Manufacturers wisely switched to surface mount circuit boards and modern designs that are engineered to draw less current, but radiant heat is the enemy when it comes to longevity. Even though some avionics have internal fans, its the build-up of standing heat that does the expensive damage in hot weather ops. Units packed tightly in the stack heat each other through convection, radiation and by direct conduction. (As an aside, internal fans may not run on backup power. Aspens EFD-series PFD and MFD displays have fans to keep them under their limit of 55 degrees C, but they dont run on the backup battery.)

Transponders, DMEs and autopilot controllers mounted at the bottom of the stack usually generate the most heat. This sets you up for radiated heat often called chimney effect. Avionics cooling fans push hot air away from the equipment chassis out of the front of the stack.

Many avionics units have ⁵/₈-inch diameter air fitting on the mounting rack designed for mating with a cooling hose. One cubic foot per minute of airflow at a pressure equal to .01 inches of water should do the job.

If youre installing the fan yourself, as with a homebuilt, or discussing it with your shop, there are some essentials to keep in mind.

The fan sucks in cabin air, so the area around in the intake should be open. We once found a fan sandwiched on the inside of an interior sidewall of a Cessna P210-a hot environment to begin with. Even with a three-port blower, the full stack of radios got smoking hot. Moving the fan out of the kick-panel made a world of difference. For twin-engine airplanes, the nose baggage area is an excellent location.

As you would expect, the fan wiring is simple: power and ground. But there are divergent schools of thought on whether to wire it to the primary electrical bus or to the avionics bus. On the avionics bus, it comes on when you throw the avionics master. But wiring it to the main bus could offer more protection because it runs immediately at startup, and continues to run during the shutdown sequence. This is the preferred interface, in our view.

Ugly fan wiring can induce motor noise into the audio system. Proper wiring technique is essential, even with high-quality EFI/RMI filtered motors. Youll also want to keep cooling hoses out of the controls, where chafing and binding can occur. Controls can tug on the hoses, pulling then off the cooling ports.

Sandia Aerospace brings heat-alerting front and center with the SAFE line. The 128 (one port), 328 (three port) and 528 (five port) units alert the pilot of impending motor failure so the problem can be addressed without continued operation and subsequent damage. The software in many Garmin G1000 suites can monitor this fault communication output and display the warning as an on-screen message alert.

Those big G1000 displays dont have integral fan systems, so the drill is to direct forced air to the back of the display chassis as many OEMs have done. For non-glass applications, the Sandia SAFE fans can be interfaced with a panel-mounted annunciator for displaying the warning.

For aftermarket retrofit, Garmin markets the three-port GFC314/328 series, which are the ACF-series made by Sandia and move 25 cubic feet of air per minute. These fans use hardy and quiet brushless motors that are rated for 78,000 hours of continuous operation. Many shops prefer the Sandia/Garmin models because they are FAA and PMA approved, eliminating the need for a silly field approval.

The Cyclone series from Lone Star is available in various port configurations and is built to high standards. Designed as a King KA-series cooling fan replacement, the $349, PMA-approved Cyclone 600 pushes 45 CFM of air and has six output ports, a brushless motor and weighs just over 13 ounces. What we like the most about the Cyclone is its flexibility for mounting vertically, horizontally, or at a 45-degree angle.

Lone Star brags about their laser-balanced impeller and high-impact Phenolic housing. The Lone Star units are proven; the Cyclone 21 three-port model has been around since 1990. Its also small, measuring 3 x 3 x 1 inches. For single-radio cooling, Lone Star has a tiny 1 x 2 x 2 model that weighs only 52 grams and costs just over $100.

If you dont have a lot of equipment in the stack but are still concerned about heat affecting a single unit, an axial fan is an option. This is a single-port fan that essentially bolts to an avionics mounting rack. These fans dont need much space, but they leave little room to grow as you add more equipment. Troll used to offer the FN100 single-port Cool Can but discontinued the unit for this reason.

The better choice is a multi-port blower even if you dont use all the outputs. The three-port Troll FN200 Icebox is a proven reliable blower and is specd to move 7 CFM out of each port. If there arent a lot of boxes to cool, many techs will cap one or more of the ports for more push. Troll also makes a five-port model (FN300) for panels that are really loaded. The fans come in 14- and 28-volt flavors, but dont carry official FAA approval. Thats left to the shop to handle. Piper uses a multi-port Troll blower for cooling the equipment in the Meridian turboprop, which says something for perceived quality and reliability.

The AmeriKing AK950-series fans seemingly mimic the Troll design but are PMA- and STC-approved. Available in one-, two-, three- and five-port flavors, the AK950 units use universal mounting schemes for interchangeability. The companys single-port AK951-F1 model is inexpensive, selling for just over $100. We wouldnt bother with the two-port model. The three-port model just makes better sense. For serious cooling, Ameri-King offers a 10-port unit, but we cant imagine having that much scat tubing behind most GA instrument panels.

Modern avionics with surface mount circuit design are a double-edged sword: They allow for a smaller footprint, but this makes them more sensitive to radiant heat in the stack. Dont assume your shop will install a traditional avionics blower during an avionics project. Look for this line item in the proposal and ask what kind of fan, and the number of ports, is being installed. On average, a three-port fan will cost around $600 installed. If you dont have one and are opening up the stack for some other replacement or maintenance, thats the time to put one in.

If your aircraft is equipped with a fan, watch for overly hot avionics or excessive noise from the fan during preflight. Fan failure could mean expensive avionics to follow.

Larry keeps his cool at Exxel Avionics in Hartford, Connecticut.