No assumed temperature for contaminated runway

[kuobin]

Hi, dear :
Does anybody know why No assumed temperature takeoff is allowed for the contaminated runway?

[john_tullamarine]

"Russian Roulette" comes to mind ... ?

[dixi188]

If in doubt, use max thrust.
They are helping you with this decision already made.

[awair]

Reduced thrust results in reduced acceleration.
The contamination further impedes acceleration.
While, conversely, the contamination is expected to reduce braking action.
Both the take-off run & accelerate-stop are adversely affected.

Plus, as above.

Good luck.

[Denti]

Well, Flex or Assumed Temperature is not allowed. However, derated thrust settings, if available, are allowed and may allow an increased performance limited TOW. However, one has to be careful during training to make sure that pilots understand that increasing thrust above the derate could lead to serious control issues.

[BANANASBANANAS]

Well, Flex or Assumed Temperature is not allowed. However, derated thrust settings, if available, are allowed and may allow an increased performance limited TOW. However, one has to be careful during training to make sure that pilots understand that increasing thrust above the derate could lead to serious control issues.

Exactly so. Under the 'right' environmental conditions on the 'right' contaminated runway a B744F could have a higher RTOW with TO2 than TO as the Reference Thrust.

[FlightDetent]

Well, Flex or Assumed Temperature is not allowed. However, derated thrust settings, if available, are allowedDare I ask, what's the logic behind forbidding one and allowing the other? Honestly clueless (only flew AST airplanes. Guess I share a reasonable insight how both of these work and what the differences are but at the first ask struggle to see the enabling difference for DRT.

So far I believed the CONTA = NO AST was a philosophical / systematic choice. Just not to try all the luck in all cases, supported by the empirical inability to correctly describe what the actual contaminant of the day really is like, let alone the aircraft's behaviour over it.

The AST calculation has some margins (density) and limits (Vmcg/a) embedded which the DRT does not, correct? Why is the ruling the other way around....

[Denti]

Derates are essentially a different engine rating, well, not really of course, but it is considered the same for the performance calculation. Which means you can legally use less thrust, but also take into account the advantages like, especially for contaminated runways, lower Vmcg/a, whereas Flex/assumed temperature cannot take that into account.

Have seen double digit V1 figures on the 737 with the use of derate on a contaminated runway (how would that work on airbus with the callouts?).

I am sure john_tullamarine can explain it in much more (historical) detail.

[tubby linton]

Contaminated runway ops do not give you the same safety nets as operating off a clean strip. The idea of using maximum thrust and a high lift flap setting it to get you airborne asap. This has to be weighed against having a reason to stop which given the braking action and operating possibly below Vmcg could also pose big problems. Do you really need to depart under these conditions or can you wait until the runway is clean?

[FlightDetent]

The questions stands.

DRT with regards to Vmcg/a is no more protected than AST. Contrary, with DRT for many installations a pilot by his own mishandling can find himself outside the certified territory, a case for calling the AST more protected.

But that is available knowledge. As well as the fact that DRT with lower rating and Vmcg/a could yield a higher PTOM compared for AST with the same EPR/N1.

On a wet runway, both are allowed to depart. The DRT configuration might provide for better TOW.

On a contaminated (frost covered): The surface deposit is the same, the drag effect and calculated friction is the same. Even the delivered thrust might be the same. In that case the AST could be less enabling (higher V1 and less stopping distance remaining) and firewalling the levers poses no risk as opposed to DRT. Yet, it is the AST which becomes illegal.

I could see a commercial / statistical risk / exposure balance in there, similar to wet screen height and TOR calculations. Okay.

But is there a technical element that is more risky with AST compared to DRT? Carrying less weight and being protected to full rated thrust is less risky, right?

Sure, a lower Vmcg/a gets you airborne while reducing TOM would not move that goalpost. Possibly unable to depart due to MNM V1 and lack of braking real estate thereafter - an inherent limitation of AST.

But in a case where the numbers do add up, what's better with DRT to warrant the additional legality? Wrong vector on the question, actually. What's more sinister about AST for it to be unacceptable?

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[john_tullamarine]

Boy, this thread looks to be taking off (pun intended).

However, derated thrust settings, if available, are allowed and may allow an increased performance limited TOW.

We know what you mean but better to cite "handling-limited takeoff". With derate, you may get a benefit from a lower Vmcg or Vmca which allows for a lower V1 or Vr which can provide for a better runway-limited set of figures. Perverse situation but there you are. I am guessing that, in most cases, the benefit will be from a reduction in Vmcg. All depends on the particular Type's performance numbers in the AFM.

Dare I ask, what's the logic behind forbidding one and allowing the other?

Derate is philosophically the same as unbolting the original engine and bolting on another, lower thrust motor. So why would we not adopt the same attitude to takeoff.

whereas Flex/assumed temperature cannot take that into account.

My take is that it is a case of philosophical navel gazing. Flex is only a money-saving animal, rated is what you are gifted by the OEM. (Or what you pay heaps for to get the gift for a derate option).

a pilot by his own mishandling can find himself outside the certified territory

Certification stuff is a set of presumptions, one of which is that the crew is competent to do whatever is required to be done. There are no guarantees in certification, just probabilities. Generally, things go reasonably fine as evidenced by a very low mishap rate overall. If, on the day, the pilot is incompetent, the bets are off.

firewalling the levers poses no risk

Danger, Will Robinson. Cavalier firewalling can introduce some undesirable dynamics into the handling. I was involved in a fatal mishap investigation years ago where the crew lost it principally due to firewalling the good motor. My take is, leave the throttles where they are unless you are sure that you are going to hit the hill.

Again, none of this stuff comes with an iron-clad guarantee that it is all going to work according to Hoyle ..... all you can do is load the dice in your favour.

[tdracer]

firewalling the levers poses no risk

Danger, Will Robinson. Cavalier firewalling can introduce some undesirable dynamics into the handling. I was involved in a fatal mishap investigation years ago where the crew lost it principally due to firewalling the good motor. My take is, leave the throttles where they are unless you are sure that you are going to hit the hill.

Exactly - firewalling the thrust levers is (or at least should be) a last resort - the "we're going to crash anyway so I might as well try this". If you're light, you can rapidly run out of tail to handle the asymmetric thrust (and if you're not light, you're probably not using much derate). On some types (mainly pre-FADEC, but it can happen with FADEC dispatched in a degraded mode) firewalling can result in a big overboost. There was a Kallita 747F that crashed roughly 10 years ago - they lost an engine around V1 and firewalled the engines (non-FADEC) - the resultant overboost rapidly failed another engine, and a third engine suffered damage and started to roll back. IIRC the pilots survived, but they came down on the house of some unfortunate farmer who wasn't so lucky.

[FlightDetent]

Gentlemen, respectfully.

The notion of 'firewalling TL is not a risk' was specifically used to compare the AST full thrust which is to a pretty much all reasonable extent covered by the certification and AFM data against over-boosting a pilot-selectable DRT beyond the arbitrary AFM limit all the way to the max-hardware setting. Where the risk of flipping over is very real. No need to capsize the topic over one word.

JT: The DRTs we talked about are pilot selectable. When the crew is allowed to save money (by using an option their airlines invested into) using DRT method or AST method, on wet runway both are allowed. On a frosty (contaminated) or watery (3mm+) AST is illegal yet DRT remains a possibility. I suspect you may have left some breadcrumbs in your post but cannot get the smell of them, apologies.

The fact that in certain ASDA scenarios DRT will pick more payload through lower control speed is understood. As much as I flex my brain (double pun intended), that is a parallel subject to the approved/not-approved case. Or?

[john_tullamarine]

So long as folks have the basic story, the sideline considerations are of academic interest ?

[FlightDetent]

You do tell, good sir. In my language 'academic' replicates for 'scientific', meaning more noble and best qualified within a framework of a discussion.

Since Old Smokey has transcended you are the best qualified, an observation made from the postings you share with kindness in these places.

Not having the core grasped was my concern, different from not allowing topic expansion.

[john_tullamarine]

My concern is that the younger folk read what we discuss and take away whatever they might make of it. I have no problem with a slow and steady throttle increase, matched by appropriate control inputs. The reality is that there have been various mishaps where the pilot has mishandled the throttle advance grossly and found himself out of synch with the yaw response. Right or wrong, I would rather emphasise the underlying philosophy that the flex setting is adequate (providing we don't end up with gross failure problems out of left field). For the derate case, the main concern with a significant asymmetric overboost, likewise, is the potential for Vmcg or Vmca problems, especially at light weights and min speed schedules. That we have a different position on the topic is fine. To get to the certification bottom of why this but not that would require some extensive digging into the literature.

I certainly would welcome more and extensive discussion on the subject as it is so endemic in operations that the more folk in the front seats know about it the better for risk control in operations.

Old Smokey - a good while now since I have had discussions with young Tiger - we go back a long way having worked together on a project around 30 years ago before he moved into performance work. As I recall he was with TAA prior to the Troubles. I was aware that he had fallen into difficult medical circumstances but haven't heard how he has since fared. Does your comment indicate that he is no longer with us ?. I fear that may be the case ?

[Alex Whittingham]

I think it is just regulatory inertia. Those of us who were flying in the '70s and '80s will remember a different relationship between company accountants and pilots, and Captains were closer to God in those days. The accounting wasn't so tight and the senior pilots of the day had been through the '50s and '60s where accidents and engine failures were more common, and the tombstone imperative had moved not so far along its track. The suggestion that operating with less than full thrust would save the company money was not universally well received, nor were concepts like intersection take-offs. The regulators, probably of the same generation, seem to have agreed and imposed a series of caveats on assumed temp take-offs such as 'not on contaminated runways', 'not with anti-skid or reverse inop', 'not when wind shear is forecast after take-off' etc., and, once written, it takes a brave regulator to remove the caveats. Derates were rare. On the L1011, for instance, nearly everyone had RB211-524B4 engines but just by chance one airline, BWIA, had ordered the the jets with RB211-22B engines, a lower rated thrust. That meant we could manually set the EPR for the derated thrust, accompanied by severe warnings about selecting full TOGA thrust in emergency and great oaths not to do so in the take-off brief - no automatic protection then. The argument that a derated take-off was a normal take-off was accepted at the time, and it was permitted by the regulators without caveat. Over time modern engines became available with many more derate settings now selectable from the FMS. I can see no reason in principle why a reduced thrust / SEL OAT / FLEX / Assumed temperature should not be also be treated as a normal take-off. I note that the AMCs to EU OPS still make it clear that reduced take-offs are always at Captain's discretion.

[Goldenrivett]

On the L1011, for instance, nearly everyone had RB211-524B4 engines but just by chance one airline, BWIA, had ordered the the jets with RB211-22B engines, a lower rated thrust. That meant we could manually set the EPR for the derated thrust, accompanied by severe warnings about selecting full TOGA thrust in emergency and great oaths not to do so in the take-off brief - no automatic protection then.

Hi Alex, The only “automatic” protection we had was the Flight Engineers hairy hand.

Unless you changed the SOPs so that crews could not select Full Power in the event of an engine failure until above AA and above X kts (VMCA), then Flex power take offs (Reduced thrust, Assumed Temp etc) can not be permitted if you are taking advantage of a reduced VMCG / VMCA.

[Sidestick_n_Rudder]

Since no one seems to have given the answer to the original question….

I believe the problem with FLEX/ATM method is that by definition it lowers the thrust as much as possible, to make use of the entire runway. So, unless you are limited by climb/2nd segment, ATM method will give you zero (or almost zero) stop margin, regardless of the weight you’re taking off at.

OTOH, FIXED de-rate is by definition fixed - as mentioned by others it’s just like bolting a smaller engine. So with fixed derate, unless you happen to be exactly on rwy-limited RTOW, you will get some stop margin…

And since rwy friction is considered unreliable on contaminated runways, the regulators want you to have some extra stop margin…

Hope it makes sense…

[common toad]

Does anybody know why No assumed temperature takeoff is allowed for the contaminated runway?

A clean runway is just that; a known quantity.

A contaminated runway may have varying degrees of contamination in its length and width. As a result its affect upon acceleration and, in the event of a rejection, the braking action cannot be known with any certainty.

That is why we use full power (or derate) and a flap/slat that will get the aircraft airborne without delay.

[simmple]

A derated take off is like said, bolting on a smaller engine, so when you go full power on the live engine after a failure you get less thrust therefore less yaw and control problems on ground and in air.
With flex you get max rated on the live engine so more yaw and control probs with a failure.
Using derate in the sim rather than flex made life a lot easier.

[FlightDetent]

Hope it makes sense…Unfortunately it does not. What sounds like agreeable common sense happens to be a lack of depth (no judging). Worthy of a different thread, apologies, and to my belief is squarely due to the fact that pilot performance training morphed from actual Aircraft Performance to demonstration of Calculation Software Features and EFB reset procedures. Which is needed on the line but somehow the performance is not trained or reviewed anymore. I don't disagree with JT about the topic content, I disagreed about what the topic was - not best seated now to introduce a new one, genuinely sorry (check the TAS effect on FLEX calculations...)

Re: Old Smokey - I have no knowledge neither ever had a personal connection. Used my word to describe lifting off from the forum but fearing to ask the reasons. My last bit of info was probably J.T. making a similar comment to the above. Lord's blessings, either way.

Alex: Thank you kindly, I lean to think the same but came here to see if perhaps there's a small rock with a creepy technical / perfo reason hiding beneath.

The same engine used to be differently rated e.g. 18.5 / 22 / 23.5 and some on the thread here indeed had flown -400 with both 22k and 23.k, or from the other shore the -214 on 27k or 23.5k. Which is bolting a different engine to the same hull, as repeatedly pointed out above. It may be only wizardry and designation and very little or no even mechanical difference at all - clear on that perfectly.

The AST is a method of operational thrust reduction for which some arbitrary constraints are set up (not pushing all the luck at once), guess constraining the reduction to no greater than 25% has not been mentioned upthread.

And then, sometime later, darn smart folk and salespeople come up with a clever if not a weasly idea: Operational de-rate. Do the hard work and publish an AFM supplement, speculate on the paper the aeroplane does have the smaller rated engine bolted on and obtain
- lower declared control speeds
- lower noise footprint
- relief from the conservative measures fenced around the AST method.
while at the start, middle and by the end of the day it is yet still just a way not to use the full-installed thrust and the engine is bolted on the same.

G.rivett: The landscape we are seeing now, to explain the bewilderment,
- flex allowed down to 40% i.s.o. 25%
- DRT becomes a pilot-selectable option during cockpit setup, even offering multiple choice of down-ratings
- DRT + AST at the same time is permitted
- some of the installations cannot be firewalled beyond the operational DRT setting (major good)

Hence the twist discussed: If the engine target is 86% N1 reduced operationally from the full e.g. 98%, what's the reason behind the RoE being so different?

Lower Vmcg/a does not explain it, because the DRT operates inside its limit and so does the AST.

[FlightDetent]

With flex you get max rated on the live engine so more yaw and control probs with a failure. Using derate in the sim rather than flex made life a lot easier.Nope. Just keep the TKOF setting and enjoy the ride. Somebody's choice to make it harder doesn't mean it needs to be.

[fdr]

PRO:
With a Derated thrust, the thrust set is the maximum that is available within the EEC/DEEC etc or by the TAT/EPRL limiting system.. etc. That precludes to an extent the firewalling of the thrust levers and introducing a greater asymmetry issue in the event of a thrust loss. Directional stability not he runway may be constrained where there is low friction coefficients.... In an Assumed Temp case the thrust can be increased to the rating limit, and the friction coefficient may exacerbate directional control on the ground. hmmm. maybe..

CON:
Derate removes the opportunity to get more thrust beyond the derate level that is selected. That is, considerable thrust potential is being removed. That shouldn't be a factor in the normal course of operations, but then blowing cowls off, having compound failures etc are a repetitive theme is aviation.

AC25-13 was issued in 1988, and was a fair GM at that time.

§4(c) defines reduced thrust takeoff,
§4(e) describes contaminated runways.
§5(f)(1) gives the prohibition.

§5 is an AMOC, that doesn't prohibit an operator making a case for using an ATM instead of DRT case.

This happens to be a point of interest in my current testing, as I am impacting thrust output of the engine considerably, for the same N1 setting the aircraft was getting ~20% more thrust output, and to get normal thrust output took a 5% reduction in N1. This then makes the case that the thrust itself has not reduced at that point, even though N1,N2, EGT, ff have been reduced, the thrust itself is being set at the same Fn output as the normal engine. That then permits up to a. further 25% reduction in thrust, within the GM of the AC, at which point the N1 etc is rather low, low enough to have to look at some cold weather issues with GPWS mode alerting.

The AC's position was to provide a rational prohibition of using an ATM reduced thrust in substantially adverse conditions. That is not a bad position. It is not rational to preclude a use of a DRT which is otherwise a legal rating that performance is predicated on, so the AC doesn't restrict the use of a DRT, an AC cannot be more restrictive than the regulation which the DRT meets in principal. The argument on the impact of increasing thrust doesn't survive cursory examination, as the contrary case is a greater concern to controllability, that is, higher thrust levels lead to greater potential control difficulty on a contaminated runway, on the sudden loss of an engine...

The balance of risks on thrust setting being higher or lower for a contaminated runway relate to the potential of an engine to fail at any time, of reduced acceleration that may be random from contamination, of reduced deceleration from variations of friction coefficients, impacting braking, and the issues of control with asymmetric T/R. On the day, the guidance needs to consider the runway width, excess runway lengths, risks of an overrun condition etc, which are bespoke to the day and location, weight and weather. And then there is the MD-80 inboard wing snow feeder system to the engine intakes... as found on CRJ's, CL600's, most business jets etc... All adding to the fun of operating under uncertainty, although, hint, anti icing HOTs on aircraft with aft engines, or without leading edge devices takes on a certain piquancy.

That's why the manglers get paid the big bucks to guess what the best answer is for the lawyers years later, and why the pilot gets paid the big bucks to take the fall of such policy and training, and to be the first on the scene of an accident.

[john_tullamarine]

Turning into a good discussion ...

So, unless you are limited by climb/2nd segment, ATM method will give you zero (or almost zero) stop margin, regardless of the weight you’re taking off at.

QF was one of the first to introduce reduced thrust. The then Performance Boss, Wal Stack (with a flying background in prior years), sold the idea to the troops with the guarantee that he would always make sure that there was a runway pad left in the kitty for Mum and the kids (1000ft, if I recall correctly). A wise old chap was our Wal and a thoroughly entertaining lecturer - he was a visiting lecturer at Sydney Uni when I were a young lad student there.

With flex you get max rated on the live engine so more yaw and control probs with a failure.

Only if you elect to shove the throttles up in the event of a loss of noise.

my belief is squarely due to the fact that pilot performance training morphed from actual Aircraft Performance to demonstration of

Indeed. Back in the 60s-80s the ops engineers ran the performance and weight control courses for endorsements and a fair bit more of the underlying stuff was passed across. I provided that for one operator during the 70s-80s and their line pilots got to know a lot more than the typical pilot has at the end of his ATPL exams ....

some of the installations cannot be firewalled beyond the operational DRT setting (major good)

.. but those that can present an ever-present worry to management and regulators.

Lower Vmcg/a does not explain it, because the DRT operates inside its limit and so does the AST.

Handling may provide some useful outcomes but engine life cycle dollar costs are what it's all about.

Derate removes the opportunity to get more thrust beyond the derate level that is selected.

Depends on the installation. If the derate is not constrained by the system, there may be a problem.

[Goldenrivett]

G.rivett: The landscape we are seeing now, to explain the bewilderment,

Lower Vmcg/a does not explain it, because the DRT operates inside its limit and so does the AST.

Hi FlightDetent,
DRT has a lower VMCA / VMCG - hence V1 min (V1 GO) is lower which can be achieved earlier during the take off run. Thus it is possible to increase TO mass despite lower thrust setting. Thrust above DRT is NOT allowed.

FLEX, Assumed Temp etc. may produce the same thrust as DRT - but crews are permitted to to increase thrust up to TOGA limit in the event of an Engine Failure. Therefore VMCG / VMCA must be higher - thus the V1 min must be higher than the V1 min associated with DRT.

[FlightDetent]

G., still sideways to the dilemma. Why DRT set to an identical N1 provides higher PTOM on ASDA limited case is well understood, although rarely seen spelled out as clearly as you just did.

The different RoE is being scrutinized. Having an engine fail at their respective min V1 will be covered on both sides by their own pre-determined Vmcg/a limits. Both methods are pure pilot manager's choice (cutting some deadwood arguments here) to save the engine costs. DRT carries an extra book of perfomance tables, alright.

FDR sheds a light, noting that (as long as) DRT is considered an equivalent full certified standard there shall be no more constraints imposed. Which is a get out of jail card.

[FlightDetent]

Yet it is exactly this which does not reasonate. a rational prohibition of using an ATM reduced thrust in substantially adverse conditions
......
It is not rational to preclude a use of DRT which is otherwise a legal rating From a risk-exposure viewpoint we don't want the pilots avoiding the full boost by an operational choice (a) and then ...
... then there really should not be any (b) if (a) is being respected.Hey, cap, we need to go full forward on this slippery piece, no flex allowed.
Oh my dear padwan. We'll call it a DRT and reduce as usual. Here, let me activate the CDU prompt for you <click>.


Okay, here's a thought. :) The exposure is less with DRT because it costs more money to purchase and is not an installed option available to completely everyone yet (AST pretty much is). Thus a large percentage of people wishing to make a reduced-take off on CONTA still won't be able to roll that dice. The concept of not pushing all the luck all the times by everyone is retained in good standing.

[alf5071h]

Re JT #11 and #25
Knowledge, know how, and wisdom; much valued, difficult to acquire, often absent or mislaid in the modern world. Thank you. :ok:
We learn from mistakes - small one's. However, in a very safe industry, increasingly intolerant of mistakes, and with rare, unique events, learning tends to revert to theory and not practice (simulators are simulation, lacking 'surprise' and poorly represent the feel in event).
There is less requirement on memory, more on the written procedure and advice.

Unfortunately investigators, regulators, and ourselves increasingly mistake hindsight for the wisdom of foresight.
So let's keep to the simple things, follow the basic guidelines, and also be prepared, because when 'it' happens to you it will not be as per the book; not the engine, just the decision stop/go. The task is to manage the outcome.

Look at the day job. Contaminated operations involve new risks, in most areas mitigated, e.g. use max thrust. Yet these could introduce, new or overlooked problems; not the emotive Vmcg, but the simple 'off the side' during engine run up because the runway is more slippery than we are familiar with, higher thrust levels, wind.

https://safetyfirst.airbus.com/engine-thrust-management-thrust-setting-at-takeoff/

Flight Detent :ok:

[john_tullamarine]

Yet these could introduce, new or overlooked problems

I was not aware of the cited Airbus article. It sums up the philosophical problem of surprising "unknowns" very well.

My take is that when we get caught up in such a situation it all happens very quickly and the risk is that our cognitive and hand-eye capabilities are overwhelmed and left behind the action at which point we tend to be along for the ride and part of the problem. Been there, frightened myself witless, with a changed outlook following.

Many have had similarly frightening experiences, think about it over an ale afterwards, and adopt a more conservative approach to the flight management problem as a mitigator.

[vilas]

https://cimg8.ibsrv.net/gimg/pprune.org-vbulletin/1080x1703/screenshot_20221202_095458_2_5d156b4744cc17077cb20557435f9e7 abb229667.png
graphical presentation

[vilas]

This is an old presentation about when thrust is not correctly set to Takeoff and Airbus cautions about it across all models. B737 doesn't have cautions. Such an event happened last year on 737-800 in Finland on 1Dec21 where on a contaminated runway the PF on derate TO was clearing engines on brakes at 70% N1 got distracted by slipping aircraft (due to his insufficient brake application) and forgot to set TO thrust. Capt was distracted by ATC query didn't check. The aircraft got airborne 400mts from RW end. So this is another problem. Why should pilot bother to use flex on contaminated runway even if it was allowed? He has enough headache as it is.

[kuobin]

Thank you so much. guys.

[AThR]

It is simpler than you think. What does assumed temperature or flex do? It actually reduces engine thrust and take the runway length as an advantage to get airborne. I am not familiar with the Boeing performance software. But if you use Airbus flysmart to calculate takeoff performance, it gives you two sets of data. The data for a TOGA takeoff and the data for max flex or your preferred flex.

One of the main differences between these two data is the remaining runway length after an accelerate stop (rejected takeoff). When TOGA is used, the margin is always higher than flex because the former gives you a better acceleration and you get airborne a lot quicker.

So, to answer your question. A contaminated runway has less braking efficiency. Thus, you need more runway length to stop the aircraft. A TOGA takeoff gives better acceleration and thus gives you more runway to reject a takeoff.

As others have mentioned you can do a derate takeoff. Here, the thrust of the engine is reduced to reduce Vmcg/a (minimum control speed). A reduced Vmcg allows you to reduce V1. And a reduction in V1 means you have more runway to stop the aircraft. Hope this helps.

[172_driver]

I operate mostly short sectors with B737, so we're light weight. De-icing pad is located at the deep end of the runway which is 3600 m. Snow clearing/chemical treatment of the runway often brings the runway conditon code up to 5 (= Braking Action Good). Performance Tool publishes our accelerate stop distance and stop margin in case of RTO at V1. Since the runway is "contaminated" we're confined to using Derate and no ATM. It's definitely annoying to see TO-2 (Derate) with stop margin of 2000+ m when we safely could've used TO-2 and ATM 59 deg. with a stop margin still well in excess of 1000 m.