Field limited or Climb limited
 

Hi!

Another nice question: What is the difference between field limited and obstacle limited TO? And what role do the runway weight charts play in this topic?

Have a nice day!

OD

Field limited: the airfield is on flat ground, but there's a river at the end of the very short runway. Doesn't matter how fast you can climb, you're not going to get off the ground before the end of the runway unless you reduce your weight.

Climb limited: the runway stretches for miles, but there's a bl00dy great tall building just off the end of it. Doesn't matter how long the runway is, it's not going to help you get over the top of that building unless you can climb faster.

FFF
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There are many factors limiting the take off. Namely the airfield(length of runway, winds, slope etc, this is the Field Limit Mass) the obstacles if any (obstacle limit mass), the climb performance based upon meeting certain minimum climb gradients (climb limit mass) the tyres exploding (tyre limit mass) etc.

So today let us assume this.
Filed limit mass 50,000kg
Climb limit Mass 45,000kg
Obstacle limit mass 65,000Kg
Tyre limit mass 70,000kg

If you are heavier than 50000kg you wont get out of the airfield. If you are heavier than 45000kg you'll fail the minimum acceptable climb performance
If your heavier than 65000 then you'll die hitting the obstacle infront of you.
If your heavier than 70000 then the tyres will shred.

Ok so, what mass do you assume as the mass to take off?
I'd suggest 45000kg if you want to keep your job and live.
So you could say that today you're take off is climb limited.
Forgive the simplicity of this explanation to those of you who might be inclined to nit pick.

Hope this helps

Steve

Flying for fun,

Sorry your explaination for climb limit is wrong. You just described obstacle limited.
Climb limit mass is based on meeting a minimum acceptable air gradient of 2.4%.
Forgive the nit pik

Steve

You are quite right, Steve - my mistake.

FFF
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Thank you very much!excellent explanation!Only one more question:
What is the runway weight chart giving me?And these weights are all calculated with the ASDA and the TODA?

Thanks

OD

I take it your refering to the Field limit take off mass graph on page 55 of the CAP manual.
If you are, then this graph ends up giving you a weight. This is the weight, above which you will not be able to comply with the take off regulations. What I mean by that is:
1) either make it out of the airfield because you might over run the runway before getting airbourne.
2) or in the event of an emergency stop, not be able too before the end of the runway.
It's a lot more complicated than this, but essentially that weight that the graph gives you, is taking into account the ASDA and TODA.
Look at the bottom of the graph, an you see it say "Field length available". Well, now look page 53 at the top is says that this is in fact a balanced field. Balanced fields are when the TODA and ASDA are the same. But in the performance manual it assumes it is also the TORA.
If you get confussed ingnore the last few lines of this post.
Steve

allright, thats ok. what i do: i take the weight on the left side of the rwyweightchart and there i see whats appropriate for my temperature and i take the right one for the climb. so now i put them all in the to data sheet and calculate with the whole corrections. then i decide what to take. the right side of the rwyweightchart is for the climb. so this is only for the climb gradient of the 2,3% path?

thanks

od