Air Conditioning Systems for the Turbo Commanders
By Hugh Davis
The air-cycle machine, also called the environmental control unit, installed in all Twin Commanders from the 690 to the end-of-production 695B (late 1000B) models, are an integral part of the airplane�s air-conditioning and pressurizations systems.
All turbine Commander models have some variance of a cooling turbine (air-cycle machine) that receives bleed air from both engines. This bleed air can exceed 650 degrees Fahrenheit and up to 150 psi. The bleed air normally is cooled by a heat exchanger and the pressure reduced to a useable pressure range prior to entering the air-cycle machine.
In the early turbine Commanders (690 to 690B), ram air is introduced to cool the bleed air from an inlet in the upper aft fuselage, and exhausted via an outlet on the lower section of the fuselage. For increased cooling efficiency when the aircraft is on the ground or flying at airspeeds below 140 knots, a ground blower switches on automatically.
In addition to directing conditioned bleed air to the turbine, ambient air is routed via jet pumps or fed directly to the heat exchanger prior to entering the cooling turbine. The cooling turbine exhausts cooler, lower-pressure air that is vented to the mixing chamber (and from there to cabin floor air ducts), and to overhead outlets in the cabin. The overhead air should always be cool to cold. Warm air coming from the overhead outlets generally indicates a failure of the air-cycle machine or the ground blower, which calls for retrieving the Abnormal checklist.
Most 690-series Twin Commanders have a Max Flow switch on the environmental panel. This switch allows additional bleed air to be introduced into the air-cycle machine for an increase in air flow for heating, cooling, and pressurization. There is a limitation in the Airplane Flight Manual (AFM) to not engage the Max Flow switch during takeoff or landing.
The 690A has a similar system called Ground Cool that directs increased bleed-air flow to the air-cycle machine during ground operations. However, there is a limitation in the AFM not to exceed 80 percent rpm. Higher rpm will affect the longevity of the air-cycle machine.
On all models of the Twin Commander, desired cabin temperature (selected using a rheostat on the Environmental control panel) is maintained by directing conditioned bleed air and cool-to-cold air from the air-cycle machine to a mixing chamber. The pilot can directly control the position of the mixing value, and thus cabin temperature, by selecting Override mode on the Environmental control panel, and using the rocker switch to operate the mixing valve.
Several changes in the air-conditioning system were introduced with the introduction of the Sunstrand environmental air-cycling machine in the JetProp models in 1980. The ram air intake on the top of the fuselage and the exhaust on the lower fuselage were removed. Ambient air is drawn via an inlet on the right side of the fuselage and used to pre-cool the bleed air prior to the pressure regulator. Conditioned air is again introduced into the mixing chamber, where warm bleed air and cold air is regulated to the desired temperature selected by the pilot. In addition, recycled air is drawn from the cabin back through the air-cycle turbine where it is cooled again and reintroduced into the cabin via the mixing chamber, therefore increasing efficiency. This is similar to the operation of the recycle switch in your automobile.
On late-model JetProp aircraft, conditioned air is directed to side and eyebrow windows in the cockpit for defogging. In addition, a direct bleed-air duct bypassing the environmental control unit is installed for emergency pressurization. Valves controlled by an aneroid switch operate at 13,000 feet and can provide near normal pressurization.
In addition to the restricted use of Max Flow during takeoff and landing, Max Flow should not be used below Flight Level 200 on the JetProp-series 840 and 980 aircraft. The Max Flow valve increases bleed-air pressure from 21 psi to 37 psi to increase the speed of the air-cycle machine turbine. Due to the limited lubrication of the bearings in the turbine, many service center maintence personnel recommend that Max Flow not be used at all.
Gulfstream Aerospace developed a new Enviro environmental control system based on proven technology to eliminate the problematic Sunstrand air-cycle machine and its short TBO times in the JetProps. In this new system, bleed air is directed to the environmental unit and again is cooled via a heat exchanger. It uses ambient air via a NACA air scoop while airborne and a ground blower similar to the 690 aircraft when on the ground. Without additional cooling, the Enviro unit will deliver conditioned air to the cabin at ambient temperature +15 degrees F when Max Cool is selected on the temperature control knob. For cooling, the system includes a Freon air-conditioning unit. Using a ground power unit, it will pre-cool the cabin before engine start. Operation of the Freon system is limited to 18,000 feet and both generators must be online.
Several STCs have been granted for add-on Freon air-conditioning systems for all models of the Twin Commander that have the original air-cycle machine. The installation of the Enviro unit in the JetProp series, or the inclusion of a Freon unit in your 690-series Commander will increase the comfort level for you and your passengers, and increase the value of your aircraft on the market.
Hugh Davis is an instructor in the Twin Commander program at FlightSafety International�s Houston Hobby Learning Center.
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