Help Your Avionics Keep Their Cool

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If your avionics are cooled by a ram air scoop (as many are), you're asking for problems and expense. A good cooling fan installation can pay for itself many times over in reduced avionics failures and repairs.

AvionicsIf you walk out across just about any aviation ramp, you'll notice few noticeable changes in the aircraft in the last decade or two. If you open a cowl or two, you also notice the powerplants haven't changed much, either. But look at a few radio stacks and you'll see a dramatic progress. In the last couple of decades, we've gone from the old tube radios to modern digital avionics. We are even starting to see glass-cockpit EFIS systems in high-performance single engine aircraft, including home-builts.

And for good reasons too. The new avionics equipment is lighter, draws less current and, believe it or not, is cheaper! In 1972 an Cessna/ARC nav-comm sold for $3,900.00. Today a King KX-155 with the same configuration sells for around $3,300.00 You know the stellar reliability of the King digital gear, compared with the horror stories related with the old ARC line. In summary, the newer radios are cheaper, more accurate, use less current, and are far more reliable.

However, modern avionics have some installation requirements that must be met, or the equipment will not be reliable. Digital avionics must be kept dry, clean and cool. I can't express enough how important this is. And the best way to keep your avionics dry, clean and cool is with a good cooling fan.

Scoops versus fans

Most aircraft have an outside air scoop that will funnel outside ram air directly into the radio stack. While this system worked okay with older radios, it definitely will cause problems with modern digital avionics. The reason is obvious if you think about it: that outside air scoop can blow dirt and moisture directly into your expensive radios.

Furthermore, when you need cooling the most—on the ground—very little air is coming in from the outside scoop. It is not unusual for avionics equipment to reach 150 degrees F on a sunny day. It really gets hot behind those black panels.

A cooling fan should be installed in such a way that the air will be pulled in from inside the cockpit, not from the outside. The fan should be rigged to blow cool air into each radio, or at least on the outside case. Most modern avionics have a cooling air port to which a cooling hose may be attached. The equipment being cooled does not require a high volume of air. All you need is a small but constant flow of air across the components of the radios.

Hooking it up

The fan will usually mount somewhere under the panel (or in the nose of a twin) and should be wired either to the main buss or avionics buss. Many shops wire the fan to the avionics buss, so when the avionics master is turned on, the fan will then run and start cooling. One argument for this method is that by the time you turn on the avionics master the voltage on the buss is steady, thus the fan will last longer.

However, my preference is to hook the fan to the main buss so the fan is on anytime the battery master is on. I personally like to get cooling air flowing across the avionics even before the avionics master is turned on. True, the fan will draw some current before the aircraft is started but it's such a tiny draw that it shouldn't be a problem. And since the cooling fan is a DC motor, the voltage swing during start won't really bother it.

A good cooling fan motor is not your run-of-the-mill motor. It is a special RFI/EMI-tested motor that is heavily filtered and shielded to keep noise out of your radios, so you won't see waving VOR needles or a LORAN that loses lock. Believe me, a cheap cooling fan will cause havoc with your nav-comms, ADF, LORAN, stereo or moving map. Why would you spend money on top-of-the-line equipment only to have it fail because of a $15.00 motor.

What does it cost?

Quality cooling fans don't come cheap. A good one starts about $175.00, and the best (from Bendix/King) is around $500.00. In addition, installation will run anywhere from four to twelve hours, depending on the type of avionics you have and how hard things are to get to. The installing agency will need to add some type of circuit protection, too.

Cooling helps a lot with older aircraft, and especially with pressurized aircraft. We've found that by adding cooling in Cessna P210s, we could triple the reliability of the ARC factory radios.

In early of 1995, central California (where I work and live) had extraordinary amounts of rain. This caused massive amounts of damage to many local aircraft whose avionics were exposed to the moisture ingested during flight through the outside scoop that was used for cooling. Our shop saw $36,000.00 of moisture-related avionics damage, and in every case the aircraft was using outside cooling and did not have a inside cooling fan.

The bottom line

Have a high-quality cooling fan installed. Don't use ram air cooling. Your modern avionics biggest enemies are dirt, moisture and heat. Keep these enemies away and the reliability of your avionics will be greatly improved. It's not unusual nowadays to install an modern digital radio stack and the owner fly for ten years or more without a radio problem. Most of this is because of the small little fan under your panel that you never see. I feel so strongly about this that I will not install an avionics package without a cooling fan. A good fan is without doubt the best insurance you can buy to protect your avionics investment.