Back in my heavy iron airline aviation days in the avionics world, I tested and worked on the aircraft’s FDR (Flight Data Recorder). The FDR employed today with its massive storage capabilities is a far cry from what I used to work on that only recorded four parameters scribed on a metal foil. The news media likes to call the FDR one of the “black boxes”, even though they have never been black. They’ve always been orange with a reflective tape strip. They also have an ULB (Underwater Locator Beacon) which emits an ultrasonic ping whenever it senses underwater operations. Everything is built to survive a crash and be found for accident investigative work.
Your SR22 post s/n 2749 and SR20 post s/n 1885 were produced with a light duty version of an FDR called an RDM (Recoverable Data Module). It’s a small, nondescript metal box, mounted in the tail of the plane. There’s no orange paint, no reflective tape, and no ULB. The data stored is supposedly the property of Cirrus. You may not have known such a thing exists or even what it does. This would be good as it’s an indication that you’ve not had a significant incident or accident in your Cirrus and don’t know someone who has.
We’ve had a couple of failures come through our shop recently with pilots reporting symptoms like a failed USB port (connected to the RDM for data download), red X fuel level indications, red X TKS level indications, and even simultaneous red X of all the engine displays. The displayed failure might even change from one flight to another. Oddly enough, the fault can be related to the RDM. It all depends on which RDM connector pins are affected and whether it’s a high resistance or a bridge between pins. Flight vibrations add to the random mix as well.
Examples of how corrosion affected the pins and sockets of the electrical connector which caused the random red Xs
What we’ve come to know about the RDM is that it isn’t well isolated electronically from the source that sends all the flight data to it. Depending on which flavor avionics package your Cirrus is equipped with, flight data is sent from the Data Transfer Unit (Avidyne) or GIA 1 (Garmin) via a RS232 data bus to the RDM. The failure mode isn’t the RDM directly, but rather the electrical connector to the RDM in the back of the plane. Apparently, the connector sits unprotected in an area where TKS fluid likes to hide out. TKS fluid happens to be very corrosive to the pins and sockets inside the connector.
The electrical connector to the RDM located in the back of the airplane.
RDM corroded by TKS fluid
As the corrosion grows it can cause high resistance between pins and sockets or create a new path for electrons that didn’t previously exist. The random nature of the connection failure also creates seemingly random failure presentations to the pilot.
There is hope, though. If you are experiencing strange red X displays or are concerned about the possibility, Cirrus has a fix. It’s simple if caught early. One just cleans the connectors and fills them with a potting compound. At the time of this writing, there isn’t a published bulletin, so you’ll want to ask your trusted Cirrus authorized maintenance provider to contact tech support to get the data for the fix. In the world of general aviation preventive maintenance, this one is very inexpensive, minimally invasive and very effective. At a minimum, I’d highly recommend making sure your annual inspection check list includes opening this connector for a good look. Let’s be safe out there and prevent those unwanted surprises.
© Paul New 2025
Recent Comments