Why 30 seconds on Rudder trim technique?
 

Boeing recommend 2 technique for trimming the rudder on the 737-NG.
Both recommend: ensure the autopilot is engaged in HDG SEL and stabilized for at least 30 seconds.
Why is these 30 seconds for?
Thank you in advance.

Might be completly incorrect but my cfi says that I must let the aircraft settle first and then trim. Now this is only in a little PA-28 so might not apply but I'm looking at the bigger picture :ok:

By stabilized do they mean, amongst other things, not accelerating or decelerating since the rudder requirement would be changing?

Why 30 seconds rudder trim technique?
 

here is the FCTM excerpts

Recommended Rudder Trim Technique

This section describes two techniques for properly trimming the rudder. It is assumed that the airplane is properly rigged and in normal cruise. The primary technique uses rudder trim only to level the control wheel and is an acceptable and effective method for trimming the airplane. It is approximately equal to a minimum drag condition. This technique is usable for normal as well as many non-normal conditions. For some non-normal conditions, such as engine ailure, this technique is the preferred method and provides near minimum drag.
The alternate technique may provide a more accurate trim condition when the roll is caused by a roll imbalance. In addition, this technique outlines the steps to be taken if the primary trim technique results in an unacceptable bank angle or excessive rudder trim. The alternate technique uses both rudder and aileron trim to neutralize a rolling condition using the bank pointer as reference.
Note: Large trim requirements may indicate the need for maintenance and should be noted in the airplane log.

Drag Factors Due to Trim Technique
If the control wheel is displaced to the point of spoiler deflection a significant increase in aerodynamic drag results. Additionally, any rigging deviation that results in early spoiler actuation causes a significant increase in drag per unit of trim. These conditions result in increased fuel consumption. Small out of trim conditions affect fuel flow by less than 1%, if no spoilers are deflected.
Note: Aileron trim may be required for significant fuel imbalance, airplane damage, or flight control system malfunctions.


Primary Rudder Trim Technique
It is recommended that the autopilot remain engaged while accomplishing the primary rudder trim technique (using rudder trim only). After completing this technique, if the autopilot is disengaged, the airplane should maintain a constant heading.
The following steps define the primary rudder trim technique:
• set symmetrical thrust
• balance fuel if required
• ensure the autopilot is engaged in HDG SEL and stabilized for at least 30 seconds
• trim the rudder in the direction corresponding to the down (low) side of the control wheel until the control wheel indicates level. The indices on top of the control wheel should be used to ensure a level wheel condition.
The airplane is properly trimmed when the control wheel is level, (zero index). As speed, gross weight, or altitude change, trim requirements may also change. In a proper trim condition, there may be a slight forward slip (slight bank angle indicated on the bank pointer) and a slight deflection of the slip/skid indicator, which is acceptable.

Alternate Rudder Trim Technique
The alternate rudder trim technique is used if the primary trim technique results in an unacceptable bank angle, excessive rudder trim, or if a more accurate dual axis trim is required.
The following steps define the alternate rudder trim technique:
• set symmetrical thrust
• balance fuel if required
• verify rudder trim is zero
• ensure the autopilot is engaged in HDG SEL and stabilized for at least 30 seconds
• trim the rudder in the direction corresponding to the down (low) side of the control wheel until the bank indicates level (no bank angle indicated on the bank pointer). Apply rudder trim incrementally, allowing the bank to stabilize after each trim input. Large trim inputs are more difficult to coordinate. The airplane is properly trimmed when the bank angle on the bank pointer indicates zero. If the airplane is properly rigged, the control wheel should indicate approximately level. The resultant control wheel condition indicates the true aileron (roll) trim of the airplane being used by the autopilot.
After completing the alternate rudder trim technique, if the autopilot is disengaged the airplane may have a rolling tendency. Hold the wings level using the bank pointer as reference. Trim out any control wheel forces using the aileron trim switches. If properly trimmed, the airplane holds a constant heading and the aileron trim reading on the wheel/column agrees with what was seen while the autopilot was engaged. Aileron trim inputs require additional time and should be accomplished prior to final approach.

Two important things to remember here:

1/ never use aileron trim with the autopilot engaged.

2/ spoiler pickup on the NG starts at approximately 4 units of control wheel rotation, as marked on the scale provided.

Cheers
EW73