wet vs contaminated runway
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wet vs contaminated runway
WET VERSUS CONTAMINATED R/W
“More stopping power is needed when the R/W is contaminated”
On 1 Jun 2008, A-320 of Guatemala went off the R/W and broke into 3 pieces. 6 dead 31 injured. On 2 Aug 2005, an Air France A-340 over-ran the R/W at Toronto. The a/c was destroyed in a post accident fire. Exactly a month earlier a Bangla Desh a/c Biman DC-10 over-ran the R/W at Chittagong. The a/c suffered extensive damage. An Air India Boeing 747 which went off the R/W. on Mumbai, on 30 July 2005 and was lucky to escape with no damage except reputation. Even before the enquiry started several arm chair pundits blamed the pilots. Question that emerged was “Were they aware of what is involved in a wet runway landing?
All the accidents mentioned took place while landing in heavy rain. All the aero planes involved experienced x-wind component and tail wind component. This combination is very dangerous. During a heavy down pour the water depths can vary along the R/W depending upon the condition and slope. The a/c wheels can hydroplane (explained in later paragraphs) if the depth is sufficient to prevent making proper contact with the R/W.This makes it very difficult to stop the a/c within the confines of the R/W.
Why do accidents happen on wet R/W?
Main reasons are that the pilots do not get the correct information on the Actual R/W condition. The only information the pilot gets is that the R/W is “WET”.
Obviously, what is a wet R/W? For a pilot a R/W is considered “WET” when the depth of water on it is less than 3mm. that is what the Flight Manuals state. If the depth is more than 3mm, the R/W comes into the category of “SLIPPERY” or “CONTAMINATED”. These differences in water depths will change the actual landing distance by a very large amount.
The a/c requires 40% more R/W to stop on a wet R/W, on a contaminated R/W the figure increases to 300%. Thus if an a/c requires 6000’ feet of R/W for landing on dry R/W, it will require 8400 ft on wet R/W and more than 18000ft if the R/W is contaminated!
Thus when a pilot receives a report that says that R/W is wet, he assumes that the depth of the water is less than 3mm and after a 40% addition to the landing distance, will make for a safe landing. In reality the actual depth of the water on the R/W can be as much as 3 inches during a heavy monsoon downpour. This would bring it into CONTAMINATED CATEGORY. In 33 years of my flying I have never heard the R/W condition report other than WET (An issue for contemplation).
Accidents on wet Runway’s are on the increase. Experience levels of the pilots in the airline are dropping due to rapid expansion in Aviation Sector. Unfortunately Training and regulations are not in pace with time. Adequate back up training on simulators is found wanting. Furthermore many accidents in recent past have involved a/c with old technology, without modern safety features that are inbuilt into new generation aircrafts.
The answer to it,
Building Grooved Runway’s, investing in Modern Runway’s friction system and proactive R/W conditions are essential for making landings safe.
Therefore it is important to understand the various aspects on a/c performance when it comes to land on wet/contaminated R/W.
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Many good points here, but IMO you just can't attribute one factor to some of the accidents you mention, don't forget sailing down most of the runway looking for a smooth touchdown 
, not using the brakes or not deploying thrust reverser's. 
D.L.
, not using the brakes or not deploying thrust reverser's. D.L.
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Are the figures obtained after 40% (wet) and 300% (contaminated) addition to the landing distance required for a dry R/W, considering using full brakes, auto brakes, spoilers and reversers?
If the R/W was actually contaminated, and not wet as reported to the pilot, wouldn't that be considered as a big safety hazard, since the difference between the distances required to stop the a/c on a wet and contaminated R/W is significant (9600 feet in the above example) ?
What is the most appropriate / recommended plan of action in this case.
Also, in the above example, if the R/W was reported as contaminated, the a/c would require 18,000 feet to bring it to stop. If the ALD was lower than this figure, the only obvious decision would be to fly to the alternate, correct?
If the R/W was actually contaminated, and not wet as reported to the pilot, wouldn't that be considered as a big safety hazard, since the difference between the distances required to stop the a/c on a wet and contaminated R/W is significant (9600 feet in the above example) ?
What is the most appropriate / recommended plan of action in this case.
Also, in the above example, if the R/W was reported as contaminated, the a/c would require 18,000 feet to bring it to stop. If the ALD was lower than this figure, the only obvious decision would be to fly to the alternate, correct?
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Yes, and rightly so too...Don't you care if a decimal point appears randomly in your calculations???? What is different with an apostrophe? Nothing to do with Police, just Professionalism, and survival...
But the original post raises a very valid point. If the ATIS or verbal report only ever states "wet", even when there is a huge CB dumping thousands of tons of water onto the runway the pilot on approach is given no information whatsoever re a possibly contaminated runway, which is actually a life-threatening event. Instead he has to extrapolate the possibility of this from non-specific implications of %Cb or +Ra in the METAR, and decide between a potentially hazardous landing or an expensive (company contentious) diversion.
Pretty unsatisfactory, really. Not so? Apostrophes notwithstanding.
But the original post raises a very valid point. If the ATIS or verbal report only ever states "wet", even when there is a huge CB dumping thousands of tons of water onto the runway the pilot on approach is given no information whatsoever re a possibly contaminated runway, which is actually a life-threatening event. Instead he has to extrapolate the possibility of this from non-specific implications of %Cb or +Ra in the METAR, and decide between a potentially hazardous landing or an expensive (company contentious) diversion.
Pretty unsatisfactory, really. Not so? Apostrophes notwithstanding.
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just Professionalism, and survival...
The problem with runway state reports on the ATIS is that they are usually there after a runway inspection. By the time the inspector has reached the end of the runway and reported wet x 3 for example, the downpour could have increased in intensity. In the example this could then make the runway 'Contaminated' or 'Slippery'. It is difficult to know how an exact runway state could be given for each landing and be completely accurate.
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Effect of grooving
Unnamed runway at an unnamed airport carrying narrowbody medium jets. The central dry part is asphalt grooved, and the outer edges are asphalt, not grooved. I'm not advocating grooving less than full width, but in this case the photos do graphically tell the story.
Last edited by OverRun; 13th Feb 2010 at 03:23. Reason: Changed picture location
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Is that central "wet" rather than "contam" section (I'd hesitate to call either "dry") not due to the crowning of the runway (and poor/slow drainage at the edges) resulting in large amounts of standing water at the sides, rather than any grooving?
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Mad (Flt) Scientist,
That central "wet" section appearance in contrast to the "contam" section is mainly due to the grooving in the central 20m. There is a central crown on the runway, and I think the crossfall drainage is at the lower limit of 1%. The grooving is very recent and part of an upgrade project. I’m guessing at the groove spacing and dimensions but it typically adds 1.1mm of macrotexture to create a macrotexture of 1.6mm. There is a very slight sheen in a few places in the central “wet” section, so it is fair to say that the runway surface macrotexture in the centre is holding the equivalent of maybe 1.5mm of water depth and leaving 0.0mm proud of the surface.
Plus we are seeing here the general poor/slow drainage of ANY wide (almost) flat pavement. The standing water at the sides is showing more prominently in the photo because (a) smooth/poor macrotexture of the ungrooved edges mean more water sitting above the asphalt, and (b) the “drainage length” affecting the runway at the edge is more the “drainage length” affecting the runway in the centre.
A model for the evaluation of the depth of rain water on a pavement is available and is shown in the following equation:
water film thickness or depth = fn (drainage length X rain intensity)^0.5/(drainage slope)^0.2.
The more the drainage length, the deeper the water film. For a runway of 45m width with the central 20m grooved, if the water film is 1.5mm at 10m perpendicular from the centreline (at the edge of the grooving), then it will be 2.3mm at the edge of the runway. However while the centre grooved “wet” section has no water sitting above the surface, the ungrooved outer edges have a texture depth of only say 0.6mm, meaning a water film on the surface of 2.3 – 0.6 = 1.7mm of water.
That is more than half-way to 3mm contaminated depth (on the edges) and the rain had stopped at the time of the photo.
I calculated the sort of depths of water that can occur on a runway in heavy rain, and did so for the YYZ overrun. At Toronto, their radar showed rainfall intensities of over 50mm/hour at the time the A340 ran off the runway, and the rainfall event was reportedly 36.2mm that day. For Australian rainfall patterns, that would be equivalent to a maximum short burst intensity of up to 120 mm/hour depending on burst duration. At that intensity, and for a 60m wide surfacing (45m runway plus 7.5m sealed shoulders), and crossfall of 1.5%, the depth of water on the runway could reach over 5mm and as high as 6.6mm.
That central "wet" section appearance in contrast to the "contam" section is mainly due to the grooving in the central 20m. There is a central crown on the runway, and I think the crossfall drainage is at the lower limit of 1%. The grooving is very recent and part of an upgrade project. I’m guessing at the groove spacing and dimensions but it typically adds 1.1mm of macrotexture to create a macrotexture of 1.6mm. There is a very slight sheen in a few places in the central “wet” section, so it is fair to say that the runway surface macrotexture in the centre is holding the equivalent of maybe 1.5mm of water depth and leaving 0.0mm proud of the surface.
Plus we are seeing here the general poor/slow drainage of ANY wide (almost) flat pavement. The standing water at the sides is showing more prominently in the photo because (a) smooth/poor macrotexture of the ungrooved edges mean more water sitting above the asphalt, and (b) the “drainage length” affecting the runway at the edge is more the “drainage length” affecting the runway in the centre.
A model for the evaluation of the depth of rain water on a pavement is available and is shown in the following equation:
water film thickness or depth = fn (drainage length X rain intensity)^0.5/(drainage slope)^0.2.
The more the drainage length, the deeper the water film. For a runway of 45m width with the central 20m grooved, if the water film is 1.5mm at 10m perpendicular from the centreline (at the edge of the grooving), then it will be 2.3mm at the edge of the runway. However while the centre grooved “wet” section has no water sitting above the surface, the ungrooved outer edges have a texture depth of only say 0.6mm, meaning a water film on the surface of 2.3 – 0.6 = 1.7mm of water.
That is more than half-way to 3mm contaminated depth (on the edges) and the rain had stopped at the time of the photo.
I calculated the sort of depths of water that can occur on a runway in heavy rain, and did so for the YYZ overrun. At Toronto, their radar showed rainfall intensities of over 50mm/hour at the time the A340 ran off the runway, and the rainfall event was reportedly 36.2mm that day. For Australian rainfall patterns, that would be equivalent to a maximum short burst intensity of up to 120 mm/hour depending on burst duration. At that intensity, and for a 60m wide surfacing (45m runway plus 7.5m sealed shoulders), and crossfall of 1.5%, the depth of water on the runway could reach over 5mm and as high as 6.6mm.
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"All the accidents mentioned took place while landing in heavy rain. All the aero planes involved experienced x-wind component"
Might I add Ryanair flight 1216, Charleroi-Limoges, 21 March 2008 to this discussion as the above factors certainly applied and the a/c left the runway. I am not aware if any report into the incident has been produced.
Might I add Ryanair flight 1216, Charleroi-Limoges, 21 March 2008 to this discussion as the above factors certainly applied and the a/c left the runway. I am not aware if any report into the incident has been produced.
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Hi All,
Interesting discussion on 'wet' versus 'contaminated' runways.
The best description I heard was back in the mid 70s.
"If you sit on it and it feels uncomfortable it is contaminated. The rest is just determining to what degree."
Good advice then and good advice now!
best regards,
Bruce Waddington
Interesting discussion on 'wet' versus 'contaminated' runways.
The best description I heard was back in the mid 70s.
"If you sit on it and it feels uncomfortable it is contaminated. The rest is just determining to what degree."
Good advice then and good advice now!
best regards,
Bruce Waddington
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Faa/jaa
Yes, under FAA, there is no such thing as 'damp'.....it's either dry or wet. JAA has a 'damp' category.
PantLoad
BTW, my home airport has excellent drainage...good crown, good grooving...so, I like 'damp'...even though it's not legal for me to use it.
PantLoad
BTW, my home airport has excellent drainage...good crown, good grooving...so, I like 'damp'...even though it's not legal for me to use it.
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That is interesting I didn't realise the FAA did not use it. 'Damp' is always a hard one to call. especially on a grooved surface. I always check the bottom of the grooves as the water is sometimes retained there even when the surface is dry.
Pilots have told me before that even if the runway is called 'Damp' then they treat it as 'Wet'. That would tie in with the FAA rulings I suppose.
Pilots have told me before that even if the runway is called 'Damp' then they treat it as 'Wet'. That would tie in with the FAA rulings I suppose.
