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Spir4
4th Dec 2018, 13:47
Hi,

A simple question: why is using flex temperature on contaminated runways not allowed?
Using flex temperature still uses the Vmcg for the full rated thrust, so it's not that you would lose control is it?

With a contaminated runway you want to get of the runway ASAP, but why is a derate allowed then?

I've searched the forum for similar questions, but none gave me a definite answer.

Thanks!

Starbear
4th Dec 2018, 15:00
Hi,

A simple question: why is using flex temperature on contaminated runways not allowed?
Using flex temperature still uses the Vmcg for the full rated thrust, so it's not that you would lose control is it?

With a contaminated runway you want to get of the runway ASAP, but why is a derate allowed then?

I've searched the forum for similar questions, but none gave me a definite answer.

Thanks!
I recommend downloading “Getting to Grips with Performance” an Airbus publication and start at section C 7 Flexible and Derated Takeoff and careful study should answer all of your queries.
You may then be tempted to explore this document further for more gems.
SB

Spir4
4th Dec 2018, 15:47
I recommend downloading “Getting to Grips with Performance” an Airbus publication and start at section C 7 Flexible and Derated Takeoff and careful study should answer all of your queries.
You may then be tempted to explore this document further for more gems.
SB

I'm familiar with that document, and it doesn't explain what I'm asking. It just states that this is the case, but not why this is the case.

Meikleour
4th Dec 2018, 16:13
Spir4:It used to be the case that a higher weight could be lifted using Derated Take-off. This differs from Flex since although both cases use less than full power the Derate case achieves a lower Vmcg since it is not allowed to use TOGA in the failure case. (in contrast to the normal FLEX situation )

Spir4
4th Dec 2018, 16:29
Spir4:It used to be the case that a higher weight could be lifted using Derated Take-off. This differs from Flex since although both cases use less than full power the Derate case achieves a lower Vmcg since it is not allowed to use TOGA in the failure case. (in contrast to the normal FLEX situation )
That is still the case no?
And that doesn't really answer my question.

Sidestick_n_Rudder
4th Dec 2018, 16:57
I would venture a guess, that the reason behind that is that contaminated runways are less predictable than dry/wet runways.

They can have non-uniform contaminant depth and/or friction coefficient.

Therefore on a contaminated runway you want to accelerate/decelerate ASAP and have an extra margin, rather than use the entire available runway, which would be the case using FLEX method...

P.S. As for why derated thrust is allowed. I think de-rated thrust is pretty much like putting a smaller engine on the airplane, so it’s like you are using TOGA with a smaller engine - kind of a rule twisting, but still...

Also, a fixed de-rate in most cases will not result in using entire runway, whereas using FLEX/ATM you decrease thrust till you hit ASDA

Alex Whittingham
4th Dec 2018, 17:17
My guess would be that back in the '80s when derate info was not often available and the contaminated runway data was mostly generic (derived from NASA's testing of a Caravelle in the '60s) it was regarded as pushing things just a bit too far to take off with less than full thrust on a contaminated runway. Enter EASA, and contaminated runway take-offs became not 'Captain's discretion' but effectively a normal take-off, and supported by better performance data. This combined with widely available derate performance data changed 'pushing things a bit too far' into 'really its just a normal take-off' for derates, which were quite a new thing, but didn't quite make the bar for the old-style flex take-offs, maybe there were some residual nerves that a flex take-off on a contaminated runway still wasn't a brilliant idea. In reality, I can't see why the logic applied to derates should not also apply to flex take-offs, if you agree with it. In fact, I would say the reverse applies. Taking off deliberately with less than full thrust on a contaminated runway still doesn't sound like a good idea to me.

Denti
4th Dec 2018, 17:51
Derate is actually quite useful on contaminated runway and can allow to carry higher weight out of that runway than full rated thrust, as it lowers the Vmcg quite a bit, allowing a lower V1. I have seen double digit V1s on 737NGs that way.

However, derate is not available on the small busses, no idea about the big ones, and come to think of it the newly acquired canadian minibus. Which does not seem to be an economic disadvantage, seeing how they sell. Be that as it may, from what i was told in training, the reason for the assumed temperature is mainly a historical one, and has never been challenged and changed since.

vilas
4th Dec 2018, 18:12
With a contaminated runway you want to get of the runway ASAP, but why is a derate allowed then? You are correct in principle for the first part. The second part is purely regulatory. It fulfills the requirement of full power takeoff since derated engine is considered a lower thrust engine with separate VMCG.

FlightDetent
4th Dec 2018, 18:57
I agree with the question. If reduced thrust is not approved due to unpredictable effects of the contaminant, and thus the regulator says - economies aside, full thrust please! ..... Derates being allowed simultaneously: it does not seem to follow the same logic.

The fact that you could lift even more with DRT compared to TOGA scenario (which is well understood and accepted) feels ironic as we get:- a heavier aircraft and
- less thrust at the same time,
rolling over a surface with characteristics so suspect the regulations demand TOGA in the AST/FLEX case. Uneven judgement?

Spir4
4th Dec 2018, 19:06
Now we're getting somewhere, and you guys/gals make excellent points!

Meikleour
4th Dec 2018, 19:38
Spir4: With a contaminated runway there is the situation where the friction available between the tyres and the runway surface is less than with a non-contaminated runway by an indeterminable amount. This will make directional control less certain due to the poorer lateral component available from the tyres - thus derating will help things (from an aircraft control point of view) but not minimise the ground run exposure - as you correctly identify.

Denti alludes to this. This was certainly available on the A330 and A340 and only really affected operations from short contaminated runways.

Denti
4th Dec 2018, 20:47
I agree with the question. If reduced thrust is not approved due to unpredictable effects of the contaminant, and thus the regulator says - economies aside, full thrust please! ..... Derates being allowed simultaneously: it does not seem to follow the same logic.

The fact that you could lift even more with DRT compared to TOGA scenario (which is well understood and accepted) feels ironic as we get:- a heavier aircraft and
- less thrust at the same time,
rolling over a surface with characteristics so suspect the regulations demand TOGA in the AST/FLEX case. Uneven judgement?

Derate is not the same as assumed temperature at all, therefore there are different rules around it. Derate is basically the same as bolting a different engine under your wing, the maximum thrust is different, the Vmcg/Vmca is different. And the lower Vmcg is actually the important point here, it allows a lower V1, which is kinda important when stopping is more difficult than continuing. Which is what is assumed in the case of a contaminated runway.

john_tullamarine
5th Dec 2018, 10:41
A couple of posts commented on contamination considerations.

I suggest that the main concern here is the adverse effects of contaminant depth (and SG) on the net accelerating force. As the aircraft staggers down the runway, first the tyres have to displace the contaminant and, second, the contaminant so flung away by the tyres may impact on the aircraft structure. Both considerations result in drag which reduces the net accelerating force. Reduce the engine thrust and, likewise, you are reducing the net accelerating force.

The usual contaminant depth constraints are there to keep the adverse drag effects in check. However, a real world concern relates to the variability of contaminant depth along the runway and the effects of grooving, porosity and so forth. To some extent, data gathered from significant operations on a particular runway can provide some engineering comfort but, overall, reducing thrust eats into the margins built into the certification data. Push your luck too far and perhaps something will eventually not work out all that well.

My concern would lie more with the degree to which flex might push the calculations toward a limiting runway performance rather than concerns with the philosophy.

GlenQuagmire
5th Dec 2018, 16:59
I don't think I agree with all of what is being said on this thread. My suspicion is that the costs involved in flight testing and approval of flex data for contaminated runway performance is prohibitively high and manufacturers simply don't consider it worth doing. I doubt very much that using flex for contaminated runway operations for the aircraft I fly would decrease margins at all - the BR710 was supplied to Bombardier on the understanding that flex would be used wherever possible because it decreased the chance of an engine failure by a small margin. So if flex operations were certified for contaminated runway conditions you may decrease the performance margin by a small amount but you may also decrease the chance of a failure so doing the maths becomes quite hard. I don't think it is vital to get off the runway as soon as possible - nor does the manufacturer otherwise they would be requiring flap 16 departures to reduce the roll. I also think flex departures at light weights in the Global are safer because controlling an engine failure is easier and I think maintaining directional control on a contaminated runway would be easier flexed than normal perf. Obviously you need the runway length to do it and you need to comply with the climb performance but I can't see a technical reason why flex take offs would be less safe simply because you get airborne slightly later. After all, its only like taking off at a slightly higher weight and you wouldn't say that decreased the safety margins would you?

This may well not be true for other aircraft and I am only talking about the airframe and engine I know..

Denti
5th Dec 2018, 17:11
I don't think I agree with all of what is being said on this thread. My suspicion is that the costs involved in flight testing and approval of flex data for contaminated runway performance is prohibitively high and manufacturers simply don't consider it worth doing. I doubt very much that using flex for contaminated runway operations for the aircraft I fly would decrease margins at all - the BR710 was supplied to Bombardier on the understanding that flex would be used wherever possible because it decreased the chance of an engine failure by a small margin. So if flex operations were certified for contaminated runway conditions you may decrease the performance margin by a small amount but you may also decrease the chance of a failure so doing the maths becomes quite hard. I don't think it is vital to get off the runway as soon as possible - nor does the manufacturer otherwise they would be requiring flap 16 departures to reduce the roll. I also think flex departures at light weights in the Global are safer because controlling an engine failure is easier and I think maintaining directional control on a contaminated runway would be easier flexed than normal perf. Obviously you need the runway length to do it and you need to comply with the climb performance but I can't see a technical reason why flex take offs would be less safe simply because you get airborne slightly later. After all, its only like taking off at a slightly higher weight and you wouldn't say that decreased the safety margins would you?

This may well not be true for other aircraft and I am only talking about the airframe and engine I know..
Well, there is a way to get the easier directional control and reduced thrust out of an engine. It is called derate and for example used routinely in 737 operation, and apparently A330/A340 as well. However, it still does not allow ATM to be used in case of contaminated runways, just derate then, on wet and dry runways both can be used together. Simply ask RR to provide the additional sets of performance data and you should be fine.

GlenQuagmire
5th Dec 2018, 17:52
wish I could but again there are no derate performance tables for the global. Bombardier would never put in the money to certify it. They barely have enough money to pay their staff!

FE Hoppy
5th Dec 2018, 21:12
I'm tempted to think it may be because contaminated perf data comes direct from the manufacturer at fixed thrust values. Rated and/or de-rated. Whereas flex calculations involve the user modifying the manufacturers data locally. This adds another layer of potential error which is outside the manufacturers control.

GlenQuagmire
6th Dec 2018, 00:59
I'm tempted to think it may be because contaminated perf data comes direct from the manufacturer at fixed thrust values. Rated and/or de-rated. Whereas flex calculations involve the user modifying the manufacturers data locally. This adds another layer of potential error which is outside the manufacturers control.

not on the global it doesn't - its manufacturers data.

Mad (Flt) Scientist
6th Dec 2018, 15:11
You're not allowed to flex on a contaminated runway because the FAA guidance (AC25-13) expressly prohibits this.

The same AC also addresses derates and imposes no such limitation for derate.

I believe the logic behind this is that aircraft performance is inherently less precise on a contaminated runway. There are usually only a few derate options available, if any, and so it is practically impossible to optimise the derate for the runway conditions. thus there is normally some margin on any given runway. Flex, however, is infinitely variable and so in principle zero margin can be achieved on every flex takeoff. This is not considered to be an appropriate margin combined with the uncertainty of contaminated runways.

Airmann
7th Dec 2018, 04:26
Look at it from a commercial standpoint. On a runway that requires derate in order to lift a higher load, a weather system which contaminates the runway suddenly costs the company direct revenue if derate is not allowed. With flex/toga it's a case of engine wear, so assuming the load doesn't require toga then choosing toga is safety related but doesn't affect the load.

So I believe that there has been a dispensation made to allow for better commercial viability. I guess when possible always use toga on contaminated runways, but if you need derate then alright. We can live with that as the increase in risk is acceptable.