Use Obstacle Clearance limited weight

[nasinferno]

I am wondering why operators are interested in caculating obstacle clearance limited weight, when the maximum takeoff weight is limited by another limitation. In otherwords, the aircraft would be limited by either WAT, or field length or any other but obstacle clearance. Some one told me they use it is used in scenarios when certain correction factors are applied on the runway....I dint quiet understood then and that is why im posting this question here......

[Rigga]

I dont remember all the details but I think this is to do with an aircraft climb out performance when an engine fails on take off runs.

International Standards for allowable climb rates assume there is an obstacle at the end of the runway that has to be cleared by a nominal amount (say 30ft).

I may be wrong.

[One Outsider]

How would you know that obstacle clearance is not what's limiting your take-off weight if you haven't calculated it?

[john_tullamarine]

The previous poster has the thrust of the answer.

There is a bunch of limitations, all of which must be satisfied simultaneously to determine the RTOW for the day. Functionally, this means that the usual approach is to run all the calculations and whichever gives the lowest weight becomes the RTOW.

Clearly, a particular runway may be simplified on the basis of prior knowledge excluding some of these calculations but the generic approach is the answer you seek.

[mutt]

Or in the event of a temperature inversion, you might have to adjust the OCH weight but not the field length/brake limiting weights.

Mutt

[muduckace]

I think this might be in reference to design spec for tail clearance on approach on g/s (thought the tail was supposed to clear the fence by 50') there have been several aircraft to challenge this depending on length of aircraft/higher aoa the tail can get very close to the ground and with more weight, slightly higher AOA.

Lost my reference material and am just taking a shot at this one so don't flame me pls.

[One Outsider]

and am just taking a shot

Quite.

so don't flame me pls.

You mean something along the lines of; "Why attempt to answer a question you don't even understand?". Was that what you had in mind?

[Capot]

The take-off weight (mass) available for any given take-off can be limted by a number of things, and the initial question refers to one of those.

Non-scientifically, and from memory here are some;

• Structural and design limits
• Obstacles in the take off path (which widens out from the runway end) which must be cleared by a stated vertical separation, with one engine inoperative
• Air temperature (WAT limits?)
• Need to stop within the available emergency distance after an RTO at V1 ("accelerate/stop"), with asymetric reverse thrust or none at all.
• Need to arrive at the other end with the weight below maximum landing weight
• Possibly on route obstacles (?)
• Others?

The maximum weight/mass allowable under ALL the limitations must be calculated for the airfield and runway in use, route, destination and alternate for the flight, using the official met data. The lowest resulting figure is then used to decide the maximum load of passengers and fuel available after taking the weight of the aircraft, catering and crew into account (APS weight).

Mostly, fuel is non-negotiable (ever tried defuelling?) so the decision bears only on the passenger and cargo weight allowed.

I think that's about it, but perhaps a pilot or dispatcher can put it better and be more up to date.

I'm sure someone will say that we use mass these days, not weight. So please read one for the other because they're right.

[muduckace]

One Outsider

No what I had in mind was keep your fingers up your A%& if you do not have constructive cryticism or if I am off topic explain why.

On topic,

I got distracted with approaches.

E.G. taking off 33L out of Bogota the hills ahead would be hard to clear in a limited bank angle due to an engine out scenario. If they were impossible to clear one engine out would this be the RTOW and OCLW?

[One Outsider]

Dear Mud,

I did explain why you were "off topic" as you put it, you clearly didn't understand what the OP was talking about.

Judging by your last question you still don't. In fact, it makes no sense. Your question is equivalent to asking a shoe salesman "if I can't tie my laces is that a size 8 or a size 9?".

And if you can't clear the hills unless all engines are running, you are not only operating illegally, you would also be dead should an engine fail on take off.

What you should have said was "If the requirement to clear the hills is what limits my TOW, is the OCLW then the RTOW?". To which the answer would be yes.

Constructive enough for you?

[nasinferno]

Use of Obstacle Clearance limited weight

Im saying that after doing some calculations. The case Im looking is a WAT limited, but the operator is interested in knowing their obstacle limited weight for the same runway and MET conditions.

[Capot]

operator is interested in knowing their obstacle limited weight for the same runway and MET conditions.

Yes, well, at the risk of labouring the point, so he should be. Especially if it turns out that the obstacle clearance weight (I'm sure there's a better term for that) for that runway in those conditions is lower.If so, a take-off with the higher weight would be dangerous, and illegal on a public transport flight.