trex8
Posts: 4618
Joined: Sat Nov 02, 2002 9:04 am

Take Off Distances

Mon May 09, 2005 6:18 am

now I understand that asking what the TO distance is, is like asking what the length of a piece of string is, there are too many variables but something has got me perplexed.

The longest runway at EWR is 3350m, the takeoff distance at sealevel, ISA+15 deg C (which should be about as "normal " conditions for EWR as anything) for a 287K MTOW 772ER according to Boeings website is 3400-3500m and that for a A345 from Janes is at least 3100m. Seems those CO and SQ planes going to HKG and SIN may be cutting it a little close, esp with the 777. Is my thinking off base??? How much 'spare" runway is it safe to try take off with a fully loaded widebody? Seems a measly several hundred meters spare or even the exact length is rather dicey.

the reason this all came up was I saw that several regional airports around the world are all boasting about how they will have the capability to fly to "cities throughout the world" due to their new and fantastically long runway of all of 3000m! Seems like you couldn't get a fully loaded 772 or A345 out of a 3000m runway period! Even some established airports like LGW have just got over 3000m.

[Edited 2005-05-08 23:19:36]
 
Goldenshield
Posts: 5008
Joined: Sun Jan 14, 2001 3:45 pm

RE: Take Off Distances

Mon May 09, 2005 6:23 am

The aircraft will we takeoff weight limited based on the runway length. Since they can't take off fuel, they will take off payload instead, which, with a 777, may not involve bumping any passengers since the plane can also hold a lot of cargo.
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CosmicCruiser
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Joined: Tue Feb 22, 2005 3:01 am

RE: Take Off Distances

Mon May 09, 2005 7:20 am

Quoting Trex8 (Thread starter):
The longest runway at EWR is 3350m, the takeoff distance at sealevel, ISA+15 deg C (which should be about as "normal " conditions for EWR as anything) for a 287K MTOW 772ER according to Boeings website is 3400-3500m and that for a A345 from Janes is at least 3100m

Anything over the REQUIRED rnwy t/o dist is what we would call "stop margin". It's "extra". The t/o dist is the longest of accel (v1) and stop or accel (v1) loose an eng. and go. If the rnwy is 3350m and your t/o dist is 3100m then if you reject 1 kt. below V1 and stop you will have 250m left when you stop. Technically speaking.

Quoting Trex8 (Thread starter):
Seems a measly several hundred meters spare or even the exact length is rather dicey.

It happens all the time.
 
trex8
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Joined: Sat Nov 02, 2002 9:04 am

RE: Take Off Distances

Mon May 09, 2005 8:34 am

would you ever routinely operate at or just below the minimum?? eg the A345 needs 3100m, and you use a 3000m runway?
 
PhilSquares
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Joined: Sun Mar 28, 2004 6:06 pm

RE: Take Off Distances

Mon May 09, 2005 8:52 am

Please remember the runway required is based on losing an engine on takeoff and either continuing the takeoff or aborting in the same distance (balanced field).

I have taken off on many occasions where we were right at the weight limit for the runway. It's really no big deal. Statistically, if you look at the odds of losing an engine on takeoff, they're pretty low.
Fly fast, live slow
 
CosmicCruiser
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Joined: Tue Feb 22, 2005 3:01 am

RE: Take Off Distances

Mon May 09, 2005 10:28 am

Quoting Trex8 (Reply 3):
would you ever routinely operate at or just below the minimum?? eg the A345 needs 3100m, and you use a 3000m runway?

No, the data rules, if it's exactly the same you're good to go ;1 ft short and you're not. Operating AT the limit (in some cases "balanced field") is normal.
PhilSquares is right.
 
trex8
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Joined: Sat Nov 02, 2002 9:04 am

RE: Take Off Distances

Mon May 09, 2005 10:44 pm

thanks guys for the info, very helpful
 
Lemurs
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Joined: Sat Mar 12, 2005 5:13 am

RE: Take Off Distances

Tue May 10, 2005 1:58 am

Quoting CosmicCruiser (Reply 5):
No, the data rules, if it's exactly the same you're good to go ;1 ft short and you're not. Operating AT the limit (in some cases "balanced field") is normal.
PhilSquares is right.

Quick follow-up Q...are the numbers based on Vr, or 35ft AGL at the runway threshold? I remember from the IL-96 post a few weeks ago people saying that operating rules in the US were 35' AGL at threshold, so you couldn't really run up to the end of the runway, could you?

[Edited 2005-05-09 18:59:40]
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OldAeroGuy
Posts: 3208
Joined: Sun Dec 05, 2004 6:50 am

RE: Take Off Distances

Tue May 10, 2005 4:35 am

Quoting Lemurs (Reply 7):
Quick follow-up Q...are the numbers based on Vr, or 35ft AGL at the runway threshold? I remember from the IL-96 post a few weeks ago people saying that operating rules in the US were 35' AGL at threshold, so you couldn't really run up to the end of the runway, could you?

Vr is the rotation speed applicable for all engine or one engine inoperative. It is selected relative to V1 (engine failure recognition speed) so that the airplane is 35 ft. above the end of the runway for an engine inoperative takeoff if the engine inoperative continued takeoff is the limiting case (usually true for twins). Note that TOFL is also determined based on 115% of the all engines takeoff distance (often more limiting for quads).

The only time you will be at the end of the runway is if the pilot doesn't recognize the engine failure at V1 and/or is slow in initiating a stop.

Stopping distance is calculated as follows:

- All engine acceleration distance to Vef (engine failure speed)
- One engine acceleration distance to V1 (allows one sec. for the pilot to recognize an engine has failed)
- Two seconds of constant speed (V1) distance.
- Stopping distance from V1 to stop with allowance for pilot reaction time to apply the brakes, chop the throttles, deploy the thrust reversers and extend the speed brakes.

The two seconds of constant speed distance acts a "pad" in the stopping calculation.

Also if the airplane was certified prior to FAR part 25, Amend. 98, no credit is allowed for use of reverse thrust. On a dry runway, reverse isn't a factor as the airplane is usually stopped before the reversers deploy and the engines spin. Reversers do improve performance on wet, snowy or icy runway though and Amend 98 was the FAA's acknowledgement of the UKCAA's provision for reverse thrust performance improvement under these conditions.

Let's not talk about the sidetrack that Amend. 45 put us down.

From the above, you can see that takeoff calculations involve a number of factors and are very complex. I've seen past postings that indicate takeoff performance is predicated on second segment climb capability (engine inoperative climb gradient from gear up to a minimum of 400' AGL) and obstacle clearance requirements. This is far too simplistic and ignores all the other factors that go into calculating a limiting field length.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
CosmicCruiser
Posts: 2049
Joined: Tue Feb 22, 2005 3:01 am

RE: Take Off Distances

Tue May 10, 2005 6:30 am

Sorry I was late getting back to this post but AeroGuy said plenty so

Just to add a little extra..V1 can "move' depending on rnwy conditions
dry
wet
cluttered

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