Some more info on the Thermawing (Rebranded Evade de-ice solution) install (actually more about the conversion to the electric A/C) that I got a while back (from Peter Wilkinson, Kelly during fly-in, CAM, etc.) but got busy with other things and failed to post. All of this is posted in the Wiki too in case people find it easier to read.


For starters, the "Kelly A/C" does not exist. It is called as such simply because the modification is done under Kelly's STC. However, all components are actually built by third parties. The Kelly STC is to convert an existing mechanical system to an electrical system (M2E) or an original equipment (OE) CAM electrical system to a reconfigured electrical system (E2E). The fundamental purpose of this STC is to configure the aircraft to accept the Thermawing de-ice solution and retain airconditioning. A by- product of the STC is to improve the reliability of the OE electrical system, reduce weight and enable Cessna to provide continued support of the electrical a/c system. The details for the E2E conversion can be found in SNL09-11- Electric Air Conditioning System Modification

The "mechanical A/C" is made by Seamech. This includes the engine-driven compressor, compressor controller, condenser coil and ECS subsystem are all made by Seamech. Seamech manufacture the refridgerant hoses which are comprised of a special to type inner hose to prevent refridgerant leakage at altitude and an external firesleeve to prevent heat damage. Note, the hoses should not be manufactured in the field using auto hose material or Aeroquip/Stratoflex materials. Cessna issued SB 09-21-01 on December 14 2009 to address the issue of the refigerant hose routing in the engine bay.

For the "electric A/C", the compressor and compressor controller are replaced with parts made by Masterflux. The model is Sierra. Peter didn't mention the part no but based on the power needs, BTU rating and coolant used, part no is SIERRA03-0716Y3. 11300BTU/h at 27A draw. Aside from the Compressor and Compressor Controller the remaining parts of the system are the same as the mechanical system.

Seamech does not publish specs online so no idea on the ratings. As per Cessna (via Steve's post), Seamech compressor is rated at 15000BTU. The compressor coil is rated at 26000BTU but this is a no-op because the electric A/C uses the same part too (ie. will keep the Seamech-built coil and only replace the compressor with the Masterflux one).

How things work

This .pdf file gives a description of the refigeration cycle, the vapour cycle system components (VCS) and pictures of the components in the mechanical system, together with an air flow schematic.diagram.

A quick walk through the setup. The compressor does the hard work of compressing the coolant and expanding it adiabatically which is what does the heat transfer. Cold air goes in the cabin (well, duh), hot air goes to the compressor coil where it gets cooled back down by ambient air pulled in by the big fans under the luggage compartment. In hindsight, it makes all the sense in the world that:

  • the BTU capacity of the condenser coil needs to net exceed the capacity of the compressor (you need to be able to remove at least as much heat as the compressor adds) and
  • the overall BTU capacity of the system as a whole is given by the smallest of compressor_BTU and coil_BTU (the system can only extract as much heat as the weakest link)


Now, the mechanic compressor is driven directly by the engine (duh). The electric draw as seen on the instruments is a bit fake as a result because it only shows the draw for the condenser coil and not the compressor itself (which still takes power from the engine of course). The compressor moves refrigerant (R134a) back and forth between the compressor and the evaporator coil under the luggage compartment so ... the mechanic A/C actually comes with very long coolant pipes going through the firewall and cross the plane length.

As you can tell, this is not actually a great mechanical setup. Long and exposed pipes will eventually leak and sure enough, there are many reports of the A/C losing coolant over time (mine was down to 20% for example). I suspect (but don't know for sure) that the location of the compressor actually robs some cooling power from the system too because the cold coolant (from compressor) goes by both the engine and the hot avionics so some of the A/C capacity is likely to get lost cooling ... the engine. The routing of the pipes avoids the hottest parts but still.

Now on to the electric A/C. The compressor in this case needs a power source too and this source is the newly added Kelly alternator which is installed where the old compressor used to live (back right side of the engine). The electric A/C will show the same draw from the coil fans plus extra draw from the compressor itself (dunno how much but see the 27A rating above and compare with about 14-16A for the mechanic version). This is net same amount of power extracted from the engine as for the mechanical A/C except this time it will be fully visible on the Amp indicator in the G1000.

Electrical wires replace the refrigerant pipes in this case for a more compact install. As a result, I would expect the electric A/C to be actually more reliable in the long run from the coolant standpoint because the hoses are compact and living in an area that is substantially more protected (easier to seal, no heat from the engine, etc). The electric A/C is more like a standard fridge basically. All parts together. The pipes carrying the cold air to the cabin remain the same of course.

Of course, this still leaves the compressor reliability itself as unknown but my take is that this should not be an issue. Masterflux is actually the company behind many of the automotive A/C systems which due to volume, are likely tested like hell by now. Ford and BMW for example use versions of the same Masterflux Sierra compressor that we are getting in the electric A/C with the only difference being the power drawn, capacities, etc. By contrast, Seamech is an aviation-only A/C manufacturer. Lots of deliveries for RV10s, Lancair, etc. so probably solid reliability too. Just not at same scale.

Weight estimates. The E2E conversion accomplished by the Kelly Aerospace STC will result in a net weight reduction of approximately 20lbs on an aircraft without Thermawing installed. This is because the third alternator and associated bracketry will be removed along with the interlock box and the upgraded 100 amp alternator installed on the front right drive is lighter than the current configuration.

The weight delta for the M2E conversion has not yet been calculated since the conversion would only be done to facilitate the installation of the Thermawing de-ice system. In practice, the aircraft will have the mechanical compressor replaced with an alternator to drive the Thermawing de ice system and the front right alternator will be upgraded with a lighter 100 amp unit. The refridgerant hoses from the compressor to the firewall will be removed. A Masterflux compressor and compressor controller card will be installed together with shorter length refridgerant hoses.

I had some hope the electric A/C will be lighter but now I kinda doubt it. The electric A/C adds a motor too (that powers the compressor. The mechanical compressor is powered by the engine directly) so seems more likely the weight of the A/C is going to be a wash.


Some more info about the alternator(s). This affects Evade upgrades, new Cessna planes and the electric A/C retrofits (from CAM style to new electric A/C) as listed in this diagram:

Engine driven ALT
Belt ALT
Rear ALT
electric A/C only (new)
Upgraded to 100A Kelly ALT
A/C + Evade
Upgraded to 100A Kelly ALT
Evade ALT
CAM -> Cessna electric A/C conversion
Upgraded to 100A Kelly ALT

Basically, regardless of combo, all (new-style) electric A/Cs will be powered through the bus and from the 2nd ALT (belt-driven). The old style was powered from a dual-use Evade/AC alternator via an "interloc box" which is where the problems came from. As per Cessna, they have no parts in stock for this component, no supplier and they believe a redesign would be more expensive than a convert so ... they are doing the convert instead.

For the new electric A/C, the A/C is now powered through the plane bus whether Evade is present or not (same for planes with TKS). When Evade is added, the third alternator now powers Evade exclusively and over dedicated circuits (ie. not through the plane bus). IMO, a much better setup. The heavy (and bursty) power user is now isolated from the sensitive avionics. Whether it works or not, the rest of the plane won't be affected which is good because the avionics are far more expensive.


Bottom line (for whoever could still follow this diatribe so far), the electric A/C solution seems rock solid. It's a simpler setup so likely at least as reliable if not more. The compressor is smaller (so you do lose some cooling power) but some of this should be compensated by the loses inherent in the mechanical setup (the cooling the engine part). I don't expect more than 500-1000BTUs to be lost from this though so we are still looking at an electric A/C that is about 2500BTUs smaller than the mechanical one.


  1. SB 09-21-01 Refigerant hose routing.
  2. SNL09-11 Electric Air Conditioning System Modification
  3. A/C conversion STC