Inch Pounds to Newton Meters for Aircraft Fasteners

Taking AN and MS Fastener Values Into a Metric Shop

Aircraft hardware documentation is stubbornly imperial. AN bolts, MS self-locking nuts and the torque tables in AC 43.13 are all written in inch-pounds, and they are not going to change. The bench you are standing at, though, may hold a digital wrench that reads only in Newton-meters — or the build log you file may have to be in SI. Every value on the card needs converting before a single fastener is pulled up.

1 in·lbf = 0.11298483 N·m. The upper end of the 1/4-28 tension band, 70 in·lbf, is 70 × 0.11298483 = 7.91 N·m, which is what a metric-reading wrench should be set to.

Bands, Not Single Values

Airframe tables publish a minimum and a maximum, because the target is a bolt tensile stress rather than a wrench reading. Convert both ends of the band; landing anywhere inside the converted window is compliant, and having the ceiling in front of you keeps a nudge from becoming an overshoot.

Running Torque Comes First

A self-locking nut resists turning before it ever touches the washer. That friction drag is measured on the actual nut and added to the table value, so the number you convert is the sum — not the figure printed in the book.

Adapters Change the Arithmetic

A crowfoot or extension that lengthens the wrench alters delivered torque. Correct for the added length first, then convert the corrected figure — doing it in the other order invites a slip.

Preparing a Torque Card in Metric

Enter the tabulated inch-pound value

Type the figure from the maintenance manual into the in·lbf field. Run the minimum and the maximum through separately so the whole permitted window is written on your card in Newton-meters.

Add the measured friction drag first

Measure the nut's running torque, add it to the table value in inch-pounds, and convert that total. A 28 in·lbf specification with 10 in·lbf of drag becomes 38 in·lbf, which converts to 4.29 N·m.

Transcribe the exact number

Copying a field puts the bare figure on the clipboard with no unit text, so what lands in the build log or the inspection record is the value itself rather than a retyped approximation.

Reverse it to answer an inspector

When a signed-off metric figure has to be shown against the imperial table, type into the N·m box or press the swap arrows — both fields are live, so the check runs in either direction.

Both unit menus are searchable and list all thirteen torque units in the app, which covers the occasional instrument-panel or trim-actuator figure quoted in in·ozf or N·cm.

Fine-Thread Tension Bands in Newton-Meters

The fine-thread, tension-type nut ranges familiar from standard airframe practice, converted end to end. These are illustrative of the scale involved; the controlling values are always those in the type certificate holder's manual for your aircraft.

Thread	Bolt series	Published band (in·lbf)	Converted band (N·m)
10-32	AN3	20–25	2.26–2.82
1/4-28	AN4	50–70	5.65–7.91
5/16-24	AN5	100–140	11.30–15.82
3/8-24	AN6	160–190	18.08–21.47
7/16-20	AN7	450–500	50.84–56.49
1/2-20	AN8	480–690	54.23–77.96

Eight Decimals for a Traceable Record

Results carry up to eight decimal places, so a converted figure entered in a logbook is the value itself rather than something already rounded twice.

Band Ends in One Pass

Minimum and maximum go through the same field one after the other, which keeps the permitted window intact instead of collapsing it to a single midpoint.

Nothing Leaves the Bench

Conversion happens entirely in the browser once the page has loaded, so aircraft registration numbers and job figures you type stay on your own machine.

Fastener Questions From the Hangar

Do I add a locknut's running torque before or after converting?

Before, and in the units the table uses. Run the nut down until it nearly meets the bearing surface with at least one thread showing, measure the drag, add it to the tabulated value in inch-pounds, then convert the total once. Converting each piece separately and adding afterwards gives the same answer but doubles the number of chances to mis-key a digit.

How does a crowfoot or extension change the value I set?

An adapter that extends the wrench along its axis multiplies what reaches the fastener, so the wrench must be set lower. The usual correction is setting = target × L ÷ (L + E), where L is the wrench's own effective length and E the added offset. Work that out in the original units, then convert the corrected number. An adapter mounted at 90° to the handle adds no effective length and needs no correction.

Why does the manual distinguish between torquing the nut and the bolt head?

Because the friction geometry differs. Standard tables assume you turn the nut; turning the head instead drags the bolt shank through the hole and changes how much of your input becomes clamp load. Where a manual permits head-side tightening it normally publishes a higher figure for it, and that variant is the one you should be converting.

The castellated slot does not line up with the cotter hole. Can I back off slightly?

Standard practice is to advance, never retreat: tighten forward to the next alignment provided you stay at or below the converted maximum for that band. If the slot still will not line up inside the window, change the washer thickness or the nut rather than settling for less clamp load than the joint was designed to carry.

Can a self-locking nut be reused once it spins on easily?

Not if the locking feature has faded. The check is the running torque itself: a nut that can be turned onto a clean bolt with your fingers all the way down has lost the drag it depends on and is scrap. Fibre-insert types also have temperature limits that rule them out near exhaust and engine installations regardless of how they feel.