Alright, with much delay (after a rainy vacation in Turkey), here is the next sneak-peak at our beloved 733. Todays video is featuring the "Speed trim" functionality. While I explain very shortly in the video, here is the lowdown on the system: The speedtrim system was implemented by Boeing because the 737 can exhibit poor speed stability at very low speeds and high thrust settings (i.e. after a go-around). The nose-up moment of the engines can be stronger than the aerodynamic nose-down tendency for speed loss, so its possible that the airplane would get into very low speed regime with an unattentive pilot. Boeing has therefore tasked the flight-control-computer with watching the speed trend in certain situations. If they detect that the speed is going up or down, they will utilize the autopilot stabilizer trim to combat this trend. The pilot will sometimes be surprised, because the plane seems to "work against" him. He pushes the nose down to accelerate, and the bleedin´ plane trims nose-up again? What gives? Well, it works as designed. To avoid fighting with the speed-trim, use the electrical trim switches on the yoke. This will send a new target speed to the speed-trim mechanism, and will also disable speed trim for 5 seconds. Another way to disable it is to push the yoke in opposition to trim movement. This will ALWAYS cut out the autopilot trim, and also the speed-trim. Of course when you release the yoke, it goes right at it again. Here are the engagement criteria for speed-trim: - Flaps not up - Airspeed 100-300 KIAS - 10 sec after lift-off - 5 sec after releasing trim switches - N1 above 60% - Autopilot not engaged - Sensing of trim requirement Here is the video:
Jan