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atc4usaf
23rd Mar 2011, 17:01
Hello everyone. I just had a question about the trim indicator on the Boeings: Is there a conversion anywhere that shows the relationship between degrees of trim and the units that Boeing uses?

And would those settings be similar between different aircraft, like the 737 and 747? Just wondering if Boeing kept the setting similar for transitions between aircraft.

Thanks in advance,

Scott B.

FCeng84
23rd Mar 2011, 17:57
Boeing's convension for stabilizer trim is that a degree of stabilizer is equal to one unit of pitch trim as displayed to the flight deck crew. The range of motion for the stabilizer on Boeing commercial jets is approximately 15 degrees. Pilot pitch trim units are 0 for most airplane nose down and about 15 for most airplane nose up.

Note that stabilizer position is positive in the trailing edge down direction that causes nose down pitching moment - this polarity is opposite that displayed to the flight deck crew. Also note that the zero stabilizer position corresponds to alignment with the fuselage. Typically the stabilizer range of travel is from about +4 deg (most trailing edge down) to about -11 deg (most trailing edge up). For simplification, the pitch trim range shown to the flight deck crew has positive values only.

catpinsan
23rd Mar 2011, 18:25
Dear atc4usaf,

The user experience for stab trim is very different on the two airplanes in Q. In my opinion this would make the comparison - 'not so useful'. The 737 stab trim wheels rotate 'violently' making quite a racket (helps identify runaway trim when using older technology trim systems). On the 744 there is no moving wheel in the cockpit and quiet operation with an electronic LCD type display of the trim scale (runaway, i.e. motion when not commanded and vice versa, is monitored via electronics, and identified by an EICAS message).

The 744, due to its vast range of possible CG figures has 3 different T/O green band ranges to help avoid a gross mistake in trim setting. Despite this, if memory serves right, the range of the trim indicator on the 744 is less than that on the 737.

It IS a reasonably common practice to memorise nominal rudder trim settings for engine out situations, however learning the nominal pitch trim settings for different conditions on the 744 is not a common practice, nor is it practical.

Conversion of trim units to degrees? It's a question of convention. The actual angular stab value is of no significant use to the pilot - just like the actual RPM has no advantage as compared to N1 measured in percent.

Correct technique for trim is to trim off the pressure on the column as has always been on conventional (non fly by wire) airplanes.

Hope that helped, though it was a bit of a ramble.

cps

atc4usaf
24th Mar 2011, 18:07
Thank you guys! Those replies were an immense help and explained what I was looking for perfectly. :ok: