737 Elevator Power Off Check Tolerances

[CaptainSandL]

What is the reasoning behind the different tolerances for the elevator power off test in the 737?

I had always assumed that the limit was there to ensure that the aircraft would not be too out of trim if you suddenly had a dual hydraulics failure. Then I saw the tolerances for the NG.

So why should the limit vary so much with type of rods (aluminium vs titanium) and series of aircraft? A look through the test schedules & AMM shows the following limits:

Series: Aluminium Rods, Titanium Rods
737-300: +1.0NU/-1.0ND, +2.0NU/-2.0ND
737-4/500: +3.5NU/-2.0ND, +4.5NU/-2.0ND
737-7/8/900: N/A, +14NU/-15ND

PS I would hate to find myself simultaneously without hydraulics and out of trim by 15 turns of the stab trim.

S&L