On Dec. 6, the FAA issued a revised Special Airworthiness Information Bulletin (SAIB) concerning failure of both new and replacement versions of the “Sensor-Oil Pressure Switch (Hobbs Switch),” part number (P/N) 83278. The revised SAIB affects Cessna 172R and 172S models. However, it removes Cessna 182 and 206 models that were previously listed, as they’re now experiencing fewer issues with the part.
In 2013, the FAA issued Airworthiness Directive (AD) 2013-11-11, which requires a life limit of 3,000 hours time-in-service (TIS) on engine oil pressure switch P/N 83278. This airworthiness concern is not currently an unsafe condition that would warrant an AD, but the revised SAIB is intended to raise awareness of an ongoing problem and to advocate preventative actions.
The oil pressure switch drives the following:
· The red “OIL PRESS” warning annunciator on pre-Garmin G1000 equipped airplanes; or
· The red “OIL PRESSURE” annunciation on the primary flight display of G1000 equipped airplanes; and
· The airplane hour meter on Textron airplanes.
There have been cases of failure in of engine oil pressure switch in which engine oil was found leaking from the pressure switch housing and/or found to have been pumped overboard, which can lead to obscured vision through the windshield with limited forward visibility, loss of all engine oil, and consequent engine failure. The Model 172S airplanes currently in production position the switch on the engine accessory housing in a more benign environment, helping to prolong the life of the switch. The FAA found that the original switch location on the crankcase subjects the switch to a vibratory environment, which is more likely to result in the premature failures noted above. Note: The equivalent of Service Bulletin SEB-79-10 has been incorporated in production on Model 172S airplanes serial number 172S12621 and higher.
The FAA recommends that owners/operators and technicians follow the previous recommendations “a” through “e” and adopt its new recommendation, “f,” to replace and relocate the pressure switch to a position less vulnerable to damaging vibration. All recommendations, including instructions for “f,” are listed in the PDF of the revised SAIB, which can be downloaded here.