“Failed Path-A data path has failed”. That is a very vague message that basically means data isn’t flowing from point A to point B. It doesn’t signify what type of data, the significance of that data, or the source of the failed path. It is a message that if it were to happen to us on our phone, tablet, or computer, we would very likely ignore it because of its vagueness. I had that message on a flight yesterday, and while I looked at it, I didn’t do anything about it. Why you may ask? Because there is no checklist for it, and there is nothing that I or anybody could do during flight to restore the data path, partially because we don’t know what path has failed. The message presented itself in the same place as other advisory messages (sometimes referred to as nuisance messages) like “airspace ahead”, or “inside airspace.” We get those two messages constantly on every flight, so we acknowledge the message button so it will stop flashing at us.
These type of messages are not what we call a CAS message and does not have a checklist associated with it. CAS stands for Crew Alerting System and it is the system that alerts us to malfunctions or failures such as “flap malfunction”, “generator failure”, “battery discharge”, “battery offline”, etc. but the CAS system can also send advisory messages. On the Phenom 300, we get a CAS message when we turn on our ice protection systems, and we expect it. There is a checklist for every CAS message and they can be as simple as saying “Crew Awareness” (turning on the ice protection is an example of when the checklist would say Crew Awareness) or they can two to three pages long.
Why all this talk about messages? Because as it turned out, the data path that failed was to a computer that controls our multi-function spoilers and our pitch trim. Here is what happened.
I saw the Failed Path message and pretty much wrote it off. At the time we were descending about 2,000FPM (feet per minute) and a couple of minutes after the Failed Path message, I start getting this gut feeling of “something’s not right”. I scan the flight instruments and we’re about nose and level and we’re descending about 600 FPM, but the flight director is still commanding 2,000 but the autopilot servo isn’t responding. Hmm, something is going on. At this point I start scanning the rest of our instruments and I notice yellow Xs on the spoiler and pitch trim indicators, which means their position can’t be determined by the system. The autopilot isn’t pitching the nose down for the descent, so I turn the autopilot off and start hand-flying the airplane and realize that the plane is way out of trim for level flight. Partially out of habit and also to troubleshoot the issue I try to the use the pitch trim switch on the yoke and nothing happens. After discussing the situation, we decide to treat it as a normal pitch trim fail and turn on the backup pitch trim. When we activate the backup trim system, we are supposed to get a PTRIM NML FAIL CAS message, but we don’t get this CAS message and the yellow Xs don’t go away. What do I do now? I fly the plane, fighting against the nearly full nose down trim the whole time.
How out of trim was the airplane? If an airplane is trimmed properly, you can fly it with your fingertips, but I was having to use both hands to hold the nose level. I comment that it feels like we are doing a sim training session, because this is the type of malfunction and abnormality we train for in the sim. Adding to this feeling of doing a sim session is the fact we are flying at night, in the clouds and rain, and going to an airport with a low cloud ceiling of about 800 feet and a gusty crosswind. Yeah, I have my hands full at this point. We shoot the instrument approach and land without incident, though we did get a “SWPS FAULT” CAS message. (This is the stall warning and protection system which is completely unrelated to the pitch trim system except that it is fed information from one of computers that controls the pitch trim) Needless to say we contact the company and as I write this they are in the process of replacing that computer.
We start talking about why the backup pitch trim wouldn’t work, when it uses a different computer and the only conclusion we could come to is that because the system couldn’t determine the position of the trim that the backup computer basically said “I don’t know where the trim is or what it’s doing, so I’m not gonna let you use it”. This is pure speculation on our part but is the only logical conclusion we can come to. We power the plane back up and the yellow Xs are gone, but the SWPS FAULT CAS message is still there. A system reboot works on airplanes too. I know a lot of people don’t like to hear this, but it is a machine with a lot of components and those components start acting up. Even though the backup system didn’t work, the airplane was still flyable and as I said in my Aircraft Training post, “We practice so this stuff becomes second nature so that if something happens in the real world, we can respond to the situation calmly and efficiently and without over reacting.”