Two elementary questions: 1) Alternate Air 2) Vacuum

spiderweb

spiderweb

Final Approach
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Ben
Reading the manuals for the C172 and C182, I note that should the air intake (not the air that cools the engine) clog due to whatever (snow, or too much dirt), the alternate air valve will automatically open. How does it do that?

Next, In G1000-equipped Skyhawks and Skylanes, I see there are dual vacuums. (There weren't in the C172N I used to fly.) Are they connected ONLY to the round-dial gauges, one to them and one to the G1000, or what?

Thanks for bearing with me!
 
in my io520, the engine alternate air is provided automatically by a spring loaded door in the airbox (where the intake air filter is housed)
If the normal intake gets blocked with ice, the intake vacuum sucks it open. Should the ice melt the door will spring closed again.
 
Let'sgoflying! said:
in my io520, the engine alternate air is provided automatically by a spring loaded door in the airbox (where the intake air filter is housed)
If the normal intake gets blocked with ice, the intake vacuum sucks it open. Should the ice melt the door will spring closed again.
Click to expand...

That's how I understand it--I just have trouble visualizing it.

BTW I don't think your avatar aircraft has enough engines. What if you lose one?
 
Let'sgoflying! said:
in my io520, the engine alternate air is provided automatically by a spring loaded door in the airbox (where the intake air filter is housed)
If the normal intake gets blocked with ice, the intake vacuum sucks it open. Should the ice melt the door will spring closed again.
Click to expand...

That's how it works in most airplanes that don't require manual effort. Some "auto" boxes have a manual lever as well.

As for the dual vacuum pumps, My guess is they both power the standbys.

I didn't think the G1000-based airplanes had two pumps though. The older "steam" airplanes did. But that's from memory so I'll need to go look at the actual airplane next time I fly it.
 
spiderweb said:
Reading the manuals for the C172 and C182, I note that should the air intake (not the air that cools the engine) clog due to whatever (snow, or too much dirt), the alternate air valve will automatically open.
Click to expand...
For those confused, this refers only to the newer 172/182's with fuel-injected engines. The older carburetor versions have no such capability, and if the air filter on those clogs, you have to pull the carb heat knob manually to restore induction air.
 
Ron Levy said:
For those confused, this refers only to the newer 172/182's with fuel-injected engines. The older carburetor versions have no such capability, and if the air filter on those clogs, you have to pull the carb heat knob manually to restore induction air.
Click to expand...


1964 C205 had the same feature.
 
Ron Levy said:
For those confused, this refers only to the newer 172/182's with fuel-injected engines. The older carburetor versions have no such capability, and if the air filter on those clogs, you have to pull the carb heat knob manually to restore induction air.
Click to expand...

Thanks for the clarification!
 
The 2006 172S we have has G1000 and one vacuum pump. That pump powers the standy attitude indicator.

Dan
 
New G1000 equipped Skyhawks and Skylanes are equipped with a single vacuum pump which provides vacuum suction to the standby attitude indicator. Dual vacuum pumps may be an option, but are not included as standard equipment. Interestingly enough, the C172 NAV III POH states on page 7-28 that the aircraft is equipped with dual vacuum pumps, but the vacuum system description (7-60) and schematic (fig 7-9) specifically state and indicate only one vacuum pump is installed.

Attitude information for the G1000 is produced via AHRS (GRS77/GMU44) and displayed on the PFD (or MFD in reversionary mode).
 
I bet the POH describing two pumps is a carry-over from when the NAV III was a KLN-94 and an MFD. I looked and our brand-new 172S G1000 has only one vacuum pump.
 
TMetzinger said:
I bet the POH describing two pumps is a carry-over from when the NAV III was a KLN-94 and an MFD. I looked and our brand-new 172S G1000 has only one vacuum pump.
Click to expand...

I believe the Bendix King stack including the KLN94 and KMD550 was considered NAV II. I only recall 1 vacuum pump on NAV II models but I would have to double check to say with certainty.
 
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HPNPilot1200 said:
I believe the Bendix King stack including the KLN94 and KMD550 was considered NAV II. I only recall 1 vacuum pump on NAV II models but I would have to double check to say with certainty.
Click to expand...

I have a 2001 model C-172R with the NavII package (KLN94 and KMD550, plus 2-axis AP) and it has the dual vacuum pumps. They are plumbed into a dual-checkvalve arrangement to a common vacuum manifold so that if either of them fails the other one still provides vacuum. The panel has dual indicators for L VAC and R VAC failure. I also have the IO-360 engine upgrade and the alternate air door is spring-loaded, directly behind the K&N air filter, so that if the filter ices up the resulting vacuum opens the door sufficiently to allow air in automatically. And no, I've never tested it! :D
 
HPNPilot1200 said:
I believe the Bendix King stack including the KLN94 and KMD550 was considered NAV II. I only recall 1 vacuum pump on NAV II models but I would have to double check to say with certainty.
Click to expand...
I believe the second vacuum pump was an option for the steam-gauge late-model 172/182's to meet a Part 135 requirement (135.163(h)).
(h) Two independent sources of energy (with means of selecting either) of which at least one is an engine-driven pump or generator, each of which is able to drive all required gyroscopic instruments powered by, or to be powered by, that particular source and installed so that failure of one instrument or source, does not interfere with the energy supply to the
remaining instruments or the other energy source unless, for single-engine aircraft in all cargo operations only, the rate of turn indicator has a source of energy separate from the bank and pitch and direction indicators. For the purpose of this paragraph, for multi-engine aircraft, each engine-driven source of energy must be on a different engine.
Click to expand...
 
HPNPilot1200 said:
. . . . Interestingly enough, the C172 NAV III POH states on page 7-28 that the aircraft is equipped with dual vacuum pumps, but the vacuum system description (7-60) and schematic (fig 7-9) specifically state and indicate only one vacuum pump is installed. . . .
Click to expand...

That's what confused me. I don't have the -actual- POH from the aircraft--just looking at store-bought one.
 
Does anyone have a schematic of the alternate air function? A drawing? A doodle? I'm frustrated by my stupid inability to "see" in my mind how it works.

Thanks.
 
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