Vacuum pump gauge

[LZ-WIL]

Hi everyone,

I was wondering, if any one have noticed, or it is just in my C172, that the vacuum pump gauge is almost always reading at the lowest limit of the green "normal" zone. This is of course when the engine is running. I have replaced it once in the hangar, despite it was "green" - same result. My question is, is this reading normal, otherwise the plane flies like a bee no issues at all. Here the plane is in high RPM and climb.

[sahararover]

It's the same on mine, always assumed it was like that on the real one. let's wait for the more knowledgeable to jump in...

[AKar]

A bit of real life stuff, the vacuum pump is a constant displacement vane-type pump that turns with the engine accessory drive. The suction it generates therefore varies with engine rpm. To take this into account, the vacuum regulator valve is used. It basically allows controlled leakage into the vacuum system to maintain the 'suction' at its proper value, nominally in between 4.5 and 5.5 inches of Hg negative gauge pressure at rpm range from 1200 to maximum static. Common vacuum regulator valves are adjustable so they should work well within the tolerance in normal conditions. The kind of indication as in the A2A 172 is likely just a bit offset adjustment.

A leak in the vacuum system or a bad regulator valve can cause low vacuum even with a good pump. The indications for clogged gyro or regulator valve filter is the opposite: a high suction.

-Esa

[LZ-WIL]

Thanks Esa,

That was a very detailed and interesting explanation for me. I was wondering since the gauge indication varies very little with rpm change and tends to stay on the lower limit of the normal operation zone. Since I have never smelled a real C172 from inside, I was guessing how it is with the real thing. That for C172. I have C182, where I've noticed that the vacuum pump gauge has more "live" readings, depending on the engine rpm. I guess it is a different plane, than C172 and comparison is not likely to be made.

--
Will

[AKar]

Will,

There is no principal differences in vacuum systems (or even practical ones!) in between the 172 and 182. The "livelier" reading, as you describe it, may be because of some individual or model-specific differences, but what I suspect it is just maybe reproduced after the glass cockpit 182 A2A apparently used for flight testing - the vacuum system measurement appears to be somewhat different in those. But I'm not sure how they show up in practice, I lack any good time "in field" with these planes.

The vacuum system is extremely simple one, and quite similar in all the common GA airplanes. Except some planes, Aerostar included apparently, which don't use suction but positive pressure to drive the gyros but that's another story.

-Esa

[LZ-WIL]

Hi,

Thanks again for the brisk response and the explanation. I just wanted to hear some other opinions whether it is normal gauge behavior. I guess it is for that model (at least in the sim) as "sahararover" confirmed from his experience. Thanks guys, I can now relax and have a beer (or two, they never come by one) that my A2A Cessna 172R is performing wonderfully.

--
Will

[AKar]

If it was my airplane, I wouldn't sweat about it. I'd just ask the maintenance shop to adjust the vacuum regulator setting a bit at the next scheduled and ask for being allowed to participate the test run.

-Esa

[speedbirda2a]

Hi, I believe A2A have simulated these errors with loss of suction. on the Cessna 182T I believe an amber alert appears on the annunication panel if there's an issue with the external system for suction gauge. You'd then have to use the static Alternate Source switch to regain an accurate reading. I'm sure the experts on this foerum will correct if wrong. Jason

[AKar]

The A2A Cessnas are equipped with dual vacuum system. In case of pump failure, the remaining pump keeps the system fully operational. It takes no pilot action whatsoever, and only a L VAC or VAC R is displayed when a switch senses vacuum being less than 3'' in the respective system.

The alternate static source selection is used instead if the static system is unreliable and blockage is suspected, this would not affect the gyro instruments, which all except the turn coordinator use vacuum system, in any way.

-Esa