Centre of Gravity After an Equipment Change
An operator wants to replace a 12 kg avionics unit in the aft equipment bay with a 9 kg unit, and add a 4 kg antenna on the rear fuselage. The aircraft's empty mass is 1,250 kg with an empty-arm moment of 3,125 kg·m (i.e. an empty CG at 2.50 m aft of the datum). The avionics unit is at 4.20 m aft of datum; the new antenna is at 6.00 m aft of datum. The aft CG limit is 2.80 m. The forward limit is 2.10 m. The maintenance manual gives no instruction for recalculating CG after this modification, and the operator's mass and balance schedule lists the old equipment.
1. Calculate the new empty mass and empty CG after the change, and state whether it is within limits.
Read the worked answer
Use mass × arm = moment, and sum the changes.
Original: mass 1,250 kg, moment 3,125 kg·m.
Changes:
- Remove 12 kg at 4.20 m: moment change = −12 × 4.20 = −50.4 kg·m
- Add 9 kg at 4.20 m: moment change = +9 × 4.20 = +37.8 kg·m
- Add 4 kg at 6.00 m: moment change = +4 × 6.00 = +24.0 kg·m
Net mass change = −12 + 9 + 4 = +1 kg Net moment change = −50.4 + 37.8 + 24.0 = +11.4 kg·m
New mass = 1,251 kg New moment = 3,125 + 11.4 = 3,136.4 kg·m New CG = 3,136.4 / 1,251 = 2.507 m (approx.)
This lies between 2.10 m and 2.80 m, so the empty CG is within limits. Note the CG has moved aft slightly (from 2.50 to about 2.51 m), so the aft margin is reduced but not exceeded.
2. The empty CG is within limits. Why is that not sufficient to release the aircraft, and what else must be checked?
Read the worked answer
Empty CG within limits only shows the basic empty condition is legal. The aircraft flies loaded, and the modification changes both the empty mass and the moment arm, which shifts the whole loading envelope.
Must also check:
- The operator's mass and balance schedule must be updated to reflect the new equipment masses and arms; the old schedule is now wrong.
- Recompute the most adverse loading cases (full fuel, aft baggage, minimum crew; and forward cases) using the new basic empty mass and CG.
- Confirm the aircraft remains within the CG envelope for all permitted loadings, not just the empty case.
- Confirm the structural mass limits (max zero-fuel mass, max take-off mass) are still respected.
A small aft shift in empty CG can push an already-aft loading case out of the envelope even though the empty aircraft is legal.
3. The manual gives no instruction for the CG recalculation. What does this mean for the mechanic's authority to proceed?
Read the worked answer
The mechanic cannot simply perform the swap and sign it off on personal calculation. The modification changes mass and balance data, so it requires approved data and an appropriate release authorisation.
Judgement points:
- The change must be covered by approved data (e.g. a minor modification approved by the competent authority, or an STC/approved design change) and the mass and balance schedule must be amended accordingly.
- The Category A mechanic may carry out and record the physical work within their scope, but release to service for a modification of this kind typically requires certification by an appropriately authorised person.
- The calculation itself is a check, not an approval: it supports the decision but does not replace approved data.
If approved data is unavailable, the correct action is to stop and escalate, not to proceed on the basis that the numbers 'look fine'.