June 2012
Q: Dear Steve,
I own a Piper PA-28-181 and I have a vacuum system problem. From what I understand, the vacuum system in my Cherokee is similar to a lot of other single engine Pipers, so this is likely a system problem rather than a Cherokee problem, but here goes.
As I climb, the vacuum pressure drops off. On the ground it’s right in the middle of the green but the needle slowly moves toward the left side of the gauge as I climb. Above 7,000 feet the pressure indicates below the green arc.
During my private pilot school I was taught that it’s important to keep the needle in the green but I’ve kept a close eye on both the DG and artificial horizon instruments when the vacuum level drops and they both seem to indicate correctly.
Can you tell me what’s going on? What do I need to do to fix it?
—Sagging Vacuum
A: Dear Sagging,
Let’s review our vacuum system basics. The PA-28-181 vacuum system consists of a dry air pump mounted on (and driven by) the engine; a vacuum regulator; a central vacuum filter, a differential pressure gauge; and the hoses and clamps that tie these components into a system.
The two main parts of a typical light aircraft dry air pump are the aluminum housing and a carbon rotor. Carbon vanes float in slots in the rotor and are thrown outward by centrifugal force during rotation to seal against the elliptically-shaped inner walls of the housing.
The dry air pumps produced today are much better and more dependable than earlier pumps—but all of these pumps have the nasty habit of committing pump suicide whenever even the slightest bit dust, dirt, grit or any other contamination is drawn in.
A healthy pump should maintain 4.8 to 5.2 in Hg (5 in Hg is 2.5 psi) vacuum at 1,200 engine rpm.
To maintain the operating-room cleanliness required, the air flow in a vacuum-type pneumatic system is drawn into the system through the central air filter, flows through the DG and AH, then moves on to the vacuum regulator before entering the inlet of the pump. After circulating through the pump, the air is exhausted into the engine accessory section.
The vacuum regulator consists of a diaphragm-controlled valve and valve seat. At low engine rpm, the valve seals against the seat. As rpm is increased, the vacuum level in the system exceeds the desired values; this excess pressure causes the diaphragm to pull the valve off the seat which lets ambient air enter the system through the valve. The addition of ambient air “dilutes” the airflow, restoring the desired value.
The regulator maintains system pressure, keeps the demands on the pump within safe values and also helps keep the pump cool. Air drawn in at the vacuum regulator is filtered through a B3-5-1 filter, also known as a garter filter.
The part of the system that is most misunderstood is the pressure gauge. Notice in the illustration that the pressure gauge is connected to both the filtered inlet air and the pump suction air. It’s a differential pressure gauge—it measures pressure drop of the airflow through the instruments. It does not measure the vacuum value at the pump inlet.
It’s common to conclude that when the vacuum gauge needle drops below the green arc during a climb that it’s caused by a drop in pump efficiency. This can be rationalized when a person reasons that a less efficient pump is still able to provide the required vacuum levels at sea level where air is dense; but at 7,000 feet, where the outside air is only 81 percent as dense as at sea level, the pump is no longer able to move enough of this less dense air. This is the easy conclusion, but it fails to take into account the effect of other system shortcomings.
The second most common conclusion reached by poor troubleshooters is that the regulator needs to be adjusted to get the gauge—and the system vacuum levels—back up where it belongs. Ralph Heysek, owner of Aerotech Components, Inc. and an expert on aircraft vacuum systems, writes, “If the regulator needs adjustment, the system needs maintenance.”
Heysek is always willing to help anyone with pneumatic system questions. Aerotech Components is headquartered in Las Vegas. (See Resources for contact information. —Ed.)
Don’t change the pump or adjust the regulator as a first response to low vacuum readings. Factors that always affect low vacuum readings are a clogged central air filter, restrictions in one or both of the hoses between the filter and the gyro instruments, a leak in one or more of the hoses between the gyros and the pump, and a sticky valve in the vacuum regulator.
The first, easiest and least expensive troubleshooting step is to replace the central air filter.
The central air filter and the garter filter on the vacuum regulator should be replaced at 100 hour or annual intervals. I’ve found garter filters that are so old they disintegrate when touched. This is especially true when the regulator portion of these valves is located up under the instrument panel on the cabin side of the firewall.
How old are the vacuum system hoses on your airplane? Don’t know and can’t find any maintenance record entries addressing system hose changes? It’s a fair bet that there are 20- and even 30-year-old airplanes flying today that have never had their vacuum system hoses replaced. (I’d also bet that many system clamps are original, too.) According to Heysek, hoses that are old enough to take a “set” are known leak sources.
What about the vacuum regulator valve? Pumps, central air filters, garter filters and gyro instruments are changed with some regularity but hoses, clamps, and regulators? Hardly ever.
In a leak-free and well maintained vacuum system, the pump should never have to pull more than 1.5 in Hg more than the gauge reading, or 7.2 in Hg. That’s equivalent to 3.3 psi.
Maintenance shops that understand pneumatic systems will either own or have access to a pneumatic system test kit. Aerotech Components occasionally loans a 343 set. It also sells the CV-700, its own portable pneumatic system test kit that includes a substitute pneumatic source, tools for testing pressures at points within the aircraft system, and a 71-page troubleshooting manual.
Aerotech Components also sells a pneumatic filter kit (CV1J4-P) that’s installed in the suction hose between the regulator and the pump. This Clearview inline air filter acts as a guarantee against pump contamination during normal operation. It also protects gyro instruments from carbon contamination in the event of a sudden dry air pump failure.
In conclusion, if a modern dry air pump doesn’t last 1,000 hours, the problem is likely to be in the hoses, clamps and regulator rather than in the pumps.
Know your FAR/AIM and check with your mechanic before starting any work.
Steve Ells has been an A&P/IA for 38 years and is a commercial pilot with Instrument and Multi-Engine ratings. Ells also loves utility and bush-style airplanes and operations. He served as Associate Editor for AOPA Pilot until 2008. Ells is the owner of Ells Aviation and the proud owner of a 1960 Piper Comanche. He lives in Paso Robles, Calif. with his wife Audrey. Send questions or comments to editor@www.piperflyer.com.
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