No assumed temperature for contaminated runway
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.
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.
Only half a speed-brake
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.
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.
Only half a speed-brake
Derate v assumed temp
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.
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.
Moderator
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.
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.
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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.
Only half a speed-brake
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.
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.
Only half a speed-brake
Yet it is exactly this which does not reasonate.
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.
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.
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
......
It is not rational to preclude a use of DRT which is otherwise a legal rating
... 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>.
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.
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.
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/engin...ng-at-takeoff/
Flight Detent
Knowledge, know how, and wisdom; much valued, difficult to acquire, often absent or mislaid in the modern world. Thank you.
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/engin...ng-at-takeoff/
Flight Detent
Moderator
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.
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.
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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.
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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.
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.
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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.
