Percentages are one of the most practical mathematical tools in an aviation maintenance technician's (AMT's) daily toolkit. Whether you are checking that a battery is charged to an acceptable level, calculating how much corrosion inhibitor to mix into a cleaning solution, verifying that a component still falls within manufacturer service limits, or computing efficiency losses in a pneumatic system, you are doing percentage work. Math, including percentage calculations, is part of the FAA AMT General Airman Certification Standards, and more importantly, the real shop floor demands that you get these numbers exactly right — errors in a torque value or a fuel mixture can have serious consequences.
This article builds percentage skills from the ground up: the core formula, the three types of percentage problems, applied aviation examples, and a focused look at the test traps that catch even careful candidates.
What a Percentage Actually Is
A percentage is simply a way of expressing a ratio as a fraction of 100. The word itself comes from the Latin per centum, meaning "by the hundred." When you say a rivet hole is elongated by 15%, you mean that for every 100 units of original hole diameter, the elongation is 15 of those same units. Percentages let technicians communicate proportional changes in a universally understood, dimensionless way.
The fundamental relationship is:
Percentage = (Part ÷ Whole) × 100
This single equation is the root of every percentage problem you will encounter. The only thing that changes from one problem to the next is which of the three quantities — percentage, part, or whole — is the unknown.
The Three Core Percentage Problem Types
Type 1: Finding the Percentage
You are given the part and the whole, and you must find what percentage the part represents. Use the formula directly: % = (Part ÷ Whole) × 100.
Aviation example: An aircraft's usable fuel capacity is 60 gallons. After a flight, 42 gallons remain. What percentage of fuel is left?
42 ÷ 60 = 0.70; 0.70 × 100 = 70%.
Type 2: Finding the Part
You know the whole and the percentage, and you need the actual value of the part. Rearrange the formula: Part = (Percentage ÷ 100) × Whole.
Aviation example: A hydraulic reservoir holds 8 quarts when full. The operating manual states the system should never be below 25% capacity. What is the minimum fluid level in quarts?
(25 ÷ 100) × 8 = 0.25 × 8 = 2 quarts.
Type 3: Finding the Whole
You know the part and the percentage it represents, but you need the original total. Rearrange again: Whole = Part ÷ (Percentage ÷ 100).
Aviation example: After a partial battery discharge, a nickel-cadmium battery contains 18 amp-hours of charge. This represents 75% of the battery's rated capacity. What is the full rated capacity?
18 ÷ (75 ÷ 100) = 18 ÷ 0.75 = 24 amp-hours.
Practice identifying which type of problem you are facing before you reach for a calculator. Read carefully: "what percentage" signals Type 1; "what is X% of" signals Type 2; "is X% of what" signals Type 3.
Percentage of Change (Increase and Decrease)
A closely related and frequently tested concept is the percentage of change — how much a value has grown or shrunk relative to its original value. The formula is:
% Change = [(New Value − Original Value) ÷ Original Value] × 100
A positive result means an increase; a negative result means a decrease.
Aviation example — wear and service limits: A manufacturer's new-part specification for a brake disc thickness is 0.500 inch. The serviceable wear limit is 0.450 inch. What is the maximum allowable percentage of wear?
Change = 0.450 − 0.500 = −0.050 inch.
% Change = (−0.050 ÷ 0.500) × 100 = −10%. The disc may wear no more than 10% of its original thickness before it must be replaced.
Important note: Always divide by the original (starting) value, not the new value. Confusing original and new values is the single most common error in percentage-of-change problems.
Percentage Efficiency
Efficiency is expressed as a percentage showing how much of the input energy or work is converted to useful output. The formula is:
Efficiency (%) = (Output ÷ Input) × 100
Aviation example: A generator produces 900 watts of electrical output while the engine delivers 1,000 watts of mechanical input to the generator shaft.
Efficiency = (900 ÷ 1,000) × 100 = 90%. The remaining 10% is lost as heat through resistance and friction.
Efficiency is always equal to or less than 100% — no real device produces more output energy than is put in. This is a quick sanity check: if your calculation gives more than 100% efficiency, you have inverted the fraction.
Working with Decimal Equivalents
When you do percentage calculations on a calculator or by hand, you will often convert the percentage to its decimal equivalent first by dividing by 100 (or simply moving the decimal point two places to the left). This step is critical and worth emphasizing:
- 15% = 0.15 (not 15 or 1.5)
- 7.5% = 0.075
- 125% = 1.25 (values above 100% are possible — for example, a measurement that has grown 25% beyond spec)
- 0.5% = 0.005 — extremely small percentages like corrosion inhibitor concentrations require care here
A sloppy decimal point move is the fastest way to turn a correct setup into a wrong answer. Double-check by estimating: 15% of 80 should be a little more than 10 (which is exactly 10%), so the answer around 12 makes sense.
Applied Aviation Scenarios
Fuel Mixture Ratios
Some cleaning compounds and corrosion preventives are mixed as a percentage by volume. If a technical order specifies a 5% solution and you are making one quart (32 fluid ounces) of solution, you need: (5 ÷ 100) × 32 = 1.6 fluid ounces of concentrate, with the remainder being the diluent.
Battery State of Charge
Lead-acid battery service often involves measuring specific gravity and relating it to state of charge expressed as a percentage of full charge. If a battery rated at 35 amp-hours delivers only 28 amp-hours before reaching the minimum voltage cutoff, its measured capacity is (28 ÷ 35) × 100 = 80% of rated capacity — a figure compared against the manufacturer's minimum acceptable value.
Control Surface Rigging and Travel
When rigging flight control surfaces, a manufacturer might specify that the maximum up-aileron travel is 25° ± 10%. That tolerance equals (10 ÷ 100) × 25 = 2.5°, making the acceptable range 22.5° to 27.5°. Expressing tolerances as percentages rather than fixed values allows the same specification to scale if the design changes.
Compression Testing
Differential compression testers express cylinder health as a percentage. If 80 psi is applied and 72 psi is retained, compression is (72 ÷ 80) × 100 = 90%. A commonly cited guideline in engine manufacturer service information (e.g., Lycoming and Continental) references around 60/80 (75%) as a minimum for serviceability, but the specific acceptable limits are engine- and manufacturer-specific rather than a single FAA-mandated numeric standard.
Key Numbers and Rules
- Core formula: % = (Part ÷ Whole) × 100 — memorize all three rearrangements.
- Decimal conversion: divide any percentage by 100 before multiplying (e.g., 8% → 0.08).
- Percentage of change: always use the original value as the denominator.
- Efficiency: output ÷ input × 100; result must be ≤ 100% for any real physical system.
- Percentages above 100% are mathematically valid and do appear in aviation (e.g., an oversize repair hole that is 110% of the original diameter).
- Approximation check: 10% of any number = move the decimal one place left. Use this to verify answers before committing.
Common Test Traps
- Confusing part and whole in percentage-of-change problems. The FAA test frequently gives you the new value and asks for the percentage change, expecting you to divide by the original. If you accidentally divide by the new value, you get a different (wrong) answer that is also one of the distractors on the test.
- Forgetting to convert percentage to decimal. Multiplying by 15 instead of 0.15 gives an answer 100 times too large — but the test will have that larger number as a wrong-answer choice to catch you.
- Treating efficiency as output ÷ input when the values are given in different units. If input is given in horsepower and output in watts, convert to the same unit first; otherwise your ratio is meaningless.
- Assuming 100% is always the maximum. Questions about oversized holes, stretched components, or growth beyond original dimensions may correctly produce values over 100%. Do not automatically reject these answers.
- Misidentifying the "whole" in mixture problems. In a 5% solution, the whole is the total solution volume (concentrate plus diluent), not the diluent alone. Adding the concentrate to the diluent volume as if that diluent were the whole will give a slightly wrong concentration.
Mastering percentage calculations is not just about passing the FAA AMT General knowledge test — it builds the quantitative instincts you will rely on every time you verify a service limit, mix a chemical, or record a system performance figure in a logbook. Practice all three problem types until solving them feels automatic, and always apply the estimation check to catch decimal errors before they count against you.