FROM THE SHOP FLOOR: PRESSURIZATION CABIN
AND SYSTEM CHECKS AND TROUBLESHOOTING
This is another in our continuing series of Twin Commander care and maintenance tips provided by highly experienced technicians at Twin Commander factory authorized service centers. This issue’s story is authored by Steven Reid of Legacy Aviation Services.
Cabin pressurization checks in a hangar are accomplished with the use of a pressurization cart (machine) that is attached to the aircraft at the air cycle machine in the tail section. Regulated air is connected to the cabin door pressure line in the baggage compartment area to inflate the door seal. With the cabin controller outflow port disconnected, an adjustable orifice is installed to control the pressurization rate. The airframe is now ready to pressurize.
With a technician inside, the “Door Lock Switch” activated, and other technicians outside the cabin, the checks and tests begin. The cabin is pressurized while technicians feel and listen for leaks. All airframes will have small leaks that are mostly inconsequential; it is the larger ones that cause an aircraft to not reach the Max Cabin Differential.
From within the cabin, lighting an incense stick is a great way to track the hard-to-find leaks. Just follow the smoke, as it will head for the leak. From the outside of the aircraft, technicians feel about the cabin with their wrists and the backs of their hands to detects leaks. As many owners and service centers know, often it is the “boots and roots” areas that are in need of repair. Rudder pedals and wing roots are commonly the culprits of a weak cabin pressure problem. New and improved boot kits are recommended to solve these problematic areas.
For shop pressure cart equipment with an amperage gauge, a reading of more the 28 amps may be an indication that a turbine section of the air cycle machine should be checked. This indication is likely caused by failing, or frozen, turbine bearings, resulting in rotation restriction.
If an aircraft passes the hangar checks and tests but will not pressurize satisfactorily while in flight, then other components need to be checked. Discrepancies may involve items such as the bleed air valves, the 717 valve, the cabin controller, outflow valves, or various associated components. Systems vary depending on the model.
On Fairchild-equipped models, a temp squawk during ground operations is often a result of a ground blower failure. The ground blower is not running to cool the heat exchange unit and the system goes into “full hot” mode.
Also on Fairchild-equipped models, an in-flight cabin temp squawk is often a result of failure of the lower-duct airspeed switch that controls the cabin temp.
A suggestion for owners and operators: Perform checks of the ground blower using the hidden “under the panel” maintenance switch. This switch is always left in the “On” position. If an amperage drop indication in the overhead panel occurs when the switch is turned to “Off” in flight, this suggests that the ground blower is running during flight. This is likely the result of failure of the upper-duct airspeed switch (actuated @ 140 KIAS). Damage to the brushes and commutator are likely to occur. The ground blower should not activate over 140 KIAS if the upper-duct airspeed switch is functioning properly.
The hidden maintenance switch may be left in the “Off” position in flight in a failed airspeed switch scenario; just be sure to put it back in the “On” position once under 140 knots. Also, having a ground blower light installed in the panel is not a bad idea.
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