Torque Conversion Guide for Aviation Maintenance

Torque is one of the most critical values in aircraft maintenance. Under-torque a fastener and it can back out in flight. Over-torque it and you risk thread stripping, bolt failure, or structural damage. Getting it right every time means knowing how to convert between the units your AMM uses and the wrench in your hand.

This guide covers every torque unit conversion an AMT will encounter on the job, plus the torque wrench extension correction formula that every mechanic should have memorized.

The Four Torque Units You'll See in Aviation

Inch-Pounds (in·lb)

The most common unit in US aviation documentation. Used for smaller fasteners, avionics hardware, and most general airframe work. When an AMM says "torque to 85 in·lb," that's a moderate torque for a small bolt — roughly equivalent to tightening a bolt firmly by hand and then a bit more.

Foot-Pounds (ft·lb)

Used for larger fasteners — engine mounts, landing gear hardware, structural bolts. The conversion is exact and simple: 1 ft·lb = 12 in·lb. This is the single most used conversion in US aviation maintenance. Memorize it.

Newton-Meters (N·m)

The SI unit for torque. Airbus documentation uses N·m almost exclusively. Many European-manufactured components and modern engine manuals also use N·m. If you work on Airbus aircraft or international fleets, you'll need to convert often.

Kilogram-centimeters (kg·cm)

Less common today but still appears in older Japanese and European documentation, and some legacy engine CMMs. You may see it on older hydraulic component manuals.

The Key Conversion Factors

Common Quick Conversions

Torque Wrench Extension Correction Formula

This is one of the most important formulas in all of aircraft maintenance, and one of the most commonly misapplied. When you use a crow's foot, open-end adapter, or any extension on a torque wrench, the reading on the wrench is not the actual torque applied to the fastener. You must correct for the added length.

• TW = Torque Wrench reading (what you set the wrench to)
• TS = Target torque (what the AMM specifies)
• L = Effective length of the torque wrench (from drive to handle center)
• E = Extension length (from drive to center of the fitting)

Example: AMM calls for 100 in·lb. Your wrench is 10 inches long. Your crow's foot adds 2 inches.

You set your wrench to 83.3 in·lb — when it clicks, the actual torque on the fastener is 100 in·lb.

Wet vs. Dry Torque

Always check whether your AMM specifies wet torque (with lubricant applied to the threads) or dry torque (clean, unlubricated threads). The difference matters significantly — a lubricated fastener can reach much higher clamp load at the same torque value compared to a dry one. Applying dry torque specs to a lubricated fastener results in over-torque and potential thread damage.

Common lubricants that affect torque: anti-seize compound, engine oil, hydraulic fluid, and thread-locking compounds. If the AMM doesn't specify, ask your supervisor or check the general torque practices section (usually ATA Chapter 20) of your aircraft's maintenance manual.

Torque Wrench Best Practices

• Use your wrench within 20–80% of its rated range for best accuracy
• Always calibrate before critical work — most shops require annual calibration minimum
• Release the tension setting when storing a click-type wrench — leaving it loaded weakens the spring
• Pull smoothly and steadily — jerking the wrench adds overshoot
• Apply torque in the tightening direction only — don't use a torque wrench to loosen fasteners
• Record the torque wrench serial number and calibration date on your work card when required