The Overdue Torque Wrench and the 90-Degree Adapter
You are installing eight 5/16-24 airframe bolts that attach a replacement engine mount to the firewall structure of a Part 91 airplane. The maintenance manual specifies 40 lb-in, plus or minus 5 percent, dry, with the bolt heads up. Your click-type 0-75 lb-in wrench needs a 3-inch rigid adapter extension, held at 90 degrees to the wrench handle, to clear a firewall flange. The tool crib log shows the wrench was last calibrated 13 months ago, and the shop's calibrated-tool program requires 12-month recall. A second mechanic tells you to 'add 20 percent for the extension' and that 'the calibration sticker is close enough.' The aircraft is scheduled to leave this afternoon, and four of the eight bolts have already been run down with your wrench.
1. Does the 3-inch adapter change the torque delivered to the bolt? Evaluate the other mechanic's 'add 20 percent' advice and state the value you would set on the wrench.
Read the worked answer
Torque is force times the perpendicular distance from the pivot. With the adapter at 90 degrees to the wrench handle, the added length is perpendicular to the applied force, so the effective lever arm does not change and the wrench indication equals the torque at the fastener. Set 40 lb-in.
An adapter placed in line with the handle does change the arm, and there the correction reduces the indicated value:
T_indicated = T_required x L / (L + E)
With a typical 12-inch wrench and a 3-inch in-line extension, that is roughly 20 percent lower, not higher. So the advice is wrong in direction, and the 20 percent figure is unsupported for a 90-degree installation anyway. Verify the manual has no special note for crowfoot or extension use, and apply the specified value with a smooth, steady pull.
2. Can you use this wrench for the job? What must happen to the four bolts already torqued with it?
Read the worked answer
No. The shop program requires 12-month recall and this wrench is 13 months out, so its error is unknown. The specification is plus or minus 5 percent; an uncalibrated instrument cannot demonstrate compliance with that band, and 'close enough' is not a basis for approval for return to service.
Actions:
- Obtain a calibrated wrench from the crib (or have this one recalibrated and re-verified) before torquing the remaining four bolts.
- The four bolts already run down with the overdue wrench must be loosened and re-torqued with the calibrated tool. Do not 'check' them with the same suspect wrench, and do not assume the error was small or in a safe direction.
- Note the tool substitution in shop paperwork so the record reflects which instrument produced the final torque.
The calibration status of tooling is part of the mechanic's responsibility for the work being signed off.
3. What records close out this task, and who owns the airworthiness of the work?
Read the worked answer
The maintenance record entry must contain, per 14 CFR 43.9: a description of the work (eight engine-mount attach bolts torqued to 40 lb-in), the date, the name and certificate number of the person signing, and a reference to the approved data used (the maintenance manual torque value and procedure). If the work is part of an annual or other inspection, the approval-for-return-to-service entry follows 14 CFR 43.11, including a list of any discrepancies not cleared.
The mechanic who signs is the one responsible for the airworthiness of that specific work, and that includes having used serviceable, in-calibration tooling. If the bolts are later found under- or over-torqued, the signature - not the tool crib - is the accountable record.