I needed a part today for my Mits; a little electro-mechanical mixing valve, a whopping ground and three wires power it. Simple little part. It's common to both Citation and MU2s.

I called a few places and found the following:

- I can get a used part of a scap bird for $600, sent direct to me Fed-Ex, or...

- I can get a refurb from Cessna for...$8,000$...cha-ching!

Cessna can eat my shorts. Everyone has to eat, but that's just ridiculous.

How was your Flt to Florida today Craig

Short. Really f'n short, lol. Thanks for asking Stan.

Started up the motors, got on the runway, put the coals to it, and it started to get HOT. In the Mits, if the mixing valve goes TU, you get whatever the setting it was on when it broke. I was at FL270 yesterday coming back from Chicagoland, so it was set for that kind of environment.

I picked up dad, we flew back to South Jersey, and I started tearing into it. The part will be here by 10:30, so we should be ready to roll a few hours later. Gotta have the A&P sign off on it after inspection and all.

As luck would have it, the woman and her son who were meant to come bailed last minute (he was sick this morning), and dad wants to make Raleigh the final so he can play with the kids, so looks like I won't get to FL for two weeks.

Stupid airplanes. It always takes a few months to a year to work out the squawks.

This is my main reason for not buying another complicated old twin anything. It takes a year to work out the old bugs and the rest of your life working out the new old bugs. You can't win in that game.

You should never ever make a statement like that in aviation

Andrew

True enough, but it's installed none the less. Almost ready for a test run...

You should never ever make a statement like that in aviation

Andrew

That's one of the big reasons we dumped the S/II. It averaged $600/hr in maintenance and we bought any used part we could find.

Beech was getting that way... over $3000 for a ruddervator skin. A sheet of magnesium.

Who knows what it will be like now that Textron owns them. I'm not going to sell my Beech, but I've certainly gotten to know the air salvage people well.

You should never ever make a statement like that in aviation

Andrew

Guys, the part is on, I paid $600ish for it. While I was in there, I saw that there was turbine oil all over the place. Further investigation uncovered a bad o-ring on the reservoir for the ACM; I had less than a 1/4" of fluid left. It's a very fortunate thing the stupid mixing valve died; I don't even want to know what a seized ACM costs to fix, and I would never have known the problem was there if I hadn't any reason to look.

A&P inspected and signed off, I flew it back to Raleigh earlier uneventfully. I got off easy.

The reason I got the used parts so easily and knew what to do is because of Joe Megna at JetAir in Green Bay. The guys at JetAir are top notch, just wanted to pass that along.

Cessna, not so much.

On a tangent, how does an air cycle machine work? Heard they work great but are costly to fix. How is this different than a freon system?

Michael,

Very different from a Freon System. Basically, an ACM is a two stage turbine and compressor run off bleed air. Air is compressed via an axial compressor that is spun by engine bleed air. This has the effect of heating it up and increasing pressure (obviously!). It then is routed through the turbine where expansion occurs and temperature drops. There are also a multiple heat exchangers where outside air is run across the air with a water mist injected...this has the effect of further cooling the air much like a non-freon (or other refrigerant air conditioner). Before being routed to a mixing valve (kind of acts like a TXV valve in a normal air conditioner), the air goes through a water separator to dry it out...and that is the water used in the mister mentioned above.

Advantages of an ACM over a Freon (or other refrigerant-based air conditioner) - they are bullet simple to operate (assuming the air cycle machine turbine/compressor is freely spinning) with basically no maintenance required except an oil change every 600 hours or so. They are very effective at putting out cold air even in fairly warm temperatures (the cold air output is in the mid-30 degree F range...as a matter of fact, it is mixed with warm air to provide the desired temperature air in the cabin. I don't think you get straight "cold ACM air" even if in full-cold mode (except manual cold which forces the mixing valve shut).

Disadvantages of an ACM over a Freon or other refrigerant based air conditioner) - You have to have at least one engine running and pull bleed air off the engine for it to work, so you can't "pre cool" the cabin before engine start (even with a GPU). Also, because they have a high rpm turbine/compressor assembly, they are fairly expensive to replace if the unit fails. Craig really dodged a bullet -- running the unit without oil would almost certainly have led to the turbine seizing and he was looking at an expensive overhaul or looking to buy a unit from a scrapped plane.

Air cycle machine uses air as the refrigerant and does not involve a phase change.

Vapor cycle (what some call "Freon") uses a liquid refrigerant and involves a phase change from liquid to gas and back again.

ACM on Wikipedia:

http://en.wikipedia.org/wiki/Air_cycle_machine

Basically, take hot high pressure bleed air, precool it to outside air first, run it through a turbocharger like compressor to make it even hotter and higher pressure, then cool it to outside air again, then expand it through a turbine (that drives the compressor) and it becomes cooler and lower pressure. Send that through a water vapor separator, then mix cool air with hot air at the desired proportion to set the temperature in the cabin.

The advantage of the ACM is that it is simple and lightweight, no heavy compressor. The moving parts are a turbine/compressor (like a turbocharger) with a mixing valve.

The disadvantage is that the ACM can't run without the engine going (so no precooling on the ramp from a GPU like vapor cycle), and when engines are at idle, doesn't cool as well.

A vapor cycle system doesn't use engine bleed air but instead works like a room air conditioner. The refrigerant is compressed into a hot gas, chilled and condensed into a lesser hot liquid, expanded back into a cold gas, which then cools cabin air, and then back to the compressor. This is the basic mechanical refrigeration cycle common to almost all cooling system you know. The compressor is usually electric, though some airplanes have compressors driven mechanically from the engine, or even via hydraulics (piston twin Cessnas had that commonly).

As for ACM being costly to fix, I don't know. I've never had to work on mine in 8 years other than routine maintenance and a separator filter change. It is basically a small turbine operating at relatively low temperatures, so not much to go wrong with it and should last many thousands of hours. The primary complaints are with the control system and the mixing valve, not with the basic machine itself.

I think the cost is that there are few places that still do that work, though I don't think there is any problem getting them serviced when needed.

Mike C.

I love the ACM system. As soon as an engine gets fired up, it gets cool. Once both are turning, it gets down right cold. The hottest I've seen is OAT of 95F; cooled me off before I could bitch about the heat.

How many ACM's does the mits have?