Vacumm Pump / instrument failure

J

JoeSerra

Climbing to Cruise
I believe My Vacumm Pump failed in flight today(lost attitude and direction insutruments)I had no suction and instruments nonoperative.
any advice (replace with new or rebuild) and what I might expect to pay to have it done. I may be lucky and only a house came lose?
any advice on troubleshooting.
thanks
Joe Serra
73' Sundower
 
Joe, if you do a BAC search on vacuum pump you will find a number of FAQs and Forum Topic posts that address this. There are too many of them for it to be practical for me to paste in all of the links here.

It is a near certainty that you have had an in-service pump failure. That means that the hoses must be cleaned at a minimum, and replaced at best, due to contamination. It is preferable to replace vacuum pumps on a scheduled basis. It avoids in-flight failures, and can save money by avoiding ancillary instrument damage and contamination of the new pump. The FAQs explain all this.

My personal rule-of-thumb is to replace new or RAPCO-remanufactured 211CC pumps at 1,000 hours, in vacuum systems (as opposed to pressure systems). The RAPCO remans are good units. They only reuse the end plates; all the wear items, including the main pump body/elliptical chamber, get replaced with new parts.
 
This is one of the topics that Mike Busch covers well in his seminar. Sometimes the question can be avoiding early failures as well as looking at cost of failures. In many cases having a back up electric gyro may make it possible to save money by reducing risk due to failure and keeping the old vacuum pump running all the way to failure. Even considering complications due to pump failure.
If anyone is trying to decide on doing a preventative air pump overhaul or replacement I suggest reading Mike Busch's articles first at Saavyaviator or the avweb archives. His seminar at BACfest will likely cover this and his ideas on "infant Mortality" and Maintenance induced failure rates make for interesting reading.
 
Just to emphasize an aspect of what Paul is saying. If you fly IFR, even if only light IFR, you can run an air pump to failure ONLY if you have at least an electric backup attitude indicator; and preferably a standby vacuum system as well. Air pump failure in IMC has a bad habit of being deadly, even with experienced pilots.

Mike Busch is generally correct in that many parts can be operated to failure, or near failure. Other than what the FAA mandates (especially under Part 23, like the Cirrus and Commander, which have a boatload of mandatory replacement items/times), under Part 91 (and CAR 3) we have a lot of latitude.

Of course, the risk and consequential cost in this approach varies wildly depending on the item and the knowledge base of the operator. There is no point in changing a cylinder because it has reached 1,000 hours; or has had one low compression reading due to ring leakage. And almost no one changes the Sierra-Duchess gear pump brushes every 1,000 hours, as is specified. You can wait until the pump fails, on the assumption that the emergency extension will be faultless (because it almost always is).

Running an Attitude Indicator until it fails to erect in a reasonable time is one thing. Ignoring the bearing noise and slow attitude correction, and running it until it rolls over in IMC, and you roll into the ground, is something else. Or how about a badly worn tire. It would be pretty dumb to wait until it blows on landing, and an off-runway excursion requires airframe, engine, and prop repair.

To sum up, you have to have the judgment to distinguish between repairs that can be deferred and parts that can be run to failure, versus those that should not be left to chance. And it is also true that there is even a risk in changing a part. Most parts have some percentage of infant mortality. And the simple fact that work was done on the plane creates some added exposure to failure; many problems occur shortly after an Annual Inspection. I do think that far too many parts get changed unnecessarily, due to unwarranted concern or simple ignorance. I would bet that 75% of cylinder replacements related to low compression are probably unwarranted, when the leakage does not involve the valves.

Please understand that I am in no way trying to contravene something Mike Busch teaches in his seminar. I have attended his training, and it is excellent. He also has much more general maintenance experience than I do, and has earned recognition for it. I am just suggesting that you have to understand the nuances in the general principle. You cannot afford to just operate everything until it fails. For those who are not mechanically inclined, or who won't recognize early warning signs, time-based replacement is wise (when such guidance is available). That's why the FAA mandates it in revenue service (or requires backups).

I personally prefer guidance that is based on field experience. For example, manufacturers say standard hoses should be changed every five years. Seems silly to me; they are often still in excellent shape (inside and outside) after as much as ten or fifteen years. But should you run your 35-40 year old original hoses until a cooler line bursts in flight? Or a brake line lets go while you are making an emergency stop, or fighting a crosswind on roll-out? Air pump manufacturers suggest a MTTR of 500 hours. My experience has been that 1,000 hours is safe on most new air pumps. Even with standby vacuum and an electric AI, I don't want to have a pump failure in IMC, if I can reasonably avoid it. I consider a cost of $0.25 to $0.50 per hour for a vacuum pump to be pretty reasonable.
 
I thought I would throw in my 2 cents on this topic. Everyone agrees that the standard dry pumps fail on average of 400-500 hours. I fly limited IFR but, I decided I wanted a system that would be less likely to dye at the wrong time. I looked at standby vacuum systems and decided I didn't want the extra plumbing and complexity of the system. I chose to install a wet pump using the M-20 stc. The set up is simple and to my knowledge there has never been a failure of a wet vac pump. They are usually rebuilt at TBO. I will have information on this system at BAC 08 for interested parties. It isn't cheap, about 2K (mostly for the paperwork as you can get the pump for about 300 bucks out of texas and an airwolf air/oil seperator for 500) But, it is STC'd and all the necessary items sans the rubber lines come with it. It is less than a Dry pump every 500 hours and a standby system inc the elect AI.
 
Thanks Mike for adding background and detail to my comments. Like all things aviation judgment plays an important role. My main point is that if you want to look into deferring replacement the effect on safety and the avilable backup systems play a role. I thought it was an interesting process to look at the cost and safety effect of a totally separate backup attitude gyro and then considering deferring preventative replgacement of vacuum system parts.
I believe that Mikes (Rellihan and Busch) have seen many of these issues and I agree fully with Mike's comments above. The judgement needed to make these decisions,for me, is greatly dependent upon input from experts like them. Our job of course, is to learn when to ask.
 
What about the Sigma-Tek AEON dry piston air pump? They have a TBO of 2,000 hrs.

I know that Mike R said it wouldn't fit on the Sierra's IO360, but what about the O360-A4K in the Sundowner's has anyone tried one on yet?

Steve Miles
77 Sundowner
N3342L
 
I just did this 2 weeks ago. A rebuilt 211C was $200 (after core charge refund) and the install was easy if you buy the special wrench. You can get both from Aircraft Spruce. You'll need your A&P to signoff.

Good Luck
 
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