Keith Air in 421

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    Hey Guys,

    Does anyone have the Keith Air system installed in their 421? I’m seriously thinking about getting one installed but would like to hear anyone’s experience before taking the plunge. Is it hooked up to the battery bus and can it be run on the ground only using GPU power?

    Thanks in advance!

    Best Regards,


    My airplane still has the factory unit, but I have flown a 421 with Keith Air. It seemed to work very well, as does the factory unit. The two bonuses are that you can plug in a GPU and pre-cool the airplane and it makes room for a 2nd nacelle tank if you wished.


    I did the big plunge a couple of years ago, installed teh Keith Air. It used to be that I started the right engine first, then turned on the A/C before I started the left engine. Now, I have to start both engines before I can turn on the A/C – one alternator can not run the system.

    The old system might cool more, but Keith Air has so far worked on every flight.

    If you give me a call on my cell, I can fill you in on all the details: 480 363 4668


    421 guys with Keith or JB air,

    DO NOT RUN on ONE alternator even for a couple of minutes with A/C on!. The load on the alternator, heat generated internally on alternator and the stress on the drive are bad news. Use with an APU o.k.; disconnect run circulate until both alternators are on line the residual cold will work in all but extreme conditions.

    The Keith system does not have the efficiency of the factory hydraulic system. Think about it, electric motor turning a compressor living in the same small space, HEAT. While I like it in humid places where a/c is need once you sit in plane, the hyd. system is easier to maintain, is more cost effective and does not shake the nose rivets loose. Over time this can and will happen just a matter of how much the a/c is used. Think about an electric motor spinning and drawing 50-70 amps to spin a hyd compressor is going to have vibrations. It is engineered much better to use a hyd. motor to drive the compressor than an electric motor by many times.

    The later Keith systems are working better than earlier versions, both get cold air to the pilot/co-pilot first, but the temp of the Cessna system wins in my experience hands down


    Some small details on the hydraulic system. The amount of power from the hydraulic motor is limited based on the pressure (regulated) and flow rate (determined by the pump – but also limited). You cannot service it like a car system (where the A/C compressor is belt driven and will just keep increasing pressure if overfilled). Getting exactly the right amount of refrigerant into the system is critical. Too little or too much and the cooling will be poor. Too much does not respond like a car would – so it is very easy to overfill by just watching gauges. By weight is the only way to go.

    Cessna also changed the expansion valve on the evaporators from a bulb system to a fixed pressure spring. Wasn’t clear in the SB, but I expect the bulb systems were not performing well as they go older.

    A hydraulic mule can be used to ground test the Cessna system and this is preferable to using the engine as the prop drives a hurricane blast even at idle 🙂


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