Vacuum System Info, esp. Filters

Bob Axsom · #1 04-01-2008, 09:43 AM

Two days before the Taylor 125 air race my Attitude Indicator rolled over and died. It had been shakey at start up and slow to erect (OK, OK, it's not funny) for sometime before this final failure. I drove down to Rudy, Arkansas and turned it in for overhaul. Yesterday they called to say it was fixed and I asked what they found. Mike told me the rotor was loaded with debries which had destroyed the bearings. He asked if I had a vacuum pump failure and I reported that I did have one last year. He said this sequence is common and to prevent the carbon dust from the vacuum pump failure from destroying the Gyro instruments you need an inline filter between the pump and the instruments with the flow arrow pointing to the pump. When I built my plane I installed a Rapco Vacuum System Kit (p.383 in the 2007-2008 catalog) which does not include an inline filter (p.385 in the same catalog). He suggested cleaning the hoses with mineral spirits (blowing then out will not work) or replacing them and replace the filters. He also said it is a good idea to replace the filters at the time of the annual condition inspection. Finally, he said to use the black woven shroud hoses instead of the soft rubber hoses which will collapse. Anyway, I thought I would pass this along. The maintenance procedure sounds conservative but I am changing my checklist to cover it.

Bob Axsom

Bob Axsom · #2 04-06-2008, 10:33 AM

I installed a new in-line filter between the vacuum pump and the regulator.

[img=http://img215.imageshack.us/img215/8512/dvc00001eh9.th.jpg]

This is intended to protect the gyro instruments in the event of a vacuum pump failure. We have around 410 hours on our RV-6A now and they say you should replace the vacuum system filters at this time so I replaced the garter filter on the regulator, the inlet filter and added this new in-line filter. There are three different in-line filters listed on the same page in the Aircraft Spruce catalog and I was getting filter poor so I bought the metal Rapco filter for around $26 instead of the others which were over $50. Yhe instrument shop suggested that I just replace them at each annual which I am considering. Of course, it would be cheaper to set up another replacement log and check list items like I have for the ELT batteries. I do not intend to just ignore them till I get a series of failures again.

Bob Axsom

Norman CYYJ · #3 04-06-2008, 07:05 PM

I have used in-line fuel filters with very good success and they are extremely cheap just a couple of dollars. The filter is fine enough to keep out any dirt. I change them out on each annual.

Bob Axsom · #4 06-01-2008, 10:39 AM

After completing the ILS Rwy 6 approach to London, KY on 5-29-08 for refueling under 1,700 ft overcast, the Attitude Indicator failed again on the next leg of the trip in the same manner as it had a couple of months ago at the start of this thread.

Bob Axsom

airguy · #5 06-01-2008, 11:22 AM

Is there a repair warranty still in effect? Even 90 days would still cover this one.

I am curious as to how the filter would stop this - unless I'm mistaken (very possible

) the vacuum pump pulls ambient air through the gyro into the pump under a vacuum and then exhausts that air out to atmosphere, correct? Ambient air pressure exists on the input side of the gyro with vacuum on the output side, the pressure differential is what drives the gyro. So if the pump fails, no air moves backward from the pump to the gyro - the normal through-gyro airflow would fill the vacuum in a few second - so why the need for the filter AFTER the gyro? I can certainly see the need for a pre-gyro filter to clean the ambient air on the way in - but not the vacuum air on the way out between the gyro and the pump.

The only reason I can see for that filter is in the case of a catastrophic pump failure that would break open the pump and allow ambient pressure to BACK FLOW in the line (which is under vacuum at the time of failure) to the gyro, but even then the ambient pressure flow would not make it all the way back to the gyro before the air in the line pressured back up to ambient and the flow stopped. In that case, wouldn't a simple check valve work just as well? I don't see a scenario that would carry pump debris into the gyro.

What am I missing here?

AlexPeterson · #6 06-01-2008, 11:40 AM

Air (and pump debris) will move backwards from the pump to the gyro in the case of the vanes coming apart on the pump. The pressure in the lines (and gyro) is less than ambient, so some backwards motion can be expected.

The air going into the gyro is not ambient - there is a pressure regulator (containing a filter) between the gyro supply side and the cabin.

So, air goes from cabin, through the filter, through the gyros, through a pressure relief valve and to vacuum pump. It is between these last two items one should have an in-line filter also. And, as Bob pointed out, it is not part of the "standard" Rapco setup. The relief valve has a small filter on it also, but not an in-line sort.

Bob Axsom · #7 06-01-2008, 04:03 PM

I have the Attitude Indicator out and I'm taking it down to Rudy early in the morning. Even more than the free repair I would like to know why it failed. Hopefully, I will get both. I will let you know.

Bob Axsom

Bob Axsom · #8 06-02-2008, 10:08 AM

I drove down to Rudi Aircraft Instruments in Rudi, Arkansas this morning and i explained the failure to Mike. From the the description he was able to determine that the vacuum was marginal for the instrument and he replaced the rotor free of charge with a "venturi rotor". He decsribed the manufacturer's test process to me which includes marking rotors that start in the "3" range as rotors that will work in low vacuum systems like those that are driven by venturies instead of vacuum pumps as "venturi rotors". I told him that my vacuum gauge reads very low at idle and since I added the inline filter it reads between 4.5 and 5 in cruise. We concurred on the need to adjust the the regulator to get the number up between 5 and 5.5 on the ground. He said that it will drop off toward 4 at 10,000 ft. and emphasized that I should keep an eye on the vacuum pump because it may be weakening. One problem that sometimes happens is when the vacuum pump is replaced the O-ring in the mount interface with the engine is not replaced and it does not reseal well. Oil is allowed to enter the pump and leads to failure. I replaced the vacuum pump when it failed earlier and I did not remove the block containing this O-ring so this could be a weakness. I will reinstall the gyros and adjust the regulator today then check it out but I'm not going to remove the vacuum pump unless this doesn't fix the problem.

Bob Axsom

frankh · #9 06-02-2008, 10:57 AM

I mean messing with these things anymore?...Why not sell all that stuff on Ebay and slap a Dynon D10 in a standard 3 1/8th hole.

Cost aside you might live longer doing IFR!

Frank

Bob Axsom · #10 06-02-2008, 12:12 PM

I'm 71 and I have been flying IFR since 1984 so I will stick with what got me to this point. I'm sure someone will tell you that your choices are not the smartest someday and you can tell them that your Dynon D10 is just great. I somehow doubt that it will be supported as my system has been for all these years. Humor me while I plod along and share with others interested in this system.

Bob Axsom