Waypoints

Certainly the pneumatic system is one of the simplest systems on an airplane. Whether yours is a vacuum system or a pressure system, as many Beech airplanes employ, there's nothing involved except moving air. The air is pulled (in the case of a vacuum system) or pushed (in a pressure system) through your gyro instruments, causing the gyros to spin up and stabilize. A small engine-driven (usually) pump pulls or pushes the air.

Sure, the system is simple, but few things scare an instrument pilot more than the prospects of a failure of one of the gyro instruments or the pneumatic pump. The NTSB reports that on average over the past 10 years, pneumatic system failures are a factor in two fatal accidents each year. According to the AOPA Air Safety Foundation's Nall Report, vacuum and instrument failures were factors in two accidents in 1998, one of which was fatal. If you're one of those two per year it can ruin your whole day, but all in all that's not a bad statistic. The reason it's not worse isn't because pneumatic pumps and gyros don't fail often (they do), it's because general aviation pilots don't spend a lot of time flying in instrument meteorological conditions. It is in IMC that a pneumatic system failure can turn deadly. In the clouds, rain, and turbulence associated with IMC, we do a lousy job of controlling an airplane when one or more gyro instruments give it up. A soap holder slapped to an instrument face doesn't exactly create a realistic representation of a pneumatic or gyro failure in flight.

In the past dozen years or so, I've experienced two pneumatic failures in flight. Both occurred in visual conditions.

Not all pilots are so lucky. An experienced Mooney 231 pilot lost control of his airplane in IMC shortly after takeoff from Mattituck, New York, en route to Lebanon, New Hampshire. A 700-foot overcast and visibility of 1.5 miles in rain and fog shrouded his departure. Less than eight minutes after takeoff he reported a loss of vacuum pressure and an inoperative attitude indicator. Air traffic control tried to help out with vectors, but the pilot was not able to maintain control in what was certainly a difficult situation. He was one of the two that year.

Likewise, an experienced Bonanza pilot and his family lost their lives just last November after he reported a gyro problem shortly after takeoff from the airport at Linden, New Jersey. Again, the weather was low: 600 scattered, 1,300 broken with visibility about 2.5 miles in light rain and mist. In this case, it is unclear whether the pneumatic pump failed, but the NTSB listed a failed horizontal situation indicator as a factor in the accident. In this case, the HSI was air driven, whereas many HSIs are electrically powered.

It is important that pilots understand which instruments before them are air driven and which are electrically driven. In most light general aviation airplanes, the attitude and heading indicators are air driven while the gyros in the turn coordinator are electric. This provides for a level of redundancy in the case of either a pneumatic or electrical failure. Also, the gyros in turn coordinators tend to turn more slowly than those in attitude and directional instruments. As a result, TC gyros tend to fail less frequently than those in the air instruments.

As noted earlier, HSIs are most often electrically powered. In that case, the pilot has the luxury of an additional electric gyro. It's a luxury for a number of reasons. For one, electrical failures occur less frequently than pneumatic pump failures. In addition, an electrical failure is usually more apparent than a pneumatic failure and when the failure occurs, the pilot typically has more time to react—if he's blessed with a strong battery. On the other hand, once the battery is dead, your gyros are gone too, whereas no electricity is necessary to power an engine-driven pneumatic pump.

A regulator controls the amount of vacuum or pressure sent through the system. The normal vacuum necessary to drive the gyros is about five inches Hg. Just as an airspeed indicator shows indicated airspeed, a vacuum or pressure gauge displays indicated vacuum or pressure. Air velocity through a gyro increases with decreasing air pressure at altitude. At 30,000 feet both the airplane's true airspeed and speed of air through a gyro are 162 percent greater than at sea level. As a result, only 2.5 inches of vacuum pressure at 30,000 feet produce the same gyro rotor speed as four inches at sea level. Fortunately for the pilot, the regulator sorts all of this out, keeping the gyros spinning at the proper speed.

These days, pneumatic pumps are most often of the "dry" variety. Dry pumps have been standard issue for the past couple of decades because they are less messy than "wet" pumps, which were the standard for decades. Wet pumps are lubricated and cooled using oil from the engine. As a result they tend to blow a small amount of oil overboard with the air being exhausted from the vacuum system. Some owners plumb the exhaust air through an air-oil separator, which separates most of the oil from the air and returns the oil to the engine. Otherwise, the oil mist gets sent overboard and usually onto the belly of the airplane. Wet pumps are very reliable and, with their metal vanes inside, are virtually indestructible, but their messiness led to the development of the dry pump. Dry pumps are air cooled and lubricated by their own carbon vanes. While creating no mess, they have proven to be less reliable than wet pumps.

The Airborne Division of Parker Hannifin is a leading manufacturer of dry pneumatic pumps. In bold, red capital letters in its service letter, the company recommends that a backup pneumatic power source be installed in every aircraft or that a standby electric attitude indicator be installed.

A standby electric AI is certainly a good backup. Other choices include a standby pneumatic pump driven either by the engine or electricity. Another alternative is a simple system that uses the differential between ambient air pressure and manifold pressure to drive the gyros. Finally, one can install a venturi tube on the side of the fuselage. Common on airplanes 50 years ago, venturis are simple and foolproof, until they ice up. Air rammed through the tube on the side of the fuselage accelerates and cools as it is forced into the tube's narrow neck. The fast-moving air sucks air from the vacuum system line, which opens into the throat of the venturi. The cool air, however, causes icing problems.

Parker Hannifin recommends that its dry pumps be replaced at between 300 and 1,200 hours, depending on the model. Some aircraft owners, as a rule of thumb, replace the pump every 500 hours or five years. Of course, pumps are not immune to infant mortality either, so a new pump does not necessarily equate to a trouble-free future.

There are installation gotchas to be wary of. One Bonanza owner elected to do the right thing and replace his aging pressure pump before it quit on its own. On the advice of his shop, he authorized the installation of a wet pump to replace the ship's stock dry pump. Unfortunately, the shop didn't do its homework and installed the wet pump without converting the aircraft to a vacuum system, which would have required some plumbing changes and a supplemental type certificate.

After a long cross-country flight, the pilot discovered a small amount of oil streaming down the side of the fuselage, despite normal indications on the gauges. It seems that the wet pump was throwing the oil into the pneumatic lines, rather than overboard—as it would have in a vacuum installation. Fortunately, the inline air filter prevented the oil from reaching the instruments. Instead the oil was ported out through the regulator and then into the slipstream and down the side of the airplane. A longer flight might have allowed the air filter to become saturated, either blocking the line completely or permitting oil to pass into the gyros.

Few pilots understand the nuances of their pneumatic system. Besides understanding it, a pilot should respect it, back it up, and train for when it fails. It's a simple system, but one that is terribly unforgiving when it fails in IMC, where you need it most.

Links to additional information about pneumatic systems may be found on AOPA Online ( www.aopa.org/pilot/links/links0010.shtml). E-mail the author at [email protected].