PPL - Private Pilot Licence
Mass & Balance
Loading the aircraft within its weight limits and keeping the centre of gravity where it must be.
Every aircraft is certified to fly safely only within a range of weights and a range of centre-of-gravity positions. Too heavy, and it may not get off the runway or climb over the hills; balanced too far forward or aft, and it may not be controllable. Mass and balance is the check you do on the ground so that never happens in the air.
1. Mass terms
| Term | What it includes |
|---|---|
| Basic empty mass (BEM) | The aircraft with its fixed equipment, unusable fuel and full oil. Found in the aircraft's mass and balance report. |
| Payload | Passengers, baggage and cargo – what you are paid (or want) to carry. |
| Zero fuel mass (ZFM) | Everything except usable fuel. Many aircraft have a maximum ZFM because weight in the fuselage bends the wing roots. |
| Ramp (taxi) mass | The mass at engine start, including the fuel for taxi. |
| Take-off mass (TOM) | Ramp mass minus taxi fuel. Must not exceed the MTOM. |
| Landing mass | Take-off mass minus trip fuel. Must not exceed the MLM. |
Fuel is measured in litres but weighed in kilograms: Avgas ≈ 0.72 kg per litre, Jet A-1 ≈ 0.80 kg per litre.
2. Why too much mass is dangerous
A heavier aircraft needs:
- a longer take-off run and landing distance,
- more power to climb – the climb rate and angle get worse,
- a higher stall speed,
- more fuel for the same trip, and it has less margin for turbulence and manoeuvring (structural limits).
Combine heavy with hot and high – Amman Queen Alia is at 2,395 ft and often above 35 °C in summer – and performance can fall dramatically.
3. Balance: the centre of gravity
The centre of gravity (CG) is the point where the aircraft would balance. To find it we use three ideas:
- Datum: a reference point chosen by the manufacturer (often the firewall or the nose).
- Arm: the distance of an item from the datum.
- Moment: mass × arm.
CG position = total moment ÷ total mass. It must lie between the forward and aft limits in the flight manual (the "CG envelope") for the whole flight.
4. Effects of the CG position
| CG forward | CG aft |
|---|---|
| Very stable, heavy to pitch | Less stable, light and sensitive in pitch |
| Higher stall speed | Slightly lower stall speed |
| More elevator needed to rotate and flare – you may run out of elevator on landing | Risk of pitching up and stalling; recovery from a stall or spin may be impossible |
| More drag from the tailplane, slightly more fuel | Slightly less drag, but out of limits it is the more dangerous side |
5. Worked example
A four-seat trainer. Limits: MTOM 1,100 kg, CG between 2.05 m and 2.40 m aft of the datum.
| Item | Mass (kg) | Arm (m) | Moment (kg·m) |
|---|---|---|---|
| Basic empty mass | 700 | 2.00 | 1,400.0 |
| Pilot and front passenger | 160 | 2.10 | 336.0 |
| Rear passenger | 80 | 3.00 | 240.0 |
| Fuel 100 l × 0.72 | 72 | 2.20 | 158.4 |
| Baggage | 20 | 3.50 | 70.0 |
| Total | 1,032 | 2,204.4 |
CG = 2,204.4 ÷ 1,032 = 2.136 m – inside 2.05–2.40 m, and 1,032 kg is below the 1,100 kg MTOM. Check the landing case too: burning fuel moves the CG, so it must stay in limits with minimum fuel as well.
6. Moving and adding mass
- Moving an item: change of CG = mass moved × distance moved ÷ total mass. Moving the 20 kg bag 1.5 m forward in the example shifts the CG by 20 × 1.5 ÷ 1,032 ≈ 0.03 m forward.
- Adding or removing an item: recalculate total mass and total moment.
- Secure everything: a bag sliding aft on take-off moves the CG at the worst moment.
7. Airliners: load sheets
Airlines use a load sheet and trim sheet with the same principles. The CG is usually given as a percentage of the mean aerodynamic chord (% MAC), and the take-off CG sets the stabiliser trim. The crew checks the final figures (ZFW, TOW, CG) before departure and enters them in the FMC.
Flying it at Levant
SimBrief works out your payload, fuel and weights (ZFW, TOW and LW) and compares them with the aircraft limits; the Levant SimBrief planner sends your passenger and cargo load. Before departure, check that the OFP weights match what you load in the simulator, and enter ZFW, CG and fuel into the FMC – a wrong weight gives wrong take-off speeds.
Check yourself
- How do you calculate the CG position? Total moment ÷ total mass.
- How much do 150 litres of Avgas weigh? About 108 kg (150 × 0.72).
- Which CG position makes the aircraft less stable? An aft CG.
- Why must you check the CG at landing as well as take-off? Burning fuel changes the mass and can move the CG out of limits.
- A forward CG makes the stall speed higher or lower? Higher.
Simulation training material. The example uses invented figures – always use your aircraft's own mass and balance data.