Pressure Decay Test
You may have to complete a Pressure Decay Test (PDT) in accordance with Airworthiness Directive 96-20-07 if you are at the 100-hour interval and you still have the B1500, B2030, B3040, or B4050 series heaters. A Combustion Differential Pressure Switch test will also have to be accomplished. The AD's PDT requirement does not apply to the Janitrol's Extended Life B-series heaters (B2500, B3500, and B4500 series) which have the CermaKote® ceramic coating used to improve the combustion tube's life to a TBO of 1,500 hours. Additionally, the 94E42 series of combustion pressure switches alleviates the repetitive testing requirement of the AD.
The PDT is used to evaluate the condition of the combustion tube. The test ensures that there are no excessive leaks between the combustion and ventilation sections of the heater. This test relates not only to safe operation of the heater but, more importantly, to the safety of the cabin occupants. What you will hear from the people who have been in the heater business for a long time, is that the AD 96-20-07 PDT (and the original AD 82-07-03) requirement should have applied to all South Wind and Janitrol combustion heaters. The FAA issues Airworthiness Directives to provide corrective action to a condition that has been identified, but does not mandate routine maintenance. And that is the real point after all. The PDT is routine maintenance for all combustion heaters. The test is diagnostic in nature and can be performed without removing the heater from the aircraft.
Carefully remove coking to assure
nozzle functions property.
A damaged tip must be replaced.
After returning the heater system to its original configuration, you move to the power-on checks. External power should be used to preclude excessive drain on the battery or misleading indications caused by a weak battery. Follow the appropriate aircraft operator's manual or the maintenance manual for this check.
Apply power to the heater system by engaging the "Master On" switch. The "Heater Out" light should illuminate. Check the operation of the blower assemblies. Look for unusual vibrations and stuck or rubbing fan wheels. Check for leaks in the combustion inlet line by using a soap and water spray. Check the airflow at the exhaust and cabin vents. If the airflow feels inadequate or slower than normal, check your ductwork connections for tight fittings and obstructions and take another look at your blower motors. Check the motors' current draw; excessive current draw is a sign of a weak motor. If you did not look at the brushes before, look at them now. Verify proper voltage at the motor as well. Remember, airflows and, therefore the blower motors, are critical to the heater's performance.
Next, you energize the "Heater Start Switch." The "Heater Out Light" should go out and the "Heater Running" light should illuminate. The heater will "light off" and continue to operate. While the heater is running, take a look at the exhaust for evidence of smoke. There should be little, if any, smoke at the exhaust. If there is, remember we are talking primarily about either a combustion air or fuel problem. The problem could be coking on the nozzle. The nozzle can be removed and cleaned, but be careful not to damage the nozzle tip. Do not use a wire or wire brush to clean the nozzle face or orifice. The nozzle is critical to the heater's proper operation. Any damage to the nozzle's orifice is grounds to replace the nozzle - it is not repairable.
Follow the manual for shutdown of the heater and note that the indicator lights operate correctly. Allow the blowers to continue to run for a couple of minutes. This cool down helps to prevent metal stresses and purge the combustion chamber of any remaining exhaust or fuel.
Always look for the interaction of the four separate sub-systems (ventilation blower, combustion blower, fuel system, ignition system) in relation to the heater's performance. The manual's troubleshooting steps begins with checking for power to the system. Use external power and make sure the master switch is on and has continuity. Then, with just a multi-meter and the wiring diagram, most problems can be pinpointed quickly.
For example, the heater fails to light. If there is power available there may be a shortage of airflow from the combustion blower. Check for duct leaks, fan operation and current draw on the motor. There could be an open in the circuit at either the combustion pressure switch, cycling switch, duct switch, or overheat switch. These can be checked using a multi-meter. The fuel system possible causes include insufficient fuel pressure (check the pump for proper operation and sufficient power), faulty regulator, fuel solenoid inoperative, or a restriction in the fuel nozzle, fuel lines, or the fuel filter. The only thing left to check is the operability of the ignition unit. The heater needs combustion air, proper fuel pressures, and good ignition to start. Those three elements must remain constant and ventilation air must be present for sustained operations.
If the heater starts and then goes out, the problem could be related to either any of the thermal switches (cycling, overheat, combustion pressure, or duct). Or, it could be caused by either low voltage (use external power) or lack of fuel at heater. It could also be a lack of vent air. If the heater has sufficient combustion air and fuel, plus spark, the combustion process will begin. If the vent blower is inoperative, then the heater will build up heat so quickly and trip either the cycling switch or the overheat switch. The overheat switch has a manual reset, but it is not recommended to keep resetting the switch. The problem must be resolved and the overheat switch should be replaced.
If the heater burns unsteadily, the potential solutions are incorrect fuel pressure, sporadic operation of the ignition unit, faulty spark plug, fluctuating combustion airflows, and either a loose or restricted fuel nozzle. If the heater fails to shut off, the culprit is either a fuel solenoid stuck in the open position, an inoperative duct and cycling switch, or a defective master switch. Problems with either of the blower motors are approached the same way. Power to the motor (switch on, circuit breaker closed, external power, and continuity) is first. Overly worn brushes, defective radio noise filter, or burned out motors result in a loss of the blower assembly. The last two potential causes of blower motor difficulty are jammed blower wheels or a bad ground connection.
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