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This is the third in a series of articles on aircraft combustion heaters. This months article will address cabin heater inspections.

By Jim Davidson, President & FAA DER
Davidson Engineering Resources, Inc.
Tucson, Arizona
August 2004

Cabin Heater Maintenance

NOTE – Your Aircraft Maintenance Manual (AMM) should be followed at all times. The following article is for reference only and is not presented as a replacement for OEM maintenance procedures.

Daily or Preflight Inspections

The ventilation air inlet, combustion air inlet, heater exhaust and fuel drains need to be inspected for obstructions and for abnormal conditions.

Abnormal conditions would be excessive soot accumulations in the area of the heater exhaust or fuel dripping from the fuel drain. The excessive soot accumulations can mean the heater is burning “rich”. Burning “rich” can be caused by a restriction in the flow of combustion air into the heater, a damaged or clogged fuel nozzle, or a problem with the ignition system.

Do a visual check of the cabin heater installation. Look for damaged or chaffed wiring and loose electrical connections. Check out the switches in the cockpit – are they loose, wobbly, or seem not to move as expected.

Every time you take a few moments to check the condition of your cabin heater you are assuring yourself a safer and more comfortable “next” flight.

Operational Checks

An operational ground check can help uncover problems with the cabin heater before you are air borne. The following procedure represents one possible of many cabin heater configurations. Again, refer to and follow your Aircraft Maintenance Manual (AMM) for the ground check specific for your aircraft.

Place the MASTER SWITCH to the ON position (or HEAT position). The ventilation air blower and the combustion air blower should operate. If equipped, the HEATER FAILURE light should illuminate.

While the ventilation air and combustion blowers are operating, listen for abnormal motor sounds and feel for abnormal vibrations. In addition, observe your amp gauge for abnormal current draw by the two blower motors. If either the motors are operating in an abnormal fashion, reference your AMM for repair procedures.

100-Hour Inspections

Refer to your cabin heater manufacturer inspection procedures for the inspection necessary and required for your type of cabin heater. Hours refer to heater operation hours and not aircraft operation hours. Some cabin heater installations have an hour meter installed; if your installation does not have an hour meter installed, refer to your cabin heater manufacturer documentation for the definition of “heater hours”.

Inspect the ventilation air and combustion air inlets and cabin heater exhaust outlet for obstructions, attachment security, and for any distortions.

Inspect the fuel drain line to verify that there are no obstructions. If an obstruction is evident, follow the AMM procedures for clearing the obstruction.

Check all fuel line connections for leakage, fuel discoloration, or for damage. You have seen leaky fuel lines; there is an accumulation of dirt and grime in the area of the leak. Also the leak area has a certain color, light red for example. Check all fuel lines for tightness and security. Do not over tighten fuel lines as you can twist and damage the fuel lines. By over tightening, you can put a twist pre-load on the tube that will eventually fail – worst case during one of your future flights. Follow the AMM instructions for proper tool usage – such as the use of two fuel line wrenches and torque wrenches when tightening the fuel lines. Also follow your AMM for installation of anti-leak tapes and compounds.

Inspect all the electrical connections for security, wire damage, terminal lug damage, and for chaffing. Many cabin heater installations make us of a terminal strip, check that the terminal lugs are properly installed; that no terminal lugs have stray strands of wire sticking out, and make sure that all wiring is properly secured. Another item to look for is wire stress. Wring should not be routed to or from the terminal strip at severe or tight angles. Wiring installed at these tight angles is stressed and can eventually break. In addition, the wire insulation on the outside of the tight angle is stretched and the insulation on the inside of the tight angle is compressed, both situations will eventually cause the insulation to fail. Failed insulation can lead to arcing and poor cabin heater performance.

The high voltage cables need to be inspected for damage and chaffing. Very high voltage is capable of arcing through damaged insulation, which causes cabin heater system problems. The excessive soot around the cabin heater exhaust could be caused by such a problem. A tell tale sign of arcing through the high voltage cable would be a discoloration or darkening in a spot on the cable. Also inspect the high voltage connections to assure they are secure. I remember working on an old Camaro when I was a kid, the engine was running rough and I was trying to find out why. A friend of mine asked if I had replaced the spark plug wires. Being a kid on a budget – (Hey Dad, can I borrow a few bucks), I hadn’t spent the money on new spark plug wires. He said to run the engine in the dark and look for blue sparks. I did. They were all over the place. Needless to say, I saved my money (thanks Dad), and replaced the spark plug wires as well as the coil lead. The car ran pretty good after that.

Inspect the combustion air blower attachment fasteners for security and for damage. Also verify that all tubing and wiring attachments are secure. Tighten any loose connections you find per the AMM.

For Information on Aircraft Electrical Design Certification:

Contact: Jim Davidson, President and FAA DER
Davidson Engineering Resources, Inc.
Phone (520) 977-9824
Fax (520) 232-3660

Davidson Engineering Resources, Inc.      Phone (520) 977-9824      Fax (520) 546-4242     

Email jimdavidson(..at..)davidson-der(..dot..)com