Performance Effects of Mass and CG Position
Mass effects
Increasing mass affects nearly every performance parameter adversely:
- Take-off and landing distances increase.
- Climb gradient and rate of climb decrease.
- Accelerate-stop distance increases.
- Stall speed increases with the square root of the load factor; Vs ∝ √mass at a given configuration.
- Cruise speed and ceiling reduce.
CG effects
An aft CG reduces the tail-down download required, reducing trim drag and slightly improving cruise performance, but reduces static stability and can make the aircraft more sensitive in pitch. A forward CG increases the download on the tail, increasing trim drag, raising stall speed slightly and increasing take-off distance because more elevator authority is needed to rotate. The forward limit is often dictated by rotation and flare capability.
Traps
- Assuming aft CG always improves performance - it can, but stability and handling suffer.
- Forgetting that a forward CG increases take-off distance.
- Confusing indicated stall speed with the effect of load factor in turns (Vs increases with √n).
- Ignoring that field-length-limited mass may be lower than structural MTOM on hot/high days.
- Density altitude compounds mass effects; both must be considered.
Worked example
An aircraft at 12,000 kg has a take-off distance of 900 m. At 13,500 kg (12.5% increase) the distance rises roughly in proportion to the square of the mass ratio for the ground run component - expect a substantial increase, perhaps 1,050–1,100 m. Always use the aircraft flight manual data rather than a simple ratio.
Rule of thumb
Higher mass → longer distances, lower climb performance, higher stall speed. Forward CG → longer take-off run and higher trim drag.
Operational Procedures
Pre-flight mass and balance procedure
Operators follow a standard sequence: determine DOM from the aircraft's weighing record; establish traffic load from the load plan; determine fuel from the flight plan; compute ZFM, TOM and LM; verify against MTOM, MLM and MZFM; determine CG from the trim sheet; check the CG envelope at TOM and LM; complete and sign the load sheet; and confirm with the commander.
Last-minute changes (LMC)
An LMC is a change made after the load sheet is completed. Procedures specify a maximum permissible LMC (often expressed as a mass and/or index change) that does not require a new load sheet, provided the commander is informed and the change is recorded. Larger changes require recomputation.
Loading supervision
Cargo must be secured, distributed within compartment limits, and checked against the load plan. Special loads (dangerous goods, live animals, heavy items) require specific documentation and stowage. Passenger seating may be controlled to keep CG within limits (e.g., not leaving rear rows empty on some types).
Traps
- Performing an LMC that exceeds the permitted limit without a new load sheet.
- Failing to inform the commander of an LMC.
- Ignoring compartment floor-loading limits.
- Not re-checking CG after fuel is added or removed.
- Assuming the load sheet is optional for small aircraft - it is required by the operator's manual.
Worked example
A load sheet shows TOM 12,450 kg, index 75.0. A late bag of 30 kg is loaded in the aft hold, adding index 1.5. New index 76.5, still within the aft limit 82. The LMC is recorded and the commander informed. If the LMC limit were 1.0 index, a new load sheet would be required.
Rule of thumb
Any change after signing must be assessed against the LMC limit, recorded, and notified to the commander.
Documentation and Regulatory Requirements
Regulatory framework
In Brunei, civil aviation is regulated by the Department of Civil Aviation (DCA) under the Civil Aviation Regulations, which adopt ICAO Annex 6 standards. Commercial air transport operators must comply with mass and balance requirements in their Operations Manual and the aircraft flight manual. The pilot in command is responsible for ensuring the aircraft is loaded within limits before flight.
Required documents
- Aircraft weighing record: establishes basic empty mass and CG; must be updated after modifications or repainting.
- Load sheet / trim sheet: completed for each flight.
- Operations Manual: contains procedures, limits and LMC rules.
- Flight Manual: contains structural limits and CG envelope.
- Mass and balance report: retained for the period specified by the authority.
Records and retention
Load sheets and mass and balance records are typically retained for a defined period (often three months, per operator/authority requirements). Weighing records are retained for the life of the aircraft plus a period.
Traps
- Assuming the flight manual and operations manual contain identical limits - they may differ, and the more restrictive applies.
- Forgetting that the aircraft must be re-weighed after major modifications.
- Misstating the retention period.
- Confusing the roles of the loader, dispatcher and commander.
- Ignoring that some aircraft require a specific trim sheet format approved by the authority.
Worked example
After a repaint adding 40 kg, the basic empty mass must be amended and the CG recalculated. The weighing record is updated, and all subsequent load sheets use the new DOM. Failure to do so could cause the aircraft to exceed MZFM unnoticed.
Rule of thumb
If in doubt, the more restrictive limit applies, and the commander signs for the final mass and balance.