Skyhawk 172/Cutlass 172
Flight Crew Instrument; Flight Crew Private
Flight Crew Last 90 Days 30; Flight Crew Total 650; Flight Crew Type 650
Confusion; Human-Machine Interface
Aircraft Equipment Problem Critical; Deviation - Altitude Excursion From Assigned Altitude; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Flight Crew Became Reoriented; Flight Crew Overcame Equipment Problem; Flight Crew Returned To Clearance
Aircraft; Human Factors
During IFR flight talking to Approach Control; I began to experience heading/altitude control issues. By correcting for perceived incorrect heading; I lost approximately 300 FT of altitude near [an] Air Force Base where active flight operations were underway. My assigned altitude was 3;000 FT MSL. I eventually determined that my vacuum pump was working intermittently and causing false readings on the directional gyro (DG) and attitude indicator; and I engaged the standby vacuum system. Also at this point; I inadvertently turned off the avionics power switch and had to restart the GPS for the route and approach. Once the GPS was reprogrammed and the standby vacuum was engaged; the remainder of the flight went as planned.
C172 pilot reports difficulty maintaining altitude and heading on an IFR flight plan in IMC; and discovers that the vacuum pump is working intermittently. The standby pump is engaged but during the troubleshooting process the avionics power switch is inadvertently turned off requiring a restart and programing of the GPS.