Originally Posted by Machinbird
I'm surprised that the aircraft would be certified without some means to recover to a good trim setting. If all you have is "Jack be nimble, Jack be quick, as a defense before it gets away from you, then that isn't too good.
The CRJ trim system, by design, should detect un-commanded motion and shutdown. The problem referred to by some earlier posts is that there was a manufacturing defect at a supplier, and the circuit boards for the horizontal stab trim control unit (HSTCU) did not have the required conformal coating applied to the boards during manufacturing, which left the traces on the circuit boards exposed to the atmosphere. The result was that after many years in service, the traces on the boards could corrode, and the corrosion could bridge adjacent traces, creating circuits that weren't supposed to be there. This resulted effectively in a circuit diagram that differed from the design, creating paths for electricity to flow where it shouldn't, resulting in uncommanded trim motion.
It is well and good to have a perfect design, but you need to ensure the delivered product actually conforms to the design, or all the bright work of the designers is for naught.
This issue was covered by
Transport Canada AD CF-2006-20. This AD, among other things, required the installation of new HSTCU, with conformal coating on the circuit boards. The accident aircraft should have had the new HSTCU installed many years ago, so accident should not be directly related to this old issue.