1.3 Weights, Measures, and Conversion Factors

Metric and Imperial Systems in Aviation

Aircraft technicians operate in a global industry that uses both the Metric (SI) and Imperial (US Customary) systems. For example, American aircraft manufacturers like Boeing primarily use Imperial units (inches, pounds, PSI, foot-pounds), while European manufacturers like Airbus utilize Metric units (millimeters, kilograms, bar, Newton-meters). Pilots and technicians must perform rapid and error-free conversions to ensure proper servicing and structural repairs.

Length Conversions

Length measurements are critical for sheet metal thickness, fastener sizes, tire dimensions, and structural repairs:

• 1 inch (in) = 25.4 millimeters (mm) = 2.54 centimeters (cm)
• 1 foot (ft) = 0.3048 meters (m)

If a repair manual specifies a clearance of 0.080 inches, and a metric feeler gauge is the only tool available, the conversion is:

0.080 in x 25.4 mm/in = 2.032 mm

Mass Conversions

Mass conversions are vital for aircraft weight and balance, cargo loading, and calculating takeoff performance:

• 1 pound (lb) = 0.4536 kilograms (kg)
• 1 kilogram (kg) = 2.2046 pounds (lb)

If a cargo load sheet lists a container weight as 1,200 kg, and the aircraft's weight and balance log operates in pounds, the technician converts the mass:

1200 kg x 2.2046 lb/kg = 2645.52 lb

Volume Conversions

Fuel and hydraulic capacity calculations require conversions between three common volume units:

• 1 US Gallon (gal) = 3.7854 Liters (L)
• 1 Imperial Gallon (Imp gal) = 4.5461 Liters (L)
• 1 Imperial Gallon = 1.201 US Gallons

Technicians must be careful to distinguish between US and Imperial gallons, as they are not equivalent. An Imperial gallon contains approximately 20% more volume than a US gallon.

Temperature Conversions

In aviation, temperatures are monitored for engine exhaust, oil, cylinder heads, and outside air. Technicians must convert between Celsius (C) and Fahrenheit (F) using the following formulas:

• Fahrenheit to Celsius: C = (F - 32) * 5/9
• Celsius to Fahrenheit: F = (C * 9/5) + 32

An outside air temperature of -40 degrees Fahrenheit is a unique mathematical convergence point:

C = (-40 - 32) * 5/9 = -72 * 5/9 = -40 degrees Celsius

For thermodynamic calculations, absolute temperatures are used: Kelvin (K) in the metric system (K = C + 273.15) and Rankine (R) in the imperial system (R = F + 459.67).

Pressure and Torque Conversions

Pressure

Pressure limits govern hydraulic systems, cabin pressurization, tires, and altimeters. Common conversions include:

• 1 Pound per Square Inch (PSI) = 6.8948 kilopascals (kPa) = 0.0689 bar
• 1 bar = 100 kPa = 14.5038 PSI
• 1 inch of mercury (inHg) = 33.864 hectopascals (hPa) / millibars

Standard atmospheric pressure at sea level is 29.92 inHg, which converts to hectopascals as:

29.92 inHg x 33.864 hPa/inHg = 1013.2 hPa

Torque

Fastener security is critical in high-vibration aerospace environments. Proper torque prevents thread stripping or structural failure. Manuals specify torque in foot-pounds (ft-lb), inch-pounds (in-lb), or Newton-meters (N-m):

• 1 ft-lb = 12 in-lb = 1.3558 N-m
• 1 N-m = 0.7376 ft-lb = 8.8507 in-lb

If a structural bolt requires a torque of 35 ft-lb, and the technician has a torque wrench calibrated only in Newton-meters, the setting is:

35 ft-lb x 1.3558 N-m/ft-lb = 47.45 N-m

Aviation Fuel Calculations: Volume to Weight

Pilots and flight dispatchers calculate fuel in terms of weight (mass) rather than volume because the energy content of the fuel is directly proportional to its mass, and aircraft performance (lift and range) is limited by weight. However, fuel trucks deliver fuel in volume (gallons or liters). Therefore, technicians must calculate the weight of fuel loaded based on its density.

Fuel density is not constant; it changes with temperature. As fuel temperature increases, the fuel expands, meaning its density (weight per unit volume) decreases. Technicians must check the current density of the fuel being pumped.

Typical Nominal Fuel Densities (at 15 degrees Celsius)

• Jet A-1 (Kerosene-based turbine fuel):
  • Metric: 0.80 kg / Liter
  • US Imperial: 6.7 lb / US Gallon
  • British Imperial: 8.0 lb / Imperial Gallon
• Avgas 100LL (Aviation Gasoline for piston engines):
  • Metric: 0.72 kg / Liter
  • US Imperial: 6.0 lb / US Gallon
  • British Imperial: 7.2 lb / Imperial Gallon

Worked Example: Fuel Loading Calculation

An aircraft requires an upload of 450 US Gallons of Avgas 100LL. The technician needs to determine the weight of this fuel in both pounds and kilograms to complete the weight and balance form.

1. Calculate weight in pounds: Weight (lb) = Volume (gal) x Density (lb/gal) Weight (lb) = 450 US gal x 6.0 lb/US gal = 2,700 lb

2. Convert weight to kilograms: Weight (kg) = Weight (lb) x 0.4536 kg/lb Weight (kg) = 2,700 lb x 0.4536 = 1,224.72 kg (or 1,225 kg rounded to the nearest whole kilogram).

If the fuel truck flow meter read in liters instead, the volume loaded would be:

450 US gal x 3.7854 L/gal = 1,703.43 Liters

Using the metric density of Avgas (0.72 kg/L), the weight in kilograms is:

1703.43 L x 0.72 kg/L = 1,226.47 kg

(Note: Minor differences in calculations occur due to rounding conversion factors, but technicians must always use the exact conversion ratios and densities specified in the aircraft manufacturer's approved manuals).