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JammedStab
19th Apr 2016, 20:36
You are at an airport with two parallel runways operating a jet airliner. A shorter one and a longer one by perhaps a couple of thousand feet. Both are available for departure. The taxi distance to the shorter one is a couple of minutes shorter.

The aircraft is near max takeoff weight and the takeoff performance calculation shows that at the takeoff weight for the aircraft, the short one is acceptable but it is right at the limiting weight for the runway length using full takeoff thrust.

So it is decided to use the longer runway. The only thing is that with the longer runway a derate and ATM thrust reduction is used for the takeoff along with a different flap setting.

Without including a bunch of variables such as the longer runway covers weather changes, payload adjustments, etc.......How much advantage have you gained by using the longer runway. Is the reduced thrust takeoff negating most or all of the advantage that the longer runway offers?

cf6-80c2b5f
19th Apr 2016, 20:54
Good quesiton, JammedStab. I guess you have gained an advantage of not having to use max thrust for takeoff. Maybe not a big deal on the 744, but on the 742 with the worn out Pratts it was something to avoid, if possible. Since ATM is used on the longer runway instead of a straight derrate (TO 1, TO 2), then you could still push the power up on the rest if you lost an engine. Plus, if you are over guarantee you get more time taxiing to the longer runway. Would management call you on it?

JammedStab
19th Apr 2016, 20:58
Thanks. Just a performance question so cost in not a consideration.

Peter G-W
19th Apr 2016, 21:22
You seem to think that a reduced thrust takeoff is a bad thing?

Mad (Flt) Scientist
19th Apr 2016, 21:48
The reduced thrust means that all other things being equal, you're probably more controllable in the event of an engine failure. (Although both cases meet minimum certification standards, the manner in which they do so may differ)

Capt Scribble
19th Apr 2016, 21:54
Reducing the take off thrust reduces the 'wear' on an engine, hence cost.

Piltdown Man
19th Apr 2016, 22:14
Assuming that you meet all performance requirements and any additional company requirements like a 15% margin on runway lengths, then either runway is acceptable. That is the point of doing the numbers. As for the economics, they have to be set in context. If you need the extra minutes to get to a closing airfield or an airborne slot, then the shorter runway is obvious choice. But if you are told "de-rate" any any cost, then it's the longer runway only. But if, like most of us here, you are not given the raw numbers you will have to make up your mind. If I'm on time, I would have prepared the numbers for both but I'd flip a coin to decide.


PM

Goldenrivett
19th Apr 2016, 22:25
Is the reduced thrust takeoff negating most or all of the advantage that the longer runway offers?
No.
There is better overall aircraft performance than you may suspect due to the real OAT is colder than the assumed temperature input for the engines. The engines would produce the same thrust if operated at the assumed temperature, but the wing is flying in the denser air at the actual OAT.

flyingchanges
19th Apr 2016, 22:47
Always work from the hearing backwards...

Intruder
19th Apr 2016, 23:17
If you use a performance computer to calculate takeoff performance, it is VERY likely V1 will be increased significantly on the longer runway, and the stop margin will not increase significantly.

In cases such as you describe, I often run several calculations, with and without derates and ATM reductions. I look for a good balance between thrust required, V1, and stop margin.

Also, I will ask for the longer runway when near the performance limits on the short runway.

Denti
20th Apr 2016, 02:48
Depends on performance software i guess. The airbus thingy throws out the highest reduction possible, right down to a 1m margin left on the runway. However, it also provides all lower flex temperatures and runway remaining margins for those figures, so take the one with the margin you personally are happy with.

That said, the company wants us to use reduced thrust as much as possible, so if there is no operational downside i would take in that situation the longer runway, reduce thrust a bit and use a larger margin.

stilton
20th Apr 2016, 04:44
Good answer and a good point by GRivet.

PENKO
20th Apr 2016, 13:58
I don't think you will ever get a clear answer to this question. A longer runway has a higher flex, which lowers maintenance. But if the extra fuel burnt to get to that runway is excessive, than who are we kidding'? If your company's beancounters does not provide specific guidance, then they probably don't care either way.

As far as safety is concerned, as long as the numbers are correct, it should not matter to take the shorter runway. Not even at the hearing afterwards.

FullWings
20th Apr 2016, 19:02
I agree with PENKO.

Many of us operate from a large selection of airfields in a variety of conditions. Unless they are particularly adverse (contamination, etc.) if the numbers work out then the quickest and easiest makes sense.

In some aircraft that use fixed derates as well as ATM I think I’d rather take the short runway and get some OEI performance!

Willit Run
20th Apr 2016, 21:13
I would compare the stop margins and use the plan that gives me the most wiggle room. As one gent said" always work from the hearing backwards".
Could you use the shorter runway? maybe yes, but is it wise, when there is a better solution.

Water Wings
21st Apr 2016, 00:18
We often had this at LAX. 24L vs 25R. 95%+ of crews would take 24L if it fit even if pushing right up on the numbers.

Rick777
21st Apr 2016, 03:03
We often had this at LAX. 24L vs 25R. 95%+ of crews would take 24L if it fit even if pushing right up on the numbers.

I would take 25R at LAX every time, but then we parked right next to it.

Derfred
21st Apr 2016, 05:01
So, your question is
How much advantage have you gained by using the longer runway.

And then you have said to ignore cost.

So I'm not actually sure what "advantage" you are referring to. If you are referring to performance advantage in the case of an engine failure, the answer is what advantage? Either runway is safe in the case of an engine failure. One will have more thrust, so will be harder to control, but will climb better. The other will be easier to control but not climb as well, but will still clear terrain.

But, are you asking which runway is safer?

I would say the long one. For various reasons, as follows:

1. An engine is less likely to fail at lower thrust. I know many pilots feel better with more thrust, but all they are doing is increasing the likelihood of an engine failure. An engine at lower thrust will also suffer less damage from bird or foreign object ingestion.

2. On the longer runway ASDA for an RTO is less likely to be the limiting factor, so less braking may be required reducing the possibility of a brake/wheel fire or tyre deflation. Again, safety improvement.

3. What may conspire to kill you on the day may not actually be the certified standards of engine failure/stop/go scenarios. In fact, none of these should kill you because the aircraft is certified not to. What may kill you is a multiple failure, an error (eg thrust calculation error), a sticking brake, tyre failure, manipulation or handling error, or other event that is not possible to plan for. In that case, having more runway to sort it out may save you.

PENKO
21st Apr 2016, 07:34
...or you may hit a dog crossing the runway whilst still slowly accelerating towards 155 knots VR when, on the shorter runway you would have been airborne ages ago with a VR of 129 and TOGA thrust.

..or your tire may fail catastrophically at those higher speeds on the longer runway.

..or you might inadvertently cross an active runway whilst taking a detour to the longer runway.

..or that careless buss driver slams into your right wing whilst taking said detour.


It is almost a philosophical question, no clear cut answers :)

FullWings
21st Apr 2016, 07:45
1. An engine is less likely to fail at lower thrust.
Now, that’s a reason I would have given and it seems obvious but is it actually true of modern (jet) engines? Are there any statistics available to back it up, genuine question? With many piston engines peak mechanical and thermal stress occurs at less than peak power (not that this has anything to do with turbines but it shows that the simple answer is not always the most correct).

2. On the longer runway ASDA for an RTO is less likely to be the limiting factor, so less braking may be required reducing the possibility of a brake/wheel fire or tyre deflation. Again, safety improvement.
With the software we use, adding runway length often results in a lower flap setting (increased speeds) and improved V2 (even more speed). As brake energy is proportional to speed squared, that can be quite significantly more in the event of an RTO.

3. What may conspire to kill you on the day may not actually be the certified standards of engine failure/stop/go scenarios. In fact, none of these should kill you because the aircraft is certified not to. What may kill you is a multiple failure, an error (eg thrust calculation error), a sticking brake, tyre failure, manipulation or handling error, or other event that is not possible to plan for. In that case, having more runway to sort it out may save you.
Agreed. Although I would add that there are many examples of aircraft failing or nearly failing to get airborne from very long runways due to too much thrust reduction, which might not have happened if full thrust or thereabouts was expected to begin with off a much shorter strip.

Goldenrivett
21st Apr 2016, 09:11
Hi FullWings,
An engine is less likely to fail at lower thrust....is it actually true of modern (jet) engines
It makes perfect sense to me that if an engine will happily run for ever at MCT or less, then it must be considerably less stressed and hence less chance of a failure than one operating at full power (10 min limit).

Although I would add that there are many examples of aircraft failing or nearly failing to get airborne from very long runways due to too much thrust reduction, which might not have happened if full thrust or thereabouts was expected to begin with off a much shorter strip.
Excellent! Please provide a link or reference to those accident reports.

PENKO
21st Apr 2016, 09:54
Is that true Goldenrivett? Does higher thrust really equate a higher chance of failure? I am no expert in engines so correct my if I am wrong, but take for example the A320 series. Practically the same engine on the 318, 319, 320, 321, just rated at a higher thrust, so it seems. So is an engine on the A321, with it's higher thrust rating more likely to fail than an engine on a 318?

One would think that even at max TO thrust these engines are far far below their breaking point.

737Jock
21st Apr 2016, 09:55
Question is not very realistic. Most airports will be using the shorter runway for landing and the long one for T/O.

From a performance perspective both runways are safe.

mustafagander
21st Apr 2016, 10:09
If I were to have that choice, I'd take the shorter, max thrust, runway all things being equal. But, and a significant but, were I going to hold for the short one for more than a minute or two, the time/cost advantage is gone so I would taxi for the long one. Take the short one if you get something back for it like reduced airframe time otherwise taxi a bit longer for what gives you better margins is my recommendation.

Goldenrivett
21st Apr 2016, 11:17
Hi PENKO,
So is an engine on the A321, with it's higher thrust rating more likely to fail than an engine on a 318?
No. It should have about the same reliability. The reason being, the higher thrust rated engine will be removed from service earlier. Hence the higher maintenance costs.


The CFM56-7B Turbofan Engine (http://www.cfmaeroengines.com/engines/cfm56-7b)
"In 2007, CFM breathed new life into the CFM56-7B fleet by introducing the Tech Insertion production standard. Compared to the base CFM56-7B, this new configuration provides operators up to 1 percent better specific fuel consumption over the engine’s lifecycle, and between 5 and 15 percent lower maintenance costs (depending on the thrust rating) through enhanced durability.

Cough
21st Apr 2016, 11:26
If everything goes well, you have burnt the engines a little more for the short runway. But thats it, we all know its safe...

But if it doesn't, we all know where we would like to be. On the short runway, eng fails at V1 you don't have extra trust to put on (SOP's allowing of course). On the long one you do, your climb out (all things being equal) on the long one will be easier (less flaps generally mean a greater climb rate and the extra thrust you could put on means that gets even better).

But there is of course a flip side. If the engine fails before V1, you stop, right! So on the short one, things are a little tight but the low V1 combined with a short taxi distance means your brakes are pretty cool prior to V1 and have less work to do. On the long one, you've taxied further, the brakes start a little warmer and the V1 is higher so they have a *load* of work to stop you.

There are always flip sides to the coin - Slopey shoulder time...

wanabee777
21st Apr 2016, 11:58
Had this scenario occur a few times at KATL headed to OMDB. Tower would instruct us to taxi to 26L with the explanation that there would be a 20 minute, or more, delay for runway 27R due to traffic.

Although we were legal by the numbers for 26L we always, (at least my augmented crew), accepted the delay and requested the longer runway, 27R.

Even then, because of our weight, a lot of Captains would use one of the higher thrust options offered by our AWABS, and some would use full power regardless.

Jumpjim
21st Apr 2016, 13:08
Some light reading for you guys....

Stopkotte, Jack. “Minimizing Costs While Maintaining Performance Margins, Part 1 — Lowering Costs and Improving Reliability.” GE Aircraft Engines, September 2003. (If you can find a copy....)

"When less is more" - Aerosafety World magazine (http://flightsafety.org/aerosafety-world-magazine/march-2011/when-less-is-more)

Statistically, utilising the minimum Perf A compliant takeoff thrust makes you 5x less likely to have an engine failure, along with significant cost savings in engine wear...

FullWings
21st Apr 2016, 16:03
It makes perfect sense to me that if an engine will happily run for ever at MCT or less, then it must be considerably less stressed and hence less chance of a failure than one operating at full power (10 min limit).

It makes sense, yes, but with the kind of monitoring we have now in 2016 the situation is close to being able to trade thrust for life (TBO). While your margins are still good, does the chance of an actual failure increase with increasing thrust or are you just bringing essential maintenance forward? I suppose only manufacturers/operators could tell you that definitively. Yes, I did read the linked article above but it didn’t really answer this particular question. Is it similar to scheduling the oil change every 10,000 miles in your car but actually having it at 5,000 because the computer thinks it’s time as you’ve been driving like a loony...?

In my airline we get the vast majority of our engine failures in the cruise - of course there’s much more cruising time than taking off but you’d have thought if an engine was on the way out, it’d have gone earlier when you were thrashing it to get away from the ground, rather than producing half the power or less in almost steady-state.

Excellent! Please provide a link or reference to those accident reports.
Here’s one: Emirates 407 ("https://en.wikipedia.org/wiki/Emirates_Flight_407”). Although the calculations were 100T off to begin with, it was the subsequent excessive derate that did for them. Once you’ve accepted that you’re doing a reduced thrust takeoff, long runway and all, there’s not an immediate trigger as to whether a certain level of reduction is too much as there are too many variables: mass, temperature, obstacles, etc.

And another: MK Airlines 1602 ("https://en.wikipedia.org/wiki/MK_Airlines_Flight_1602”). Shorter than above but still nearly 3000m TODA.

I’m sure there are many more, some probably known only to the FOQA departments of many of the World’s airlines...

Escape Path
22nd Apr 2016, 01:57
Some light reading for you guys....

Stopkotte, Jack. “Minimizing Costs While Maintaining Performance Margins, Part 1 — Lowering Costs and Improving Reliability.” GE Aircraft Engines, September 2003. (If you can find a copy....)

"When less is more" - Aerosafety World magazine (http://flightsafety.org/aerosafety-world-magazine/march-2011/when-less-is-more)

Statistically, utilising the minimum Perf A compliant takeoff thrust makes you 5x less likely to have an engine failure, along with significant cost savings in engine wear...

That was quite an interesting reading. Thanks for pointing us to that article. It particularly explains what Derfred first said: engine life improves using flex/derate thus reducing likelihood to failure.

Personally, given the choice and all other things being equal, I'd take the longer runway. The aircraft is designed to fly safely out of the short or the long runway, but more concrete to get off the ground will always be more appealing to me, even if using flex. If the engine fails you still have more thrust to use (assuming you're using flex) even though the aircraft should meet OEI requirements just using flex. If you reject the takeoff, the performance calculations already have you covered; if you're not comfortable with the margin given by the highest flex, you can use one that's a bit more conservative. Plus, I reckon the couple of extra minutes of taxi will be far offset by the reduced wear using flex on the longer runway (even if the question was not considering economics)

seen_the_box
22nd Apr 2016, 09:14
I'm struggling to think of anywhere I operate to where you get a choice of runways for departure so, for me at least, the question is moot.

PENKO
22nd Apr 2016, 09:22
Statistically, utilising the minimum Perf A compliant takeoff thrust makes you 5x less likely to have an engine failure, along with significant cost savings in engine wear..
That is NOT what is written in the article.
I can only interpret the figures in that article as that fixed derates cater for lower maintenance cost, not 5x more likely engine failure. Sure, if you do not perform any maintenance then yes, the engine will fail sooner. But that is not how we operate.


I'm struggling to think of anywhere I operate to where you get a choice of runways for departure so, for me at least, the question is moot.
It's not moot at all. Substitute 'a choice of runways' for 'a choice of intersections' and you have the same age-old discussion wether it is safe to depart from an intersection.

777AV8R
22nd Apr 2016, 11:17
Interesting discussion. After reading all of this, where is the discussion about Airmanship? -and- Making a decision to select the runway that poses the least risk to your operation?

Sure, we can 'fit' the numbers to the runways but at the end of the day, experience should lead a crew to make a decision to select the runway that poses the least risk to the operation. Airmanship isn't taught. It comes with experience and the desire to weigh all of the facts before departure, then do the right thing.

As an accident investigator, we do not look for fault, but we do look for the reasons that events happen and all of the details that led up to the event. Ultimately we search for clues to help mitigate the problem from happening again.

To attend an accident site or serious incident site, take the measurements of the area and make a few calculations, sometimes becomes disturbing. If in this case, we determine that the length available of the long runway would have provided a different outcome, the questioning becomes one of: Could you explain why you elected to depart from the short runway knowing that the longer was available?

One cannot take back the actions and outcome of the past however; we can all learn from it and prepare ourselves in better ways with a new mindset.

In the end, Fuel left in the bowser and runway behind you is of no use to you.

Just my experience anyways.

Safe flight!

PENKO
22nd Apr 2016, 11:46
Could you explain why you elected to depart from the short runway knowing that the longer was available?

This is pure intimidation. You could ask the same question for just about every decision made in the flight deck. Could you explain why:
-you derated
-you flexed
-you took an intersection
-you loaded that extra tonne of cargo
-you flew a visual approach when there was an ILS available
-why you did not upload an extra tonne of fuel on a CAVOK day
-why you did not land yourself on said CAVOK day
-why you came out of bed in the first place


In the end, Fuel left in the bowser and runway behind you is of no use to you.
A nice but absolutely useless quote.
In commercial aviation you have a responsibility as a captain to operate as safely AND as efficiently as possible. As such the above quote is not applicable. As a pilot and as a captain, you weigh the risks and take a responsibly informed decision. It takes as much airmanship to depart from the longer runway as it takes to depart from the shorter one.

seen_the_box
22nd Apr 2016, 12:36
Could you explain why you elected to depart from the short runway knowing that the longer was available?

Yes, very easily. The performance calculation that we performed showed that we were able to depart safely from the shorter runway; if it hadn't, we wouldn't have attempted to depart from it.

I'm really not sure what point you're trying to make.

Sure, we can 'fit' the numbers to the runways but at the end of the day, experience should lead a crew to make a decision to select the runway that poses the least risk to the operation.

Nobody is fitting the numbers to runways. Again, we will only depart from a runway if the performance calculation shows it to be safe to do so. To give an example: In Barcelona, departures are from 25L with a TORA of around 2600m. 25R is available for "performance reasons" with a TORA of around 3300m. Taking your simplistic approach, we should always request 25R for departure, because it is the "least risk" option, even though the performance software shows that we can perfectly safely depart from 25L. Never mind the huge delays that would be incurred by requesting the non-standard runway for departure; it is "less risk" and therefore it would be poor airmanship to accept the shorter runway! I think that you're ignoring commercial realities.

In the end, Fuel left in the bowser and runway behind you is of no use to you.

As Penko has already said, part of the job of a captain is to operate efficiently, while still maintaining adequate safety margins. I could fill the wings and put the trip in the centre on all sectors I operate, demand the longest runway and take off with TOGA thrust on every departure, but that's not what the company is paying me to do. Therefore, I make sensible fuel decisions taking all relevant factors into account, and use the software provided to optimised performance.

oceancrosser
22nd Apr 2016, 13:13
If you use a performance computer to calculate takeoff performance, it is VERY likely V1 will be increased significantly on the longer runway, and the stop margin will not increase significantly.

In cases such as you describe, I often run several calculations, with and without derates and ATM reductions. I look for a good balance between thrust required, V1, and stop margin.

Also, I will ask for the longer runway when near the performance limits on the short runway.

You appear to be affored a lot more time than most of us. Several calculations?

Escape Path
22nd Apr 2016, 15:05
I have to agree with what the other are saying. As a commercial pilot you operate with safety, punctuality and economy as factors to our decisions. The optimal decision for a flight using "the seemingly safest course of action" would be taking off with a plane full of fuel and no payload and only flying on CAVOK days, but that's no way to make money. Standards have been created to operate as safely as it's deemed possible in the myriad of scenarios we might find everyday. From a safety perspective, I reckon it's as safe to take the short or the long runway, so long as the numbers are okay and there aren't other overriding factors (weather, terrain, runway asphalt condition, etc). Otherwise I think the statement we would be giving is that it would be unsafe to operate out of any single runway airport which is shorter than the longer runway in the question we are discussing.

Having said that, and understanding that realistically you aren't always given the choice of which runway would like to depart from (or it's not operationally/economically sound to do so on every single flight), if I'm given the choice I'd take the longer runway just because I reckon it's giving me a bit more wiggle room, in a nutshell.

Denti
22nd Apr 2016, 17:15
You appear to be affored a lot more time than most of us. Several calculations?

Easy enough if you use an EFB. Usually i run somewhere between two and 10 calculations each flight. Each one takes a few seconds.

RAT 5
22nd Apr 2016, 17:32
Perf A calculations assume an engine loss of thrust at V1-Vr. What are the chances? When did it last happen? I might discuss that the stopping case is more critical than the go case on a limiting runway, but I wonder if that is behind the question. To those who say the longer runway I ask if they have ever/never taken an intersection? Consider you are approaching the hold, 2 a/c at the holding point and an acceptable intersection coming up with a close slot and immediate takeoff possible. Performance already done: what would you do?
Now consider the landing options. If the longer runway is always your choice for takeoff, what about if there is a choice for landing? How many take the short taxi after landing option? Assuming all other things equal, i.e. acceptable performance.
It's not quite so basic as "do you feel lucky, punk, well do you?" It's a little more science, skill & judgement.
PENKO asks all the relevant questions. It would be an interesting question & dilemma on your command upgrade final line check. I wonder what the TRE would conclude?
However, as an escape clause; if the runway was contaminated or a significant tailwind, or windshear a possibility, or...or...then I'd think again.

FullWings
22nd Apr 2016, 20:19
Interesting discussion.

I think one of the key points is the use of the word “safer”. Like others, I’m paid to manage a safe *and* efficient operation - complete safety would be to lock the hangar doors and throw away the key, so there has to be a compromise somewhere.

Compared with all the other dangers out there which need to be avoided, shunning runways that are demonstrably acceptable performance-wise gives such a minuscule improvement in overall safety that it’s debatable you could even calculate it. It’s almost the equivalent of a lucky rabbit’s foot or four-leafed clover in that you may feel much safer but the underlying reality is little changed.

Incorrect responses to windshear, GPWS, TCAS, UAS; the wrong flap setting, the wrong thrust reduction, the wrong MSA, the wrong runway. Not de-icing properly, not being proficient in unusual attitude recovery and so on... These are some of the biggies that can really spoil your day. Just thinking about the possibility of any of the above before you set off could increase “safety” by far more.

Intruder
22nd Apr 2016, 20:30
In cases such as you describe, I often run several calculations, with and without derates and ATM reductions. I look for a good balance between thrust required, V1, and stop margin.
You appear to be affored a lot more time than most of us. Several calculations?
We use Aerodata; it doesn't take that much time. The default calculations for 2 runways can be done at one time. After that, I can vary the thrust and flap settings to do a "what if". I can do the entries while waiting for the initial results, so there's a "delay" of a minute or less...

Setright
22nd Apr 2016, 23:11
''Should I take the longer runway''

Yes.

C_Star
23rd Apr 2016, 03:45
My previous outfit was an LCC and we did a lot of ops every day, sometimes on short-ish runways and most of the airports were not very busy - we normally had a lot of discretion from the ATC as to which runway/intersection to chose.

The weights were seldom (if ever) performance-limited, so most of the time performance calculations were a trade off between taxi time and Flex.

The company policy was to encourage accepting intersections and use lowest flap setting that allowed to keep the FLEX above certain temperature (60 IIRC). The reason cited was that first degrees of flex/ATM were much more important then deeper reductions i.e. increasing FLEX from 50 to 60 had higher impact on engine wear, than subsequent increase from 60 to 70...

I have always enjoyed juggling with the performance calculations to get the optimum results for myself and the company.

My current employer OTOH - we are only allowed to use one flap setting out of 3 available and intersection takeoffs are generally frowned upon.

Then again it's wide body/long haul ops, so runway length is much more often an issue and performance margins are usually lower. Also, each aircraft does 1-2 ops per day, not 8 - so maybe the financial impact is not significant...

JammedStab
23rd Apr 2016, 05:59
Thanks for the responses,

One reply that is important stated this "There is better overall aircraft performance than you may suspect due to the real OAT is colder than the assumed temperature input for the engines. The engines would produce the same thrust if operated at the assumed temperature, but the wing is flying in the denser air at the actual OAT".

I have read this before and I believe the more the ATM, the more the benefit from this fact.

I have to admit, it is the high speed RTO scenario that makes me post the original question. So from an RTO point of view, is the somewhat longer runway helping me with the derated thrust and different flap setting? What about when the runways are wet where reverse is in the calculations for an RTO?

Goldenrivett
23rd Apr 2016, 06:12
So from an RTO point of view, is the somewhat longer runway helping me with the derated thrust and different flap setting?
Yes.
The ATM method calculates an accelerate stop distance within the Runway length available (& often shows a very small stop margin). Provided the real OAT is less than the assumed temperature, then your indicated V1 will occur at a lower TAS (hence lower ground speed) than calculated. Therefore you will be rejecting the take off from a lower ground speed and have considerably more stop margin than the calculations suggest.

PENKO
23rd Apr 2016, 08:20
I have to admit, it is the high speed RTO scenario that makes me post the original question. So from an RTO point of view, is the somewhat longer runway helping me with the derated thrust and different flap setting? What about when the runways are wet where reverse is in the calculations for an RTO?
Again, it depends.
On one of our more interesting runways we have a choice of intersection or full length.

Intersection:
TODA 2550m
VR 139
flex 48
stop margin 200m (+ a bit due to the assumed temperature)

Full length
TODA 3500
VR 155
flex 67
stop margin 700 (+ a bit due to the assumed temperature)

As a contrast, if you used TOGA to depart from full length with the highest flap the figures would look like:
TODA 3500m
VR 123
stop margin 2300m


So you ask if the longer runway is helping.
Again, the answer is: it depends what you are looking for.
If you take the longer runway for stop margin, then who are you kiddin' if you blindly accept the flex, reduced flap and the much much higher VR? The added potential problems overshadow the few extra meters of stop margin in my opinion.

Don't get me wrong. I'm not a maverick who likes to operate at the edge of the envelope. But what I dislike more is the mentality you see more and more on the line and on these forums where people are afraid to take a safe informed decision, just because that decision might be scrutinized in some imaginary court case should things go wrong. This kind of thinking causes more harm overall than the few meters of stop margin it gains.

I think FullWings wrote an excellent reply yesterday which sums it all up:


I think one of the key points is the use of the word “safer”. Like others, I’m paid to manage a safe *and* efficient operation - complete safety would be to lock the hangar doors and throw away the key, so there has to be a compromise somewhere.

Compared with all the other dangers out there which need to be avoided, shunning runways that are demonstrably acceptable performance-wise gives such a minuscule improvement in overall safety that it’s debatable you could even calculate it. It’s almost the equivalent of a lucky rabbit’s foot or four-leafed clover in that you may feel much safer but the underlying reality is little changed.

Incorrect responses to windshear, GPWS, TCAS, UAS; the wrong flap setting, the wrong thrust reduction, the wrong MSA, the wrong runway. Not de-icing properly, not being proficient in unusual attitude recovery and so on... These are some of the biggies that can really spoil your day. Just thinking about the possibility of any of the above before you set off could increase “safety” by far more.

To illustrate what this kind of thinking leads to I'll give the following example.
The above mentioned runway is prone to fog.
Nobody departs from the intersection when it's foggy, which is quite understandable for safety reasons.
But strangely enough, most are happy to accept the higher flex and speeds that the performance module calculates for the full length departure.
So they think they are taking the safe decision by accepting the longer TODA, but all they really achieve is a much longer, and much faster take off run in almost zero visibility. Who are they kidding?


.

framer
23rd Apr 2016, 08:35
I like the gist of what Fullwings has said above but I think the OP is on the right ' professional development' track in that he or she is trying to determine if there is any appreciable gain in safety, which will in turn enable them to determine where runway choice fits into their overall management of the safety of their flight.
I've learnt something from this thread so thank you to the OP and contributors.

seen_the_box
23rd Apr 2016, 08:53
But what I dislike more is the mentality you see more and more on the line and on these forums where people are afraid to take a safe informed decision, just because that decision might be scrutinized in some imaginary court case should things go wrong.

Exactly this. There seems to be a perception that we can, as captains, take sensible, well informed, objectively reasonable decisions and still be hung out to dry "at the subsequent board of enquiry" if something goes wrong. I would respectfully submit that this is bollocks. As long as the decisions you take are objectively reasonable, and according to the SOPs of the company you work for, you're covered. (At least in countries with first world legal systems).

TypeIV
23rd Apr 2016, 09:44
I would go for the longer runway unless there's a very big difference in taxi time. A couple of minutes is negligible unless wife says something else :E

Derfred
24th Apr 2016, 09:00
One reply that is important stated this "There is better overall aircraft performance than you may suspect due to the real OAT is colder than the assumed temperature input for the engines. The engines would produce the same thrust if operated at the assumed temperature, but the wing is flying in the denser air at the actual OAT".


That may have been the case with the old paper charts, but it doesn't appear to be the case with the Boeing OPT software that I currently use. I have just performed a sample calculation which produced an ATM of 34C with OAT of 30C, then repeated the calculation with OAT of 0C. The N1 was over 5% lower at the lower OAT. So I'm thinking the OPT "knows" about the improved performance at the lower OAT. Can anyone verify this? Of course, it may be that it is lowering the N1 to provide an equivalent thrust at the given OAT, and may not necessarily be considering improved lift.

Goldenrivett
24th Apr 2016, 13:00
Hi Derfred,
The N1 was over 5% lower at the lower OAT.
Please see Page 31 of section 71-00-00
http://www.air.flyingway.com/books/engineering/CFM56-3/ctc-142_Line_Maintenance.pdf
You should see there is about 5% N1 difference for the engine giving the same thrust operating at temperatures 30 degrees apart.

When using ATM calculations, for approximately every 30 degrees C (i.e. 10% change in air density) difference between assumed temperature and actual OAT, then the actual Accelerate Stop Distance will be reduce by approximately 10% of the assumed ASD.

Piltdown Man
1st May 2016, 08:50
Unfortunately, this discussion appears to show that too many people who fly public transport aircraft have a poor grasp of safety, performance, economics and common sense. Fact: Modern performance is designed so that there is a margin built into the numbers to ensure the average pilot, in the average aircraft (actually I believe it to be the 1 in 1,000) with half the reported headwind or twice the tailwind etc. But exactly how the numbers are actually generated is not our concern, we just have to make sure we are up to date with version numbers. But whatever numbers are produced, some companies insist on an additional margin. But with or without an additional margin, if your performance calculations show you can depart from runway X with a given set of conditions then you can, legally and more importantly, safely. Furthermore, the real world clearly demonstrates that additional DIY margins add nothing to safety because if they did, they would already have been mandated the manufacturer and/or your regulatory authority. I'll give you that performance computers often come up with interesting numbers but dealing with those is a different subject.

Therefore, your decision on which runway to use must depend on your company policy on derated/flex usage balanced against operational considerations. And achieving that balance is what we are paid for.

While I'm here, this discussion shows an interesting paradox. There appear to be some posters who will only use the long runway, for "safety reasons" and people like me who are more than prepared to use the short runway. So if I flew with one of the former, we could end up arguing over "safety" and actually compromise the real safety of the aircraft due to an argument. It's the same argument as minimum fuel one and I bet the same players are the same!

PM

JammedStab
2nd May 2016, 02:28
Thanks,

So just to confirm, with no wind and slippery runway at Max TOW with the shorter runway right at runway limit, you are just as safe with the full thrust, short runway takeoff as with the derate 2000 foot longer runway.

Do us a favour by the way and keep your statements about how unsafe we are to yourself.

Derfred
2nd May 2016, 06:27
What's the minimum fuel argument?

Piltdown Man
3rd May 2016, 10:01
Jammed stab - You are either an amateur, a troll or just having a laugh. You are also changing the OPs initial question because contaminated runways did not come into their question. Furthermore, my understanding is that you have slippery runways, you will always be using full thrust, no matter what the length of the runway. The important thing is that it is safe enough and whilst the margins might not be as fat (we were not provided with obstacle data nor we we told how close we were to that perf. limit of the derate), they are adequate. So unless I'm given values to these margins, I'll take whichever runway fits in with my company's operation requirements. And are you also saying that if faced only being able to use the shorter runway, you wouldn't go even if the performance figures say you can?

We are paid to do what is possible. We are selected, trained and checked and then as crew trusted to operate aircraft with known performance charactics in a regulated environment adminstered by a similar teams of professionals. In addition to all of this, margins for equipment failure, measurement inaccuracies and poor handling etc. are added. So you can do us a now do us favour by telling us how much safer you would be by using the longer runway. Is it 20% safer or 32%? Lets have some numbers please. Or are you saying you are not prepared to fly to the legal limits?

I'll say it again. I the numbers say you are good to go, then you can go.

Derfred - My tongue was in my cheek. I was just trying to guess the direction of the thread.

PM

RAT 5
3rd May 2016, 10:51
So what would the 'always longest brigade' do if the difference in taxi time is 5 minutes? Very possible at some airports, or if there is a queue. You have a slot time and it's running out. You can just make the slot from the shorter, but legally acceptable runway. Taking the longer runway you miss the slot. What would they do?

gus_eng
4th May 2016, 13:20
When we design airplanes, we do it to safely operate at various runway lengths. And we do that by providing those various speeds that you find in your AFM/CAFM/Performance software.

V1 is your safety speed defined based in a multiple factors. So it doesn't matter if you have more or less rwy length left. Respecting the calculated V1, it doesn't matter what engine rate/rwy length/whicheverfactoryoudesiretojustifyyournonsense you chose.

So, as it was stated before, both runways are safe. If your AFM/CAFM allows you to TO from a particular rwy, than you should go with the company's policy of cost efficiency.

Repeating what @Piltdown Man said "If the numbers say you are good to go, then you can go."

JammedStab
5th May 2016, 04:45
Jammed stab - You are either an amateur, a troll or just having a laugh. You are also changing the OPs initial question because contaminated runways did not come into their question. Furthermore, my understanding is that you have slippery runways, you will always be using full thrust, no matter what the length of the runway. The important thing is that it is safe enough and whilst the margins might not be as fat (we were not provided with obstacle data nor we we told how close we were to that perf. limit of the derate), they are adequate. So unless I'm given values to these margins, I'll take whichever runway fits in with my company's operation requirements. And are you also saying that if faced only being able to use the shorter runway, you wouldn't go even if the performance figures say you can?


PM
If I am the amateur, then why are you the one saying that we will be using full thrust on a slippery runway no matter what the length of the runway.

Suggest you head back for PPL school for starters and stay there while we use a TO2 derate thrust for takeoff. I do admit that I am "having a laugh" though about your earlier statement stating "unfortunately, this discussion appears to show that too many people who fly public transport aircraft have a poor grasp of safety, performance".

Jonty
5th May 2016, 06:39
If I am the amateur, then why are you the one saying that we will be using full thrust on a slippery runway no matter what the length of the runway.

Probably because both Airbus and Boeing require the use of full thrust on a contaminated runway.

What type do you fly?

Jonty
5th May 2016, 06:54
The only thing I would add, is that while legally speaking if the performance says you can go then you can go. There are a number of failures that you would probably not want to get airborne with, yet you would be unable to stop with on a limiting runway.
The takeoff performance we calculate is only based on an engine failure at V1.
The short runway is safe, the long runway is safer. All other things being equal.

Derfred
5th May 2016, 09:03
The short runway is safe, the long runway is safer.

Yes. I'll take the short one. The long one is just too safe. ;)

JammedStab
5th May 2016, 12:41
Probably because both Airbus and Boeing require the use of full thrust on a contaminated runway.

What type do you fly?
Thanks,

We use up to a 20% reduction in thrust on the 777 and 747 on slippery runways. I am looking at the performance chart right now in the Performance Inflight section called TO2 Slippery runway takeoff, 20% Thrust Reduction, Maximum Reverse Thrust, Weight Adjustment(1000 kg). Similar charts are given for slush/standing water.

If I remember correctly, full thrust seems to be required for windshear conditions on takeoff.

Unfortunately, somebody that I have never heard of before called Piltdown Man has barged onto a decent thread discussing performance with two posts that started with insults about a "poor grasp of safety, performance" and calling people amateurs when it appears that he is actually the one that fits the category.

How about from now on we just stick to professional discussion.

Jonty
5th May 2016, 13:45
JS, is that a derate? Or an assumed temperature calculation?

JammedStab
5th May 2016, 19:03
That is a derate. We have 10 and 20% options although I believe that the setup is company specific with 1% reduction options available possibly to a maximum of 20(and probably after paying Boeing for the performance figures).

RAT 5
6th May 2016, 11:05
Which brings in the ever spiralling discussion. B737NG: 22k, 24k 26k 27K are all full power. If the engine is rated 27K, and you can make an acceptable performance calculation, then a 22k takeoff is allowed as this can be considered full power; but not an assumed temp takeoff as well. (some a/c have only 22K engines anyway). Somewhere in the thrust graph 27K + assumed temp = 22K. There are some authorities that do not allow operators with >22K engines to use derates: everything is assumed temp from the max available power. Thus, on a contaminated runway, or for any other of the relevant reasons, no matter what the ATOW, you blast off with a huge surplus of power. We know about the effect of power on VMCG & VMCA; we know about the power pitch couple - more apparent with light a/c - we know about the exaggerated swing this would give with an engine failure at V1 and a long pause while you wait for Vr.
Question. Is this method of takeoff thrust calculation a good idea or not? Especially on a contaminated runway?

BEL1000
9th May 2016, 14:46
In order to reduce weight from close to MTOW (Maximum Take Off Weight) I would taxi to the longer runway cause it is far away and use Flex Take Off (It's de-rated thrust but called like this in Airbusses) if needed otherwise to reduce a bit more weight TOGA (Take off - go around power) which puts maximum take off thrust available.
If the RWY is let's say 3300 meters and I am close to full weight with the Airbus A320 I would choose TOGA if bad conditions such as rain, etc.. or Flex Detent in good conditions. Longer RWY's are better as you most of the time do not need to brake maximum or close to maximum in order to stop incase of an aborted take off. Actually it's more "recommended" to use the RWY that is said in the flight plan paper.
Taking off with MTOW almost always causes structural damage.

RAT 5
9th May 2016, 19:33
Taking off with MTOW almost always causes structural damage.
Surely you mean max structural takeoff weight. But is it not the case the a/c owe designed with margins in their design? Max structural weight will have a buffer before the undercarriage collapses. For those who operate long-range cargo flights to squeeze every cent/penny out of the flight will clock up huge numbers of max structural takeoffs. It is a very sweeping statement.

No Fly Zone
15th May 2016, 00:37
Short of some obvious reason to use the shorter runway, in the case you describe I would almost ALWAYS use the longer one. Why? More runway gives you more options if/when something goes sour. I'm a huge fan of more options and yes, I'd pick the longer rwy in in a light airplane, for the same reasons. I've never been in such a rush that 'buying' more options was not possible. If pressure to take off quickly becomes excessive, perhaps it is a great day to NOT go flying.

B2N2
15th May 2016, 16:34
I've kinda scanned through 4 pages here.
If taxi time differences are negligible ( and I were PIC) I would always go for the longer runway.
Why?
All the unforeseen stuff that the performance doctors didn't think about.
The day all the holes in the cheese line up.
Finding out the airplane won't fly at Vr is not a good thing.

Cockpit transcript released from jet crash that killed Lewis Katz, 6 others - philly-archives (http://articles.philly.com/2015-04-08/news/60947597_1_preliminary-ntsb-report-bauke-de-vries-teresa-ann-benhoff)

https://c.o0bg.com/rf/image_960w/Boston/2011-2020/2014/06/02/BostonGlobe.com/Metro/Images/ryan_planecrash5_met.jpg

https://en.wikipedia.org/wiki/2008_South_Carolina_Learjet_60_crash

http://i.telegraph.co.uk/multimedia/archive/00995/travisPlane-460_995644c.jpg

The above are all examples of things that "shouldn't happen" but they did.
Trim is not supposed to freeze either..had that happen...in a CJ2
Longer is better..if in doubt..ask y'er wife..:cool:

RAT 5
15th May 2016, 19:23
If taxi time differences are negligible ( and I were PIC) I would always go for the longer runway.

I'm not disagreeing with you, but you invite the questions:

1. You are taxying // to the RWY an there is a queue at the threshold Hold. ATC asks if you can accept the intersection for a no-delay departure. Company SOP is to calculate performance for intersection, but it is captain's discretion to use it or not. YES/NO? What have you done; ever?

2. The runway entry point requires a back-track for full length. Company SOP is to calculate performance for the entry point, but captain's discretion to use it or full length. Busy period on finals. Immediate takeoff at entry point or wait for 3 landings and then back track. YES/NO? What have you ever done?

3. Various airports have the apron near one end of the single runway: let's say the southern end. You wish to depart to the north. The wind is 7kts on the runway from south. Runway is your choice. Do you calculate for a 10kt tailwind, short taxi, short SID? Or calculate for zero headwind, long taxi and long SID? Oh, and you're late? What have you ever done?

Piltdown Man
16th May 2016, 22:44
B2N2 - Would a longer runway have really helped in this event? The choice of runway depends on what your company's requirements are. If they say flex at any cost, then so be it. But if are going to run out of duty time, run out of MEL time, miss your slot, run out of HOT, miss connections, hit noise fines for late arrivals then maybe not. To decide, you need more information than just runway length.

PM

Escape Path
17th May 2016, 00:54
If the RWY is let's say 3300 meters and I am close to full weight with the Airbus A320 I would choose TOGA if bad conditions such as rain, etc.. or Flex Detent in good conditions. Longer RWY's are better as you most of the time do not need to brake maximum or close to maximum in order to stop incase of an aborted take off. Actually it's more "recommended" to use the RWY that is said in the flight plan paper.
Taking off with MTOW almost always causes structural damage.

I'm sorry; I'm having a hard time figuring this one out. As far as my understanding goes, flex speeds will be considerably higher (the higher the flex) than TOGA speeds, therefore more brake energy will be needed to stop the aircraft than using TOGA. The longer runway might help you with more margin, but then again if you're using a high flex to take advantage of that longer TODA, your margins compared with using TOGA on the shorter runway might even be the same...

I'm missing the whole "taking off at MTOW almost always causes structural damage" point though... :confused:

If taxi time differences are negligible ( and I were PIC) I would always go for the longer runway.

I'm not disagreeing with you, but you invite the questions:

1. You are taxying // to the RWY an there is a queue at the threshold Hold. ATC asks if you can accept the intersection for a no-delay departure. Company SOP is to calculate performance for intersection, but it is captain's discretion to use it or not. YES/NO? What have you done; ever?

2. The runway entry point requires a back-track for full length. Company SOP is to calculate performance for the entry point, but captain's discretion to use it or full length. Busy period on finals. Immediate takeoff at entry point or wait for 3 landings and then back track. YES/NO? What have you ever done?

3. Various airports have the apron near one end of the single runway: let's say the southern end. You wish to depart to the north. The wind is 7kts on the runway from south. Runway is your choice. Do you calculate for a 10kt tailwind, short taxi, short SID? Or calculate for zero headwind, long taxi and long SID? Oh, and you're late? What have you ever done?

Even though I said earlier than I would take the longer runway, I said I would if all other things being equal.

As your options pose a significant increase in taxi or otherwise taking a significant delay where we could just be departing now (time costs money too, even if operational costs where not in the "initial question"), I'd be more than happy to depart now from an intersection, or the shorter runway for that matter, if the performance numbers add up and no other significant factors come into play. Sitting 5, 10 or 15+ minutes just to take a longer runway just doesn't make any sense to me if there's an option to depart now with enough safety margin. Plus, missing a slot, leaving flight stranded because of duty time expired and that sort of things are far more expensive than using TOGA or a lower flex for one single flight.

I'm all for efficiency if safety isn't impacted. If the numbers are correct and no other factors come into play, all of the 3 cases which RAT 5 poses are acceptable to take. It's just not possible/feasible/sensible to always take the seemingly safest course of action on a commercial aircraft when there are other just-as-safe options to take.

B2N2
10th Aug 2016, 23:41
B2N2 - Would a longer runway have really helped in this event? The choice of runway depends on what your company's requirements are. If they say flex at any cost, then so be it. But if are going to run out of duty time, run out of MEL time, miss your slot, run out of HOT, miss connections, hit noise fines for late arrivals then maybe not. To decide, you need more information than just runway length.

These are all items that (ideally) should not be taken into account to cut corners on what is basically a safety decision.

RAT 5
11th Aug 2016, 04:42
1. airport has // runways. The longer one has been designated for landing and the duty takeoff runway is the shorter, but adequate one. The first runway to arrive at is the duty takeoff runway. There is no landing traffic. You are in sequence for takeoff. Which do you use?

2. airport, for local reasons, is using one runway direction. The wind shifts and results in 8kts tail wind. You have performance for 10kts. What do you do? Use the duty runway or request the reciprocal with the incurring delay?

Safety: a good concept. Remember the days before 2 engine ETOPS. Shock horror. Why would you do this when there are 3 & 4 engine a/c around. Then 90mins ETOPS, then 120, now 180. Not saying it's comfortable, but......once you leave the 60 mins your senses become very alert; like the first time I crossed open water in a single piston. OMG, the engine would quit as soon as I left gliding distance.
But here we are, possibly the end of B747 in sight and the world reverts to twins everywhere. Are guys requesting non-ETOPS routes or flying the 180mins routes? Similar philosophy. You can take the argument of safety to any length and decide to stay in bed.

JammedStab
14th Aug 2016, 23:55
2. airport, for local reasons, is using one runway direction. The wind shifts and results in 8kts tail wind. You have performance for 10kts. What do you do? Use the duty runway or request the reciprocal with the incurring delay?

I do see 777's in Istanbul taking off southbound when runway 35L/R is the runway in use for takeoff. But perhaps all of them do not meet their performance requirements.

bucks_raj
22nd Aug 2016, 02:12
I don't see any difference between using any of the the runways.The longer one or the shorter one.
The Fact that I am using Flex/ATM on the longer runway means that I am going to consider the entire length to be usable in case of a reject. Hence giving it a time line... Longer runway.. Lesser thrust... More Time to V1.. More Time to Vr... More Time to Get airborne... The amount of runways required to stop in-case of a reject on a longer runways will not necessarily be more.

Shorter Runway .... More thrust.... Less time to V1... less time to Vr... now incase I reject... I have lesser speed to bleed but lesser runway as well.

My performance Characteristics on both the runway short or long are going to be the exact same for the simple reason that I have used ATM/Flex on the longer runway. Some where on the first page some one wrote about the actual density and temp that being the only +ve side of ATM/Flex in terms of performance and Abslutely nothing else (Disregarding cost)

Also company specific performance usually has optimized V1.. to use the full runway ..Puting it simply just because you have a longer runway does not mean you can liftoff heavier.The longer the runway the more are the chances of an obstacle infringement. OTHH (Doha Qatar) rwy 34R/L have obstacles, in the case one would use improved V1 speeds for takeoff the weight penalty is more than unimproved v1.

JammedStab
23rd Aug 2016, 02:28
Thanks guys, that was exactly what my original question was about. Like I already said, skip the considerations for time to taxi/duty day and all the other endless excuses. The question is based on using the max derate and ATM on each runway length.

Subject to confirmation, may feel better to use the longer runway but now you are reducing the thrust even more for the takeoff. So assuming that you are not at maximum derate/atm, it appears that there is no more margin for an RTO or accelerate-go as there was on the shorter runway.

Then again, if it is a really long runway, then perhaps it is safer.

Is this correct?

Ex-Brazilian
23rd Aug 2016, 03:19
Totally agree with bucks raj

I can't see any improvement into a "safety margin" choosing the longer runway while using assumed temperature, instead of shorter rwy/full trust. Maybe, since you are not field limited, you will get a higher V1 for the longer runway, and that could be useful, but once in the air, in both situations the airplane will perform equally.

But I also agree that taking the longer one and applying full thrust could be the best option, if you really want to improve your safety margin, since you are at MTOW.

Regarding to cost effectiveness, you should consider the longer taxi time to the longer runway, and maybe request more taxi fuel if the longer runway is not the runway considered in your flight plan, to not compromise your trip fuel, and requesting extra fuel at the last minute can lead to a little delay.

Tourist
23rd Aug 2016, 10:19
Yes there is a significantly lower chance of an engine failure at lower thrust. The damage to the engine is exponential(ish) with temp.

http://www.b737.org.uk/assumedtemp.htm#Some_Facts_About_Assumed_Temperature_Thrust_ Reduction


http://www.smartcockpit.com/docs/CFM_Flight_Ops_Support_B737.pdf

About slide 50 onwards

Capn Bloggs
23rd Aug 2016, 12:47
Sounds fair, JS.

Chu Chu
24th Aug 2016, 22:26
As SLF, one reason I'm comforted to have humans upfront is that they could potentially recognize that seemingly minor issue are actually the first signs of something serious. As I result, I like the idea of a lower-thrust takeoff with a higher V1 giving more time for that recognition to occur. So I have no idea if a longer runway is safer, but if I knew you were taking one, I'd feel better.

Piltdown Man
25th Aug 2016, 21:47
It is interesting that the assumed reason for rejecting a take off generally appears to be an engine failure. In 11 thousand (Perf. A) flights or so I have rejected about half a dozen. Only one was a high speed reject, the rest were at a relatively low speed. But not one was due to an engine failure. I think this is quite typical for most pilots in so much that you rarely stop for an engine failure, it is normally something else.

So diving back in, in a very amateurish way, unless our performance calculations have a "Safety Quotient" against each performance calculation, you can never determine which is the safer. I believe it is far too easy assume that a greater performance margin equal a greater safety margin, but this is too simplistic because it only assumes an engine failure. There is more to it than that. So unless I'm given that information, I'll stick to my company's guidelines and operational preferences.

PM

vilas
27th Aug 2016, 08:58
The original post:
So it is decided to use the longer runway. The only thing is that with the longer runway a derate and ATM thrust reduction is used for the takeoff along with a different flap setting.

Without including a bunch of variables such as the longer runway covers weather changes, payload adjustments, etc.......How much advantage have you gained by using the longer runway. Is the reduced thrust takeoff negating most or all of the advantage that the longer runway offers?
Because of the derate/Flex, the TORR/TODR on the longer runway will be longer than that off TOGA take off on the shorter runway but it will meet all regulatory requirements of take off. As GR stated using flex produces better performance than that of TOGA at that OAT because although the engine power is same the TAS at lower actual OAT is less due to higher air density. Since there is limit of 25% to 40% on Flex the advantage of using longer runway is not totally negated. Off course lower EGT results in reduced maintenance costs. That is the very purpose of flex take off. Flex take off uses more fuel than TOGA. All companies want Flex and lower thrust reduction altitudes for reducing fuel cost and only weather, wind shear is valid reason to use TOGA when Flex is possible.