If you carry out enough practice questions you will get to know the definitions of the various masses and the relationships between them. But if that fails you can always use the key facts that are provided in the CAP697. Remember that you will a CAP 697 in the exams.
Question: Given that: -
Maximum structural take-off mass (MTOM): 146 000 kg,
Maximum structural landing mass (MLM): 93 900 kg,
Maximum zero fuel mass(MZFM): 86 300 kg,
Trip fuel: 27 000 kg,
Taxi fuel: 1 000 kg,
Contingency fuel: 1350 kg,
Alternate fuel: 2650 kg,
Final reserve fuel: 3000 kg,
Determine the actual take-off mass
The key to answering this type of question is to note that you must not exceed any of the limiting masses.
So for example your take-off mass must not be so great that having burned the trip fuel you will exceed the Maximum structural Landing Mass.
This means that you must carry out a calculation to find the maximum take-off mass that will not exceed each of the limits.
Max Take-off Mass based on MTOM
Firstly you must not exceed the MTOM so the maximum possible take-off mass is 146000 kg. But other limits may prevent you from using this mass.
Max Take-off Mass based on MLM.
Your landing mass will be equal to your take-off mass minus your trip fuel so to find the maximum take-off mass that will not exceed the MLM just add the trip fuel to the MLM.
This gives Take-off Mass = 93 900 kg,(MLM) + (27 000 kg) = 120900 kg.
So the maximum take-off that can be used without exceeding the MLM is 120900 kg.
Max Take-off Mass based on MZFM
If we load up the aircraft with traffic load until we reach the MZM, than add the take-off fuel we have the maximum take-off mass (with the planned take-off fuel) that will not exceed the MZFM.
Take-off fuel is all of the fuel that will be on board at the start of the take-off run. This does not include the taxi fuel.
So in this case take-off fuel = (Trip fuel: 27 000 kg) + (Contingency fuel: 1350 kg) + (Alternate fuel: 2650 kg) + (Final reserve fuel: 3000 kg) = 34000 kg)
Adding this to the MZFM gives a take-off mass of
(MZFM 86 300 kg) + ( take-off fuel 34000 kg) = 120300 kg.
We now have three maximum values for take-off mass.
146000 kg based on not exceeding the MTOM.
120900 kg based on not exceeding the MLM.
120300 kg based on not exceeding the MZFM.
We cannot exceed any of these limits so the maximum possible take-off mass for this flight is the lowest of the three, which is 120300 kg.