More failures

In reply to:


From the user perspective, I don’t think I’ve ever turned the prop backwards…


I have often seen FBO line personnel place a two-bladed prop in a horizontal position, so even if you don’t do it someone else may do it without your knowledge.

Mike:
I think in the long run that may make the most sense. In a “C” package SR20, there is a lot of hardware and backup to support just one instrument, albeit an important one. But a back up electrical model is a simpler solution. This is likely why Cirrus is going in that direction.
Brian

The club I was in had 5 planes and they flew a fair numbe of hours. With 75 pilots, each with their own idea of how the plane should be preflighted, started, leaned, and landed. They were probably all subjected to every thing you could list as a possible cause of vacuum failure. From what I remember vacuum failure was not a regular occurrance (since there was not backup it would not be a ho hum type of event and would certainly be noted on the club email list). The planes were 20+ year old Cessnas.

Mike,

Your photo is an eye-opener, as are your comments. I ALWAYS turn my prop backwards when moving it because I was told the magnetos couldn’t fire in that direction, preventing a hot mag from kicking over the prop. It feels as though I now have to choose between the risk of damage to my vacuum pump vanes or to myself (a remote possibility, granted).

As a further confuser, one of the mechanics at my service center stated that some of the vacuum pump failures were sheared shafts. His theory was that the higher temps in the SR20 engine compartment weakened the shaft, causing it to fail prematurely. Since I didn’t see an analysis of any of my three failures (in about 300 hours), I don’t actually know what the failure modes were, but now I’m curious enough to go find out.

I would welcome others’ thoughts about turning the prop one way or the other.

Regards,

Michael

I had a new Sigma-Tek DG installed in my former plane when the autopilot was put in. Sortly thereafter, the DG had a problem.

I contacted Sigma-Tek to arrange for a repair. I first asked whether the local FBO could pull the DG out and submit it for warranty repair (I did not purchase the autopilot or DG from my local FBO, although they were happy to do it for me.) Sigma-Tek said no, even though the local FBO is a Sigma-Tek dealer. So, I had to fly my plane with a bad DG to another location, swap the DG, go home, wait for Sigma-Tek to do the repair (they took their time), fly back, swap out the DG, and go home.

Sigma-Tek has a “contact the President” on their web site, and I wrote him a respectful letter communicating my problems. No answer. About a month later, I sent him a respectful reminder that I hadn’t heard back from him. Still waiting for a reply.

I’d use a venturi for vacuum before I’d use a Sigma-Tek product!

Andy

In reply to:


I’d use a venturi for vacuum before I’d use a Sigma-Tek product!


Andy,

Just want to make sure I’ve got this straight… you’re saying that Sigma-Tek vacuum pumps DON’T suck, right? [:D]

Mike.

Mike, your humor is…uh…uh…very funny.

Michael, although I would suggest you always turn the prop in the correct direction, I doubt that this is the cause of the vacuum pump failure rate on the SR20 - other aircraft using the same pumps, which undoubtedly have the prop turned backwards just as often, do not have the same failure rate.

Regarding sheared shafts, the shaft is designed to shear when the pump siezes due to broken vanes - this is to protect the pump drive in the engine. All the pumps I have seen fail have had sheared shafts, but have also had broken vanes, so as far as I know the vanes broke first, then the shaft sheared.

Clyde

In reply to:


Your photo is an eye-opener, as are your comments. I ALWAYS turn my prop backwards when moving it because I was told the magnetos couldn’t fire in that direction, preventing a hot mag from kicking over the prop. It feels as though I now have to choose between the risk of damage to my vacuum pump vanes or to myself (a remote possibility, granted).
As a further confuser, one of the mechanics at my service center stated that some of the vacuum pump failures were sheared shafts. His theory was that the higher temps in the SR20 engine compartment weakened the shaft, causing it to fail prematurely. Since I didn’t see an analysis of any of my three failures (in about 300 hours), I don’t actually know what the failure modes were, but now I’m curious enough to go find out.

I would welcome others’ thoughts about turning the prop one way or the other.


Michael,
Taking the highlighted comments one at a time…
“I was told the magnetos couldn’t fire in that direction”

  • So was I. I regard it as a tiny risk – my mag wires are grounded (I check that periodically by doing a “dead cut check” on shutdown), and the engine has no fuel, because we shut down by starving the engine. On the other hand, the pump manufacturer says specifically that turning the pump backwards is harmful.
    “…some of the vacuum pump failures were sheared shafts”
  • My A&P has replaced plenty, and he agrees with your A&P, except “more so”, in that he says that the failure is almost always a sheared shaft. As he explained things, there’s a good reason for that – the shaft has a built-in “shear link”, designed to sacrifice itself if things start to bind inside the pump. That’s to prevent damage to the gear, and possibly to the engine. I’ve attached a photo that shows the view from the front of my failed pump. You can see the shear link clearly - it’s the white plastic part. You can see the cog sticking out behind the small piece in the front, and you can also see the manner in which the manufacturer has attached the pump mechanism and cog to the shear link.

(In the picture, the broken end of the shaft points toward you on both the pump and the cog.)

Hope this helps.

Mike.

In reply to:


(In the picture, the broken end of the shaft points toward you on both the pump and the cog.)


After a lot of research, I have found a substitute pump that should be a lot more reliable, although perhaps slightly more expensive.

Note the built-in redundancy provided by the dual-chamber design. High-quality components used throughout. Also useful for survival first-aid.

I’m not sure whether the FDA can issue an STC, but maybe one of you could pursue a 337 for this pump.

   ![](http://www.gifs.net/animate/heart.gif)

In reply to:


I’m not sure whether the FDA can issue an STC, but maybe one of you could pursue a 337 for this pump.


Gordon,

It’s a great idea, but it’s been done. If I’m not mistaken, an original model of the pump you found was used by the other aviator involved in Bob Price’s “midair collision” a few months back. I think it’s fair to say that the pump in question failed on impact, while Bob kept his cool and landed safely.

  • Mike.