AD 74-06-02    Avcon Mufflers

March 17, 1974

This AD affected 39 owners that installed defective Avcon mufflers under an STC.

To prevent carbon monoxide from entering the cabin, the AD allowed the aircraft to be flown for a total of 25 hours. However, the muffler had to be inspected for cracks every 5 hours, and the Cabin Heater had to be safety wired OFF. After 25 hours the muffler had to be replaced.

74-06-02 CESSNA AND PIPER: Amendment 39-1798. Applies to Cessna Models 150, 170, 172 and 175 and Piper Model PA-28-140 airplanes modified in accordance with STCs SA750CE, SA806CE, SA807CE, SA777CE or SA 793CE respectively utilizing Avcon Industries, Inc., kits incorporating defective mufflers.

Compliance: Required as indicated, unless already accomplished.To prevent possible leakage of carbon monoxide into the cabin heater system, accomplish the following:

A) Prior to further flight, except that the aircraft may be flown in accordance with FAR 21.197 to a base where the inspection may be performed provided that the cabin heater system is in the “Off” position and the cabin fresh air vents are open, and at repetitive intervals not to exceed 5 hours’ time in service thereafter, inspect the muffler inner shroud as follows:

1) Remove muffler outer shroud assembly.

2) Visually inspect the inner shroud flare and tail pipe weld area for evidence of cracks or leakage. If cracks or leaks are found, replace the muffler prior to further flight with a serviceable unit.

3) If no cracks or leakage are found during the inspection, safety wire the cabin heat control in the “Off” position.

B) Within the next 25 hours’ time in service after the effective date of this AD, replace the existing muffler with a serviceable unit, at which time compliance with Paragraph A is no longer required.

C) Any alternate method of compliance with this AD must be approved by the Chief, Engineering and Manufacturing Branch, FAA, Central Region.

Avcon Industries Service Letter No. 1, dated February 19, 1974, pertains to this subject matter.

NOTE: The defective mufflers are contained in Avcon Kits of the following serial numbers: 556, 557, 562, 565, 580, 583, 584, 585, 588, 589, 591, 596, 597, 598, 599, 603, 604, 605, 608, 612, 618, 621, 623, 624, 627, 628, 632, 640, 642, 643, 644, 649, 650, 656, 659, 668, 669, 674 and 690. Confirmation of affected aircraft can be obtained by comparing the Avcon serial number stamped on the STC Kit I.D. tag with the Avcon serial number noted above. STC Kit I.D. tags are mounted near the aircraft manufacturer’s I.D. tag.

This amendment becomes effective March 18, 1974, to all persons except those to whom it was made effective by letter dated February 22, 1974