Every aircraft has a weight and balance record that must reflect reality. When an aircraft undergoes a major repair, major alteration, or installation of new equipment that could shift its center of gravity, a new official weighing is often required. Even when not strictly mandated, periodic reweighing ensures that the data in the aircraft's weight and balance report accurately represents the actual aircraft — a discrepancy between paperwork and reality can lead to dangerous loading errors in flight. For an Aviation Maintenance Technician (AMT), understanding the proper procedure for weighing an aircraft is both a regulatory responsibility and a foundational safety skill.
This article walks through the equipment needed, how that equipment is set up, and the procedural steps that ensure the resulting weight data is accurate enough to trust with lives. The principles apply broadly across general aviation aircraft, though specific procedures for any given aircraft always defer to the manufacturer's maintenance manual.
Equipment Required for Aircraft Weighing
Accurate weighing begins with the right tools. The FAA Weight and Balance Handbook (FAA-H-8083-1) identifies several categories of equipment that are essential to the process.
Scales
The most critical piece of equipment is, naturally, the scales themselves. Aircraft weighing scales may be either platform scales (sometimes called floor scales) or load cells. Platform scales are placed under each wheel or jack point, and the aircraft rests directly on them. Load cells are electronic devices placed between a jack and the aircraft's jack points; they measure the compressive force applied at each point and report it as weight.
Regardless of type, scales must be calibrated and certified. Calibration certificates should be current, and the technician performing the weighing must verify the calibration dates before beginning. Scales that are out of calibration can produce readings that appear reasonable but are systematically off — there is no way to detect the error from the number alone. Most facilities require scales to be calibrated by a certified facility at regular intervals. The scale capacity must also be appropriate: each scale must be rated to handle at least the maximum load expected at that weighing point, with sufficient margin.
Plumb Bob and Leveling Equipment
A plumb bob is used to project reference points on the aircraft down to the floor or a measuring tape below. This allows the technician to accurately locate the datum and reference stations horizontally, which is critical for calculating arm distances. Without accurate arm measurements, even perfect scale readings produce meaningless balance calculations.
Aircraft leveling requires a spirit level (bubble level) and, for most aircraft, reference to specific leveling points or lugs described in the manufacturer's maintenance manual. Some aircraft use the top of the door sill, a specific structural tube, or a leveling lug on the fuselage. The level must show that the aircraft sits in its flight attitude — or more precisely, in the attitude specified in the maintenance manual — before any weight readings are recorded. Leveling affects weight distribution between the scales, and an aircraft even slightly out of level can produce fore-and-aft weight readings that are noticeably wrong.
Jacks and Jack Stands
When load cells are used, the aircraft must be raised on jacks positioned at manufacturer-specified jack points. Jacks must be appropriately rated and stable. Jack stands may be used for safety once the aircraft is raised, though the actual weighing occurs with the aircraft supported by the load cells on the jacks. When platform scales are used under the wheels, jacking is not required for the weighing itself, but may be needed for setup and positioning.
Chalk, Tape, and Measuring Tools
A steel tape measure or similar measuring device is needed to determine the distance from the datum to each weighing point. These distances are the arms in the weight-and-balance equation (moment = weight × arm). Chalk lines or tape on the hangar floor mark the plumb bob contact points so they can be measured accurately and referenced later. Accuracy here matters: an error of just one inch in the arm measurement for a main gear weighing point can introduce a moment error large enough to shift the computed empty center of gravity by a measurable amount on a light aircraft.
Preparing the Aircraft
Before placing the aircraft on scales, a systematic preparation sequence must be followed. Skipping steps here is a common source of error.
Fuel: The aircraft should be weighed with either full fuel tanks or completely drained fuel tanks, depending on what the manufacturer specifies. Practice varies by aircraft type — some light general aviation aircraft are weighed with fuel drained to unusable fuel only, while others are weighed with full fuel tanks, per the specific aircraft's type certificate data sheet or maintenance manual. Unusable fuel remains in the aircraft during all normal operations and, if the aircraft is weighed in that configuration, is included in empty weight. Whatever the specified fuel state, any deviation from it must be drained prior to weighing, or its exact weight and arm must be mathematically accounted for in the scale readings.
Oil: Whether oil is included in empty weight depends on the specific aircraft. Many small aircraft are weighed with full oil, while many large/transport aircraft and some general aviation aircraft are weighed with the oil system drained or with only undrainable (residual) oil retained. The technician must check the aircraft's type certificate data sheet or maintenance manual to confirm which condition defines that aircraft's empty weight, and service or drain the oil system accordingly before weighing.
Other fluids: Hydraulic fluid reservoirs should be at their specified level. Residual water or waste in any tanks must be noted and accounted for.
Equipment: All equipment that is part of the aircraft's licensed empty weight configuration must be installed and in its normal operating position. This includes permanently installed avionics, seats, safety belts, and required equipment. Any removable items that are not part of the normal licensed configuration should be removed. The technician must carefully compare the aircraft's current configuration against the equipment list in the weight and balance report to identify any additions or removals since the last weighing.
Flight controls: Control surfaces should be in their neutral, streamlined position unless the manufacturer specifies otherwise.
Doors and windows: These are typically closed and latched in their normal flight position.
The Weighing Environment
The hangar or weighing area must be enclosed and free from drafts. Even a moderate breeze can cause scale readings to fluctuate, making it impossible to obtain stable readings. Doors should be closed, fans turned off, and work should not proceed during windy conditions with the hangar open. Air currents moving across the aircraft can produce inaccurate scale readings, introducing variable errors that cannot be corrected mathematically.
The floor under the scales must be level and smooth. An uneven floor causes a platform scale to rock slightly and may not allow the scale to read accurately. If the floor has a slight slope, shimming the scales may be necessary, though the aircraft must still be independently verified as level.
Why This Procedure Matters
The purpose of all this precision is to produce an accurate empty weight and an accurate empty weight center of gravity (EWCG). These two values are the foundation of every weight and balance calculation the operator performs before each flight. An incorrect empty weight means the operator starts every loading calculation with a wrong number. An incorrect EWCG means even perfectly calculated loading could place the aircraft outside its actual certified CG envelope.
Operating an aircraft outside its CG envelope — even if it is within its weight limits — can produce handling characteristics that the aircraft was not designed to manage safely. Forward CG beyond limits can make rotation on takeoff extremely difficult or impossible. Aft CG beyond limits can make the aircraft statically unstable and unrecoverable from certain upset conditions. The accuracy of the weighing procedure is therefore not an administrative formality; it is a direct safety input into every subsequent flight.
Key Numbers and Rules
- Calibration: Scales must have a current calibration certificate before use; out-of-calibration scales produce legally and practically invalid results.
- Fuel and oil condition: Whether an aircraft is weighed with full or drained fuel, and full or drained oil, depends on that specific aircraft's type certificate data sheet or maintenance manual — always verify rather than assume.
- Drafts: The weighing area must be free of air movement; this is a hard requirement, not a preference.
- Level attitude: The aircraft must be in the attitude specified by the manufacturer — confirmed with a calibrated spirit level at the manufacturer-specified leveling points — before scale readings are recorded.
- Scale capacity: Each scale must be rated to exceed the maximum expected load at its position with adequate margin.
- Documentation: All weighing data, equipment used, calibration references, and the resulting calculations must be entered in the aircraft maintenance records.
Common Test Traps
- Assuming any level surface is good enough: The aircraft must be level in the attitude specified by the manufacturer, verified at the correct leveling points — not just visually approximated or assumed based on the floor.
- Assuming a single fuel or oil convention applies to all aircraft: Test questions often present scenarios where fuel or oil is not in the condition specified for that particular aircraft. Always check the type certificate data sheet or maintenance manual for the required condition rather than assuming full or drained is standard.
- Ignoring drafts: Students sometimes underestimate how much a door left open affects scale stability. The FAA emphasizes that even small air currents can produce inaccurate readings.
- Forgetting to verify scale calibration: Using an uncalibrated scale is not just a procedural violation — it produces results that cannot be relied upon for airworthiness decisions.
- Confusing tare weight: When equipment such as jacks, adapters, or chocks rests on or contacts the scale during weighing, their weight (tare) must be recorded and subtracted from the gross scale reading to obtain the actual aircraft weight at that point. Forgetting to subtract tare is a classic error on both the test and in practice.