LED landing lights causing radio interference.
 

We have fitted LED landing lights into a Robin DR400 and find that they interfere with the radio. I believe that is because they have ‘switched mode’ or ‘buck converter’ power supplies integrated that can, if not properly designed, emit radio frequency noise.

Has anybody else experienced this? If so, what was the cure?

Since no make or model of the lights is mentioned, only some generalities:

* consider adding filtering in the supply line - typically a coil in series, and at each side a C to ground; the C's with good HF properties, ceramic might do well. One can buy these as "black boxes", they are sometimes called "pi-filter"s
* Install this/these filter(s) as close as possible to the (rightly!) suspect power switcher(s). It might even be useful to add such a filter at each end of the supply line - that line making a fairly effective transmitter antenna
* the Robin being wood built, check grounding. Ideally, all ground lines go to one central point - some implement that as a strip/bar of brass, others use a bolt that stacks the cable eyes terminating the various ground wires.

Whenever I heard this complaint during last year, it turned out to be a non-certified/non-PMA LED with "sub-optimal" electronics. Which brand is in question?

They were fitted by licensed engineers during an annual, I'll find out the brand.

I don't think the brand is very relevant, though I did hint so - sorry! Except if someone would have the exact same, but these things are so easy to design that there must be dozens of makes and models on the market.

Perhaps - as you guess - the switching regulator is crap; but it is also possible the installation is, err, less than perfect.

Sounds like bad earthing or a bad aerial connection with your radio that's letting in interference.

So many folk just can't properly terminate a BNC - d'oh!

I was at a gliding club the other day and they could only use one of their DR400 tugs as the weather required flying with the LED landing lights on and one of them suffered from similar radio interference - the other did not.

I would concur with the "bad earthing" theory - it could be a well-implemented bad design rather than poor workmanship though, especially in a wooden aeroplane. The switch-mode supplies and the radios need to have their ground-returns "star-pointed" to minimise power-rail-borne interference. It's unfortunately common practice in aircraft installations top "daisy chain" the power cabling; just tapping into the nearest available ground point. This is just about acceptable in a metal airframe because the overall airframe point-to-point impedances are very low, but in a wooden airframe the situation is very different.

Does the ground-return wire from the power supplies go all the way to the main power distribution point (or even the battery bus), or just to a nearby cable for (say) the nav lights?

PDR

The alternative would be to use a linear regulator to drive them.