Weigh the aircraft, preserve the record

How do wheel readings reveal CG?

Each scale reaction acts at its own known arm.

Prepare the condition

Aircraft attitude changes the horizontal distance between wheel reactions and the datum. Level the aircraft using the specified reference, establish the required configuration and fluid state, and use calibrated weighing equipment. Wind, trapped water, temporary equipment or uneven settling can corrupt the baseline.

The baseline must state what was installed and which fluids were present. A correct weighing of the wrong configuration is still the wrong operational input.

Remove tare before taking moments

Tare is equipment carried by a scale but not part of the aircraft, such as a pad or adaptor. Subtract each station’s tare from that station’s gross reading, then multiply the net reaction by its arm. Subtracting all tare at one arbitrary arm corrupts the moment.

Add all net reactions for aircraft mass. Add their moments for total moment, then divide. Multiple main-wheel readings may be combined only when their longitudinal arms are the same or their individual moments have been retained.

Check station geometry and residual errors

A tricycle aircraft usually places the combined CG between the nose and main-wheel reaction lines, although the permitted flight CG envelope is a separate aerodynamic and structural matter. An unexpected negative net reaction or a CG beyond the supporting stations should trigger a data and configuration investigation. Do not force the result back into an envelope by changing a sign until it looks plausible.

An error on an aft scale changes both total mass and aft moment. Its effect on CG depends on how far that scale lies from the true CG, so equal kilogram errors at two stations do not necessarily have equal balance consequences. Record the individual station readings and tare values rather than keeping only their sum. The detailed record makes both the calculation and an uncertainty investigation reproducible.

Why can yesterday’s empty-mass record be wrong today?

An equipment change alters both the baseline mass and its moment.

Reconstruct the baseline

A weighing report establishes a reference configuration and CG. Subsequent approved installations, removals, repairs and modifications require the mass-and-balance record to be updated according to the applicable maintenance and operating system. Keep the equipment list and the mass record consistent.

For a removal, subtract both mass and moment; for an installation, add both. Do not subtract the removed equipment’s arm from the CG: arms are coordinates, not independent CG changes.

Reweighing and uncertainty

Follow the applicable reweighing requirements and the approved programme; do not invent a universal interval. Accumulated uncertainty or an undocumented configuration can invalidate a paper reconstruction even if the arithmetic is correct.

A last-minute cargo change is an operational load amendment; a permanent equipment installation changes the aircraft baseline. Confusing those two records can either double-count or omit the equipment.

Keep a running moment ledger

A practical equipment ledger has one row for each change: item identity, installed or removed mass, station arm, signed moment and the resulting baseline mass and moment. Calculate the new CG after the totals have been updated. Rounding the CG after every small modification and then reconstructing moment from that rounded value can accumulate avoidable error; retain the required calculation precision.

For a replacement, show a removal and an addition even if the net mass is zero. Replacing a 20 kg unit at 4 m with a 20 kg unit at 10 m leaves aircraft mass unchanged but adds 120 kg·m of moment. The aircraft becomes more aft-balanced. A mass-only equipment record misses this change entirely, demonstrating why configuration control and balance control cannot be separated.

Can you explain and apply the mechanism?

Four original questions. Work from the stated facts, then compare your reasoning with the explanation.