Vacuum pump or other failures?

[motoadve]

Since we need to turn off failures in Prepar 3d for ther 172 A2A to work realistically, are other failures modeled apart from the ones that show in maintenance hangar?

Can the vacuum pump fail? what else can fail apart from thos maintenance items (which really cover most of the plane items)

Failures are very realistic with this 172, I got a magneto failure between Catalina island and the coast and to be able to run smooth I had to shut one magneto off, great stuff

[Scott - A2A]

Simulated vacuum failure system behavior is one of the last areas we plan to overhaul, at some future point. On one hand it may seem easy, but doing it properly is far from simple.

Scott.

[motoadve]

The interesting part is the gauge has to be simulated and also many times the failure is slow, the horizon starts tilting slowly which is tricky in IFR if you are not monitoring the gauge. Then you can follow the horizon and spiral the plane.

[AKar]

Such a feature (combined with perhaps not so über accurate instruments that FSX/P3D has per default) would make IFR flying rather interesting. I'm in.

The interesting part is the gauge has to be simulated and also many times the failure is slow, the horizon starts tilting slowly which is tricky in IFR if you are not monitoring the gauge. Then you can follow the horizon and spiral the plane.

Because of this, I think that low vacuum warning is very important feature in any IFR airplane using vacuum gyros. This, along with dual vacuum, makes 172R clearly the IFR platform of choice from the two that are simulated so far.

The issue with vacuum systems is that the vacuum pump itself is inherently unreliable. While they've got rather short replacement interval (or, as in Rapco's pumps, one of wear measurement), they withstand no contaminants whatsoever, and may be damaged during handling even before they are installed. When the pump fails, it is usually an instant event. They have a shear coupling for the very purpose to 'disconnect' the pump from the engine in a controlled manner when it fails so that no damage is caused to the accessory drive. Obstructions in gyro filter or an issue with regulator may cause more gradual loss of vacuum.

I understand that the good old vacuum pumps that were lubricated by engine oil were much "better" than the current dry carbon vane pumps, but I don't think I've ever even seen one - they're rare nowadays.

-Esa

[motoadve]

I had a vacuum pump failed, last couple of flights the gyro needed to be re adjusted all the time, was my first plane and new to me, vacuum gauge showed in the green still, then while taxiing one day the horizon tumbled and no vacuum.

They replaced and it failed in an hr( installation is critical and easy to damage it)

[AKar]

last couple of flights the gyro needed to be re adjusted all the time [...] vacuum gauge showed in the green still

That makes an interesting condition, almost sounds like combination of somewhat erroneous vacuum indication (they're not even supposed to be that accurate, but more like indicative), and of a pump problem. Not sure what would cause a "partial pump failure" - due to their design (*), they are rather binary in their working. If the regulator and the filters were found satisfactory, my guessing would be that the pump issue was caused by a leaking line or similar - that kind of problem could also lead to a premature pump failure and be rectified unnoticed as a side effect during the pump change. But just guessing.

They replaced and it failed in an hr( installation is critical and easy to damage it)

An absolutely important point that all operators should be aware of. If I had a plane with dual vacuum system, with vacuum pumps having a typical 500 hour replacement interval, if at all practicable I'd schedule another pump to be replaced 100 hour before the other (e.g. n+450 hrs; n+550 hrs .... n+950 hrs; n+1050 hrs .... and so on) for that very reason. It's not necessarily the installer who damages it - they are easily damaged during the shipping too. Also, if the graphite dust/debris from the failed pump was left behind in the system, it could damage the new pump enough to cause it to fail.

One additional point. The vacuum pump's rpm and therefore it's effect depends on the engine rpm. However, the system is designed so that the pump provides necessary suction for the instruments to operate even at idle rpm. If there ever was a low vacuum indication or symptoms of such on ground that go away when the rpm is increased, it would make a regulator or filter problem an immediate suspect.


-Esa


(*) The pump's idea is very nicely demonstrated in this photograph. Note the six vanes, which are made of graphite quite like the stuff used in pencil. During pump's operation, the vanes are held against the outer wall centrifugally. As the rotor turns with the engine, each successive two vanes form a chamber with the outer wall, a volume of which varies as the thing rotates due to the shape of the outer wall. With appropriately positioned inlets and outlets, the assembly turns into a pump that sucks air in and discharges it under pressure to/from that chamber. Suction line comes through the vacuum gyro instruments and a pressure regulator that allows necessary 'leakage' into the line to account for the varying engine & pump rpm so that pressure differential over the instruments (="vacuum") remains where it should be. The positive pressure, or discharge side is used for de-ice boots in small planes so equipped. The vanes you see are extremely brittle, and very often if any of them shatters due to any reason, its remains break all the other vanes as well.

[Nick - A2A]

Interesting stuff! My experience of vacuum pumps has been limited to the rather robust laboratory variety. Although the ones I use (a type of diaphragm pump) aren't particularly powerful they do seem to survive a lot of abuse on small boats including inadvertently sucking-up seawater now-and-again.

After reading Esa's comments, I found this AVweb article: Why Vacuum Pumps Fail. It does seem to suggest that the modern 'dry' vacuum pumps aren't necessarily such an improvement on the previous generation of 'wet' ones. ('Wet' in this case not meaning full of seawater though!)

For someone with little real-world experience of aviation, it's quite good to learn about these more prosaic aspects of flying. (Not that I'm suggesting a vacuum-pump failure is boring, just that the mechanics of why they fail is something I hadn't even considered until now.)

Cheers,
Nick

[AKar]

Nick,

Yep, that's a good article. I'm not sure why the wet pumps were retired originally and replaced with carbon vane pumps. In most airplanes the discharge pressure is "wasted"; it's not used for anything. The small(?) amount of oil that gets blown overboard in a lubricated pump installation shouldn't be of a problem, I think. In those that employ de-icing boots, 'clean' discharge air is a benefit, of course. Perhaps there are other reasons I haven't thought of. The article mentions pressure gyros (that use positive "pressure" instead of "vacuum" suction to drive them). I've got no knowledge whatsoever about those. I wonder why such a system was made up anyway.

it's quite good to learn about these more prosaic aspects of flying. (Not that I'm suggesting a vacuum-pump failure is boring, just that the mechanics of why they fail is something I hadn't even considered until now.)

Personally, I find it to be the most interesting side of aviation - how the things are put together, how they're supposed to work and how/why they don't. Designs and implementations vary between airplanes enough to fill hours and hours of studying of even very simple GA airplanes.

-Esa