I keep hearing people say not to turn my prop backwards, to keep from hurting the vacuum pump. But I see the prop bounce backwards fairly often, especially during shut-down. What’s the true story on this? Does it really affect vacuum pump life?
As with most Old Wives’ Tales, this one is rooted in a tiny bit of truth, expanded to unwarranted extremes. The pumps manufactured by Airborne (who has since halted aircraft vacuum pump production), were designed specifically for ClockWise (CW) or CounterClockWise (CCW) rotation. This design was intended to reduce shear loads on the carbon vanes, as the pump neared the end of its useful life. If the pump remained in service past the recommended service life (which varied by model and application), and the vanes became much too short, backwards prop rotation could indeed break a vane. Of course, at that stage of their life, they could just as easily break during the next startup, cruise, or shutdown, as opposed to hand prop movement. In other words, there is no risk of pump breakage due to moving the prop backwards by hand, if the pump remains within its service life window.
Most, if not all, of the “dry” vacuum pumps currently on the market have vanes that are oriented perpendicular to the bore. They are designed to rotate in either direction. While they are not sensitive to rotation direction, they do have a limited service life and should be proactively replaced when warranted.
There is also a new “wet” vacuum pump now back on the market. It is sold by M20 Turbos, Bill Sandman’s outfit in Punta Gorda, FL. The STC covers our planes, and the package includes a matching air-oil separator. These pumps are warranted to TBO, and wet pumps have only rarely been known to fail in service (dry pumps fail regularly). The wet pump option isn’t inexpensive, but most of us do not have the option of going all-electric; and the electric backup pumps have their own price constraints and installation issues, as well as retaining the failure-prone dry pump.