FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 7. Aircraft Hardware, Control Cables, and Turnbuckles | 5. Washers | 7-89. Taper Pin Washers (AN975)
9/8/98
SECTION 5. WASHERS
AC 43.13-1B
7-85. GENERAL. The type of washers used
in aircraft structure are plain washers, , and
special washers. Typical washer types are
shown in table 7-14.
7-86. PLAIN WASHERS (AN960 AND
AN970). Plain washers are widely used with
hex nuts to provide a smooth bearing surface,
act as a shim to obtain the proper grip length,
and to position castellated nuts in relation to
drilled cotter pin holes in bolts. Use plain
washers under lock washers to prevent damage
to bearing surfaces. Cadmium-plated steel
washers are recommended for use under
boltheads and nuts used on aluminum alloy or
magnesium structures to prevent corrosion.
The AN970 steel washer provides a larger
bearing surface than the plain type, and is often
used in wooden structures under boltheads and
nuts to prevent local crushing of the surface.
7-87. LOCKWASHERS (AN935 AND
AN936). Lock washers may be used with ma-
chine screws or bolts whenever the self-
locking or castellated type nut is not applica-
ble. Do not use lock washers where frequent
removal is required, in areas subject to corro-
sion, or in areas exposed to airflow. Use a
plain washer between the lock washer and
material to prevent gouging the surface of the
metal.
CAUTION: Lock washers are not to
be used on primary structures, secon-
dary structures, or accessories where
failure might result in damage or dan-
ger to aircraft or personnel.
7-88. BALL SOCKET AND SEAT
WASHERS (AN950 AND AN955). Ball
socket and seat washers are used in special ap-
plications where the bolt is installed at an an-
gle to the surface or when perfect alignment
with the surface is required. These washers
are used together as a pair.
7-89. TAPER PIN WASHERS (AN975).
Taper pin washers are used with the threaded
taper pin. NAS143 and MS20002 washers are
used with NAS internal wrenching bolts and
internal wrenching nuts. They may be plain or
countersunk. The countersunk washer (desig-
nated as NAS143C and MS20002C) is used to
seat the bolthead shank radius, and the plain
washer is used under the nut.
7-90.7-100. [RESERVED.]
Par 7-85
Page 7-15 (and 7-16)