Hey Guys; I have completed studying the Bob Tait for performance and hit a couple of speed bumps on the way and was hoping for some assistance before I attempt the practice exams. I have also booked in the performance exam for the 22nd.

I have memorized the formulas and understand what most of them mean; here are my questions:

So I understand that when the question says Shifting Weight a calculator is always the answer! When Adding or Subtracting weight we may use the calculator for the Aft Limits. However with the forward limits we plot the points on the graph in order to determine the answer.

My 1st question is how do I tell apart these questions both in the shifting and adding or subtracting section? Is there a specific key word in which it determines if we use the Req. CG for the forward limit or the aft limit? I get the question will include Shift/Add/Subtract but in order to find out the weight to shift or to add i must first find out the Req. CG which could be 2680 or the forward limit which would be determined by using the formula. So the base question would be; How would I know when to use the Aft CG or the forward CG and when to draw it out if it is indeed the forward limit or is the forward limits' weight to add or subtract always drawn out?

My 2nd question is If the question asks how much weight we may add or subtract how do we know to use a formula at all? ie some of the answers require us to determine the lowest of MTOW-2950, Max Landing-2725+Flight Fuel, MZFW-2630+FOB and the lowest of these minus what the questions given us is the answer.

Please Help Thanks!

Hi JooJong
That's a common problem students have. When faced with a particular question, is the problem the aft limit or the forward limit? I encourage students to always have the Centre of Gravity envelope beside them as they read the question. The examiner will always have to give you a starting point with a weight and moment index. If you take that weight and moment index and check it out on the envelope (even just with your finger tip) you will get a good idea of where the starting point is. You may not be going to use the envelope to solve the problem, but you will see whether the problem is the forward limit or the aft limit: then you can use either the calculator or the envelope as required.

If you are asked the maximum weight that can be carried for a particular take-off (that's what I call the 'beetle diagram' type problem), you will always be told to ignore balance considerations. So it is just a matter of using the 'beetle diagram' to find the least of take-off limit, landing limit + burn- off and zero fuel weight + all fuel on board at take-off. That is also what you would have to do in practice. First find the maximum weight that can be carried ignoring balance then, as a separate calculation, consider how that weight will be distributed to satisfy the centre of gravity requirements. Those two considerations are unrelated.

All the best with your study

Bob

Thanks Bob your a Legend! Drawing the points does help the interpretation of the question at hand!