The International Cessna^® 170 Association

...for lack of a quicker description. Anyone have an explanation for vacuum gauges rising and falling?
I am searching past posts for someone else's experience.
Half way to Colony Glacier from Merrill Field, the vacuum instruments slowy rose, then quickly fell. Airspeed rose to 100, then fell to 80, sometimes fell to 90.
VSI rose & fell also; or maybe it fell, then rose. Suction gauge stayed constant, but higher than normal.
Its been ~25 hrs since the belt-driven vacumm pump was removed and venturis installed this winter on our 53 170B. No problems until Saturday morning's sunny & smooth flight. Only change since the previous flight was our first big rain of the year occured.
I'll take it into Merrill Field Instruments this week (their schedule allowing), but I'm just curious until they look at it.
I discovered an accurate airspeed indicator on final is more reassuring than my uncalibrated rump.
Thanks
Dwayne

VSI, altimeter, airspeed indicator are not part of a vacume system. They have nothing to do with your venturi system.
Do not pass go.

Dwayne,

Mod Cessna is correct. The vac system only runs vac gyros. The instruments you refer to are operated by the pitot/static system(ASI pitot and static, Alt and VSI static only). Sounds to me like some moisture found its way into your static system causing erronious readings. With a blocked static system changes in altitude, pressure or temp will cause the error to vary. If there is sufficient water in there, any 'sloshing around' say from turbulence will also cause the error to vary quickly. If only partially blocked the error will vary as above but will slowly right itself. A giveaway will be if you get moisture condensing on the inside of your ASI glass although this won't always be the case. A few warm days should fix it or else you can have your mechanic blow out your static line with compressed air.

Hope this helps,

Noel

Thanks for spending the time to decipher & answer my extremely inept description.
After instrument folks study it this week, I’ll hear a separate explanation why the DG didn’t stay on heading for more than two seconds. Could be part of the higher than normal suction displayed on the gauge? I didn’t study the artificial horizon to remember its attitude or personal issues. I believe the vacuum side’s issue is coincidental, but not directly related to the altimeter’s rise & fall.
Tomorrow I’ll look at how rain passes through the cover and enters the pitot, static, or elsewhere. I’d really like the rain to drain out the pitot hole like it’s supposed to. I’ll let instrument guys look at the vacuum system tho. They gave me the old filter after vacuum filter change during venture install. For the sake of curiosity, I’ll see if the old one has water marks.

deleted by the author

Dwane,
As Miles pointed out the problem may be with your vacuum filter.....But what you indicated is more or less your regulator.....(if you have one), this part has a stainless steel screen that is open to the cockpit, when the screen gets plugged you will get a higher than normal reading on your vacuum gauge, and actually more suction thru the instruments....something you don't want.... you might check this out......Brad

Thank you Brad. For the answer. And taking for granted I know the difference betwixt the systems.
Gotta love a patient professional when listening to a bumbling layman. Reading between the lines of a bad explanation for the true gist of the problem.
Now I have two good answers for two different systems. The second one is worth waiting until the shop solves the vacuum problem.
From Noel's answer, it sounds like water may eventually evaporate/leave/blow out the static system and not create damage, but a disfunctional vacuum system (from whatever cause) may damage other items on a subsequent flight.
I'm anxious to hear from insrument folks why both systems acted up at the same time - the coincidence.
Dwayne

db99577 wrote:...Gotta love a patient professional when listening to a bumbling layman. ...

Many here have learned patience since we have both laymen rocket-scientist and Vespa hi-way-rocket test-pilot members.

I have never had any symptoms of trouble with either the pitot or static systems on either of my two Vespa GT 200 Motor Scooters. The speedomters on both scooters, however, are wildly optimistic, being about twenty per cent high at a cruising speed of about sixty MPH.