Jacking Points, Scales, and Tare Weight Considerations During Weighing

Accurate weight and balance data is foundational to safe aircraft operation. Every time an aircraft is officially weighed — whether after major repairs, alterations, or when records are lost — the technician must follow a disciplined process that accounts for three critical elements: placing the aircraft on approved jacking points, using properly calibrated scales, and correctly handling tare weight. A mistake in any of these areas can produce incorrect empty weight and center of gravity (CG) values that silently compromise every subsequent weight and balance calculation the pilot or dispatcher performs. For AMT General candidates, this topic is both a knowledge-test staple and a genuine day-one shop skill.

This article walks through each element in depth — what it is, why it matters, how it is performed correctly, and where technicians most often go wrong.

Jacking Points and Aircraft Positioning

Before a single scale reading is recorded, the aircraft must be placed in a level flight attitude. Manufacturers specify this requirement in the aircraft maintenance manual (AMM) and Type Certificate Data Sheet (TCDS), and it is non-negotiable. Even a slight nose-high or nose-low pitch attitude shifts the apparent weight distribution among the weighing points and produces a false CG calculation. Most aircraft are leveled using a spirit level placed on a designated leveling lug or longitudinal datum reference inside the cockpit, cabin, or on the fuselage skin.

Jacking points — also called weighing points — are structurally reinforced locations specifically designed to accept the vertical load of the aircraft's weight through a jack or a scale platform. Using any location other than those specified by the manufacturer risks structural damage and, more critically for weighing purposes, introduces measurement error because the load path to the scale is not through the designed geometry. For tricycle-gear aircraft, weighing is typically performed with scales placed under each main gear and the nose gear. For tailwheel aircraft, the scales go under the main gear and the tailwheel. Some large or unusual aircraft use a combination of jacks and scales simultaneously.

When scales are placed directly under the wheels, the technician must ensure the scale platform is level with the hangar floor and that the aircraft can roll freely onto the scale without cocking the gear sideways, which would introduce friction errors. Chocks and brakes must be released (or used very carefully per the AMM) so that no side load is transmitted through the gear that might lift one scale and compress another falsely.

Scale Types and Calibration Requirements

Two broad categories of scales are used in aircraft weighing: platform (floor) scales and electronic load-cell systems. Platform scales look much like industrial floor scales and are adequate for smaller general aviation aircraft. Electronic load cells are used under each jack pad or gear contact point on larger aircraft, feeding readings into a digital display or computer that can sum the values and even compute CG automatically.

Regardless of type, scales used for certificated aircraft weighing must be recently calibrated and within their certification interval. Calibration is typically performed by the scale manufacturer or a certified metrology laboratory, and the calibration certificate should be on hand during the weighing operation. The FAA's Weight and Balance Handbook (FAA-H-8083-1) emphasizes that inaccurate scales are one of the most common sources of weight and balance error. Calibration intervals are set by the scale manufacturer or the operator's approved procedures rather than by a single FAA-mandated period, so the technician must verify and document the current calibration status before relying on a scale for an official weighing.

Scales must also be zeroed (tared for their own platform weight, when applicable) before placing any aircraft weight on them. Digital load cells should display zero with no load applied. If a scale does not zero properly, it must be serviced or replaced — proceeding with an offset scale produces a systematic error in the final data.

Understanding Tare Weight

Tare weight is one of the most important — and most frequently mishandled — concepts in aircraft weighing. Tare refers to the weight of any equipment or material that is resting on the scale platform along with the aircraft but is not part of the aircraft itself. Classic tare items include wheel chocks, jack adapters, ground locks, sling hardware, and even drip pans placed under the aircraft to catch fuel or oil drips during the weighing process.

The principle is straightforward: the scale measures the combined weight of the aircraft and any tare items sitting on the platform. To obtain the true aircraft contribution at that weighing point, the technician must subtract the tare weight from the scale reading. This is done independently for each scale position, because different tare items of different weights may be present at each gear location.

Here is a concrete illustration of the concept (the specific figures are for illustration only, not a value published in FAA-H-8083-1). Suppose a main gear scale reads 1,840 pounds, and the technician placed a wheel chock (4 lb) and a drip pan (2 lb) on that scale platform. The tare weight at that station would be 6 pounds, and the net aircraft weight at that station would be 1,840 − 6 = 1,834 pounds. If the technician forgets to subtract tare, every subsequent moment calculation is slightly wrong, and the CG shifts accordingly — small errors that compound across three or four stations can push the computed CG outside its actual location by an inch or more on large aircraft.

Tare items must be weighed on the same scales being used for the aircraft whenever possible, or on a calibrated shop scale, to get an accurate tare value. Estimating or guessing tare weight is not acceptable practice.

The Complete Weighing Procedure in Summary

A proper aircraft weighing sequence ties all three elements together:

1. Review the AMM and TCDS to identify the correct weighing points, leveling method, and any specific conditions (fuel state, ballast, doors open or closed, control surfaces positions, etc.).
2. Prepare the aircraft: drain or fill fluids to the required state, remove or install required equipment per the operating empty weight definition, and position the aircraft in the hangar away from drafts that could affect scale readings.
3. Position and zero the scales at each weighing point. Record any residual zero error if the scale cannot be perfectly zeroed.
4. Roll or jack the aircraft onto the scales using approved jacking points only, then level the aircraft in both pitch and roll to the manufacturer's specification.
5. Record tare items at each station and weigh them. Document tare weights carefully on the weighing form.
6. Record gross scale readings at each station once the aircraft is stable and level.
7. Calculate net weights by subtracting tare from gross scale readings at each station.
8. Compute empty weight and CG using the moment arm distances specified in the TCDS or AMM for each weighing point.

Why Accuracy Here Matters So Much

The empty weight and empty-weight CG that come out of an official weighing become the baseline for every weight and balance computation performed during the life of that aircraft — by every pilot, dispatcher, or operator until the aircraft is weighed again. An error of just a few pounds or a fraction of an inch in CG can accumulate through subsequent alterations and equipment changes until the aircraft is operating with a CG that is dangerously outside its envelope, while all paperwork appears to show compliance. This is why a new weighing is required whenever the effect of an alteration or repair on weight and balance is not accurately known, per 14 CFR Part 43, Appendix A, and why the entire weighing procedure must be documented on FAA Form 337 or equivalent aircraft records. Many alterations can instead be handled by computation when their weight and balance effect is already known, but a reweighing is the safest course whenever that effect is uncertain.

Key Numbers and Rules

• Level flight attitude is mandatory — the AMM specifies the exact leveling method and tolerance, typically within fractions of a degree.
• Scales must be calibrated — check the calibration certificate; calibration intervals are set by the scale manufacturer or the operator's approved procedures, so always verify current status rather than assuming a fixed interval.
• Tare subtracted at each station independently — net weight = gross scale reading minus tare weight at that station.
• Fuel state matters — aircraft may be weighed with tanks full, empty, or at a specified fuel load depending on the AMM; many manufacturers specify weighing with only unusable (undrainable) fuel remaining, while hydraulic fluid and engine oil are typically at full levels unless the AMM states otherwise. Always verify the specific requirement in the AMM.
• Record everything — weighing data, tare weights, scale serial numbers, calibration dates, and leveling method must all appear in the aircraft weight and balance record.
• Moment arms come from the TCDS or AMM, not from measuring in the field; only the scale readings are field-measured values.

Common Test Traps

• Forgetting to subtract tare at each individual station. Test questions often give a gross scale reading and a tare value and ask for net weight — many students add instead of subtract, or subtract from the wrong station.
• Assuming scales auto-correct for tare. Unless a digital scale is specifically zeroed with the tare items already on the platform, it will not automatically correct for them. Always confirm the zero procedure used.
• Confusing fuel state requirements. The exam may describe an aircraft weighed with a particular fuel state and ask whether the procedure was correct — knowing that the AMM specifies the required fuel state (which may be full, empty, or unusable fuel only) is essential; do not assume a single universal rule applies to every aircraft.
• Using non-approved jacking points. Test scenarios may describe a technician using a convenient structural member rather than the designated jack pad. This is incorrect both structurally and for measurement accuracy.
• Ignoring pitch/roll leveling. A question may give scale readings and ask why the CG calculation is wrong when the aircraft was not level — the answer is that the weight distribution among stations changes with aircraft attitude, making all readings inaccurate.