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

[Terraplaneblues]

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

[CaptainSandL]

Hi Terra,

It does help thanks, I was not aware of that SL.

It explains the thermal matching difference between the two types of rods and the improved mach trim schedule on the 4/500’s, which is presumably better again on the NG’s.

But surely the big issue here is achieving a “predictable pitch upset during the in–flight hydraulic power–on to power–off transition” (Boeings terminology), ie reducing the out of trim condition that you have in the event of a total hydraulic failure.

Leaving aside the oh-my-god scenario above, getting an old aluminium rod 737-300 through an elevator power off test is almost impossible because the limits are so tight. Yet the newer aircraft with titanium rods always do much better and they give us broader tolerances. It all seems the wrong way around.

On a practical level, I would have thought that 1.5 turns was neither here nor there and not worthy of a re-fly and 15 turns would be excessive to the point of uncontrollability but this can be a pass. Go figure???

S&L