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...

''Should I take the longer runway''

Yes.

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...

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?

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.

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:
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?


.

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.

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).

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

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.

Hi Derfred,
Please see Page 31 of section 71-00-00
http://www.air.flyingway.com/books/e...aintenance.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.

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

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.

What's the minimum fuel argument?

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

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?

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."

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".

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

What type do you fly?

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.

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

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.

JS, is that a derate? Or an assumed temperature calculation?

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).

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?

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.

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.

Longer is Better
 

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.

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

https://c.o0bg.com/rf/image_960w/Bos...crash5_met.jpg

https://en.wikipedia.org/wiki/2008_S...arjet_60_crash

http://i.telegraph.co.uk/multimedia/...60_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:

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?

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

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:

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.

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

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.

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.

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.

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?

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.

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.h...rust_Reduction


http://www.smartcockpit.com/docs/CFM...pport_B737.pdf

About slide 50 onwards

Sounds fair, JS.