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Engineering, Maintenance, Certification Bits & Pieces

This is a first in a series of articles on aircraft combustion heaters. Right now in the hot summer months you should be paying attention to the combustion heater. It hasn’t been used since that last winter or spring flight when the air was just too nippy for comfort. It’s now sitting in a very harsh environment – vibration, fuel, oil, and of course heat. This article will go over the Combustion Heater Blower Motor; other articles will follow addressing the other components. I sincerely hope you find this article interesting and informative.

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

It' 104F in Phoenix - What Combustion Heater, In My Aircraft?

Well, yes. If you own and operate any number of Piper, Cessna, Beech, Pilatus or Canadair and other aircraft you have a combustion heater. You may not have thought of your combustion heater for a while, but it is there, sitting and waiting for cooler temperatures. Maybe you should take an afternoon and give some thought to your combustion heater.

Combustion heaters consist of a heater assembly, a ventilation air blower, a combustion air blower, a nozzle holder, solenoid valve assembly, spark plug, high-voltage ignition system, various switches (air pressure, over-heat and cycling), and RFI suppression components. All these components operate together to provide that warm air you so much look forward to and need during cold weather operations. But, what happens when one of the team-members starts to lag, to slow down, and to skip a beat. The warm air gets a bit cooler, get a bit less forceful, and could ultimately stop all together. Not good if you are IFR, at 20,000 ft and the OAT is minus a bunch of degrees. Actually this situation could cause the pilot to become distracted and endanger safe and continued flight. Needless to say - you want your combustion heater to be operational in all cases.

What can you do to assure with the greatest probability that your combustion heater will operate properly throughout the winter months? Attention to the details, an understanding of what each component does and that each component has a finite operational life. Failure modes of each component are different and are exhibited in different ways. A blower motor RPM may begin to slow down due to bearing wear or contamination, but it won't necessarily stop dead in it tracks – right away. An high-voltage igniter may start to loose energy and then quit suddenly.

An understanding of what each component does will help you understand how the heater works and how it fails. With this knowledge you can perform maintenance checks and prevent that one very cold flight in the dead of winter you are not planning for.

Combustion Air Blower

This unit is a 12 or 24-volt motor with a squirrel-cage type vane assembly attached to one end of the motor shaft. The form of the metal exterior covering the squirrel-cage type vane assembly is designed to pull in fresh air and then the rotating squirrel-cage type vane assembly accelerates the air into the combustion chamber. The function of the combustion air blower is to provide the air (oxygen) necessary to support the combustion of the fuel, which then generates the heat that is ultimately transferred to the ventilation airflow. If you are performing maintenance in the area of the combustion air blower you should take a few minutes and check out the following.


Combustion air blower wiring should be inspected for chaffing, excessive bends, cracks, and loose connections. Remember, it is those electrons that make the combustion air motor go around. If the electrons find an easier path to ground, they don't get to the motor. Check out all power and ground connections, make sure they are secure, clean, and all wire crimps are good ones. Wire crimps with strands of copper sticking out are no good! Replace them. Wiring that has rubbed against airframe structure should be inspected – but if you see copper the wire needs to be repaired. Using your AMM or AC 43.13-1B, determine if the wire can be covered by an anti-chaffing material, or if the wire has to be repaired by a splicing. Since the combustion blower motor is exposed to water, fuel, oil and other fluids the splice should be an environmental type splice. This type of splice assures that the wire connection is not degraded over time by fluids, dirt and grime. Make sure the connections of the radio noise suppression device, which is in series with the combustion blower motor, are good connections.

Look at is how the power and ground wiring is routed. If any wiring has chaffed, check out where the closest wire clamps are. Are they too far apart? Is the wire excessively long between the clamps? Chaffing could be a result of several things acting singularly or in concert. The wire clamps may have loosened over time and the wiring has slipped. Check the tightness of all wire clamps. A modification may have been performed that moved the wiring. Wire length between clamps is very well explained in AC 43.13-1B, follow the guidelines there and chaffing will be minimized or avoided. An anti-chaffing covering could be used to prevent direct wire to structure contact. If the anti-chaff over cover is not porous, you will have to make sure that fluid drain holes are placed in the proper location. A drain hole is placed at the point where the wiring sag is maximum, usually right in the middle of the wiring clamps. (Remember that excessive sag is to be avoided – again refer to AC 43.13-1B). This drain hole is necessary to avoid the anti-chaff covering from filling up with fluids. Over time the anti chaffing covering can wear through and most likely in areas that don’t see the light of day. The wiring under the anti-chaff covering may then become damaged. Mirrors and flashlights and a few yoga-type maneuvers will help you find these problems.

Connections secured with nuts and lock washers can back off due to time and vibration and should be checked for adequate tightness. Don’t reuse lock washers that have been installed for years, they have weakened and won’t provide the necessary spring to properly function. Replace these lock washers with approved parts for your aircraft. Check your AMM for torque values for these nut connections, and make sure you never exceed these torques settings. If you cannot find torque settings in the AMM or AC 43.13-1B, use good judgment when tightening these connections. If you over tighten the nut you run the risk of actually twisting the stud or screw and causing internal damage, ultimately resulting in a failure from all the future hours of vibration.

Use AC 43.13-1B as a guide for proper wiring practices, or use your manufacturer Aircraft Maintenance Manual (AMM). Good wiring practices will ensure many hours of safe and trouble free combustion heater operation.

Attachment Fasteners

The combustion blower motor is attached to something and what attaches it to that something are fasteners. Vibration is the worst environmental hazard these fasteners have. Over time the fasteners can loosen and back off, or even the screws could crack and fail. A loose combustion blower motor could very well cause the ducting to the combustion heater to separate. A total loss of the combustion air results in the heater flame going out, a reduced combustion airflow will cause the heater to burn rich. Check all combustion blower motor fasteners for security and tightness. Make sure the threads of the screws or studs are not crossed or damaged. Check the attachment nuts for rounded flats. The lock washers, if used, can loose their spring. All attachment hardware should be inspected when the combustion motor assembly is removed for maintenance. Replace all damaged hardware with approved hardware for your specific aircraft. During reassembly, follow your AMM torque requirements, making sure the screw or stud threads are free of grease or other contaminants.

Brushes and Bearing

Are your combustion motor brushes and bearings replaceable? The trend towards throwaway motors has many pros and cons. Of course, if your mechanic cannot change the brushes or bearings you have to buy a new or reconditioned blower motor. How much motor life are you throwing away anyway – think safe operation first and then dollars? What are the costs and benefits associated with a throw away motor versus the costs of replacement parts and mechanic repair time? Personally, I would want to replace brushes and bearings for the simple reason that the other blower motor components could inspected and determined to be in good condition. By disassembling the combustion blower motor a mechanic would be able to check for wear and damage of the other parts. The wear can tell a mechanic volumes about how that motor has been operated on that specific installation. If the electrical components are not damaged by heat or by other intruding material (grit, metal shavings, etc.) why throw them away when they could be replaced with new brushes and bearings and extend the useful life of the combustion blower motor. The squirrel-cage type vane assembly, the metal exterior covers, the rotor, the wiring the attachment studs and fasteners could all be in condition that allows for additional safe operation. All those aside, follow your AMM and make sure you have a combustion blower that will make it through the next cold winter.

Bottom Line

In my experience wiring problems have to be the most probable culprit for failures. Give the wiring a good once over. Check clamps for pinched wiring, check wire for chaffing, and check terminals for tightness and damage. All of these checks will pay off next winter. Check all the mechanical fasteners for looseness and damage. Check the ducting between the combustion blower assembly and the heater assembly – any leaks, loose connections? Fix them.

Follow the AMM and run the combustion motor separately, listen to the motor. Can you feel vibration if you can reach the blower motor case? Listen to the sound the combustion motor makes, does it change pitch, does it change volume, or is it steady? All these things are what the combustion blower motor is “telling” you about how it “feels’. Mechanics provide “health care” to all the systems on the aircraft, so you must know what each “patient” is saying about its’ “health”. Read the OEM data for your combustion heater, it offers diagrams, parts lists, and most of all descriptions of healthy and not-so-healthy operations. Learn what to listen for and look for during proper and not-so-proper operations.

Now is the time to give your combustion heater a bit of time. It’s a lot better to spend a hour every once in a while during the blazing heat of summer making sure your heater is in good condition, than to divert on that first cold flight because the heater has failed due to a loose electrical wire or a failed bolt or nut.

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