AC 43.13 Torque Values for AN Bolts and Self-Locking Nuts: A Reader's Guide for UK Engineers and Owners

In AC 43.13-1B, the FAA publishes recommended installation torques for AN, MS and NAS hardware in Table 7-1 (in inch-pounds, for oil-free cadmium-plated threads) and the minimum prevailing torque a used self-locking nut must still develop in Table 7-2. Example: a 1/2-20 bolt with an MS20365/AN310 tension nut is 480-690 inch-pounds to install, and a reused self-locking nut on that thread must still show at least 10 inch-pounds. These are FAA "acceptable" methods for when the manufacturer gives no value. They are not UK or EASA law.

If you work on an aircraft whose hardware is specified to AN, MS or NAS standards, Chapter 7 of AC 43.13-1B is written around exactly that hardware. This guide sets out what its Chapter 7 says about torque, using the exact figures and table numbers, and what it does not tell you. The Kopik AC 43.13 base indexes the circular together with 14 CFR Part 43 and the FAA AMT General Handbook.

An FAA advisory circular, not a UK requirement

It's worth being precise about status before quoting any number. In the American system, 14 CFR 43.13(a) requires maintenance to follow the current manufacturer's maintenance manual or Instructions for Continued Airworthiness, "or other methods, techniques, and practices acceptable to the Administrator". AC 43.13-1B is a collection of such acceptable methods. The FAA AMT General Handbook adds that advisory circulars "do not create or change a regulatory requirement", and that the AC 43.13 series applies to non-pressurised areas of civil aircraft of 12,500 lb gross weight or less.

And even within the FAA system, AC 43.13-1B neither approves a major repair nor replaces the manufacturer's manual. Table 7-1 says so itself: it is the recommended torque "when specific torque is not supplied by the manufacturer".

Reading Table 7-1: four columns, two thread series

Table 7-1, "Recommended Torque Values (Inch-Pounds)", is organised by thread size in a fine thread series (8-36 to 1-1/4-12) and a coarse thread series (8-32 to 7/8-9). For each size there are two installation ranges, for bolts loaded primarily in shear, and two maximum values:

• Installation range with tension-type nuts MS20365 and AN310 (40,000 psi in bolts).
• Installation range with shear-type nuts MS20364 and AN320 (24,000 psi in bolts).
• Maximum allowable tightening torque with MS20365/AN310 nuts (90,000 psi in bolts) and with MS20364/AN320 nuts (54,000 psi in bolts).
• A caution that values are derived from oil-free cadmium-plated threads, and a note that they apply to all cadmium-plated steel nuts with approximately equal numbers of threads and equal face bearing areas.

Compare the fine-thread equivalents and the difference is significant: a 1/4-28 fine thread with a tension nut is 50-70 inch-pounds, against 40-50 for a 1/4-20 coarse thread. A 7/16-20 fine thread is 450-500, against 235-255 for 7/16-14 coarse. Identify the thread before reading the row.

The procedure in paragraph 7-40

The table is only half the job. Paragraph 7-40 sets out how torque should be applied:

1. Torque for the size of the bolt shank, not the spanner size.
2. Calibrate the torque wrench at least once a year, or straight away if it has been abused or dropped.
3. Threads clean and dry, unless the manufacturer specifies otherwise.
4. Run the nut down to near contact and check the friction drag torque; whenever possible, torque the nut rather than the bolt.
5. Add friction drag to the desired torque to obtain the final torque.
6. Apply a smooth, even pull; if the wrench chatters or jerks, back off and retorque.
7. For castle nuts, start aligning at minimum torque plus friction drag and never exceed maximum plus friction drag; if the slot won't line up, change the washer or nut.
8. Correct the setting when an adapter changes the effective length of the wrench (Figure 7-2).

Table 7-2: is that self-locking nut still serviceable?

Paragraph 7-64 describes two families of self-locking nut: all-metal (primarily for high-temperature areas) and fibre or nylon insert, which are not installed where temperatures exceed 250 ºF. Its instruction is clear: do not reuse a fibre or nylon locknut that cannot meet the minimum prevailing torque values of Table 7-2. Paragraph 7-122c adds that repeated removal and installation causes self-locking nuts to lose their locking feature.

Worked example: a 1/2-20 bolt

You are refitting a used nylon-insert nut on a 1/2-20 bolt, the manual gives no torque, and the nut is an MS20365/AN310 tension type. First, the nut must still show at least 10 inch-pounds of prevailing torque (Table 7-2). If it doesn't, replace it. If it does, run it down, note the friction drag, and add it to the Table 7-1 range of 480-690 inch-pounds, staying within the maximum of 1100. Finally, check that at least one thread of the bolt protrudes through the nut (paragraphs 7-37 and 7-64f).

Table 7-2 stops at 7/16 inch. For smaller nuts the circular gives no prevailing-torque figure, and none should be invented.

Single-use hardware and placement rules

• Cotter pins should not be reused (7-38, 7-103, 7-127), and neither should safety wire.
• Pal or speed nuts should never be reused (7-122d).
• No self-locking nuts on parts subject to rotation, on access panels or doors, or on parts routinely disassembled before or after each flight (7-64).
• On turbine aeroplanes, no self-locking nuts where a loose nut, bolt, washer or screw could be drawn into the engine air intake (7-64c).
• Lock washers are not to be used on primary or secondary structure, or on accessories where failure might cause damage or danger (7-87).
• Washers used to make up grip length: 1/8 inch maximum combined height (7-37).

Five errors the tables won't catch for you

• Reading the wrong nut column. A 3/8-24 bolt is 160-190 inch-pounds with an AN310 tension nut but 95-110 with an AN320 shear nut (Table 7-1). Applying the tension-nut figure to a shear nut puts it far above its 95-110 range, most of the way to its 240 maximum.
• Using spanner size instead of shank size. Paragraph 7-40 is explicit that torque is for the size of the bolt shank.
• Forgetting the friction drag. The AC's final torque is the desired torque plus the friction drag measured just before seating.
• Treating Table 7-2 as a tightening value. The 27 inch-pounds listed for 3/4-16 is a test of whether a used nut still locks; the installation torque for the same thread with a tension nut is 2300-2500 inch-pounds.
• Lubricating by habit. Table 7-1 values are derived from oil-free cadmium-plated threads, and paragraph 7-40 asks for clean, dry threads unless the manufacturer specifies otherwise.

In the base you can ask, for instance, "What's the minimum prevailing torque for a reused self-locking nut on a 3/4-16 fine-thread bolt?" (answer: 27 inch-pounds, Table 7-2). The full circular is on the FAA website: AC 43.13-1B.