Stabilizer override switch B 737-3/4/5
Join Date: May 2002
Location: USA
Age: 73
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The logic is really pretty simplistic. (Making a distinction from simple)
It was put there in case the mechanical disconnect switches fail open thus rendering the trim inop. Problem is that certification requirements are written such that once you have the pitch trim shut down they don't want you trying to get it working again (ala Alaska's broken screwjack).
Bottom line is you have a switch which will restore the system to operation in a very narrow set of circumstances but it has the potential to cause a runaway stab in a slightly broader set.
All of the national regulatory agencies agree that they don't want pilots troubleshooting a potentially deadly problem in flight so you don't have the switch in the QRH.
It was put there in case the mechanical disconnect switches fail open thus rendering the trim inop. Problem is that certification requirements are written such that once you have the pitch trim shut down they don't want you trying to get it working again (ala Alaska's broken screwjack).
Bottom line is you have a switch which will restore the system to operation in a very narrow set of circumstances but it has the potential to cause a runaway stab in a slightly broader set.
All of the national regulatory agencies agree that they don't want pilots troubleshooting a potentially deadly problem in flight so you don't have the switch in the QRH.
Join Date: Mar 2003
Location: Europe
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Well actually... my QRH has a checklist for 'Jammed or Restricted Flight Controls' which includes the following: "If electric stabilizer trim is desired, position the Stabilizer Trim Override Switch to OVERRIDE". (The NNC has been company-tailored by Boeing so it might have a different title in your QRH.)
