T/O B-737 Contaminated RW

[Santa737]

We disputed with colleagues about contaminated RW T/O (B-738) The subject of dispute why ATM is not allowed when RW is contaminated by slush, snow, standing water or ice. Any ideas?:rolleyes:

[despegue]

Can you please explain what you mean with ATM? I can only think of Automated Teller Machine?...:\

One thing about Contaminated runways is the fact that your V1 can be higher (closer to Vr)than when calculating wet RWY speeds, and indeed, no reduction.
Reason for this is the failure to attain adequate acceleration after an engine failure due to the increased drag of the contamination on the RWY.

[Santa737]

Thank you despegue (http://www.pprune.org/members/59407-despegue). ATM is assumed temperature method.

[lederhosen]

If you have access to the FCTM (sorry Boeing Flight Crew Training Manual) you will find a section called abbreviations at the front. If you look under ATM sure enough it defines ATM as assumed temperature method.

Hulshof's Performance reference handbook for the 737 does not agree with Despegue's suggestion of increased V1 speeds when operating on contaminated (note the difference from wet) runways. To quote: 'since the reduced stopping capacity is more significant than the reduced acceleration capability, V1 needs to be reduced to a lower value.'

Common sense and experience with events like Air Berlin at Dortmund suggest that runway friction measurement is also an inexact science. Reducing margins on contaminated runways is not a good idea when you do not really know how much you have in hand.

Judging by his chosen name Santa737 probably operates somewhere more often contaminated than I do. On the relatively rare occasions I cannot use assumed temperature I find the restriction no big deal. In fact I like it.

[Denti]

Indeed, not being able to use ATM is rather a positive thing in my view, however one is still able to use fixed derates which is a very good idea indeed. As lederhosen wrote usually V1 values on contaminated runways are very low, some of them even in the below 100kts area on the 737. V1 to Vr spreads can be rather big and are something one is not really used to on this type so we do brief that in those situations.

However why we are not allowed to use ATM is something where i would have to guess, so i'm waiting on input from more knowledgeable persons there :)

[despegue]

Denti,
Can you please explain me how you can have a V1 that is below 100kts (so around 16kts. below Vmcg on a max thrust T/O on B737-300 for example) and no reduction?
In my company, this is a BIG no-no and we have been demonstrated the result of such a low V1 combined with an engine severe damage in the sim... indeed a big no-no...

[de facto]

Despegue, the question is about the -800 not the jurassic park.
V1 can go below 100kts as stated,lowest vmcg by memory is about 93kts,for sure 99kts.

Common sense and experience with events like Air Berlin at Dortmund suggest that runway friction measurement is also an inexact science. Reducing margins on contaminated runways is not a good idea when you do not really know how much you have in hand.
Agree,therefore ATM not allowed but Derate ok if vmcg limited.

[PEI_3721]

Because AC 25-13 prohibits the use of reduced thrust on contaminated runways.

http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/list/AC%2025-13/$FILE/AC25-13.pdf

“Reduced takeoff thrust setting are not authorized on runways contaminated with standing water, snow, slush, or ice, … …. and are not authorized on wet runways unless suitable performance accountability is made for the increased stopping distance on the wet surface.”

Although the AC does not provide a reason for this it is most likely due to the reduced stopping capability, thus the aircraft should accelerate and reach V1 as soon as possible to maximize the total distance available. This is best achieved with full thrust.

[300-600]

Agree with PEI....probable reason is to reduce max stopping distance. Also correct the poster who said measuring runway coefficient can be an inexact science....especially true where slush is involved close to the zero degree point.

[Breakthesilence]

Using ATM, you prolong the time on the ground during the takeoff roll increasing the risk associated with contaminated runways (difficult control of the airplane, damages by ice, slush or pieces of snow blowed to the plane structures etc.) together with increased takeoff roll distance.

Compared to the ATM, Derate method is better on contaminated runways because you have a V1mcg lower than using the first one (due to the Derated max N1 limit that is used to compute the Vmcg: less thrust = less yawing moment in case of 1 Eng out); on the other hand, using ATM doesn't prohibit to increase thrust (windshear, engine failure) on the remaining engine up to the max takeoff thrust increasing control difficulties (while with Derate you can't without being "out of limitation" and getting the situation worse with a low Vmcg).
Derate method could reasonably let you take a higher maximum takeoff mass than a full thrust normal rating takeoff on contaminated runways. (the airline I work for doesn't use the Derate method, those are just some informations I recovered deep in my odd mind :E)

[lederhosen]

There is obviously a difference in the characteristics of runways with a hard contaminant and a fluid contaminant. The former being typically ice or compacted snow and the latter being slush or standing water.

With hard contaminant the main problem is stopping distance, whilst with fluid contaminant both acceleration and stopping distances are increased.

As has been pointed out you are allowed to derate which increases controllability in the engine failure scenario and may permit higher take off weights. It depends of course whether you see engine failure as the principal risk. If you think that all the other possible reasons to reject a take off outweigh the very rare engine failures on the runway (except of course in the sim!) then going full thrust would seem to give the best safety margin by getting you airborne in the least possible distance and time.

However the first question is can you delay take off until the runway is cleared? After that comes the question how can you give yourself the best margin of safety. Clearly reducing weight is a good idea. In reality this means keeping extra fuel to a minimum, of course absolutely no tankering. Using the longest runway is also surprisingly overlooked on snowy days e.g. the number of people using 26L in Berlin Tegel when 26R is right next to it and considerably longer. Everyone knows this stuff, but compliance is at times patchy at least when watching from the holding point and observing where people are rotating along the runway.