All For The Want Of…

LIFE CAN BE HARD AS A PILOT. Take Bob for example — he had a problem. It was a minor problem, but one that comes up from time to time and just slaps the living crap out of an unsuspecting pilot. Basically, it sucked … or didn’t really, as the case may be.

Bob’s vacuum pump was getting lazy. You know how it is, the pump has around 500 hours on it, and the vacuum gauge stops reading 5″, and starts down the slippery slope towards 3. HERE’S A TIP – 95 times out of 100, this means your DRY PUMP IS ABOUT TO DIE ON YOU.

Bob was pretty smart and knew this, so he took the plane in to his local A&P to have the vacuum pump replaced. The A&P did the right thing, and ordered up a new pump of the proper series for Bob’s plane. With the pump in hand, the A&P pulled a gasket off the shelf, and put the pump on to the correct pad on Bob’s engine.

WAS THAT THE RIGHT GASKET? Well, it fit okay, but the gasket was made of cork. There is a mandatory service bulletin out on cork gaskets for dry vacuum pumps. You see, all engines are set up for WET vacuum pumps, with the option to install a dry pump. This means that there is an oil port so that the wet pump, which uses engine oil to function, can get it’s wet stuff.

THE PROBLEM WITH DRY PUMPS IS THAT THEY DON’T NEED THE OIL, IN FACT, IF THEY GET OILY, THEY GET TRASHED. Bob had no worries as he launched skyward in his plane. The plane climbed with great enthusiasm, the vacuum gauge was back at a solid 5″, and Bob was proud that he had dodged a bullet by replacing his vacuum pump before it failed and caused him to lose vacuum.

TICK, TICK, TICK. Around 15 minutes into his flight, the engine on Bob’s plane starts acting up. A quick look down to the oil pressure finds that the pressure is at ZERO, and Bob presses the NEAREST button on his GPS receiver. Lucky for Bob, there is an airport nearby, since his engine selected that same moment to stop turning and SEIZE.

What happened. The sequence on this one is sad.

  • Bob’s vacuum pump was dying, so he took it in for work.
  • The mechanic replaced the old pump with a new one, but put in the wrong gasket.
  • Bob flew the plane, which pressurized the vacuum pump cavity with oil.
  • The oil pressure caused the cork gasket to pop.
  • Despite the small size of the oil port, the 75 pounds of pressure emptied the engine of oil in just a few minutes…

… the engine seized and an emergency landing was made, all for the want of the right gasket!

Lesson (Mechanics). Those old cork gaskets are no longer any good for your dry vacuum pump replacement needs. See the vendor information for the details, but get the right gaskets. Don’t fall into the trap of using what was there before — review the vendor information, and make sure you get the right gaskets for the vacuum pump!

Lesson (Pilots). Just because a cork gasket was installed previously doesn’t mean it is okay to install again. Bob got lucky with an airport underneath him as his piston engine plane suddenly became a glider. Are you going to count on your luck?

THE BOTTOM LINE: Bob got off lucky – he lost an engine, but he was able to land safely, WHICH IS REALLY ALL THAT COUNTS. What kind of gasket do you have under your dry vacuum pump? Is it a “cork time bomb,” waiting to pick the wrong time to empty your engine of oil? If you have had a vacuum pump changed recently, check with your mechanic and make sure the right gasket was used. The forced landing and engine damage that you avoid will be well worth the effort!