No assumed temperature for contaminated runway
 

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

"Russian Roulette" comes to mind ... ?

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

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.

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.

Dare 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....

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.

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?

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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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.

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.

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?

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

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.

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 ?

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.

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.

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…

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.