Accessory Power – Smoke in Your Cockpit

The accessory plug is an often-overlooked part of your airplane. In most airplanes, this was called the “cigar” plug or “cigarette lighter” plug, before pilots and maintenance personnel started to figure out that smoking was bad for you … and just as bad for your airplane. Eventually we realized that the stuff in the smoke was playing havoc with the interior of the airplane — and learned it wasn’t doing good things to the instruments either. Heaven forbid it should get dragged through them (no pun intended) by the vacuum pump. This is not to mention the toll smoking takes on the human body in terms of the increased risk of cancer and lung disease. But I digress…

In this case, we’re talking about the accessory plug in 24-volt aircraft, and some assumptions that could cause you to experience something that should be a rarity – an IN FLIGHT FIRE OR SMOKE in your cockpit. Here is the setup:

  • The lighter in most airplanes is 12-volts, even in many 24-volt airplanes!
  • The voltage in many 24-volt airplanes is dropped through a resistor, which when properly matched to the lighter plug, sends the right voltage to the lighter plug, which in electrical terms is a pure resistive load.
  • If the impedance of the power cord you are plugging in to your airplane to power your daughter’s boom box doesn’t exactly match the resistance of the original lighter, the voltage the power cord will see will be quite different, and very likely a lot higher than the box is expecting.

Fuses are usually installed in the tips of such accessory plug devices to interrupt the current flow before the wires smoke. However, those fuses are set up for 12 volts, not a 24-volt system. Thus, they may not provide adequate protection.

If the fuse installed is the wrong rating or type, the wires between the boom box and the lighter plug can burst into a smoky mess in a second. WOOF goes the wires, followed by acrid smoke and coughing by the passengers and pilot. If the fuse doesn’t break the circuit, the wires can actually glow red-hot, and start afire anything combustible they come in contact with.

As my friend Bob explained, once you let the smoke out, it’s impossible to get it back in.

While of lesser consequence, whether the wires smoke or not, the voltage surge may blow out the electronics of the boom box, wrecking the 12-volt input or the whole unit, depending on how it is protected internally. Heaven forbid if someone takes out the fuse and replaces it with something conductive — in this case, you are hoping that the aircraft circuit breaker or fuse will protect you!

Even if the electrical device being plugged in to your airplane’s accessory plug is designed for use in aircraft, you still need to check that the voltage rating of the device matches your airplane. While most aircraft electronics will work at any voltage, some are voltage specific, and can be damaged if plugged in to the wrong voltage.

THE BOTTOM LINE: Use caution when plugging items into the “lighter socket” lest it become just that. Make sure the voltage is correct, and if you are flying a plane with a 24-volt electrical system, make sure you know how the voltage is provided to the accessory plug. Only by checking these items before you make the connection will you be sure that your wires and electronics will stay in good condition — and your flights will stay safe from these electrical gremlins!