Hi Bob,

With reference to Exercise 5.12, Question 5, though the present Cof G is 2643, the required C of G is 2680 given that when fuel is burnt from the main tanks, the C of G moves aft? This relates to the fuel burn off and balance as described on page 5.52 and 5.53. Is my understanding correct?

Also, can you please explain the last paragraph on page 5.48 regarding forward limit problems occurring at low ZFW.
I don't understand what you mean by the only compartments of ZFW which are ahead of the centre of gravity are the nose and row 1 while the main tanks are well ahead.

Thank you.

When fuel is burnt from the main tanks the centre of gravity moves aft. You are presently in the envelope so as you burn fuel from the main tanks, the centre of gravity will approach the aft limit. When the centre of gravity reaches the aft limit, no more fuel can be burnt from the main tanks.

In the echo, the centre of gravity can never be further forward than 2400 mm and it can never be further aft than 2680 mm. The centre of gravity permitted range is shaded yellow in the figure below, which is drawn to scale.





The only stations that are ahead of the centre of gravity are the nose locker, row one and the main tanks [green]. All other stations are aft of the centre of gravity [red].

If the Echo is heavily loaded, the centre of gravity will be well aft since most of the stations you can load are aft of the centre of gravity. Also if the Echo is lightly loaded, especially with fuel in the mains, the centre of gravity is likely to be closer to the forward limit.

Clearly, if the centre of gravity is presently beyond the aft limit, there would be no point is putting ballast anywhere other than in the nose, row one or in the main tanks. Any weight placed anywhere else, would make the situation worse because those stations are aft of the centre of gravity range.

Bob

Thank you Bob. I do understand it now.

In a situation where the centre of gravity is beyond the aft limit and the nose and passenger seat in row 1 is empty, can both be utilized for ballast to create a new centre of gravity and then add fuel as ballast if needed?

Or would it be better to just add fuel as ballast? Of crouse there would need to be capacity for flight fuel and ballast fuel.

Because the exam is multi choice, there can be only one correct answer. Therefore the examiner has to tell you exactly what you are going to do to remedy the problem. It would be possible to put ballast in the nose and also on the co-pilot's seat in row 1. However, you will find that in most cases that seat will already be occupied, or you will be able to put ballast in the nose until you reach the zero fuel weight limit of 2630 kg. When that happens, you can't put any more ballast on the aircraft anyway.

Bob

Thanks Bob. Possibly a finaly clarification.

In Question 7 on page 5.55, the question asks about the minimum ballast to be added with only the pilot on board. It does not say where to add the ballast.

Since it did not state where to add the ballast, I decided to add 50 kg to row 2 and 50 kg to row 3. When I plot the points on the graph, I got about 40 kgs ballast to be added.

Your method used 100 kgs in the rear compartment and ended up with 20 kgs of ballast.

Do I take it that both methods are correct?

When the question says MINIMUM ballast, you are being told where to add the ballast. If you want to find the MINIMUM ballast required to move the centre of gravity aft, it would make sense to add the ballast to the most rearward compartment.

I do see what you mean. Thank you for the clarification on this topic.