Every aircraft maintenance technician works in a world where two major measurement systems coexist: the International System of Units (SI), sometimes called the metric system, and the US customary system. Aircraft components, service manuals, torque specifications, fuel capacities, tire pressures, and fastener dimensions may be published in either system—or occasionally both. A technician who cannot confidently convert between the two risks misreading a critical specification, applying the wrong torque, or calculating an incorrect fuel load. Unit conversion is therefore not just a math exercise; it is a fundamental safety skill.
This article covers the most important unit families an AMT encounters—length, area, volume, mass and weight, pressure, temperature, and force/torque—explains the underlying logic of each conversion, and provides the testable numbers and practical tips you need to pass the FAA Aviation Mechanic General knowledge test and work accurately on the shop floor.
The Two Systems at a Glance
The US customary system uses inches, feet, yards, and miles for length; pounds and ounces for weight; gallons and quarts for liquid volume; and degrees Fahrenheit for temperature. The system is deeply embedded in older American-built aircraft, hardware specifications, and many FAA regulatory references.
The SI system is built on seven base units. For practical AMT work the most important are the meter (length), kilogram (mass), pascal (pressure), newton (force), and kelvin/Celsius (temperature). SI uses a consistent decimal (base-10) structure: prefixes such as milli- (one-thousandth), centi- (one-hundredth), and kilo- (one thousand times) make scaling straightforward. Modern imported aircraft, many avionics systems, and international maintenance data frequently use SI.
Length Conversions
Length is the conversion family that appears most often in structural and dimensional inspections.
- 1 inch = 2.54 centimeters (exact, by international definition). This is the single most important length relationship to memorize.
- 1 foot = 30.48 cm = 0.3048 m
- 1 yard = 0.9144 m
- 1 mile = 1.609 km (statute mile); 1 nautical mile = 1.852 km
- 1 meter = 39.37 inches = 3.281 feet
- 1 millimeter = 0.03937 inches (useful for gap and clearance checks)
Worked example: A structural repair manual specifies a minimum edge distance of 38 mm. Converting: 38 × 0.03937 = 1.496 inches, so you would verify the edge distance is at least 1.50 inches on your ruler. Rounding errors can be critical—always carry enough decimal places and round conservatively toward the more restrictive limit.
Area and Volume Conversions
Area conversions are simply length conversions squared. If 1 inch = 2.54 cm, then 1 square inch = 2.54² = 6.452 cm². This matters when calculating fuel tank cross-sections, panel surface areas for weight estimation, or heat-transfer calculations.
Volume conversions appear constantly in fueling and lubrication work:
- 1 US gallon = 3.785 liters
- 1 liter = 0.2642 US gallons
- 1 US quart = 0.9464 liters (there are 4 quarts per gallon)
- 1 fluid ounce = 29.57 milliliters
- 1 imperial gallon (UK) = 1.201 US gallons = 4.546 liters — important when servicing aircraft certified to British standards
Worked example: An engine oil capacity is listed as 12 liters. To purchase oil measured in US quarts: 12 liters ÷ 0.9464 L/qt ≈ 12.68 quarts, so you need at least 13 quarts on hand.
Mass and Weight Conversions
In everyday shop practice, mass and weight are often used interchangeably, though they are physically distinct. Mass is a measure of matter (kilograms in SI); weight is the gravitational force acting on that mass (newtons in SI, pounds-force in US customary). For AMT purposes the key relationships are:
- 1 kilogram = 2.2046 pounds (commonly approximated as 2.2 lb)
- 1 pound = 0.4536 kilograms
- 1 metric ton (tonne) = 1,000 kg = 2,204.6 lb
- 1 short ton (US) = 2,000 lb = 907.2 kg
- 1 ounce = 28.35 grams
Worked example: A component is listed as having a mass of 34.5 kg. For weight-and-balance records in pounds: 34.5 × 2.2046 = 76.06 lb. Always verify which unit a maintenance manual uses before recording weights.
Pressure Conversions
Pressure appears in tire inflation specs, hydraulic system pressures, oxygen system pressures, and pitot-static system calibration. Several pressure units are in common use:
- 1 psi (pound per square inch) = 6,894.76 pascals (Pa), or approximately 6.895 kPa
- 1 psi = 0.06895 bar
- 1 atmosphere (standard) = 14.696 psi = 101.325 kPa = 29.92 inches of mercury (in Hg)
- 1 in Hg = 0.4912 psi = 3.386 kPa
- 1 bar = 100 kPa = 14.504 psi
- 1 millibar (mb) = 1 hectopascal (hPa) — these two are numerically identical and used interchangeably in weather and altimetry data
Worked example: A nose gear tire requires 185 psi. A technician using a gauge calibrated in bar needs to set: 185 × 0.06895 = 12.76 bar. Using the wrong unit and inflating to 185 bar would be catastrophically dangerous—illustrating exactly why unit awareness matters.
Temperature Conversions
Temperature is unique because conversions involve both multiplication and addition, not just multiplication. The three scales an AMT encounters are Fahrenheit (°F), Celsius (°C), and occasionally Kelvin (K) in thermodynamic and gas-law calculations.
- °C to °F: multiply by 9/5, then add 32. Formula: °F = (°C × 1.8) + 32
- °F to °C: subtract 32, then multiply by 5/9. Formula: °C = (°F − 32) ÷ 1.8
- °C to Kelvin: K = °C + 273.15 (absolute zero = 0 K = −273.15°C)
- °F to Rankine (°R): °R = °F + 459.67 (Rankine is the US customary absolute scale)
Worked examples: Engine oil temperature limit of 245°F in Celsius: (245 − 32) ÷ 1.8 = 213 ÷ 1.8 = 118.3°C. Conversely, a battery electrolyte check at 15°C in Fahrenheit: (15 × 1.8) + 32 = 27 + 32 = 59°F.
A quick mental check point: −40° is the same in both Fahrenheit and Celsius. This is a useful sanity-check anchor.
Torque Conversions
Torque specifications are critical in fastener installation. Errors here can mean under-torqued joints that loosen in flight or over-torqued fasteners that fail in tension.
- 1 foot-pound (ft·lb) = 1.356 newton-meters (N·m)
- 1 inch-pound (in·lb) = 0.1130 N·m
- 1 N·m = 0.7376 ft·lb = 8.851 in·lb
- Note: 1 ft·lb = 12 in·lb (unit within the US system)
Worked example: A manual specifies 35 N·m torque on a cylinder head stud. In ft·lb: 35 × 0.7376 = 25.8 ft·lb. Set your torque wrench accordingly.
Why It Matters: Safety and Regulatory Implications
The FAA's Aviation Maintenance Technician Handbook—General (FAA-H-8083-30) stresses that a technician must use the units specified in the applicable maintenance manual. Substituting equivalent values is acceptable only after a verified and documented conversion. Maintenance record entries must clearly identify the unit used.
Historical accidents in aviation and aerospace have been traced directly to unit confusion—fuel quantities expressed in pounds vs. kilograms, pressure specs in psi vs. kPa. The AMT knowledge test tests conversion ability partly because the FAA recognizes this real-world hazard. When in doubt, write the unit next to every number you record and double-check conversions before applying any torque, pressure, or measured quantity.
Key Numbers and Rules to Memorize
- 1 in = 2.54 cm (exact)
- 1 kg = 2.2046 lb
- 1 US gal = 3.785 L
- 1 psi = 6.895 kPa
- 1 atm = 14.696 psi = 29.92 in Hg = 101.325 kPa
- °F = (°C × 1.8) + 32; °C = (°F − 32) ÷ 1.8
- 1 ft·lb = 1.356 N·m; 1 ft·lb = 12 in·lb
- −40°F = −40°C (the crossover point)
- 1 imperial gallon = 1.201 US gallons (do not confuse when servicing British-certificated aircraft)
Common Test Traps
- Imperial vs. US gallon confusion: The test may present a volume in imperial gallons; remember that 1 imperial gallon ≈ 1.2 US gallons. Treating them as equal produces a roughly 20% error.
- Forgetting the +32 in temperature conversion: Students frequently multiply °C by 1.8 and stop, omitting the addition of 32. Always apply both steps.
- Mixing in·lb with ft·lb: A torque spec in in·lb applied as if it were ft·lb delivers 12 times the intended torque—enough to snap studs. Always confirm which torque unit the manual uses.
- Using 2.2 instead of 2.2046 for kg-to-lb: For estimation this is fine, but exam questions may require the precise factor, and structural weight calculations may require more decimal places.
- Confusing kPa and psi directions: Because 1 psi ≈ 6.895 kPa, a number in kPa is always larger than the same pressure in psi. If your converted answer is smaller in kPa than in psi, you have the conversion inverted.