CRJ900
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Take-off Runway Length Vs Length Of Runway

Tue Feb 12, 2008 11:02 pm

Is there a standard safety margin length that airlines must adhere to when planning a flight to an airport?

Like, if the aircraft needs 2,000 metres to take off with MTOW, must the runway be minimum 2,500 metres or something like that?
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timz
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RE: Take-off Runway Length Vs Length Of Runway

Tue Feb 12, 2008 11:22 pm

I'm no expert, but basically it goes like this: as the aircraft accelerates along the runway, there is some speed that is the highest speed from which they can brake to a stop, still on the runway, at their weight that day. Say that speed is 140 knots. So if an engine fails before they reach 140 knots, they hit the brakes. If an engine fails at 141 knots, they continue the takeoff on their remaining engines. They're supposed to be able to cross the end of the "runway" at ... 35 ft altitude or something like that. If they can do both of those things-- stop on the runway from 140 knots, or continue the takeoff from 141 knots-- then they're legal for that runway.

The complications involve clearways and stopways and obstacles and who knows what else.
 
PGNCS
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RE: Take-off Runway Length Vs Length Of Runway

Wed Feb 13, 2008 3:01 am



Quoting CRJ900 (Thread starter):
Is there a standard safety margin length that airlines must adhere to when planning a flight to an airport?

Like, if the aircraft needs 2,000 metres to take off with MTOW, must the runway be minimum 2,500 metres or something like that?

To answer your question directly, no.

There are a myriad of things that go into the mix including runway conditions and low visibility which come with a defined set of assumptions, but there is no standard margin as such. The runway IS guaranteed to be long enough for safe operations within the assigned set of assumptions for every takeoff and landing, but margins are generally handled in terms of the weight of the aircraft versus the length of the runway from an operational point of view. The US military more rigorously examines actual runway length versus aircraft performance. This isn't perhaps the answer you wanted, but it's as short and generalized as I have time to give right now. Hope it helps!
 
bond007
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RE: Take-off Runway Length Vs Length Of Runway

Wed Feb 13, 2008 3:29 am



Quoting PGNCS (Reply 2):
To answer your question directly, no.



Quoting PGNCS (Reply 2):
and landing,

Whoa, there is a margin for landing I beleive. If it's a Part 121 flight, then you have to be able to come to a full-stop within 60% of the runway length.


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PGNCS
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RE: Take-off Runway Length Vs Length Of Runway

Thu Feb 14, 2008 3:37 pm



Quoting Bond007 (Reply 3):
Quoting PGNCS (Reply 2):
To answer your question directly, no.



Quoting PGNCS (Reply 2):
and landing,

Whoa, there is a margin for landing I beleive. If it's a Part 121 flight, then you have to be able to come to a full-stop within 60% of the runway length.

But that's not "a standard safety margin length" which is what the question asked. My point is that there is no rule that says that you have to have the TO distance + 500 feet (or whatever), which is what he was asking. All our LANDING data is, in fact, based on percentage safety factors (which increase further for wet/LLM situations), but there is not a standard additive required. That's the principal point I'm trying to convey; sorry if it was confusingly presented. I probably shouldn't have mixed TO and Landing distances in the same thread, either.
 
oly720man
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RE: Take-off Runway Length Vs Length Of Runway

Thu Feb 14, 2008 4:14 pm

Take off length = runway length.
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I think it's safe to say that every metre will be used if it can be.
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CosmicCruiser
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RE: Take-off Runway Length Vs Length Of Runway

Thu Feb 14, 2008 4:25 pm



Quoting Oly720man (Reply 5):
I think it's safe to say that every metre will be used if it can be.

I'm not sure where you're going with that but as others have said the rnwy rerquired will be a function of wgt and wx. The absolutely shortest dist would be a "balanced" field where t/o to 35' and accel/stop dist are equal to rnwy length. As a rule you have a margin that's going to give you a little extra padding. I'm not going to use any more rnwy than is needed for me to reach Vr and rotate. You don't really know looking at those pics whether or not the pilot may have been a little slow rotating which can add considerable length to lift off. We're taught 2 deg/sec. Any faster you run the risk of a tailstrike and any slower you use up more runway.
 
mandala499
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RE: Take-off Runway Length Vs Length Of Runway

Thu Feb 14, 2008 4:33 pm



Quoting CRJ900 (Thread starter):
Like, if the aircraft needs 2,000 metres to take off with MTOW, must the runway be minimum 2,500 metres or something like that?

It depends on how you define "need"...

There are a few definitions to use:
ASD = Accelerate Stop Distance... Aircraft goes to V1, then stops.
TOD = Take Off Distance... Aircraft goes to V1, has 1 engine failing, then goes to a particular altitude... 50' or 35'... forgot.

The "oops" factoring is inputted into the figures.

So, generally speaking, if the aircraft needs ASD of 2000m, or TOD of 2000m, it only needs 2000m...
If you got a 2500m runway and it's dry conditions, go ahead and derate until one of those two numbers hit 2500m.

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Starlionblue
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RE: Take-off Runway Length Vs Length Of Runway

Fri Feb 15, 2008 3:52 am



Quoting Oly720man (Reply 5):
I think it's safe to say that every metre will be used if it can be.

Assuming they calculated properly and the aircraft performed nominally, those aircraft are still within the margins. While they may only rotate when they're almost at the end, they passed V1 waaaaay back when there was still enough runway to brake (+50% pad). Rotating later allows use of less flap and increases climb speed.
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tb727
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RE: Take-off Runway Length Vs Length Of Runway

Fri Feb 15, 2008 4:10 am



Quoting Bond007 (Reply 3):

Whoa, there is a margin for landing I beleive. If it's a Part 121 flight, then you have to be able to come to a full-stop within 60% of the runway length.

I am not sure about 121, but 135 landing distances it has gotten a little more complicated recently due to part 91 subpart K. 135 is stop within 60%, now with Eligible Crew On-Demand being part 135 and meeting certain crew requirements, it can be 80%. Then you add in wet/contaminated/icy and it gets all crazy. Luckily we have a chart Smile Part 91 is also in there, so we have all kinds of numbers to look at when we fly depending on what we are doing. When you get light and run the numbers, it's surprising how short of a runway a jet can get into. The problem is usually getting out.
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heinzmahrer
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RE: Take-off Runway Length Vs Length Of Runway

Fri Feb 22, 2008 3:23 pm

Take Off runway requirements are:

accelerate the aircraft to a certain speed (decision speed, V1), loose an engine and come to a complete stop on the runway,
or, if it is a go decision,
loose an engine at V1 and continue the take off and be able to clear the departure end of the runway by 35ft.

Factors affecting take off performance are:
- pressure altitude and temperature (air density)
- configuration (i.e. how much flap setting is used)
- runway conditions (dry, wet, snow etc. affecting acceleration or stopping capability),
- engine/wing anti-ice systems on or off (affecting engine power)
- and aircraft weight.
The only factor the pilot can really control is the weight (reduce fuel and/or payload) unless he and his passengers are prepared to wait for cooler temperatures in the evening. Offload passengers and you make them unhappy, offload fuel and you may have to make a fuel-stop somewhere and your passengers complain again . . .
And then of course there are climb requirements: It is not enough to get off the ground, you must also be able to clear that mountain 3 miles after take off (in a one engine out condition!) and that could mean another restriction in weight.

For landing:
cross the runway threshold at a calculated speed on a 3° slope, land and stop the aircraft using brakes only (thrust reversers are not used in this calculation!) and multiply this distance by 1.67: This is your required runway length (dry runway, for wet add another 15%)
Again weight is the main factor affecting distance but the pilot has less control over it. Once you are down to your minimum reserve fuel you cannot get any lighter.

What I have described applies for commercial operations on transport type aircraft, i.e. airlines, charter operations, commercial operations with business jets etc.

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