FAA Aviation Mechanic Airframe (AMA)Airframe StructuresHard
A technician needs to apply 100 in-lbs of final torque to a fastener but must use a torque wrench with an extension that increases the effective lever arm. The torque wrench handle length (pivot to grip) is 10 inches, and the extension adds 2 inches beyond the wrench's drive, giving a total effective length of 12 inches. Using the formula TW = TA x (L / (L + A)), what torque wrench reading (TW) should the technician set to achieve the correct 100 in-lb final torque (TA) at the fastener?
- A90 in-lbs
- B120 in-lbs
- C83.3 in-lbs
- D100 in-lbs
Show answer & explanationAnswer & explanation
Correct answer: C. 83.3 in-lbs
Using TW = TA x [L / (L + A)] = 100 x [10 / (10 + 2)] = 100 x (10/12) = 83.3 in-lbs. Because the extension increases the effective lever arm, the wrench must be set lower than the desired final torque so that the actual torque applied to the fastener equals 100 in-lbs.
Why the other options are wrong
- A. 90 in-lbs does not match the correct ratio calculation of 10/12.
- B. 120 in-lbs would result from dividing incorrectly (using L+A over L) and would severely over-torque the fastener.
- D. Setting the wrench to 100 in-lbs would over-torque the fastener because the extension multiplies the applied force.
Torque Wrench Extension Calculation
When using an extension on a torque wrench, the wrench setting must be adjusted using TW = TA x [L / (L+A)] to compensate for the increased effective lever arm, ensuring the correct final torque is applied to the fastener.
- L = wrench handle length, A = extension length
- TA = desired actual torque at the fastener
- Wrench reading (TW) is set lower than TA when using an extension
Memory trick: Longer arm, lighter reading — extension multiplies force, so dial down the setting