it is assumed that the additional 15% margin (1.92 factor) provides equivalent safety to dry, this is not always true.
A wet runway is considered to be contaminated if the water depth is greater than 3mm (think about how you determine that). Conversly less than 3mm of slush is ‘wet’, but how do crews determine what ‘slush’ is? (Near 0 deg C, transient conditions, SG 0.85)
Contaminated operations now require the use of specific data, this is obtained either by testing or by calculation; see CS 25.1591 (page 1-G-6) and particularly AMC 1591 (page 2-G-25). (www.easa.eu.int/home/rg_certspecs.html)

The certification requirements for contaminated data make many additional assumptions which operators should be aware of: e.g Operators are expected to make careful and conservative judgments in selecting the appropriate performance data to use for operations on contaminated runways. Appropriate data is not defined, but I assume that it refers to that in an approved AFM.
… it is not possible to produce performance data that will precisely correlate with each specific operation on a contaminated surface.
Note that aquaplaning speed is defined by 9*_/P for calculation purposes, but other research sources indicate 7*_/P is the more important value (tyre spin up speed vs spin down speed).
Contaminated data is probably (usually is) based on the use of reverse thrust – a major difference from dry/wet operations.
Operation on runways contaminated with water, slush, snow, ice or other contaminants implies uncertainties with regard to runway friction and contaminant drag and therefore to the achievable performance and control of the aeroplane during take-off, since the actual conditions may not completely match the assumptions on which the performance information is based. Where possible, every effort should be made to ensure that the runway surface is cleared of any significant contamination.
and …
The provision of performance information for contaminated runways should not be taken as implying that ground handling characteristics on these surfaces will be as good as can be achieved on dry or wet runways, in particular following engine failure, in crosswinds or when using reverse thrust.
Regulatory passing the buck to the operator?

There is an interesting technical review of wet takeoff performance in the presentations in FLIGHT OPS - aerodynamics & performance section (www.smartcockpit.com)
See “Wet Runway (Physics - Certfication & Application)” N.B asphalt / concrete comparisons, page 11-.
Also see “Understanding Range Of V1”

OverRun
2nd Apr 2008, 02:38
safetypee,

And I share your general observation that concrete runways have reduced friction characteristics in comparison to asphalt. I was going to say it earlier, but I couldn’t find any data to support it so I crossed it out of my draft post.

Thanks for the link to NASA’s (Yager’s) Factors influencing aircraft ground handling performance. That is clearly one of the foundation documents of wet runway knowledge.

In NASA/Yager’s document, on page 7 and again in Figure 5, he seems to indicate that in his tests, the ratio of wet/dry stopping distance is worse for ungrooved asphalt compared to ungrooved concrete, as “a result of lower surface macrotexture”. I think that this result is specific to the particular runways tested in that programme, and that the real difference in wet/dry stopping distance between concrete and asphalt comes back to the macrotexture provided.

Across the world, concrete is popular at busy airports (meaning lots of rubber deposits), military airports (meaning little maintenance until the next war), and in countries/continents where maintenance is less prevalent (meaning little runway cleaning is done). That may be why the observation can be made that concrete runways have reduced friction characteristics in comparison to asphalt.

Sir George Cayley
3rd Apr 2008, 21:27
Tom Yager at NASA Wallops Test Facility, nr The Hamptons Virginia IS Mr Friction in the USA.

He has been conducting tests on specimen surfaces using a variety of carts for many years. He's forgoten more than the rest of us will ever know!

The contibution textue makes to runway braking performance appears to be gathering support. Avinor, the Norwegian Airport operator has just published a paper on using lasers to measure texture and relate it to wet friction characteristics.

ICAO has recently formed a group of experts to review current guidance and make recommendations for change.

Sir George Cayley

edosharon
17th Dec 2010, 08:49
Hi
Do you know where in the JAR is says that in emergency we are not required to add 1.67 to the landing dist?

Thanks

bluk
17th Dec 2010, 09:44
Getting to grips with performance says:
In the event of an aircraft system failure, known prior to dispatch and affecting
the landing distance, the available runway length must at least be equal to the
required landing distance with failure. This distance is equal to the required landing
distance without failure multiplied by the coefficient given in the MMEL, or to the
performance with failure given by the Flight Manual.

This calculation is only used for dispatch.

Suppose that you have flaps locked and your actual landing distance is 2300m
and then find rwy with distance 1.67*2300 ;)