737NG Rudder Trim

[NSEU]

Can the maximum amount of rudder trim available on a 737NG -800/-900 be expressed as a percentage of full rudder movement? Eg. the limit of rudder trim on a 767-200 (with certain engines) allows up to 80% of full rudder travel in trim motion.

Thanks in advance.
Rgds.
NSEU.

[DDG]

B737Ng rudder trim is measured in UNITS of TRIM,up to 16 units left,zero units(neutral),or up to 16 units of right trim.
I can`t speak for some NG aircraft that use flight control position information as displayed on the lower Display Unit as that OPTION is not in use on the aircraft i maintain.
It can be referred to in inches of travel whilst carrying out rigging of the rudder as per the manuals.

Regards DDG

[NSEU]

Thanks, DDG.

I must have overlooked it earlier, but I did find degrees in our manual also. For full trim, 18 degrees is quoted as compared to 29 for full rudder travel (This is without any rudder ratio input).

So a ballpark figure would be 62%. Whether this ratio is held, I still don't know.

The original problem I had was estimating how much turn I could get on the nosewheel with just rudder trim application: I assume Rudder Trim moves the nosewheel as trim backdrives the rudder pedals and the rudder pedals operate nosewheel steering. This is a big assumption as I have not seen any warnings in the 737NG manual about using Rudder Trim on the ground (particularly important on those aircraft which don't have nosewheel lockout pins). I seem to recall there are warnings for other, larger, Boeing aircraft in their manuals.

Full nosewheel steering is 7degrees, so 62% of 7 degrees is about 4.3 degrees. This is not much in terms of tyre travel on an NG, but if you attach a long towbar to the nosewheel, you could cause some serious damage to personnel and equipment (IMHO).

The pilots/engineers I have asked about this seem oblivious to this potential risk. Have I overlooked something obvious here?

Thanks.
Regards.
NSEU

[alexban]

I maybe wrong ,but the rudder trim doesn't move the nosewheel.
When you check flight controls on ground you are required to hold the stering wheel when aplying full rudder (to avoid movement of the nosewheel).
You are not required to do this while checking full motion of the rudder trim.
This was just a empirical explanation,but I can tell you that the nosewheel is not moving while selecting full ruder trim motion.(the rudder pedals are moving though due to reseting of the rudder feel and centering unit)
Brgds Alex

[NSEU]

"the rudder pedals are moving though due to reseting of the rudder feel and centering unit)"

Thanks for the feedback, Alex.
I'm just wondering if your trim checks are usually made when you are taxying or at the gate when the steering pin is usually inserted? And would you notice such a small deviation from the taxyway centreline during low speed taxy? (remembering that, theoretically, the angle on a 737 nosegear would only be 4.3 degrees at max trim).

I still don't understand how you can backdrive the rudder pedals through the cable system and not move everything attached to the rudder pedals. I'm wondering if nosewheel steering does not begin the moment you push the rudder pedals (normally)? Do you have to use large inputs during takeoff, to correct any deviations from the runway centreline?

Anyway, I'm inspired to run some tests of my own on an aircraft over the weekend (under controlled conditions). I'll let you know the outcome.

Regards.
NSEU.

[avioniker]

The pedal steering is limited to 7 degrees. It does begin immidiately upon displacing the pedals but with the large pedal movement needed to attain the relatively small 7 degree displacement it seems to be delayed or lagging.
Inserting the steering bypass pin has nothing to do with anything in the rudder system. It merely causes the hydraulic fluid that would be displacing the steering collar to be bypassed and circulated in the system without ever getting to the steering actuators.
As for
"backdrive the rudder pedals through the cable system and not move everything attached to the rudder pedals"
everything attached to the rudder pedals DOES move.
It isn't really all that complicated. Honest. Really, You can believe me...

[NSEU]

Thanks, gentlemen.
I've just been told that a 757 driver tried it and the airplane weaved about the taxy centerline, but the effect was very subtle he said (perhaps one degree of nosewheel turn). Not quite as subtle as I had expected, but nevertheless it does affect the nosewheel steering on this particular aircraft.

Cheers.
NSEU.

[alexban]

As I said ,I maybe wrong.
I am now on holiday,but I'll try it on a 737 when I'll get back to work next month.I'm talking about triming while on stand,no pin inserted.
Brgds Alex

[NSEU]

Tried the rudder trim on a 767-300 last night, and to say the least, it was very inconclusive.

The nose wheel tyres were on a smooth metal plate, so I would have imagined it would have moved more easily than on tarmac. One Center hydraulic pump was switched on and the trim was set to max.

It was very difficult to tell if the nosewheel was moving at all. A few times it looked like it moved a fraction of a degree, but you couldn't be sure your eyes were playing tricks on you. Either one pump was having problems moving the rudder and the nosewheel steering or the movement of the nose wheel was so slow and limited, it would not present any kind of danger.

At one point, I put my hand on one of the actuator rams and I felt it twitch, but that may have been when the hydraulics were switched off.

Will try again with two pumps on next time... or try on an aircraft with more trim authority (the 767-300 only has about 65% trim authority).

Rgds.
NSEU

[B73567AMT]

Adjusting Rudder Trim DOES have an effect on Nose Wheel Steering, albeit very little on the B737 Series. You have to stare at the tiller very closely to see it move...but it does indeed. Oddly enough though, on the B757 and B767 aircraft, moving Rudder Trim moves the Tiller alot more noticeably.

[NSEU]

"Oddly enough though, on the B757 and B767 aircraft, moving Rudder Trim moves the Tiller a lot more noticeably."

This is probably simply a matter of Rudder Trim authority, B73567AMT. Some 767 rudder trim systems can move the rudder up to 80% of full rudder travel, whereas on the 737NG, for example, rudder trim is limited to around 62%.

Rgds.
NSEU.