How to calculate ZFWCG/ZFW

The first thing we need to factor is the "empty weight" of the aircraft (in the case of the A320-211 this is 42,300kg). Each passenger is assumed to be just 70kg and carrying 5kg of cabin baggage (total = 75kg/pax).

Note: That is something that will need to be reviewed in the near future for safety reasons. It's obviously absurd to think we can average out the weight per pax to just 75kg, because it does not take into account the increasing rate of obesity, or that many pax try to cheat the cabin bag allowance. From a safety point-of-view, it would be much better to assume average weight per pax at about 100kg, but so far nobody seems to want to face up to that inconvenient truth.

For tutorial purposes, we will stick with the "official" weight per pax figure (75kg), and assume there is a full load of 150 pax (the aircraft in the tutorial is an 320-211) plus 8 crew, this gives us a subtotal figure of 158 × 75 = 11,850.

Now we need to add that to the 42,300kg of empty weight, giving a new subtotal of 54,150.

Our ZFW limit is 60,500kg so we need to hope that cargo does not exceed 6,350kg. For now, let's assume that we have about 3,500kg of cargo. This means our total ZFW is now 57,650kg. We must round this total up to 57,700 and this is our final ZFW total.

That was the easy part!

Now we need to explore the even more complex problem of Center of Gravity, or "CG".

First thing to understand is that the CG is expressed as a percentage of the aircraft body relative to the center. "Forward CG" is CG where the balance is closer to the nose of the aircraft, and "Aft CG" is NOT the opposite of Forward CG!!!

CG greater than 50% would be abnormal and have a severe effect on performance. This would result from something dramatic happening, such as something breaking loose during the flight, or all the passengers running towards the tail of the aircraft and stopping there.

So true Aft CG is not something you'd normally have to think about. For most pilots, the term Aft CG refers to CG values greater than 25%, and Forward CG refers to CG values below 25%. Obviously the default CG of 25% is simply a point of reference and not necessarily the ideal CG, as some people erroneously think.

The concept of ideal CG is way too complex to give it a fixed value. The ideal CG will vary for each flight depending on the conditions for that flight. It also depends on who is making the decision about what is ideal. For the airline, it is about what gives the most efficiency. Other people with different values will not necessarily agree. But the CG is not decided by the pilots, it is merely reported by them to the MCDU.

Other people do their best to distribute the load according to airline policy of "load profile", which is effectively a managerial decision rather than operational. Operational factors should always take priority over managerial policy (and if they don't then you work for a bad airline!), so everyone on the operational side of things will be making decisions constantly right up until the moment the doors are closed. That also includes cabin crew redistributing passengers if necessary.

Pilots will be given information about how the load is distributed relative to the CG and they can then make decisions taking this value into account. CG isn't even a fixed value. We actually should think of it as a "CG Envelope" because things happen in flight than can momentarily shift the CG slightly.

Your job will be to examine all the data you are provided with and then give an appropriate CG value to the MCDU so that the correct information is fed to the FMGC and all of the calculations are based on actual operating conditions and not just the default value.

Certainly for use in home simulator games such as Microsoft Flight Siumlator, Flight Gear, or X-Plane, you don't have to worry about the CG factor in a realistic way, but in any real world aviation your figures need to be based on actual conditions and not just estimates in this case.

We will cover the topic of the effect of CG on performance in more detail in the book, as it just requires too much detail for the online learning environment.