737-800 Electric Trim Puzzle

[little-paddy]

Hi all,
Came across a strange on the other morning and can't explain it.

Scenario:

First flight of day, early AM, Cold aircraft on GPU.

Electric trip on Control column will trim back but not forward.

Electric Hydraulic pumps off.

Switch on Electric hyd pumps and bingo trims both directions.




Land at destination and try to simulate same scenario but trim fwd and back works perfectly.



Using the FCOM and Cockpit Companion, still don't know why. Surely trim on control column is electrically driven and Hydraulics should have no bearing?

Confused L-P!

[Flight Detent]

Hi LP,

I'm curious too!

Did you try both sides select switches?

After the initial fix, did you turn off the hyds and try again?

Did you try OVERRIDE selection?

Sounds like a switching problem at the outset!

Cheers...FD...:bored:

[ampclamp]

Was the trim near either of its limits?

Hyd should have nothing directly to do with it.
Only things I can think of are a sticky limit switch in the stab compartment that perhaps was given a jolt by the elevators losing / getting hyd pressure or you had a column cutout switch stuck.These are under the floor in the fwd e&e compartment.

[little-paddy]

Hey Guys,

No to both questions. Six units of trim were set. I manually wound it nose down a unit or two to loosen it but it did nothing until I put on the Pumps. I put them both on together so not sure was it A or B that cleared it.

Landed at other side after pondering it in the cruise and tried to recreate with the GPU and pumps off and the trim worked fine in both directions.

I guess there are just some things on a cold Airplane that cant be explained.

L-P

[FCS Explorer]

ok folks, don't make me pull out the books so early in the morning, but think about this:
you can't trim forward/nose down while pulling back on the control wheel.
and when the hyds is off the stick is usually leaning way towards the seat. thats why you could trim backwards but not the other direction.
pretty simple, huh?

[Flight Detent]

...and the OVERRIDE selection would have picked that up!

That's what it's for!

Cheers...FD...:)

[av8r76]

I wasn't there but maybe you missed the position of the control column. The Column might have been in a pitch up position hence not allowing nose down trim. Switching on either elec hyd pump brings control column to neutral position hence allowing trim both ways.
Even a negligible pitch in any direction inhibits trim in the opposite direction. When it happened to me the first time, completely threw me off until the tech guy told me about the feature.

Try it next time. Cheers.

[ampclamp]

Ok I think I misunderstood your first post


First flight of day, early AM, Cold aircraft on GPU.

Electric trip on Control column will trim back but not forward.

Electric Hydraulic pumps off.

Switch on Electric hyd pumps and bingo trims both directions. I thought you initially had the pumps on then off.

If starting from a cold ship no hyd pressure the column may well have been displaced therefore inhibiting trim in one direction via the trim cutout switches under the cockpit floor I mentioned before.
It only takes approx 4 degrees of opposite elevator to actuate the cutout.Pressurising the hyd will centre the column and deactuate the switch.

I am confident that was the "problem".

[little-paddy]

Yup, that could well be the case. Never thought of that. Plus for sure, putting off the pumps trying to recreate the scenario would have left the control column centered hence not recreating the same actual situation.

I knew the smart folks on here would have they answer:ok:

Thanks alot guys 'n' dolls.

Safe flying,

L-P

[ewolfman]

Hi,
I've also experienced the same a while back, and it seems to be on the 700 and 800.
Strange isn't it.

[little-paddy]

Hey Guys,

Tried it out today on a cold airplane and it seems to be as explained above. Unless the control column is centered the opposite sense trim will not work (Including when the pumps are on and you manually push the column). Makes sense when ya think about it........it was just the thinking about it that got me!!

Happy landings,

L-P

[ampclamp]

well done paddy.best try it out.
I'm a little surprsed it is not covered in your training.
Having a runaway stab and knowing how it is stopped is quite important.

If you wish to run the trim without pressurising just give the column a wiggle either side of neutral and it should centre and deactivate any cutout situation.

[Denti]

Agreed, seeing for oneself how it works is allways the best. That said im equally surprised it isn't covered during the initial training anymore. During my typerating it was a mandatory topic, however that was 10 years ago, training might have slimmed down even more since then. With the new generation of MPL "pilots" we do see some marked differences in daily operation and some of them are, well, questionable.

[Checkboard]

It's part of the trim system check for the daily inspection - and I used to have to do it as part of the initial start, along with a host of other checks (standby power switching, mach overspeed etc etc). In the last 8 or 9 years, though, this has been left to the engineers and pilots have been actively discouraged from this level of systems understanding.