Pneumatic Headaches

Pneumatic systems, while simple, can prove to be difficult to troubleshoot. If you’re ever host to a pneumatic gremlin, getting the aircraft back on the ground may be only the beginning of your problems. You see, the relative difficulty of tracking down and correcting a pneumatic system problem makes the task somewhat daunting for your A&P — who has to work through a maze of system components. The work takes time, and time is money … especially when it gets near airplanes.

Example: An owner had his wet vacuum pump overhauled at annual due to an oil leak. The wet pump had been performing well — in fact, too well and pulling more than the right amount of vacuum. Based on the diagnosis of the A&P, the decision was made to replace the vacuum regulator along with the vacuum pump.

In a perfect world, that would have solved the problem. The owner would have climbed into the plane, started the engine and witnessed vacuum gauge readings back where they belong — at 4.5 inches. Instead, the owner started the engine, and found the vacuum at 2.0 inches — too little vacuum to drive the autopilot, causing the ADI to act lazily, too. Clearly, something was not right, so the A&P had to be called again to troubleshoot the problem.

The components of the pneumatic system are simple. It consists of:

  • A vacuum or pressure pump, which supplies the drive for the system.
  • Filters, which keep the air being fed or drawn through the gyros clean and free of grit. In wet vacuum systems, there are usually air/oil separators, which pull any oil out of the discharge and route it back to the engine.
  • Hoses, which connect all the components.
  • A regulator, which adjusts the pressure or vacuum to keep it in spec.
  • And finally, T’s to connect multiple instruments, which sometimes contain a flow restrictor.

When you develop trouble in your pneumatic system, all of the components need to be troubleshot in order to find the root cause of the problem. Many mechanics fail to find a leaky hose or a leak in the system and focus instead on the vacuum regulator, which they ‘clamp down’ to remedy the situation. This is a critical mistake that can mask the real problem and cause new ones.

Unable to find an obvious leak, your mechanic elects to just clamp down the regulator. Result: Your vacuum pump is now pulling double-duty — pulling against the regulator to make up the deficit in the system. Why? There was no leak in the system; the real problem was just a bad gauge! Your system is now pulling 7 inches of vacuum and your gyro instruments are spinning their expensive innards silly. Since the gauge is broken, you can’t tell the difference, and you usually can’t hear the gyro sound over the sound of the engine.

Some ten or a hundred hours later (usually at the worst time) your artificial horizon … or directional gyro … or turn coordinator — or ALL OF THE ABOVE — will start to malfunction. Of course, you’ll be flying in instrument conditions, or flying an approach at the time. If only one instrument fails, you will be challenged to identify it and land partial panel. If more than one fails … well, your chances of surviving this little event will drop substantially.

When you have a pneumatic system problem, it is your call as the aircraft owner to make sure it is corrected the right way. Talk over the problem with your mechanic, and make sure that he or she will go as far as they need to go, to properly test the system. A proper test should include pressure testing the system hoses, a test of the regulator, a parallel test of the vacuum gauge, and even a test of the filters and air/oil separator.

BOTTOM LINE: While the thorough approach will cost more, you will end up with a pneumatic system that you can count on. When you consider that your safety may depend on it, having the necessary testing done on your pneumatic system becomes a lot more affordable.