EAA Sport Aviation - December 2009

However, they were heavy and had a tendency to exhaust oil onto the pressure side of the pump. This oil vapor in the exhaust is messy, and if the pressure side is to be used for de-icing boots, it must be separated out before the air reaches the boots, since oil can cause the rubber boots to deteriorate.

Eventually, dry pumps were created as an alternative solution. Dry pumps are a lighter and simpler form of vacuum power than wet pumps. Dry vacuum pumps consist of an elliptical chamber, within which a circular rotor spins and has floating vanes to seal against the sides of the chamber. The vanes ride up and down in their slots, following the contour of the pump chamber, pulling air out of the intake ports and pushing it out of the exhaust ports. In most pumps, both the rotor and vanes are made from a self-lubricating carbon material.

Dry pumps have become the standard source of vacuum power in modern piston aircraft. However, they have also proven to be a weak link in a very critical aircraft navigation system.

Since vane wear is the single, predictable failure mode, some pump manufacturers have made improvements in the ability to inspect the length of the vanes. This way, pumps can be removed from service before the vanes become critically short. One example is the Tempest Tornado line of vacuum pumps, which include a wear-indication port (WIP) on the back of the pump housing that allows a mechanic to check the wear of the rotor vanes. It’s a simple check that does not require removing the pump from the engine. You simply unscrew the WIP and conduct a visual check of the remaining length of the vanes, making a simple go/no-go decision.

Contamination—Aside from pump wear, failures can often be traced back to some form of pump contamination. Anything that gets inside the pump can cause the vanes to jam and fracture. This includes tiny pieces of rubber hose, Teflon tape, oil, water, or even carbon shards from a previous pump failure.

When a pump fails, it can send carbon pieces back upstream into the vacuum lines. It is critical to clean or replace all vacuum hoses in order to eliminate carbon pieces that could quickly destroy the new pump. All hoses should also be inspected to ensure that they have no internal deterioration that could cause rubber pieces to be ingested into the pump. If you do find significant carbon in the system, you might want to have the gyros overhauled at the same time. The same contamination that can cause a pump to fail can cause early gyro failure

Why Pumps Fail

As I mentioned earlier, the challenge with a dry pump is that when it fails, it fails catastrophically and without warning. Vacuum pumps are designed to extremely tight tolerances, and the carbon rotor and vanes that make up the power section of a dry pump are very brittle. Anything that puts abnormal stress on the rotor of vanes can cause them to fracture and fail.

That said, dry vacuum pumps not only fail spontaneously, but also due to one of four things: excessive wear, contamination, mishandling, or overheating.

Excessive Wear—Wear is the one predictable, and preventable, factor in vacuum pump failures. The carbon vanes slide in and out of very tight tolerance slots in the carbon rotor as they follow the curvature of the pump chamber. They are designed to wear at a predictable rate, providing slick carbon dust that serves as lubrication for the system. Over hundreds of hours, the vanes become shorter until they reach a point where they are no longer properly supported by the rotor. Once this happens, the vanes can chatter and become cocked at an angle in the slot, jamming in place and shattering as the pump continues to turn. The vanes can also wear in the slot, allowing play, and contribute to the problem. This is known as flank wear, and it progresses as the vanes get shorter. Excessive pressure from either poor pressure regulation or from high-demand applications can also contribute to flank wear.