Runway length help.

[EGNMCharlie]

Hi,

Right this is a really really simple question yet I cannot make an educated guess at what is the right answer! The question that is puzzling me is:

Would you expect an aircraft to use all the length of the available runway for its ground run and if so why?

So firstly I though no, as you use the required length to get Vr. However then I though say that you needed a take off run of 1500m and 2500m was available would it be possible to use up to the full 2500m and perhaps reduce engine wear/fuel?

I don't know if i'm thinking about this too deeply but what would anyone else say, yes or no and why? Strange how seemingly obvious and simple questions can often be the hardest!!

Cheers.

[ACMS]

Ahhhh

NO

Unless you want to eat the opposite end approach lights in a normal T/O

And have no runway left in following an engine failure at V1.

Myself, I'm pretty average at perfomance stuff, but I still wouldn't ask this question.

[dwshimoda]

EGNM,

Are you aspiring to be a professional pilot, or just an interested onlooker - it makes a difference to how to phrase the response!

If the former, then get your performance manuals out sharpish and start readinding about TORA / TODA / ASDA/ and all the other factors you have to take into account for any take off.

If the latter, then put simply, we need enough distance to accelerate to decision speed (we are taking off pretty much regardless of any emergency) and through to rotation speed, and climb to a screen height above the end of the runway. Before decision speed (V1) we need to be able to stop the aircraft before the end of the runway - this is what normally determines V1.

No aircraft uses the full length of the runway, etc perhaps with the exception of aircraft carrier launched aircraft!

Look here for more detail on a balanced field take off, and for great links to other take-off related terms:

Balanced field takeoff - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Balanced_field_takeoff)

DW

[Pontius]

EGNM,

I don't think you want too complicated an answer, so I'll not give you the benefit of all my performance knowledge (otherwise we could be here for over 3 minutes).

The simple answer to your question is 'no'. Basically, there are two cases; one has you getting airbourne after V1, the other has you stopping. In the stopping case you must, obviously, be able to stop before eating the lights at the opposite end of the runway (and for the purists, yes, I know about stopway, clearway, EMDA, ESDA etc but I said I'd keep it simple). So, there must be enough runway to accelerate to V1 and then stop. In the 'flying' case you must be able to continue the takeoff after V1, roatate and be at 35ft (15ft if it's wet) at the end of the runway at V2 (safety speed). So those are the basic requirements and leads to the calculation of your field length requirements.

Now, to the 2nd part of your question. Yes, you can reduce engine wear. Assuming for the moment that it's nice and flat around your airfield and there's not a great big aerial on the extended centreline, then we'll keep it simple and say the field length requirements are the limiting factor. So, let's imagine for the weight of your aircraft you need 1500m to either get airbourne after V1 or stop at V1. With your 2500m long runway you can reduce thrust, thereby saving on wear and tear, fuel and noise (although the greatest gain is in wear and tear). This is normally done by 'fooling' the thrust management computer (TMC)into thinking it's hotter outside than it really is, so it thinks it hasn't got as much thrust available. During your performance calculations you, as an example, used 15 degs to come out with your 1500m. Now you go back into the graphs/tables/computer and extract the temperature at which you'd actually be able to lift your aircraft weight using 1500m and it may come out with something like 45 degs, which you then tell the TMC that's it's 45 degs and it gives you the takeoff reference thrust for that temp (which will be less than that at 15 degs). This method of reducing thrust is known as Assumed Temperature and is used every day by many ailrines etc. Equally, many just use a fixed derate (10 or 20% of thrust) but this does not give the same sort of engine life savings taken over a long period.

As I said, this is not all there is to it by a long chalk and I've tried to keep it simple, so no barracking from the cheap seats. There are more calculations that go into determining max takeoff weights and the amount by which you can reduce thrust for takeoff but that's the basic idea......I think :)

[EGNMCharlie]

Right, thanks chaps! :ok:

After reading how I phrased my question, I can see my fatal error. I have made myself look a complete dim wit. I should have said:

Would you expect an aircraft to use all the length of the take off run available for its ground run and why?

So, my little calculation was assuming the TORA was 2500m, NOT the full length of the runway being 2500m would it be appropriate for an aircraft that requires a minimum take off run of 1500m to use a longer take off run if it is readily available on a day to day basis. I am fully aware that if the runway was 2500m, using the full 2500m to take off on would be a rather silly idea.

Thankyou Pontius for answering what I was meaning to say, apologies for being unclear on what I was asking.

[fly-dad]

Would you expect an aircraft to use all the length of the take off run available for its ground run and why?

Perhaps these pilots have an answer for you.

YouTube - Russian Airplane Departing Australia (http://hk.youtube.com/watch?v=8BY7-LgF7d4)

:ugh::ugh::ugh::ugh:

[john_tullamarine]

Would you expect an aircraft to use all the length of the take off run available for its ground run and why?

No reason why a takeoff can't be TOR limited. However, keep in mind that this doesn't mean that the wheels are on the ground until the end of the TOR. Although there are different standards in play, the usual requirement is that half of the air distance to screen height has to be over the TORA .. ie the aircraft has to be airborne this distance prior to the end of the TORA.

[Lookforshooter]

Generaly NO as a rule, but there are exceptions.

[FullWings]

Generaly NO as a rule, but there are exceptions.
...Such as you have an aeroplane on a strip in the middle of nowhere which is only *just* long enough to get airborne from. This would probably have been an emergency landing, so you have the choice (let's say it's a heavy jet) of leaving $???M of aeroplane to rot there or getting some volunteers to take it out with no real options in the event of a post-V1 failure. As a one-off, non- public transport operation the risks would probably be deemed acceptable. I'm pretty sure that full power would be set on the brakes and left there for a while until all were happy!

There are three-engined ferry flights done every now-and-then on four-engined aircraft and these sometimes involve a 'period of uncertainty' where another failure, especially on the same side, would leave you unable to stop or get into the air. Nasty. Most operators crew these on a volunteer basis as well.

Then I suppose there is the single-engined aircraft. As you're planning to force-land (crash) straight ahead in the event of a power plant failure below a certain height anyway, then having very little/no runway left at rotate is of academic importance (assuming there's not enough to land back on...)

[chornedsnorkack]

Consider the logic of having such a thing as V1.

There used to be planes which had no V1 and no Vr. Classical props and the first jets. The plane accelerated on ground to what was thought to be V2, rotated and climbed - or else aborted.

However, for all planes having V1 less than V2, once they are past V1, they have run out of runway to abort. If they attempt a high-speed abort, they overrun and crash - they are committed to accelerate and hope that the plane can rotate, lift off and climb before runway ends.

Given that you are risking your lives on the hunch that the plane can rotate, lift off and accelerate (having passed your chance to abort after checking it cannot), what is the point of actually rotating with plenty of runway ahead? You could accelerate on ground instead of accelerating while airborne.

[Junkflyer]

1. Tires have speed limits.
2. Acceleration is better in flight with a clean airframe.
3. You want altitude sooner for climb limits/obstacle clearance.

[SNS3Guppy]

The answer to your question really depends on what it is that you're doing with the airplane, and why, as well as the type of airplane in question.

If you're flying a light single engine airplane from a grass field, there's really no refusal speed, no decision speed, nothing of the kind. Simply one needs enough room to get the airplane into the air and go fly. If one has an engine failure, it's really little difference between having it on the runway, and gliding to a runway; it's part of planning for a takeoff in a light, single engine airplane. In general one should plan on having ample room to reject the takeoff...but at what point? In a light single, the action of the airplane, and of the pilot is fairly much the same at any point should the engine fail...the airplane is going to return to earth and stop, no matter what. Planning a takeoff in a single engine airplane, then, includes having available places to go beyond the runway, as well as on the runway itself, should an engine fail...the world becomes a forced landing site, or one big runway, if properly planned.

Not so for multi engine airplanes...but not for all multi engine airplanes. As airplanes get bigger they land faster, are heavier, and fare less well with both a forced landing and a rejected takeoff. In fact, as airpalnes begin to get larger, their brakes don't get much bigger, but they get a lot heavier. Decisions to reject become more and more critical, and as airplanes get bigger and more complex, and have more power, the options with regard to what to do after takeoff increase.

In a light twin engine airplane, failure of one engine often means the airplane will be descending to earth just like the single engine airplane...80% of it's performance is gone when one of two engines quits...because much of the remaining engine is left fighting drag and assymetry issues, and because climb performance is based not on the thrust required to fly, but excess thrust...it's just lost all the excess, and much of what was required to maintain level flight. It's coming down, too.

More importantly for the light twin, it has assymetry issues...should an engine fail, the other engine tries to push the airplane into a turn or yaw, and also into a roll. This force can be quite powerful, and can result in a loss of control. In some cases, the only choice may be to remove the power from the "good" engine, and land anyway. Due to the increased landing difficulty, and control difficulty, and stopping difficulty with the increased weight of the multi engine airplane, allowing excess runway is prudent. It's prudent for two reasons, and these directly apply to larger airplanes, as well.

Longer runway than needed is prudent for stopping, obviously; if one reaches a given point on the runway and needs to reject the takeoff, then one still needs the distance to do so...but just as importantly, if one elects to continue the takeoff, one may need a clear path ahead with which to do so. In comes the takeoff distance and clearway which permits continuing the takeoff...nearly always the safest thing to do in large, capable airplanes, nearly always the foolish thing to do in a light twin (and certainly a single engine airplane, in which one has no options).

I've flown several types of operations in which we regularly used all the pavement. Or gravel or dirt. All of it...from one end to the other. I've done this in single engine and multi engine airplanes, both small airplanes and large. The reasons for doing so have varied from very limited terrain in which to place a runway (small, one-way dirt strip in a box canyon, for example; tall cliff rising at one end, deep canyon cliff falling away at the other...and nowhere to increase the runway length), to operational necessities such as emergency operations, etc.

In transport category operations such as corporate and airline, it's a little different, and considerably more structured. One plans at a minimum to be able to stop in an emergency, and to be able to go and reach a minimum height and have a minimum climb gradient with obstacle clearance for all takeoffs. Considerable regulation applies, and the practices, policies, and performance requirements are nearly universal globally in this arena.

This is not necessarily the case for other types of operations. Some may wince at the concept of using all the runway in a firefighting operation, or a relief operation, or a back country or bush operation...but the truth is that very often that's all you've got, and you use what you've got. When there's a beach to land on and takeoff, you're never going to get more beach. If it's a single engine airplane with the engine failure on takeoff, there's little difference if it fails thirty seconds into the takeoff or 30 minutes...and one isn't getting a longer beach, a paved beach, or one with facilities. It's just the beach.

In hauling heavy loads off short runways in firefighting, we always had the advantage of a disposable load, and I've had to jettison it before during an engine failure or other emergency. Doing so on the runway isn't practical as it tends to coat the brakes and make stopping nearly impossible anyway. Once one starts the takeoff, one's committed, and the result is that the only concern during the takeoff is obstacle clearance at the end. I've flown tankers on fires out of relatively open fields such as McCall, Idaho, or Hemet, California (USA), where much, if not all of the pavement was used, and in the case of McCall, one had to maneuver to avoid trees and cows off the end of the runway. Stopping wasn't an option. I've done it in single engine and multi engine airplanes.

I've also flown passengers out of those locations, too. In each case, I took into account the full takeoff performance regime, including with and without all engines operating, and planned accordingly. I didn't use all the runway, allowed for stopping, and allowed for a continued takeoff with an engine failure. Just last year I operated out of McCall in a single engine turbine tailwheel airplane, hauling retardant onto wildfires...and I again used all the runway and just cleared the obstacles with a turn, due to the weight and requirements of that particular assignment. I used less than my maximum weight, because the performance of the day wouldn't permit it...but used all the field none the less due to density altitude (reduced performance with increased temperature and altitude).

Bottom line is that sometimes it's acceptable and appropriate, and sometimes it's not...one needs to consider what it is one's doing, with what, and why.

[Lookforshooter]

My .2 cts. If you have the choice to get off the ground earlier, single engine or otherwise...then should something happen at rotation or after, you might still be able to land the aircraft...certainly we have seen small planes do touch and goes down a long runway... So it begs to ask the question...why would anyone extend thier ground roll voluntarily as far down the runway as possible, putting more pavement behind them then in front making it much tougher to land the aircraft.?? As the saying goes..the three most worthless things to a pilot....runway behind you...altitude above you, and fuel you left on the ground.....Sorry Guppy...had to throw that in there.....

[mutt]

Dont feed the troll................
SSG reborn........

Mutt

[SNS3Guppy]

He is...he's trying hard not to get thrown off the board again...was last here as glockshooter and was banned...but was seen supporting himself again, in a recent thread.

[SNS3Guppy]

You realize that you're replying to a thread a month and a half old, right?

[galaxy flyer]

Just to throw red meat out there...in the mighty C-5 Galaxy V1 limiting take-offs were predicated on lift-off occurring on the last brick. That's right NO height at the DER!! Our OIS for military (USAF & USN) began at the DER, not 35 feet above it. A limiting take-off at max gross was something to watch, esp from the front seats!!

[Old Smokey]

Single Stroke Gong (SSG) is alive and well, albeit with a new pseudonym:yuk:

Regards,

Old Smokey

[bucket_and_spade]

Given that you are risking your lives on the hunch that the plane can rotate, lift off and accelerate (having passed your chance to abort after checking it cannot), what is the point of actually rotating with plenty of runway ahead? You could accelerate on ground instead of accelerating while airborne.

There is one scenario in which I would (slightly) delay rotation after the PNF's "rotate" call on a nice, long runway and that would be if, after an engine problem, I hadn't managed to get the thing tracking towards the centre of the end of the runway yet. If it happened in real-life - I'd spend a few more seconds (if needed) getting that bit right and then locking my feet on the pedals before hauling it off.

[BelArgUSA]

We just do that, use increased VR and V2 for Improved Climb, when we are second segment limited. That makes you use a little more of the available runway. For the 747 Classic, sometimes used for 20 flaps takeoff...
xxx
Va va voom...!
:ok:
Happy contrails

[Self Loading Freight]

FullWings wrote:

I'm pretty sure that full power would be set on the brakes and left there for a while until all were happy!

Is this because engine failures are more likely while they're running up to full power and just after they get there? Sitting in a Saab doing the on-the-spot disco dance at the end of 27 at Kirkwall, watching the windsock on Viagra pointing directly across the runway, I've often wondered why there was such a wait between hitting full oomph and actually getting going.

R

[ChristiaanJ]

I've often wondered why there was such a wait between hitting full oomph and actually getting going. This has been discussed elsewhere. The usual reason is that engines don't all take the same time to run up to full power. So you wait for them to stabilize to the same power, rpm, etc. before releasing the brakes.

[javelin]

The Mini Busses use optimized performance to increase energy in the event of an EFATO, so they try to use increased V2 which sometimes gives some very high speeds on a long runway.

Not always good when the tyre shreds around V1 and you end up with a hyd leak and massive vibration as happened to one of our chaps years ago :{

[Sir George Cayley]

Anyone mentioned declared clearway yet?

Can't be bothered to read all the thread, sorry.

Sir George Cayley