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

[john_tullamarine]

After discussion with CaptainSandL ... thread re-opened and back to the top. Following is from the second post (Terraplaneblues) in the Flight Test thread ......



Boeing Service Letter 737–SL–27–069–A explains it all.

Here is the gist of it.

-100 -200 aluminium structure elevators & tabs

Classics have composite elevators & tabs - titanium tab rods are better thermally matched to the composite elevator tab structure - results in less tab trailing edge deflection due to variations in temp. (than an aluminium rod set) so the limits were increased as a result.

The other factor classic to NG is mach trim compensation differs for aerodynamic reasons, reflected in type of FCC.

The -600 -700 -800 tolerances are not all the same as each other in the AMM.

Hope this helps