AD 68-05-01
(a) For all airplanes except Models J3, J4, J5, PA-11 and those referenced
in paragraphs (i) and (j), which have exhaust mufflers with 950 or
more hours time in service on the effective date of this AD, comply
with paragraph (e) within the next 50 hours time in service and
thereafter at intervals not to exceed 50 hours time in service from
the last inspection.
(b) For all airplanes except Models J3, J4, J5, PA-11 and those referenced
in paragraphs (i) and (j), which have exhaust mufflers with less than
950 hours time in service on the effective date of this AD, comply
with paragraph (e) within the next 50 hours time in service, and
thereafter at intervals not to exceed 100 hours time in service from
the last inspection. After the exhaust muffler has accumulated 950
hours time in service, comply with the inspection requirements of
paragraph (a).
(e) Inspect in accordance with paragraph (f), and paragraph (g) if applic-
able, the engine exhaust muffler and shroud assembly (including the
internal baffle tube and tail pipe), carburetor heat shroud and air
duct, support braces, clamps and brackets, exhaust stacks and
manifolds. Do not alter those mufflers incorporating an internal
baffle tube to remove the tube without prior FAA approval.
(Piper Service Letter No. 324B describes the critical areas.)
(f) Remove muffler assembly, disconnect air ducts, stacks, and shrouds as
necessary, and visually inspect exterior and interior surfaces with a
probe light and mirror for signs of cracks, corrosion, burn-throughs,
heat damage, collapsed stack, or weld separations. For carburetor type
engines, special attention should be given to the exhaust stack under
the carburetor heat shroud. Except during the initial inspection, the
muffler need not be removed from the airplane, provided visual
inspection with probe light and mirror is made through the muffler
tail pipe outlet and one end of the muffler at the stack connection.
(g) If the inspection specified in paragraph (f) shows that the exhaust
stacks and internal baffle tube are in good condition, but there are
areas inside the muffler which cannot be adequately inspected with a
probe light and mirror, accomplish one of the following:
(1) Accomplish a submerged pressure check of the muffler and exhaust stack
at 10 psi air pressure.
(2) Conduct a ground test using a carbon monoxide indicator by heading the
airplane into the wind, warming the engine on the ground, advancing
the throttle to full static r.p.m. with cabin heat valves open, and
taking readings of the heated airstream inside the cabin at each
outlet (including rear seat heat outlet, if installed). Appropriate
sampling procedures applicable to the particular indicator must be
followed. If carbon monoxide concentration exceeds .005 percent or if
a dangerous reading is obtained on an indicator not calibrated in
percentages, inspect in accordance with (f), and perform a submerged
pressure check of the muffler and exhaust stack at 10 psi air pressure
before further flight.
(3) Close and secure cabin heat valves at the firewall until a complete
muffler inspection in accordance with paragraph (f) is accomplished.
