Boeing 737 trim wheels

[Johnny Tightlips]

Hi all,
I am in the middle of a 737NG type rating at the moment. Just as I was starting to get comfortable in my seat today in the sim I got a smack off the trim wheel, the first one so far-and it hurt! Afterwards I was thinking why are the trim wheels so big and what exactly are they connected to? I would love to see a proper maintenance manual drawing or a good picture of the whole trim wheel system to see what lurks underneath the control pedestal. If anyone can help me out I would be very grateful.

Thanks in advance.

[Desk Jockey]

The Boeing 737 Technical Site

Hope it works.

Fold the handle back next time!

[Johnny Tightlips]

Yes the 737 technical site is excellent and I have been using it with a while and during the type rating. However the section on trim is nearly identical to the FCOM which does not give much detailed info. I am looking more for a proper technical drawing from a maintenance manual so I can see the actual "workings" of it.

[Desk Jockey]

From memory because I don't have access to maint manuals anymore and the loft was groaning under the weight of course notes. The mechanical trim wheel turns a chain which under the pedestal translates to a cable which eventually turns a drum connected to the gearbox which is connected to the screwjack which is connected to the front of the stabiliser and moves the stabiliser up and down. Many a happy night shift winding this up and down doing config warning checks! I had a look for a diagram for you but all gone now I'm afraid. DJ.

[Desk Jockey]

Sorry minor side step - Wasn't it re calibration (maintanence miss-rigging)of the trim wheel settings that led to the violent aerobatics on the Ezy 737 test flight over the east coast last year? 19000ft a min decent at one point!

More likely a manual reversion test. Done after maint on the elevator syst. Turn flt ctl power off. The a/c should continue to fly level. If it doesn't log how much trim is needed and it gets adjusted again. It's not mis rigging it's a rigging check. Adjustment is often needed.

[Johnny Tightlips]

Thanks Desk Jockey that's the answer I was looking for. So I presume it's the chain under the pedestal that makes the "clicking" sound when the trim wheels turn? If anybody else can find a diagram of the system it would be great.

[mvsb1863]

Hey JOHNNY TIGHTLIPS

Try this Throttle Quadrant

[Tee Emm]

Just as I was starting to get comfortable in my seat today in the sim I got a smack off the trim wheel, the first one so far-and it hurt!

For the very reason you painfully discovered, and ever since I first flew the 737 which was in 1977, it was considered good manners and SOP for the pilot who was about to adjust the stab trim setting during before start checks, to warn the other pilot to keep his hands and knees clear of the stab trim wheel on his side. . This was particularly relevant when the flap lever was out of the up position where the trim moves faster and especially if the trim handle was extended and you can't see it from the other side. . The warning usually consisted of a verbal warning and the placing of the back of your hand against the other chaps knee to encourage him to get his knee out of the way. Just be careful though, if the other pilot is female or bats for the other side, Your kind gesture could be misconstrued!

[rmm]

Here's a video of the whole thing in action.

Some of the noise you hear is actually from the rear screwjack transfered thru the cables.
The rear unit has a clutch and brake mechanism which is quite noisy.

Aircraft trim system, 737-200 | Free Educational & How-To Videos - Watch Educational & How-To Videos Online | Veoh

[ampclamp]

Have not lived until having part of yor knee removed by that thing.
hope the flaps were up. damn thing flies around with flaps not up.

why so big? leverage.
it would be hard work doing it manually without that circumference.
plenty of turns though!
and yes a big splined shaft joins the 2 wheels its a chain drive to a cable drum.

That I suggest is as close as you want to get.Bugger of a place to work.

for one of the other posters, remembering the ng has the one trim motor not two like the classics.

[Tinwacker]

Clicking

Johnny Tightlips:

So I presume it's the chain under the pedestal that makes the "clicking" sound when the trim wheels turn?

Going back to similar old designs it's not the chain making the noise it is a mechanical clicker or clacker like a peg on a bicycle wheel spoke that makes the noise to indicate trim wheel rotation.

TW

[CAT1 REVERSION]

I vaguely remember on my TR something about the two wheels being manufactured by two completely different companies!!!!!????? Mad I know, but I'm sure that's what we were told.....

Also something about the F/O's wheel being the master wheel and should be used to set trim as it is more accurate if the two wheels are slightly out????!!!!!!

Again, can't say this is 100% correct so I stand corrected if someone knows different!

[Johnny Tightlips]

Thanks everyone, some really good stuff there. As for the difference in the trim wheels we were told to set the trim from the captains side as that's the calibrated one and I have also heard the story about the two trim wheels being made by two different manufactures.

[ampclamp]

both wheels operate off a common spline and the limit s'/w's (on the dirty big machine down the back) are common.I have not heard the story but sounds like a fairytale to me.

[Swedish Steve]

Going back to similar old designs it's not the chain making the noise it is a mechanical clicker or clacker like a peg on a bicycle wheel spoke that makes the noise to indicate trim wheel rotation.

Under the pedestal is a brake arrangement so that the pilot can arrest stab trim runaway. If one of you sets the stab trim trimming, then the other can pull on the control column and the stab trim stops with a big clang. It is this mechanism you can hear.

But please don't break it. I once spent a weekend changing a stab trim control cable. There are only two of them. It is wrapped around a big drum at the front about 40 times, then goes back through the floor beams and is connected to the rear drum. One single cable. The second is wrapped around the rear drum 40 times and attached to the fwd drum. It was not a pleasant experience. Every time you got it correct, it moved and all the turns fell off the drum!
Glad I work on Airbusses now.

[spannersatKL]

Recently had the lot apart at the front end....bearing in the shaft between the stab trim mechanical indicator and the stab trim shaft was worn....I always thought the handle was the 'audible' warning of trim run away? The handles are made loose with several sections so as to 'rattle' when moving?

The 747 had a mechanical 'clicker' to sound when the trim was moving and warn of run away. (would sound a bit like a kids bike with a lolly stick in the wheels).