Yesterday afternoon I sat at my desk using Microsoft Flight Simulator, attempting to generate an image of the instrument panel of a single-engine airplane in flight, after it had suffered the failure of its vacuum pump. I needed this image for a training module I’m developing. The timing of this project could not have been more perfect, because today I got to witness what it looks like in the cockpit when a vacuum pump fails in flight in a real airplane.
For those of you who don’t appreciate the significance of such an event: The engine driven vacuum pump in an airplane provides the suction that allows the gyroscopic flight instruments — the heading indicator and attitude indicator — to function. These instruments are necessary for a pilot flying in the clouds to maintain control of the airplane without the benefit of any visual reference to the natural horizon. So if the vacuum pump fails, the suction disappears, the gyroscopes stop spinning and the instruments can no longer provide accurate information about the airplane’s pitch, bank and direction of flight.
My student and I were returning to the Baltimore area from Lancaster, Pennsylvania this morning on a routine instrument training flight when, about 15 miles from the airport, he noticed that the heading indicator displayed its orange system failure flag. “That can’t be good,” I said out loud as we descended through the bases of the clouds toward the runway. I looked lower on the panel and saw that the suction gauge was reading zero, meaning that the vacuum pump had failed. If we were in the clouds at that moment with no easy way out, we would have had to declare an emergency to ATC.
Fortunately, we were already below the clouds with the airport in sight, and Tom could control the plane easily by orienting it visually with the natural horizon. What I found interesting about the failure was how long it took for the symptoms to fully develop. If I had to guess, I’d say about 3-4 minutes elapsed from the time Tom noticed the flag on the heading indicator until we landed with the heading indicator deflected about 40 degrees and the attitude indicator wobbling around like a beach ball floating on the water.
While I’m sorry that Tom has to spend money to replace the vacuum pump, I’m so glad that he got to experience this failure in his airplane in a relatively safe environment, clear of clouds and near his home airport. Hopefully he’ll never see this problem in this airplane again, but at least now he knows what it looks like. The best I can do during a training flight is to cover up the gyro instruments with sticky pad notes and say, “OK, your vacuum system just failed.” But there’s no way for me to simulate the slow death of this critical system in the airplane during flight. That’s the real value of computer simulation.