Runway Contamination

[Roadtrip]

Was in NY on Sunday and saw a Virgin 744 and a Korean 742 takeoff on Rwy 31L intersection KA where the conditions were 1/2" slush/ice on the SE 5,200' and 3" packed snow/ice on the NW 5,300'. Some smaller planes took off too, but they probably didn't need the last 5,300' to make their takeoff data good. I think US carriers generally cannot have more than 1/2" of contamination. Would hate to have witnessed a heavy weight abort by one of those 74's once into the packed snow/ice on the NW end. Interested to know what the rules are for some Euro/UK carriers about runway contamination above 1/2".

[This message has been edited by Roadtrip (edited 02 January 2001).]

[mutt]

The FAA and CAA have pretty similar rulings about the maximum depth of contamination. FAA = 0.5 inch (13 mm deep) CAA = 0.6 inch (15 mm deep).

We had a discussion about this during the last JFK snow storm, the consensus was that 3 inches of PACKED SNOW which was SANDED wasn’t classed as a contaminated runway at all! This was in consultation with Boeing and the NY Port Authority.

I don’t work for Virgin or Korean so I don’t know what their rules are, but I do know that we cancelled our Sunday night B744 flight, so this topic didn’t come up this year.

Mutt

[Roadtrip]

Mutt - How interesting. Boing doesn't consider 3" of snow contamination? I'm going to need to talk to my company experts about that to see what they think. Void any guidance, I still think it would have been stupid to attempt takeoff with that packed snow (sanded or not) on the last part of the runway, due to abort considerations. I would think it invalidated V1 speed.

Do I gather that if the flight hadn't been cancelled that you would have taken off? How would you have adjusted your takeoff data for the unknown braking action on the NW end?

[CaptainSandL]

If dry snow, max allowable is 60mm (CAA rules)

[quid]

Unfortunatly, (and I first fought this battle with my Chief Pilot over 30 years ago), the FARs and Performance Data makes NO allowance for decreased braking action on aborted takeoffs. (Other than the new rules for the 15 foot screen higth.)

The "Clutter" or "Contamination" adjustments are only made to reflect the reduced acceleration during the takeoff roll.

If there is 3" of hard packed snow and ice, no adjustments would be made.

It would have to be the PIC's decision at this point. Just 'cause it's legal doesn't mean it's safe.

[flying-bits´nbytes]

The TO Performance Program from Airbus doesd consider this. When limited on the runway length you´ll get a lower max TO Weight on a RWY contaminated with compacted Snow.

FBN

[CaptainSandL]

Further to my prev post, for the 737 if dry snow then runway is not considered contaminated until there is 10mm. If wet snow/slush/standing water then must be 3mm until the runway is considered contaminated.

Our company performance engineer tells me that the decreased braking action IS accounted for in the contaminated (cf slippery) figures. Therefore if a runway is both contaminated and slippery, only the contaminated calcs need to be done.

S & L

[mutt]

Roadtrip,

Please read my message again, 3 inches of PACKED SNOW WHICH WAS SANDED
The last time that I looked in the FARs, you weren’t required to make any corrections for contaminated runways.
This doesn’t make it right, but what other guidance is available to you? FAA AC91-6B?


Quid,

The data available to you should also account for reduced stopping ability. You mentioned the 15 feet screen height, this assists with your accelerate go, they have also allowed the use of symmetric reverse thrust to assist in the accelerate stop. If you take a look at some charts (particularly the B747) you will see that your V1=VMCG weights will actually increase as the depth of contamination increases, this is primarily due to the fact that the contaminant builds up in front of the wheel during the stop, this ploughing action assists you with stopping, hence the ability to take a greater weight.

This should also prove to you that there is an allowance for decreased braking action on aborted takeoffs.


Mutt


[This message has been edited by mutt (edited 04 January 2001).]

[Canuck_AV8R]

Mutt:

I was under the understanding that the ploughing effect you mentioned was more applicable to the Accelerate-Go calculation. I understood that as the level of contamination increases the VI speed actually increases also. This is due to the decreased ability of the aircraft to accelerate from V1 to VR in the event of an engine failure at V1. For a level 3 contaminated runway the V1 is higher and therefore closer to VR than for a level 1 contaminated runway.

This was the way it was explained to me by one of our trainig captains and in our operation we are no strangers to contaminated runway operations.

As always JMHO
Cheers
Canuck

[mutt]

Morning Canuck,

If This is due to the decreased ability of the aircraft to accelerate from V1 to VR in the event of an engine failure at V1. For a level 3 contaminated runway the V1 is higher and therefore closer to VR than for a level 1 contaminated runway.

Then why do you get higher weights for deeper levels of contamination?

If you have a problem accelerating from V1 to VR, wouldnt it make sense to reduce the weight?

Mutt

[quid]

Just to make sure we're talking the same terms here:

"Contamination" or "clutter" is any measure of standing water, slush, dry or wet snow that impedes acceleration.

"Wet" or "slippery" is degraded stopping ability.

1" or 3" (or more) of HARD packed snow (or ice), will not slow acceleration, therefore no "clutter" or "contamination" penalty is made. The same holds true for "wet" or "slippery" runways with NO measure of clutter.

The "wet" 15 foot screen height model is only for a/c that come under the new Part 25 rules for about the last 3 years. And...that was changed to allow new models of two engined a/c to take off at heavier weights. (Symmetrical reverse doesn't help much on a 777.) All it does, in effect, is lower the V1 to get it stopped in case of a "slippery" abort. This lower V1 will require a greater distance between (the lower) V1 and (the same) Vr which in turn will put you lower over the screen. Any a/c certified under SR422A, SR422B, and many under Part 25 over the following 38 years do not consider the degraded braking for RTOs.

(There is nothing, however, from preventing your carrier from instituting more realistic performance data, and some do.)

mutt-

Perhaps the above is the case at your carrier. Or, perhaps it's unique to your model a/c, which I haven't flown.

You said: >>If you take a look at some charts (particularly the B747) you will see that your V1=VMCG weights will actually increase as the depth of contamination increases, <<

I have in my office, (and I looked at all of them today), the performance sections for 767, DC-9, and DC-8 from many different major airlines including SAS, UAL, NWA, JAL, SwissAir, etc. NONE of them had an adjustment for V1 in clutter. Granted, none of them were 747 manuals. I'm not saying that some don't, but it's certainly not universal. (And for what it's worth, I've never heard of that before.)

Also, you said: >>the contaminant builds up in front of the wheel during the stop, this ploughing action assists you with stopping, hence the ability to take a greater weight.<<

You're not suggesting that you can takeoff at heavier weights on slush covered runways than you can on a dry runway, are you?

Also, regarding your above statement: Once the throttles are at idle, spoilers are deployed, and you get on the brakes, the a/c really doesn't care if it's being asked to stop from 130 kts. after an abort, or from 130 kts. after a landing. If the "ploughing" effect will help it stop after an abort, it will also help it to stop after a landing - meaning you can stop it in a shorter distance (in either case) with contamination than you can with a dry runway.........I don't think so.

Is that what you meant? Perhaps I read it wrong.




[This message has been edited by quid (edited 05 January 2001).]

[CaptainSandL]

Just to back up Mutts answer, on the 737, the V1mcg limit weight does increase with increasing depth of contaminant. The official reason is as follows:

“Vmcg field length is limited by the accelerate-stop calculation. The benefit of the slush on stopping the airplane outweighs the retardation effect during acceleration, so the more slush the higher the Vmcg limited weight. The reason is that the drag force created by the slush accounts for a smaller proportion of the total forces acting on the airplane during acceleration than during braking. The engine acceleration force is quite large compared to the wheel braking force on slush during the stop, so proportionally, the slush drag has a greater impact during the stop.”

Incidentally, the V1 adjustment is also highest at low contaminant depths, up to –21kts in 3mm snow when on a short field. However at 13mm snow and max TOW, the V1 correction can be up to +4kts ie a positive V1 adjustment!!!

S & L

[Ignition Override]

Very interesting discussion-on contaminated runways (a new limitation requires our DC-9s to use APU and 'high' ignition on takeoff/landing) we are not to use a slippery V1 reduction on takeoff.

[mutt]

Quid,

I guess that the worst thing about contaminated runway operations is the lack of distinct guidelines under the FAA system. We still operate some aircraft with very basic contaminated runway corrections, which are “reduce runway limit weight by 10% and V1 by 15 knots”.

But on other aircraft we have the ability to define the exact type of contaminant, the braking action / friction as a MU factor and the number of thrust reversers that can be used.

We obviously can’t get 2 engine symmetric thrust on a B777 following an engine failure, :) but we can base data on the use of 1 thrust reverser. (This comes with a MEL restricting operations from contaminated runways with a reverser inop.)

I said that the B747 V1=VMCG limited WEIGHTS (not speeds.) increased with contaminant depth. CaptSandl has confirmed the same thing happens with the B737, I also noticed it in both B777 and B757 manuals this afternoon. I’m very surprised that the B767 doesn’t have the same data?

The reason for this increased weight is as follows.

Acceleration on a dry runway = thrust minus drag and friction

Acceleration on a contaminated runway = thrust minus drag, friction and slush drag

The same applies for deceleration. So although the reduced runway friction and increased slush drag slows down your acceleration, it greatly assists your stopping.

You're not suggesting that you can takeoff at heavier weights on slush covered runways than you can on a dry runway, are you?

No I’m not, the runway friction levels are different, but if you were to compare the same friction level, i.e. slush, and compare 6mm versus 12 mm, you would see that the greater slush drag will slow down your acceleration, but it will it definitely assist you in stopping faster, therefore allowing you to use a greater weight for 12 mm.

The same goes for landing.

CaptainSandL

You didn’t sat which model B737 you flew, but this applies to most of them and should answer your query about the higher decrements for lower Vspeeds. Most of the existing wet runway performance adjustments are based on an assumption that the wet braking mu is one half of the dry mu. However at higher weights and speeds the dry mu starts to decrease so the margin between the dry mu and the wet mu (which is established at lower wts and speeds) gets smaller because the wet mu is constant.

Ignition Override

I’m surprised that you don’t adjust the V1 on slippery runways, do you reduce the weight?


Mutt :)



[This message has been edited by mutt (edited 06 January 2001).]

[max lenz]

We all use simulators on a regular basis but still depend on calculation results done by our performance specialists. How exactly he calculated is at least hard to find out if not impossible.
How about this:
They give us their performance program, runway and obstacle data and we complete those with contamination data and run the program ourselves...

Here is another variable I would like to hear more of: On long enough runways - and here contaminants impeding acceleration are taken into account - a big Swiss Airline is using 1.4 times V stall on the Airbus family for a given weight because this shall give better climb angles than 1.2 times V stall and thus higher obstacle weights.
Only with braking impeding conditions as bad friction coefficient or allways with water or slush deeper than one mm - considered to result in aquaplanning - they are willing to reduce V2 to "normal value".
They swear to do a realistic calculation for every runway condition but still I would like to enter the variables myself just to see what happens on "my performance simulator"...

[quid]

OK, I think we're making progress here.

Different operators offer runway analysis data to the crews in different formats. (And some of the more modern a/c have more detailed data.)

There are some basic limitations that must be met, no matter how the data is presented, or what words are used.

These basic items (in this discussion) are:
1. V1 must never be less than Vmcg. 2. The takeoff weight must be limited so that accelerate stop/go (balanced field length) must be equal to (or less than) the runway available.

Wmc (yes, that is a "W") charts are seldom used by flight crews, but the performance engineers have to consider this value. It is a weight that will gurantee Vmcg considerations built into the Takeoff weights and V speeds presented to the crews.

If Wmc is greater than the Field Length Limiting Weight, NO TAKEOFF IS PERMITTED! So, no matter what words are used for this value, be they "boxed weights", "limit speeds", or perhaps "V1=Vmc Weights", the net effect is - AS THIS WEIGHT IS INCREASED, THE FIELD LENGTH LIMITING WEIGHT MUST ALSO INCREASE! So, an INCREASE in the LIMITING WEIGHT makes the takeoff more restrictive, not permissive.

(I'm not a 737 pilot, so cut me some slack here in the illustration numbers I use.)

100,000 lb takeoff, Vmcg 115 kts, V1 115 on a 5000 foot runway, max t/o weight 100,000 so we are in a balanced field length limited t/o. Critical, but still legal.

Now, introduce some clutter and the WEIGHT V1=Vmcg increases, we can't go.

If we had 6000' in the example, we had an unbalanced situation, and the excess 1000' can be used to make the t/o legal. The V1=Vmcg weight increases, but our field length limiting weight is ALSO increased. When we run out of excess runway, we must start reducing our takeoff weight. We can reduce it to the Wmc (or V1=Vmcg weight), but no further.

So, in a nutshell, contamination reduces our max takeoff weight. Agreed?

If not, knowing the manufacturers desire to sell payload capacity, and the airlines desire to maximize it, rather than plowing runways, they would employ snowblowers to cover them all the time! <grin>


max lenz-

Climb gradients are a whole different issue. I'm going to answer you question by starting a new topic "climb gradients". See you there.

[mutt]

Quid,

You are totally correct, a contaminated runway has to give you a lower takeoff weight. There was never any doubt in that.

Max Lenz,

I "think" that you are talking about Airbus's optimized Vspeed policy, (its called overspeed or improved climb by Boeing. But as Quid said, thats another topic.)

As for allowing the pilots to calculate their own performance on the runway surface available, this option is used by a number of airlines, SAS, FEDEX, the crews carry a laptop/handheld computer, they can input whatever conditions they desire IF THE CONDIDITION IS CERTIFIED.

If you have problems finding out how your performance people are calculating takeoff data, GO ASK THEM! Remember that they are there to serve you!

Mutt