March 18, 2019

Safety Corner Tid-Bits – Backup Vacuum?

Here’s a few extra safety items that I think are pertinent about our planes and club. We’ve had a few maintenance issues with the planes lately, including losing the vacuum system in our Piper Arrow PA 28R-201. This obviously is a critical part of our plane, especially if our members are flying in instrument conditions. Although instrument-rated pilots are trained to handle such emergencies, losing your attitude indicator and directional gyro in IMC is always a stressful situation. The Arrow also “boasts” a standby vacuum that can make these situations a non-event . . . . . right?

Well, yes and no.

First of all, it’s important for everyone flying the Arrow to understand the standby vacuum IS NOT a redundant backup system in the way we think a backup system should work.

The SVS operates on the differential between the intake manifold and ambient air pressure, which is then directed through shuttle valve system to drive the flight instruments. It also means specific power settings – in relation to pressure altitudes – must be maintained to produce ADEQUATE vacuum to be somewhat effective – according to the AFM the SVS system Supplement.

Unfortunately, those settings are in the logbooks and usually not in the plane. It also states that if the primary vacuum system is lost, the SVS IS NOT designed for continued IFR flight, and that the PIC should land as soon as possible. Obviously, the autopilot is vacuum directed from the gyros and should be turned off when losing the primary system.

Yes, that sucks if you’re in IFR conditions! Keep in mind the vacuum pressure will be more around 4.0 vs. a standard 4.8-5.1, unless the correct power settings and altitudes are applied.

If that sounds a little nebulous or confusing, you’re right. So . . . . what’s the bottom line?

To be honest, most small planes don’t have standby vacuum’s because they are just an additional cost and are not a redundant system and not totally reliable. For our instrument rated pilots in the club, I believe it’s a whole lot more critical to practice no-gyro flight and approaches rather than rely on the SVS system. The emergency procedures in the AFM Supplement support that by saying to not continue IMC flight and land as soon as possible. Using your blue-toothed Foreflight (to the Garmin 345) to receive directional and attitude information will be preferable.

Kevin Broderick, ATP, CFII, Skyhawk Flying Club Safety Officer


Leave a Reply