If you walk out across just about any aviation ramp, you’ll notice few noticeablechanges in the aircraft in the last decade or two. If you open a cowl or two, you alsonotice the powerplants haven’t changed much, either. But look at a few radio stacks andyou’ll see a dramatic progress. In the last couple of decades, we’ve gone from the oldtube radios to modern digital avionics. We are even starting to see glass-cockpit EFISsystems in high-performance single engine aircraft, including home-builts.
And for good reasons too. The new avionics equipment is lighter, draws less currentand, 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 thestellar reliability of the King digital gear, compared with the horror stories relatedwith the old ARC line. In summary, the newer radios are cheaper, more accurate, use lesscurrent, and are far more reliable.
However, modern avionics have some installation requirements that must be met, or theequipment will not be reliable. Digital avionics must be kept dry, clean and cool. I can’texpress enough how important this is. And the best way to keep your avionics dry, cleanand cool is with a good cooling fan.
Scoops versus fans
Most aircraft have an outside air scoop that will funnel outside ram air directly intothe radio stack. While this system worked okay with older radios, it definitely will causeproblems with modern digital avionics. The reason is obvious if you think about it: thatoutside 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 comingin from the outside scoop. It is not unusual for avionics equipment to reach 150 degrees Fon 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 frominside the cockpit, not from the outside. The fan should be rigged to blow cool air intoeach radio, or at least on the outside case. Most modern avionics have a cooling air portto which a cooling hose may be attached. The equipment being cooled does not require ahigh volume of air. All you need is a small but constant flow of air across the componentsof the radios.
Hooking it up
The fan will usually mount somewhere under the panel (or in the nose of a twin) andshould be wired either to the main buss or avionics buss. Many shops wire the fan to theavionics buss, so when the avionics master is turned on, the fan will then run and startcooling. One argument for this method is that by the time you turn on the avionics masterthe 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 thebattery master is on. I personally like to get cooling air flowing across the avionicseven before the avionics master is turned on. True, the fan will draw some current beforethe aircraft is started but it’s such a tiny draw that it shouldn’t be a problem. Andsince the cooling fan is a DC motor, the voltage swing during start won’t really botherit.
A good cooling fan motor is not your run-of-the-mill motor. It is a specialRFI/EMI-tested motor that is heavily filtered and shielded to keep noise out of yourradios, so you won’t see waving VOR needles or a LORAN that loses lock. Believe me, acheap 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 fromfour to twelve hours, depending on the type of avionics you have and how hard things areto 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 theARC factory radios.
In early of 1995, central California (where I work and live) had extraordinary amountsof rain. This caused massive amounts of damage to many local aircraft whose avionics wereexposed to the moisture ingested during flight through the outside scoop that was used forcooling. Our shop saw $36,000.00 of moisture-related avionics damage, and in every casethe 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 modernavionics biggest enemies are dirt, moisture and heat. Keep these enemies away and thereliability of your avionics will be greatly improved. It’s not unusual nowadays toinstall an modern digital radio stack and the owner fly for ten years or more without aradio problem. Most of this is because of the small little fan under your panel that younever see. I feel so strongly about this that I will not install an avionics packagewithout a cooling fan. A good fan is without doubt the best insurance you can buy toprotect your avionics investment.