AD 68-05-01 PIPER: Amdt. 39-726. Applies to Piper J3, J4, J5,
PA- 11, PA-12, PA-14, PA-15, PA-16, PA-17, PA-18, PA-19, PA-20,
PA-22, and PA-24 type airplanes, except PA-24-400 and PA-24-260
aircraft serial numbers 24-4783, 24-4804 and subsequent.
Compliance required as indicated.
(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).
(c) For all Models J3, J4, J5, and PA-11 airplanes
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.
(d) For all Models J3, J4, J5, and PA-11 airplanes
which have exhaust mufflers with less than 950 hours' time in
service on the effective date of this AD, comply with paragraph
(e) prior to the accumulation of 1000 hours' time in service
and thereafter at intervals not to exceed 50 hours' time in
service.
(e) Inspect in accordance with paragraph (f), and
paragraph (g) if applicable, 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.
(h) Replace or repair parts found to have the defects
listed in paragraph (f) before further flight, and thereafter
comply with the inspection requirements of paragraph (b) or
(d), whichever is applicable. Make welding repairs in
accordance with Advisory Circular AC 43.13-1 or an
FAA-approved equivalent. Pressure-check mufflers and stacks
that are repair-welded before reinstallation. (Care should be
exercised when reinstalling the exhaust system components to
prevent distortion or preloading of parts.)
(i) The repetitive inspection of paragraph (a) and (b)
may be discontinued when hollow muffler P/N 24506 or P/N
26385 is installed on Model PA-24 aircraft; and on Model
PA-24-250 aircraft when installed in combination with muffler
support Kit. No. 756775 (Service Letter No. 412A) or Kit No.
757058 (Service Letter No. 481) as applicable, or an equivalent
modification approved by the Chief, Engineering and Manufacturing
Branch, FAA, Eastern Region.
(j) For applicable PA-24-260 airplanes, the
repetitive inspections of paragraph (b) must be accomplished
at 50 hour intervals in lieu of 100 hour intervals until a
barrier device is installed in each muffler in accordance with
Piper Service Letter No. 518 or an equivalent modification
approved by the Chief, Engineering and Manufacturing Branch,
FAA, Eastern Region. Upon installation of the barrier devices,
the repetitive inspections of paragraph (a) and (b) may be
discontinued. (Piper Service Letters Nos. 324B, 324C, 412A, 481
and 518 cover this same subject.)
Effective March 31, 1968.
Revised March 5, 1969.
Here is the AD. Note the provision for using a carbon monoxide detector. I usually shine a flashlight up the tailpipe to check the flame tube and then pry the shroud open as far as possible, squirt a mixture of kids bubble soap/water all over it while someone blows shop air up the tail pipe.
Steve
I'd put on a new one to get away from the reoccuring AD for 1000 hrson muffler testing. 
that a Cub would have a little more power and never have to worry about a flame tube falling apart and clogging the tailpipe.
