Your opinion on contaminated runways

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pelmet
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Your opinion on contaminated runways

#1 Post by pelmet » Thu Oct 06, 2016 2:59 pm

I am just reading up on the new FAA policy on runway condition reporting for contaminated runways.

https://www.faa.gov/other_visit/aviatio ... O16009.pdf

They have a table that gives braking action equivalents for different types of contamination. I was surprised to see that 1/8" of slush on a runway is considered to be a runway with good braking action where "Braking deceleration is normal for the wheel braking effort applied AND directional control is normal".

Compacted snow at -15 or below is considered to be worse at good to medium.

So my question is....based on you personal experience, would you consider 1/8" slush as equivalent to normal braking conditions and a better situation than being on compacted snow?
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Re: Your opinion on contaminated runways

#2 Post by goingnowherefast » Thu Oct 06, 2016 6:37 pm

The tires push though 1/8" of slush pretty easily. It's probably comparable to a puddle.

Although, that slush freezes easily and turns into 1/8" rough ice easily, and then you might as well have brought ice skates. Even worse when there's ridges in it from previous vehicles that passed before it froze. Then it's like trying to maneuver on a washboard with no traction.
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Re: Your opinion on contaminated runways

#3 Post by wallypilot » Thu Oct 06, 2016 10:17 pm

goingnowherefast wrote:The tires push though 1/8" of slush pretty easily. It's probably comparable to a puddle.

Although, that slush freezes easily and turns into 1/8" rough ice easily, and then you might as well have brought ice skates. Even worse when there's ridges in it from previous vehicles that passed before it froze. Then it's like trying to maneuver on a washboard with no traction.
I agree...1/8" slush = reasonable braking action.
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Re: Your opinion on contaminated runways

#4 Post by complexintentions » Sat Oct 08, 2016 3:30 am

Your opinion on contaminated runways
I don't like them.
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Re: Your opinion on contaminated runways

#5 Post by ahramin » Sat Oct 08, 2016 9:23 am

Depends on the aircraft. That table is designed for airliners with spoilers and tire pressures over 200 psi. 1/8" slush is the same as wet. I don't think a high performance GA aircraft with 35 psi tires would have Good braking.
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Re: Your opinion on contaminated runways

#6 Post by PilotDAR » Sat Oct 08, 2016 5:28 pm

Your opinion on contaminated runways
Generally I like them, as I avoid the application of brakes anyway. It's fun to slide a wheel along while landing fully slipped, and holding one wing up and wheel off.

Okay, I'll try to be serious.......
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Re: Your opinion on contaminated runways

#7 Post by Meatservo » Sun Oct 09, 2016 4:49 pm

PilotDAR wrote:
Your opinion on contaminated runways
Generally I like them, as I avoid the application of brakes anyway. It's fun to slide a wheel along while landing fully slipped, and holding one wing up and wheel off.

Okay, I'll try to be serious.......
complexintentions was funnier. Sorry. There's something sublime about getting a laugh with one sentence. I can rarely pull it off myself.
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Re: Your opinion on contaminated runways

#8 Post by pelmet » Sun Oct 09, 2016 9:00 pm

Meatservo wrote:
PilotDAR wrote:
Your opinion on contaminated runways
Generally I like them, as I avoid the application of brakes anyway. It's fun to slide a wheel along while landing fully slipped, and holding one wing up and wheel off.

Okay, I'll try to be serious.......
complexintentions was funnier. Sorry. There's something sublime about getting a laugh with one sentence. I can rarely pull it off myself.
Maybe so...but Ahramin may have provided the answer that can prevent an accident.

Thanks.
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Re: Your opinion on contaminated runways

#9 Post by complexintentions » Sun Oct 09, 2016 10:32 pm

Ah c'mon. I always think of silly short answers to post titles, hardly ever write 'em.

But as to the topic, it does make sense that slush would create drag, and thus be a positive for braking, not a negative. It's why there are large performance penalties for takeoff with slush on a runway. I chucked some numbers into the OPT and with all other variables constant, you lose about 60,000lb of t/o weight for the difference between wet and 1/8" of slush. (B777)

That's a lot of drag.

I don't think this would be a huge deal for g/a aircraft, depending on the category. If a preceding airliner lands and reports braking as "good" on a slushy runway I doubt a C172 going to have difficulty stopping in the same distance.

And yeah, slush is way better for braking than compacted snow. If the snow is compacted hard enough and polished up by previous aircraft it'll be almost like ice. A bit of sun to melt/refreeze the top and look out. I'll take wet slush any day.
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Re: Your opinion on contaminated runways

#10 Post by pelmet » Mon Oct 10, 2016 1:19 am

complexintentions wrote:Ah c'mon. I always think of silly short answers to post titles, hardly ever write 'em.

But as to the topic, it does make sense that slush would create drag, and thus be a positive for braking, not a negative. It's why there are large performance penalties for takeoff with slush on a runway. I chucked some numbers into the OPT and with all other variables constant, you lose about 60,000lb of t/o weight for the difference between wet and 1/8" of slush. (B777)

That's a lot of drag.

I don't think this would be a huge deal for g/a aircraft, depending on the category. If a preceding airliner lands and reports braking as "good" on a slushy runway I doubt a C172 going to have difficulty stopping in the same distance.

And yeah, slush is way better for braking than compacted snow. If the snow is compacted hard enough and polished up by previous aircraft it'll be almost like ice. A bit of sun to melt/refreeze the top and look out. I'll take wet slush any day.
Thanks,

I am thinking more about the landing scenario. You will notice in the original post, I mentioned what the FAA has as a category of compacted snow at -15 and below. So they have eliminated much of the melt/refreeze scenario. I found compacted snow at cold temps to provide reasonable braking action for aircraft with no reverse capability but it was on gravel runways. The new FAA policy is only for paved runways.

Slush worries me but I don't have a lot of experience on it especially in a situation where landing distance is limited for type and weight. Back when we were using level I and level II clutter for takeoff, I believe that level I for slush(1/8" or less) mostly affected accelerate-stop due to reduced stopping performance while level II slush(more than 1/8") mostly affected accelerate-go distance due to increased drag. But I wouldn't count on slush at any legal depth as being able to provide more drag for landing.

Take a look at your limitations section on the 777 and see what the max crosswind is on slush. Probably half of a wet runway limit and in the same region as poor conditions. We used to land and takeoff in 30 knot crosswinds on compact snow. I would hesitate to do that on slush.
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Re: Your opinion on contaminated runways

#11 Post by complexintentions » Mon Oct 10, 2016 11:40 pm

Sure. Crosswind limits drop on slush. But you've kinda shifted the discussion from stopping distance, to crosswind limits. My comments were solely in reference to the former, since your original post was about braking action.
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Re: Your opinion on contaminated runways

#12 Post by pelmet » Tue Oct 11, 2016 7:11 am

complexintentions wrote:Sure. Crosswind limits drop on slush. But you've kinda shifted the discussion from stopping distance, to crosswind limits. My comments were solely in reference to the former, since your original post was about braking action.
Thanks. You are definitely correct that the original question was about landing and that is my area of most comcern(rightly or wronly). I just mentioned the restriction on crosswind limits as an example of degraded performance in what appears to me to be an obvious effect due to the increased slipperiness of slush(i.e. reduced crosswind limits).

I only brought it up because you had mentioned about increased drag from slush in the takeoff scenario with 1/8" slush but I am not sure I agree that it is due to increased drag but is instead due to increased slipperiness and its effect on deceleration potion of the takeoff scenario in the event of an RTO resulting in the 60,000 lb penalty.
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Re: Your opinion on contaminated runways

#13 Post by complexintentions » Fri Oct 14, 2016 12:45 am

My earlier comments were based only on my own experience. So here's a safety letter that explains it better than I.

https://www.tc.gc.ca/eng/civilaviation/ ... 4-3879.htm

Takeoff:
Tests were conducted using catapult-driven test carriages as well as full-scale aircraft. These early tests gave a clear picture of what slush does to an aircraft that takes off or lands. It was found that the acceleration during takeoff was reduced due to an increase in total drag acting on the aircraft. This increase in drag was caused by the tires displacing the slush and the impingement (interference, intrusion) of the spray of slush on the airframe thrown up by the tires. It was shown that the additional drag increased with increasing slush depth.

Let us have a look at some typical numbers with respect to the effect of slush on take-off performance. Just 13 mm (0.5 in.) of slush can subject a large jumbo jet to a drag that is equal to approximately 35% of the thrust of all its four engines. This number increases to 65% for 25 mm (1 in.) of slush, making it impossible to take off. In general, for a multi-engine transport aircraft, just 13 mm (0.5 in.) of slush can increase the take-off distance by some 30-70%.
As I said: it's a lot of drag.

Landing:
Slush can have an adverse effect on the landing performance. Braking friction can be low because aquaplaning is likely to occur on slush-covered runways. This will increase the landing distance compared to a dry runway. Although it sounds strange, a thicker layer of slush can be better than a thin layer because the drag from the slush helps stop the aircraft. The more slush you have on the runway, the higher the drag on the aircraft. This also applies to rejected takeoffs and can lead to strange performance restrictions when taking off from slush-covered runways. For instance, more slush can give lower take-off weight penalties.
The article does make reference to loss of directional control when operating in crosswinds on slush. I would be more concerned with that than stopping distance.
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Re: Your opinion on contaminated runways

#14 Post by rigpiggy » Fri Oct 14, 2016 7:49 am

Slush has sort of a velocity squared effect on drag.
https://en.m.wikipedia.org/wiki/Munich_air_disaster

Mythbusters also did a thing on slushies going thru a windshield. slush has a cohesiveness that is surprising.
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Re: Your opinion on contaminated runways

#15 Post by pelmet » Fri Oct 14, 2016 8:04 pm

complexintentions wrote:My earlier comments were based only on my own experience. So here's a safety letter that explains it better than I.

https://www.tc.gc.ca/eng/civilaviation/ ... 4-3879.htm

Takeoff:
Tests were conducted using catapult-driven test carriages as well as full-scale aircraft. These early tests gave a clear picture of what slush does to an aircraft that takes off or lands. It was found that the acceleration during takeoff was reduced due to an increase in total drag acting on the aircraft. This increase in drag was caused by the tires displacing the slush and the impingement (interference, intrusion) of the spray of slush on the airframe thrown up by the tires. It was shown that the additional drag increased with increasing slush depth.

Let us have a look at some typical numbers with respect to the effect of slush on take-off performance. Just 13 mm (0.5 in.) of slush can subject a large jumbo jet to a drag that is equal to approximately 35% of the thrust of all its four engines. This number increases to 65% for 25 mm (1 in.) of slush, making it impossible to take off. In general, for a multi-engine transport aircraft, just 13 mm (0.5 in.) of slush can increase the take-off distance by some 30-70%.
As I said: it's a lot of drag.

Landing:
Slush can have an adverse effect on the landing performance. Braking friction can be low because aquaplaning is likely to occur on slush-covered runways. This will increase the landing distance compared to a dry runway. Although it sounds strange, a thicker layer of slush can be better than a thin layer because the drag from the slush helps stop the aircraft. The more slush you have on the runway, the higher the drag on the aircraft. This also applies to rejected takeoffs and can lead to strange performance restrictions when taking off from slush-covered runways. For instance, more slush can give lower take-off weight penalties.
The article does make reference to loss of directional control when operating in crosswinds on slush. I would be more concerned with that than stopping distance.
Thanks,

As you quoted, thicker slush can have more drag and therefore more stopping/slowing action than thin slush. Which goes right back to my original post. According to the TALPA chart, thinner slush is associated with good braking action(which I assume means good for deceleration and better performance) while thicker slush is associated with poor braking(which I assume means poor deceleration and poor performance) which seems to be the opposite of what you quoted and what is also widely quoted in performance articles.
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Re: Your opinion on contaminated runways

#16 Post by pelmet » Thu Dec 28, 2017 8:35 pm

It still can't be trusted if the conditions are changing...

"The Providence Field Condition (FICON) was 5/5/5 with thin snow, and ATIS was [reporting] 1/2 mile visibility with snow. The braking report from [the] previous B757 was good. Upon breaking out of the clouds, we saw an all-white runway with areas that looked as if they had previously been plowed in the center, but were now covered with snow. Landing occurred with autobrakes 3, but during rollout I overrode the brakes by gently pressing harder. However, no matter how hard I pressed on the brakes, the aircraft only gradually slowed down. Tower asked me if I could expedite to the end.… I said, “NO,” as the runway felt pretty slick to me. I reported medium braking both to the Tower and via ACARS to Dispatch. A follow-on light corporate commuter aircraft reported good braking. I was a member of the Takeoff And Landing Performance Assessment (TALPA) advisory group…and am intimately familiar with braking action physics as well as the Runway Condition Assessment Matrix (RCAM). There was no way the braking was good or the snow was 1/8th inch or less in depth. I would [suggest that] data…be collected from the aircraft… to analyze the aircraft braking coefficient.… It would also be of value to ascertain the delivered brake pressure versus the commanded pressure for this event, as there can sometimes be a large disparity in friction-limited landings. I think that pilots do not really know how to give braking action reports, and I don’t think the airport wanted to take my report of medium braking seriously. I also think pilots need to know how to use the RCAM to evaluate probable runway conditions that may differ from the FICON. Additionally, there is no such description as “thin” in the RCAM. None of the FAA Advisory Circulars that include the RCAM have thin snow as part of depth description."

From ASRS #451
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