Monday, October 12, 2009

If You Must Have a Failure...

This morning I departed VKX for the weekly trip up to Connecticut - twice. The first time, N631S's wheels left the runway at 1155Z and we were back on the ground by 1220Z. The entire flight looked (courtesy of FlightAware.com) like this:

I departed VFR from runway 6 and turned to the south from the left downwind. Staying below the 1500 foot MSL floor of the Washington Class B airspace, I contacted Potomac Approach. Once radar identified, I was cleared to turn to a 120 heading and to climb to and maintain 5000 feet. I was handed off to another sector and cleared to turn left toward OTT and thence to POLLA, my first enroute fix.

That's when the vacuum pump failed.

In May 2005, when N631S's avionics were upgraded, I had the shop install a Precise Flight Vacuum Failure Warning Indicator. This morning, it earned its place on the panel as it shone in all its amber glory announcing "LOW VAC"!

I quickly cross-checked with the suction gage. It wasn't low, it was on zero. Zip, nada, no vacuum at all.

The vacuum pump had failed abruptly and completely. Meanwhile, the associated gyros were still spinning but that wouldn't continue for long. The good news was that I was in excellent VMC. So, I transmitted, "Potomac Approach, Skylane 631 Sierra needs to return to VKX. We've had a vacuum pump failure. It is not an emergency." The immediate response was, "31 Sierra, stand by," followed in about half a minute by, "Skylane 631 Sierra, descend and maintain 2000, turn left heading of 260. You are cleared present position to VKX via radar vectors."

The rest of the flight was a straightforward visual return to Potomac Airfield. Although I will admit that watching the vacuum driven attitude indicator roll over and die was disconcerting. It made clear the importance of covering failed instruments to avoid distraction in IMC.

Back on the ground, I called Phil McClanahan, A&P mechanic par excellance. He listened to my tale and said, "I think I've got one of them in my hangar." So I opened up my hangar again, removed my automobile, and pulled N631S back inside. As I was about to start removing the upper cowl, Phil showed up with a toolbox and a new Rapco RA216CW dry-vane vacuum pump.

In September of 2004, when I took over the care and flying of N631S, I was not comfortable with the fact that the vacuum pump had accumulated 573 hours. There was no warning annunciator, no backup, and a failure in IMC could be most inconvenient. So I asked the folks at Lindner Aviation in Keokuk, IA (who had taken care of the airplane for the previous owners) to put in a new pump. The tach showed 3245.0 hours.

This morning the pump failed at about 3821.6 hours, having lasted in service for 576.6 hours.

It took Phil about 45 minutes to remove the failed pump, install the new pump, and examine the filter for any evidence of contamination. We ran up the airplane and got "ops normal" from the vacuum system and the instruments, buttoned up the cowl, and were, as it's said, good to go.

OK, there was a failure. But that was the extent of the bad news. The good news included:

  • It was certainly not a premature failure. The pump had given good service.
  • It was good that it failed in day VMC rather than at night in the clag (as would have been the case if it had gone "tango uniform" one hour sooner - on Friday night.
  • It was reassuring to see the Precise Flight annunciator system perform its intended function.
  • It was good that Phil was available and that he had a new pump to install.
I hope (not expect) that all of my equipment failures are similar to this one.

I took off for KBDR the second time at 1445Z and had an uneventful 1.9 hour flight.


Anonymous said...

Good on you for catching the low vacuum light. I have the Precise Flight system too but think I might replace my vacuum pump at 500 hours as a precaution.

-Roy at VKX

Frank Van Haste said...


Thanks. Given your intent to change out your pump at 500 hours, you may be interested in my next post on the subject. Stand by.