Should I take the longer runway

[Intruder]

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]

''Should I take the longer runway''

Yes.

[C_Star]

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]

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]

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]

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]

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]

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]

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

[Derfred]

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]

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/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.

[Piltdown Man]

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]

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]

What's the minimum fuel argument?

[Piltdown Man]

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]

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]

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]

Originally Posted by Piltdown Man

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]

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]

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.