Airframe and Powerplant

Old-fashioned gyro power

None of Cirrus' SR22s is equipped with this tried-and-true system. It's also missing from the Lancair 350 and the Liberty XL-2. This system isn't required in VFR airplanes, but it is a must when flying IFR (unless you're flying a Cirrus, Lancair, or Liberty mentioned above).

One of the instruments that was originally powered by this system has, for the most part, been replaced by an electrically driven instrument. If you know that we're talking about the vacuum system, taxi into position and hold — you get to go to the head of the line.

Vacuum maintenance tips

A well-maintained vacuum or pressure system is very reliable. Here are some tips to increase the reliability of your system:

• Change the filters at recommended intervals unless someone regularly smokes in the plane or it is operated in dusty conditions. If either pollutant is present, halve the normal change intervals.
• When a dry pump fails, troubleshoot the system for carbon, restrictions, and contaminated gyro instruments. Repair all discrepancies before installing a new pump.
• Specify low-restriction vacuum system fittings in the system. They're hideously expensive but they're much less restrictive than A.N.-style fittings. The most common A.N.-style fittings are identifiable by their blue color.
• Change the hoses in your vacuum or pressure system every 10 years. — SWE

The removal of the engine-driven vacuum pump, regulator valve, and snake pit of large and bulky rubber hose-style plumbing behind the firewall by twenty-first century airplane builders such as Cirrus, Liberty, and Lancair is an indication that the vacuum system in many GA airplanes may be tiptoeing toward extinction. Since changing an older airplane to an all-electric gyro instrument system would be very expensive and would, at a minimum, require the installation of a second alternator, voltage regulator, and battery, let's look a little harder at what makes the prosaic vacuum system work, and what steps are needed to get good service life out of these systems.

Quirky for me — good for you

A basic system consists of a source of vacuum (or pressure in the case of Beechcraft products), a suction- or pressure-regulating valve, plumbing to connect the gyro instruments to the source, a gauge indicating the amount of vacuum or pressure at one of the gyro instruments, and filters to prevent dirty air from entering the system.

Most, but not all, twin-engine and a few single-engine airplanes have two vacuum pumps that are connected to the gyro instruments via a common vacuum manifold. If one pump fails, a simple one-way check valve located in the manifold assembly closes automatically to prevent leaks into the failed side of the system, thereby automatically allowing continued system operation on the remaining pump.

Gyro instrument systems certified under current regulations must provide pilots with warning information when the system is unsafe. Traditionally, this role has been fulfilled by the vacuum gauge, but the vacuum gauge is a poor indicator of system performance. New airplanes have improved warning systems such as annunciator lights or low vacuum warning flags in the instrument itself.

Vacuum systems are very simple but do have a quirky reputation — one airplane owner may never have any problems while his neighbor may be so wary of vacuum system failures that he has decided to avoid IFR flying until his budget permits the installation of a backup vacuum system (see " Airframe and Powerplant: Standby for Safety," April 2001 Pilot).

Venturi power

Before the advent of engine-driven vacuum pumps for light aircraft, vacuum was provided by a venturi that was bolted onto the side of the airplane. Bernoulli's theorem states that an increase in velocity creates a decrease in pressure. As ram air accelerated when squeezed through a venturi, the low pressure that resulted was tapped and, through simple hose connections, plumbed to the gyro instruments. A venturi provided a simple low-maintenance and elegant source of vacuum. But venturis had their drawbacks — it takes a big one (11 inches long) to generate enough vacuum to drive all three gyro instruments, and they're worthless for IFR takeoffs into low ceilings because not only does the airplane have to be moving fast enough to develop an adequate amount of vacuum, but also it takes some time for the rotors in the gyro instruments to spin up. Finally, venturis were ice magnets.

Before long, engine and airframe manufacturers started installing engine-driven vacuum pumps as optional and finally as standard equipment.

Pumps — wet ones

Initially all vacuum pumps were lubricated by engine oil — oil from the engine accessory case was drawn into the vacuum pump through a pair of aligned holes in the accessory case and pump mounting pad. The oil lubricated the internal moving parts of the pump. These wet pumps continuously exhausted a film of oil at the pump exhaust port. To recover the oil, a small swirl chamber called an air-oil separator was connected to the pump exhaust port. Most of the oil was separated from the exhaust air and returned to the engine. Hoses then carried the exhaust air to the bottom of the firewall, where it was vented into the slipstream. These wet pumps are simple, sturdy devices that eventually wear to the point that an overhaul is required. The wet pump's predictable (and therefore preferable) failure mode — a slow, gradual loss of efficiency — is offset by the film of oil that is continuously vented on the airplane belly. Modern airplane owners don't like oil on the belly of their airplanes so many owners removed their reliable wet pumps in favor of dry pumps.

Unfortunately, the typical failure mode of a dry pump is much more dramatic than that of a wet pump — the dry pump often works fine for hundreds of hours and then fails catastrophically.

Dry pumps

All the instrument gyro pumps manufactured today are dry pumps, although it's still possible to get a wet pump overhauled. Dry pumps, because they require no oil, permit manufacturers (at the pump's debut, Beechcraft and Aerostar) to turn things around and use the exhaust from the pump to drive the gyros in what's called a pressure system.

This system was adopted because a vacuum-type system depends on the differential between atmospheric pressure and pump vacuum to drive the gyro instruments. Since atmospheric pressure drops off as an airplane climbs, vacuum system pumps must work harder (thus heating up) as the airplane climbs. This is rarely a problem below 12,000 to 15,000 feet.

It's probably time to differentiate between a pressure system and a vacuum system. A pressure system uses pressurized air from the exhaust side of the pump to "push" air into the gyro instruments. This air is filtered before entry to the pump. After leaving the pump the air is filtered again before being plumbed to the gyro instruments. Upon entering each gyro instrument, the stream of pressurized air impinges on tiny "buckets" located on finely balanced rotors in each gyro instrument. When the rotors have attained a stabilized rotational speed the spinning mass exhibits the gyroscopic properties of rigidity in space.

A vacuum system creates a lower-than-atmospheric pressure at the pump inlet port to "pull" air past the buckets on the rotors of the gyro instruments. Prior to being pulled into the instruments the air is filtered by a central vacuum filter that is typically located behind the instrument panel near the gyro instruments.

Each system has a regulator, which should be adjusted to keep the vacuum level between 4.8 and 5.2 inches Hg. This simple valve opens when the pressure or vacuum value overcomes an adjustable spring pressure. This is the only adjustment in a modern gyro instrument system. As will be seen, rarely does this valve need adjusting in a healthy dry-pump system.

A little ground-carbon garnish on that gyro?

One of the challenges of the dry pump was how to lubricate the rotating internal parts. One solution was to make some or all of the internal parts out of carbon-based or impregnated materials. During operation a fine powder of carbon continually exits the pump exhaust port. This is relatively harmless in a vacuum-type system because the carbon blows out into the engine accessory compartment. But in a pressure system this carbon contamination exits the exhaust port of the pump into the pressure system. Pressure systems require a large carbon-capturing filter between the pump and the firewall. Does that mean the vacuum-type system is better because there's no possibility of carbon dust getting into the expensive gyro instruments? As first it seems so, but let's look further.

Vacuum back-flush

The vacuum system is always surrounded by atmospheric pressure. There's atmospheric pressure "pushing" air into the system to replace the air that the pump has "pulled" out. And there's atmospheric pressure air, or in some cases higher pressure ram air from the engine cooling inlets, surrounding the pump in the engine accessory section. The only "low" pressure area in the system is between the inlet filter at the instrument end of the system and the exhaust port of the pump. If, or when, the pump fails by breaking a carbon vane, the relatively high-pressure atmospheric air will rush into the system, sweeping a cloud of carbon dust into the system. As AOPA Pilot columnist Rod Machado says, "This is a bad thing."

Because of the possibility of carbon contamination, dry vacuum-pump systems require more than just a pump removal and replacement (R&R) when the pump fails. If there's any evidence of excess carbon, or if contamination is suspected, the system must be inspected further before pump installation.

Preinstallation inspection

An inspection for carbon contamination is a step-by-step process. The first step is to remove the system hose at the pump and wipe a white lint-free cloth around the inside of the tube. If there isn't any carbon in the hose, count your blessings, then go ahead and install a new pump.

If carbon is found, disassemble the system in a logical order: i.e., at the firewall, then at the regulator, then at the instruments to determine how far the carbon dust has been backed into the system. If the system hoses are older than 10 years, it's often easier to cut the hoses off the fittings and replace them than it is to wrestle with them behind the instrument panel. If there's evidence the gyros are contaminated, they must be removed and sent to an instrument shop for cleaning. All the hoses also will have to be cleaned before reassembly.

Carbon-capture filters

Those who have been paying attention have realized that the pressure system has a filter between the pump and firewall that will capture any destructive carbon dust. In the pressure system the only maintenance that's required when a pump fails is to replace the pump, clean the hose between the pump and the filter, and replace the filter.

Fortunately, AeroTech Components Inc. has STCed filter kits for Piper and Cessna aircraft that permit the installation of its ClearView air filters between the pump and the firewall connection in vacuum systems. These clear plastic filters also have pump-wear indicators. Installation of one of these filters will save many hours of troubleshooting and cleaning in the event of a catastrophic pump failure in a vacuum system. The ClearView filters are also STCed to replace the expensive sealed can-type filters in pressure-type systems. ClearView filters are available from parts houses and instrument shops.

This "destruction from the carbon cloud" scenario is less prevalent than it used to be because most modern pumps have done away with the fragile carbon rotors and vanes and have upgraded to tougher aluminum or composite materials. For instance, Sigma-Tek Inc., a Kansas company that Cessna selected to supply vacuum system components for its new single engine aircraft, produces pumps with aluminum rotors and tough carbon composite vanes. Other pump manufacturers are using aluminum vanes with good results.

Vacuum system pointers

In a well-maintained vacuum or pressure system, pumps may provide fault-free service for hundreds, sometimes thousands of hours, especially on lower-powered airplanes. Because of accessory section gearing differences, pumps that are installed on six-cylinder engines rotate at higher speeds than pumps on four-cylinder engines — this plus the likelihood of greater stress on the pumps from high-altitude flying and higher accessory section temperatures is almost a guarantee of shorter pump life on higher-performance airplane installations. For this reason, and because pump dependability is the weak point in these systems, many serious IFR pilots choose to change pumps after a set time in service — 500 hours is a common interval.

Scheduled maintenance requirements are minimal — regular replacement of vacuum and relief valve filters is normally done at 100-hour or annual inspections. Since the amount of pressure or vacuum the pump is required to supply in a healthy system is very low (4.8-5.2 inches of Hg is only 2.3 to 2.7 psi) failures are almost always systemic.

According to Ralph Haysek, an expert on vacuum and pressure systems who spent 17 years as the service manager of a well-known manufacturer of vacuum pumps and vacuum system components, a drop in vacuum pressure indicates a system problem, such as a split or kinked hose, a dirty filter, or a clogged orifice.

A well-intentioned but misguided solution to the onset of low vacuum readings is to "jack up" the vacuum regulator. According to Haysek, if the vacuum level indicated on the gauge has dropped, there's a system problem. A quick check of system health is to compare the vacuum (or pressure) readings at the pump and at the gyro instruments. There should never be more than 1.5 inches Hg (0.75 psi) difference between the two readings. A higher reading indicates system restrictions that cause overworked (and overheated) pumps to fail prematurely. Installing an STCed vacuum-cooling shroud may grant some additional time between failures but doesn't solve the source of the problem.

Vacuum and pressure systems are simple, but hard to troubleshoot. They are low maintenance, until they require maintenance. Then it's important to find a mechanic who knows where to look and what to look for when a dry pump fails.

Owners can lessen the possibility of gyro instrument system woes by replacing hoses every 10 years, keeping the system clean through regular filter changes, and installing an inline ClearView filter to lessen the impact of dry-pump failures on expensive and sensitive gyros.

E-mail the author at [email protected].