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ECHO Forward Limit
becb
Topic Author
becb created the topic: ECHO Forward Limit
G'day Bob,
I had two questions like this in my recent exam:
Aircraft has a gross weight of 2550kg and respective moment of 605 index units (or something very close to this). You have 10kg ballast bars loaded in the front and rear compartments. What is the minimum number of ballast bars need to be moved to the rear compartment? (Enter the number in the box below).
Recognising that this is a forward limit problem, I plotted the first weight and moment on the graph. It was outside the FL. Knowing that 50kg is the maximum amount of 10kg ballast bars that can be in the forward compartment, I moved these to the rear. And then I got confused, because it was still outside the forward limit, and i didn't know how many ballast bars were in either compartment to begin with so I just thought that I would try 150kg in the rear and then plot this new weight and moment, draw a line, see where it intersects and minus the two weights from each other and divide by 10 to get number of bars. I didn't get the answer right.
It then got me thinking about the CAO that states that an aircraft must also be in balance at ZFW and I had no idea about how to calculate this if i didn't know how much fuel was on board in the gross weight figure, or if this was even relevant to this question?
I've since discovered some questions very similar to this in your textbook on page 180 and will be reading through this today. If you can please clarify the above, that would be much appreciated. Thanks heaps!
Strange question. Firstly, there is no way the Echo could be out of the forward limit at ZFW. If it was you could never put any fuel in it since the main tanks must be filled first and they are on a arm of 1780mm and that's well ahead of the forward limit. So we must assume that condition given is at take-off.
Since you are asked to shift weight, the weight will not change, so you can calculate the forward limit at 2550kg. That's the one you are after. You can use the flow chart to solve the problem and there's no need to plot anything on the envelope. If you do that you get a silly answer, so I suspect that the figures you have given for the initial weight an moment are not correct. The question needs rephrasing.
