FAA Maintenance Alert

Cessna: 421C; Corroded Oxygen Line; ATA 3520
(The following mechanic’s narrative provides a greater range of supportive detail than is typically offered in defect reports. How a particular mechanic wends his or her way through myriads of details has to be an unappreciated art form making real contributions to safety every day. – Ed.)
“While in process of checking the individual O2 (oxygen) masks during an Annual/5 year O2 bottle re-certification, I heard a very faint sound of a gas leak in the overhead (interior paneling) of the aircraft. It was nearly inaudible, almost disappearing when you turned your head from side to side. I began the search for the leak by pulling the O2 outlets in the suspect area and checking the B-nuts —all were secure. The only course left to me was pulling the headliner for a closer inspection. By plumbing in low pressure nitrogen (using for troubleshooting) I was able to narrow down the ‘hiss’ to a line in the overhead, just forward of the cabin door which carries the O2 from the L/H side of the aircraft to the R/H side. This line is P/N 5100107-46; item 30; figure 06-40; page 1 of the Cessna 421C IPC (internal parts catalog). At this stage of disassembly, all I could see was the B-nut on the L/H side, but by wiggling it I could (effect and detect...) a change in the sound of the hissing. To access this line (which passes through a structural beam in the overhead) it was necessary to drill out approximately 40 CherryMax Rivets securing a thin honeycomb panel to the overhead (structure). This panel is about nine feet long by 18 inches wide, running the length of the aircraft cabin. Above this panel are the Scat ducts (P/N F229900) that supply the cabin Wemacs (duct/vents). (These ducts are made like Scat but have an oval cross section.) With the honeycomb panel and insulation bats removed I was able to extract this O2 line (P/N 5100107-46). The corrosion on the ‘humps’ of the line occurred where the ducting passed under it. The ducting was removed to facilitate line removal and was found to be totally deteriorated. It pretty much exploded in a cloud of fibers and rusted wire when removal was attempted. The spiral wound wire supporting the duct was completely rusted to fragments. It came out in small chunks (I estimate 30 feet of ducting). Everywhere this ducting touched an O2 line there was products of corrosion. Some very light, some [heavier]. These lines were replaced by new parts from Cessna where required, and all the Wemac flex ducting was replaced with new. As bad as this line was, I just barely heard it hissing (given it’s position above all the insulation and headliner material) and the fact the O2 is regulated down to a low pressure in these lines. This could have been a bad deal had O2 been really needed, as this aircraft has the small, 11 cubic foot bottle. I can't (envision) how it would last very long, not to mention the danger of leaking O2 in a confined area near an ignition source (reading light switches and lamps). I (suspect) moisture condenses in the overhead due to body respiration, and at altitude it condenses, yielding numerous points of droplet formation, enhancing... [creating] corrosion everywhere the ducts touch the line. I would think this problem exists in every pressurized Cessna twin (in the field), including the 425 and the 441. Might they suffer even greater line deterioration defects since they fly at higher altitudes? The area this discrepancy was found in is extremely time consuming and invasive to access. Even with the headliner removed (like during an interior refurbishment) you can’t see the ducts or all the O2 lines. That long honeycomb panel must come down, as does a shorter section in the aft cabin. This aircraft is 30 years old now, and a total aircraft time of 5,341.0 is probably a low time for its age. (Truncating two digits off the part number allows the FAA Service Difficulty Reporting System database to find at least five such oxygen/corrosion defects—all on Cessna 400 series aircraft.) MORE