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Boeing has issued a reminder to operators in a Fleet Team Digest (737NG-FTD-02-18001) to pass on to their crews about the correct procedure for setting takeoff thrust. It follows 8 confirmed aborted takeoff events in 2017 resulting from advancing the throttles before engines were allowed to stabilize at 40% N1.

An example of the consequences of not following this procedure can be seen in the Jet Airways 737-800 overrun at Goa on 27 Dec 2016, in which the final report states that: “Probable cause of the Accident: The PIC pressed TOGA when the thrust on no.1 engine was 40% and no.2 engine was 28% in deviation from SOP, which caused the No.1 engine thrust to increase at a faster rate than no.2 resulting in aircraft yawing towards right. In the absence of timely desired corrective actions including reject takeoff, the aircraft veered off the runway and continued to move in a semicircular arc on the undulated ground resulting in substantial damages to the aircraft.”

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Boeing say that following a review of the flight data by CFM or Boeing, the root cause for these events was the engines not being stabilized at 40 percent N1, prior to advancing the thrust levers to takeoff thrust (autothrottle TO/GA).

The recommended procedure for setting takeoff thrust begins by manually positioning thrust levers to approximately 40% N1, which allows the engines to accelerate and stabilize at an intermediate thrust setting. This minimizes asymmetrical thrust when TO/GA is selected. If the intermediate thrust setting overshoots 40% N1 speed, it is not necessary to reduce N1 down to 40%. TO/GA can be selected after the engines stabilize at 40% N1.

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*** Updated 23 Nov 2020 ***

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Full details are in FCOM Vol 1 - Normal Procedures - Amplified Procedures - Takeoff Procedure

The FCTM (Chapter 3 Takeoff and Initial Climb - Initiating Takeoff Roll) also states:

“Allowing the engines to stabilize provides uniform engine acceleration to takeoff thrust and minimizes directional control problems. This is particularly important if crosswinds exist or the runway surface is slippery. The exact initial setting is not as important as setting symmetrical thrust. If thrust is to be set manually, smoothly advance thrust levers toward takeoff thrust.

Note: Allowing the engines to stabilize for more than approximately 2 seconds before advancing thrust levers to takeoff thrust may adversely affect takeoff distance.”

737 Engine Stabilization for Takeoff

Crews should set approximately 40% N1 and allow the engines to stabilize before pressing TOGA

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