Air Conditioning on a 172?
By Greg Napert
July / August 1998
ant to surprise a pilot next time you see them stepping out of a Cessna 172 with sweat dripping off their forehead? Ask them if they want you to install an air conditioning system.
Although it's been tried before, a system has finally been developed for installation at the Cessna factory for new order airplanes, or as an STC'd kit for installation by any certified technician.
According to Mike Fitch, sales manager for Keith Products, the Cessna 172 air conditioning installation weighs 58 lbs and shifts the center of gravity aft about one inch. With this little of an effect on the aircraft, there is no change in the aircraft weight and balance limitations or significant performance degradation in the airplane. "Obviously," says Fitch, "you have to decrease the amount of baggage you can carry by what the evaporator weighs, which is about 18 lbs. This is because the compressor, mounted out in front by the engine, balances out a portion of the installation weight and the resultant change is approximately 18 pounds."
The air conditioning installation is fully integrated. Fitch says the company ran engine tests with the compressor installed. Additionally, there is no cowling modification needed to accommodate the compressor.
The only change in appearance to the aircraft is a condenser exhaust cutout and a condenser cooling air intake towards the rear of the aircraft.
The interior modifications include ductwork along the floor; a small reduction in space in the baggage compartment (about 1 1/2 cubic feet), to accommodate the evaporator and evaporator blower; and a few changes to the pedestal to accommodate cooling ductwork and two air conditioning outlets. The evaporator in the baggage compartment is designed with a cover that allows you to stack baggage right on it, and to protect the evaporator coil and cooling fan. There is no thermostat in this system; it is either "on" or "off," Fitch says. "The reason for that is the addition of a thermostat just increases the installation time, the kit complexity, and the kit cost, and we're trying to keep the price down to where the owner operator can have the entire system installed in the $10,000 to $11,000 range.
Fitch says the system works exceptionally well in the 172 as there is a relatively small amount of space to cool. "It immediately starts blowing air that is about 30 degrees lower than the ambient air temperature," he explains. "Even with the engine at idle, the unit maintains a 30 degree drop in temperature at the outlet." Additionally, as the unit continues to run, it recirculates the cabin air to continue to drop the temperature. "In fact, you typically have to turn the unit off after about 30 minutes even at very hot temperatures," Fitch says.
In operation, Fitch explains that the unit cools you down as you taxi down the runway — then you turn the unit off before take off, which disengages the clutch on the compressor; roll down the runway and take off, and, as you're on climb out and firmly established in the climb, then you can re-engage the air conditioning.
Fitch comments, "Typically, one of the first questions we get asked regarding this installation is "Where is the scoop?" That's because many people who associate air conditioning with general aviation aircraft, remember the days when some manufacturers would have a scoop that lowered down into the air stream which allowed air to flow over the condenser coil. Well, that's fine and good, except that when you lower a scoop into the airstream, you're adding quite a bit of drag to the airplane, to the extent that you would lose five to seven knots of your cruise speed. That is not a good situation, particularly on climb out, when performance is marginal on many small aircraft.