Prop overspeed. How serious?
Hi all. I was reading the SK197 TSB report. I found it interesting that they only treated the problem half way leading to secondary problems.
I fly the SF34 and our procedures are a little different from the Q400, but I tried to translate their situation into something that could happen in the little Saab. I've been trying to figure out the mechanics behind a prop overspeed.
I don't have the AOM/AFM available to me so I'm unable to read up on the systems, but I've concluded that either the HMU or the PCU needs to be in fault for a Np overspeed to occur. Am I correct? With a HMU fault that drives the Ng to a higher than normal RPM, the prop would (if not correctde by the PCU) go into an overspeed situation. I know our overspeed protection system works by flaming out the engine temporarily until the Np has been restored below the limit value. If the PCU would be in fault, it can only overspeed by setting a finer pitch than commanded and in this case I cannot see any immediate dangers as the torque and overall loads on the system ought to decrease (like using a high gear to use the engine as a break when driving).
Am I completely lost? I'd appreciate it if anyone with experience on the Saab could help me with explaining what happens during a Np overspeed and what the possible causes could be!
Best regards/ LnS