Narrative:

Twice in the past week I have witnessed an asymmetric engine spool up (more and more common as our next generation (ng) aircraft engines mature) when my first officer (first officer) was pushing the thrust levers forward at the beginning of the takeoff roll. In both cases; the first officer (different first officers) did not use the technique found in the company B737/ng aircraft operating manual takeoff and climb section table 9.1 takeoff procedures; which says to allow the engines to 'stabilize momentarily at approximately 40 percent N1 before promptly advancing the thrust levers toward takeoff thrust while simultaneously selecting to/GA.' the result; in both cases; was a significant yaw in the direction of the slow spooling engine. In both cases; if we did not take appropriate action; immediately reduce thrust on the faster spooling engine. The aircraft likely would have exited the runway (in the grass on the side of the runway) or; at best; would have left noticeable skid marks on the runway as control inputs (rudder inputs; nose wheel steering inputs; braking inputs) were used in an attempt to maintain directional control. As an 18-year captain at company; I have seen this issue three times; once about 10 years ago and twice this week. In all three cases; after debriefing the first officer on the importance of adhering to the guidance in takeoff procedures about 'stabilizing the engines at approximately 40 percent N1 before promptly advancing the thrust levers toward takeoff thrust.' I was convinced that my first officer would never repeat this mistake. In all three cases; my first officer probably had only 100 to 200 hours in the B737. The new first officers are coming out of the training department very well trained. However; although I believe that the training department is certainly discussing this issue academically; I do not believe that the new first officers are getting adequate practical experience (hopefully in the simulator) to experience what happens when this procedure is not followed. Also; I do not believe the training department and the check airmen are showing the first officers the technique(s) required to ensure 100 percent of the time that when engines spool up asymmetrically (and they will); that they never lose directional control on the runway at the beginning of the takeoff roll. Let me explain. One technique which works very well; 100 percent of the time; is to advance the thrust levers such that the leading edge of the predictive 'N1 command sector (momentary difference between actual N1 and value commanded by thrust lever position)' never exceeds 40 percent N1 until both engines are positively moving to or through 40 percent N1 together. Then advance the thrust levers toward takeoff thrust. The old guys all know the importance of this procedure; and we do it now without even 'thinking' of the procedure. It has become automatic for the experienced B737 pilots; but not the new guys.

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Original NASA ASRS Text

Title: B737NG Captain reported a trend in First Officer techniques in advancing thrust to take-off power which has been resulting in asymmetrical thrust.

Narrative: Twice in the past week I have witnessed an asymmetric engine spool up (more and more common as our Next Generation (NG) aircraft engines mature) when my First Officer (F/O) was pushing the thrust levers forward at the beginning of the takeoff roll. In both cases; the F/O (different F/Os) did not use the technique found in the Company B737/NG Aircraft Operating Manual Takeoff and Climb Section Table 9.1 Takeoff Procedures; which says to allow the engines to 'stabilize momentarily at approximately 40 percent N1 before promptly advancing the thrust levers toward takeoff thrust while simultaneously selecting TO/GA.' The result; in both cases; was a significant yaw in the direction of the slow spooling engine. In both cases; if we did not take appropriate action; immediately reduce thrust on the faster spooling engine. The aircraft likely would have exited the runway (in the grass on the side of the runway) or; at best; would have left noticeable skid marks on the runway as control inputs (rudder inputs; nose wheel steering inputs; braking inputs) were used in an attempt to maintain directional control. As an 18-year Captain at Company; I have seen this issue three times; once about 10 years ago and twice this week. In all three cases; after debriefing the F/O on the importance of adhering to the guidance in Takeoff Procedures about 'stabilizing the engines at approximately 40 percent N1 before promptly advancing the thrust levers toward takeoff thrust.' I was convinced that my F/O would never repeat this mistake. In all three cases; my F/O probably had only 100 to 200 hours in the B737. The new F/Os are coming out of the Training Department very well trained. However; although I believe that the Training Department is certainly discussing this issue academically; I do not believe that the new F/Os are getting adequate practical experience (hopefully in the simulator) to experience what happens when this procedure is not followed. Also; I do not believe the Training Department and the Check Airmen are showing the F/Os the technique(s) required to ensure 100 percent of the time that when engines spool up asymmetrically (and they will); that they never lose directional control on the runway at the beginning of the takeoff roll. Let me explain. One technique which works very well; 100 percent of the time; is to advance the thrust levers such that the leading edge of the predictive 'N1 Command Sector (momentary difference between actual N1 and value commanded by thrust lever position)' never exceeds 40 percent N1 until BOTH engines are positively moving to or through 40 percent N1 together. Then advance the thrust levers toward takeoff thrust. The old guys all know the importance of this procedure; and we do it now without even 'thinking' of the procedure. It has become automatic for the experienced B737 Pilots; but not the new guys.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.