FAA A&P Weight & Balance: The Calculations the Exam Tests

Why Weight and Balance Shows Up on Almost Every A&P Test

Weight and balance is a documented subject area in the FAA's Aviation Mechanic Airman Certification Standards, and it shows up in three places during your certification: the General written test (AM.I), the Airframe test (AM.II) when the question involves equipment installation or rigging, and the oral and practical exam, where a Designated Mechanic Examiner can hand you an equipment list and ask you to work the numbers on the spot. The FAA's Weight and Balance Handbook (FAA-H-8083-1) and Chapter 10 of AC 43.13-1B are the source documents, and every question you will see traces back to one of them.

The good news is that weight and balance is the most mechanical topic on the test. There is no judgment involved: you add, you multiply, you divide. Once you have the vocabulary and the two formulas, the questions become arithmetic under mild time pressure.

The Vocabulary the Questions Assume You Know

• Datum — an imaginary vertical plane the manufacturer chooses as the zero point for measurement. It is not always the nose, and it can sit ahead of the aircraft entirely.
• Arm — the horizontal distance in inches from the datum to the center of gravity of an item. Arms aft of the datum are positive, arms forward are negative.
• Moment — the product of a weight and its arm, expressed in pound-inches (lb-in). A moment index is the moment divided by 1,000, used on larger aircraft to keep the numbers short.
• Empty weight — the aircraft with all fixed equipment, unusable fuel, full oil and full hydraulic fluid. It is the starting point of every calculation.
• Useful load — maximum gross weight minus empty weight. That number is the total you may put in the aircraft: pilot, passengers, baggage and usable fuel together.
• Maximum gross weight — the maximum weight the aircraft is certificated to operate at, found in the AFM/POH limitations section and on the weight and balance data sheet.
• Ballast — weight added specifically to move the CG into the approved range, always recorded with its location in the aircraft records.

The Two Formulas That Answer Everything

Every weight and balance problem, forward or backward, is built on two relationships:

If you can remember those two lines and keep your units straight, you can solve the General test's weight and balance questions without ever memorizing a chart. The chart questions are simply these formulas applied to a template that already lists the arms.

A Worked Loading Problem

Suppose the weight and balance data sheet for a light trainer reads: empty weight 1,500 lb at an arm of 37.8 in, maximum gross weight 2,300 lb, forward CG limit 38.5 in and aft CG limit 47.3 in at maximum gross weight. The proposed loading is a 170 lb pilot, a 150 lb passenger in the rear seat, 50 gallons of fuel (avgas weighs 6 lb/gal, so 300 lb) and 30 lb of baggage in baggage area 1 at an arm of 95.0 in.

The total is 2,150 lb with a moment of 91,190 lb-in. Dividing gives a CG of 42.41 in aft of the datum. Check both limits: 2,150 lb is under the 2,300 lb maximum gross, and 42.41 in sits between 38.5 in and 47.3 in, so the aircraft is legal as loaded.

The arms and limits above are illustrative. On the exam and in the hangar you always use the actual weight and balance data for the specific aircraft, not a generic chart.

The Reverse Question: Which Limit Hits First?

Certification questions rarely stop at "compute the CG." They ask how much more weight you can add, or how little fuel you can carry and stay in the envelope. With the same loading, you have 150 lb of weight room to reach 2,300 lb. If all of it went into baggage area 1 at an arm of 95.0 in, the CG would move to about 45.8 in, still inside the 47.3 in aft limit. Solving the aft limit instead shows you could add roughly 200 lb of baggage before running out of CG — but the weight limit stops you first at 150 lb.

That distinction is exactly what these questions test. The aft CG limit allows more weight than the airframe can lift, so maximum gross weight is the binding limit in this example. Reverse the situation with a light fuel load and a heavy nose, and the forward CG limit becomes binding instead, which is why loading a nose-heavy aircraft often means adding fuel rather than removing weight.

Reading a CG Moment Envelope

A CG moment envelope is a graph with weight on one axis and moments (often moment indexes) on the other. The shaded area is the approved range. Plot your total weight and total moment, and the airplane is legal if the point falls inside the envelope. Two features trip people up:

• The forward and aft limits are usually slanted lines, not vertical ones, meaning the allowed CG range narrows as the aircraft gets heavier.
• The envelope frequently has a notch or extension at the bottom for light weights, which is where minimum-fuel operating limits live.

Six Traps That Cost Marks

• Mixing units. If a question gives moments in kg-mm or arms in metres, convert before you add. Answers in the wrong unit system are always among the distractors.
• Using the wrong arm. Baggage areas, seat rows and fuel tanks each have their own arm. A rear seat passenger at an arm of 73 in moves the CG far more than the same weight in the front seat.
• Forgetting to subtract the tare. When you weigh an aircraft on scales, you subtract the tare weight of each scale before you record anything. Questions on towing and jacking equipment test this.
• Confusing empty weight with zero-fuel weight. Unusable fuel and full oil are already inside the empty weight — counting them again inflates your total.
• Assuming fuel is neutral. Fuel has an arm, and on most light aircraft it is aft of the empty-weight CG, so burning it moves the CG forward. "Minimum fuel" limits exist for that reason.
• Signing off an aircraft out of limits. A maintenance release under 14 CFR 43.9 says the aircraft is approved for return to service, and 14 CFR 91.9 makes operating outside the flight manual limits illegal. If the loading does not work, the fix is ballast, redistribution or paperwork — never a signature.

How to Practise Weight and Balance

Work problems in both directions: from a loading to a CG, and from a CG limit back to a maximum weight at a station. Practise with the AFM/POH limits for two or three different aircraft so you get used to slanted envelopes and moment indexes. Time yourself at roughly 90 seconds per problem, because the General test gives you two hours for 60 questions and weight and balance is only one of nine subject areas. For the rest of the AM.I subject areas, see our FAA A&P exam structure guide, and for a study routine that covers the full ACS, see the A&P study plan.

Frequently Asked Questions

How many weight and balance questions are on the General test?

The FAA does not publish an exact item count per subject area. Candidates typically report a handful of weight and balance questions on the General test, plus related item types on the Airframe test and a practical calculation during the oral and practical exam. Weight and balance is consistently described as one of the easiest subject areas to secure because the method never changes.

What is a moment index?

A moment index is a moment divided by a constant, usually 1,000, so that large aircraft loading charts can show manageable numbers. When a problem is stated in indexes, multiply the index by the same constant before converting back to a CG in inches.

Can a mechanic legally return an aircraft to service out of CG limits?

No. The maintenance release you sign states that the work was done and the aircraft is approved for return to service. If changing equipment, ballast or configuration leaves the aircraft outside its approved weight and CG limits, the discrepancy must be corrected — and the weight and balance records revised — before the aircraft goes back into service.

Which handbook should I study from?

The FAA's Weight and Balance Handbook (FAA-H-8083-1) is the primary reference, and Chapter 10 of AC 43.13-1B covers the repair-related weight and balance practices, including weighing procedures and the records you must keep. Both are free from the FAA.