I've been reading about it, but i still can't quite understand it :( At first i thought that it was similar to the Airbus auto-trim, but that can't be right because you have to trim the 737 manually, so how do it work? Does it only work with the autopilot engaged, or also when hand flying?

Hi, Speed trim adjusts the trim of the aircraft in pitch, because the faster the aircraft flies, the greater the nose-down tendency. There is also a Mach trim, that is there for the same reason, but functions separately. Look up "Mach tuck" and why it occurs. It is related. Best regards,Robin.

Hi, Speed trim adjusts the trim of the aircraft in pitch, because the faster the aircraft flies, the greater the nose-down tendency. There is also a Mach trim, that is there for the same reason, but functions separately. Look up "Mach tuck" and why it occurs. It is related. Best regards,Robin.

Thank you for your help!! looking it up now

Thank you for your help!! looking it up now

Both speed trim and mach trim are automatic in their operation, but they operate at opposite ends of the airspeed regime. Speed trim is active at low airspeeds and high power settings, with the autopilot off, with a positive pitch, and (especially) when the aircraft is lightly loaded - i.e., primarily during takeoff and climb-out. It automatically adjusts the stabilizer pitch trim to provide the most optimum trim for the current airspeed - which is, of course, constantly increasing during the climb-out. It also serves to add resistance to nose-up or nose-down movements of the yoke - in other words, to help prevent any tendency to over-control in pitch during this phase while hand-flying. As I understand it, the lighter the aircraft, the more aft the CG, and the more power used on takeoff, the more active the STS is. Mach trim, by contrast, is most active at high speeds and high altitudes - primarily during cruise. As the other gentleman mentioned, almost all jet aircraft have a tendency towards dropping the nose as speed increases because the center of pressure on the underside of the wings (where maximum lift is developed) moves farther and farther aft as speed increases. The mach trim system applies small amounts of stab trim to counteract the pitch-down moment. The pitch-down tendency gets stronger as the aircraft approaches its maximum rated mach number.

Robin, actually, the faster you get, more nose up tendency. Kristoff, trim serves to relieve the pilot workload on the yoke column too (obviously in manual mode). When in autopilor regime, you're not going to touch yoke, so the FMS takes care of it for you. Did you notice a sound like "woooshh woooshh" when you see a video inside the flight deck? That's the trim. Gabriel

Yeah, i know that sound is the trim :) Thank you so much guys, i understand this now. Anticipating the NGX so much right now. Trying to learn as much as i can so that once i have it I can fly it (of course i'll read the manuals though) So it is similar to autotrim in that it works with speed rather than pitch (and only under certain conditions rather than all the time)?

So it is similar to autotrim in that it works with speed rather than pitch (and only under certain conditions rather than all the time)?

Speed Trim is active with the autopilot disengaged (while hand-flying) and mainly at low airspeeds. Mach trim is always "on", but only really starts to have an effect as the aircraft accelerates beyond mach 0.61 Mach trim is active at higher speeds whether the autopilot is engaged or not. (When flying within RVSM airspace between FL 290 and FL410, the crew is not supposed to hand-fly the aircraft in any case.)

Robin, actually, the faster you get, more nose up tendency. Gabriel

Mmm, not quite true. The 737 has decreasing stability at high speeds. Mach trim makes sure that if you accelerate the plane pitches up and doesn't tuck.

Nevermind. I couldn't word it correctly.

Hey guys, since my mother language is not english, i'd like to know what is the "tuck" word in aviation. I managed to use a dictionary, but the translation seems a little dirty in portuguese. Sure it has a different meaning. Anyone?Thank you in advance.

Hey guys, since my mother language is not english, i'd like to know what is the "tuck" word in aviation. I managed to use a dictionary, but the translation seems a little dirty in portuguese. Sure it has a different meaning. Anyone?Thank you in advance.

A tendency to pitch down, in this context. I can tuck my shirt into my pants, showing the belt. I can also tuck the blankets of my bed under the mattress. EDIT: You got ninjad Kristoff.

Edited August 1, 201114 yr by ZachLW

"Mach tuck is an aerodynamic effect, whereby the nose of an aircraft tends to pitch downwards as the airflow around the wing reaches supersonic speeds." —Wikipedia But the actual work 'tuck' means, push, fold, or turn

Speed trim is modelled properly will all bells and whistles... Target value depends on IAS, mach, flaps and N1 with radio altitude, N1 and Mach cut-offs. Will not operate for 5 seconds after last push of electrical trim. PS. That's for manual flight Kristoff. Auto trim for semi-automatic flight is part of the CWS (control wheel steering) mode that maintains attitude as soon as force on the control column is less than a "detent". This is also modelled.

Hey guys, since my mother language is not english, i'd like to know what is the "tuck" word in aviation. I managed to use a dictionary, but the translation seems a little dirty in portuguese. Sure it has a different meaning. Anyone?Thank you in advance.

"Tuck" as in "Mach tuck" is the tendency as you approach limiting mach for the nose to pitch down or "tuck under." The word with an "F" as the first letter is indeed a dirty word in English. Edit: Sorry, I left the computer for a few minutes and when I finished the reply, I see others said the same thing.

@Kristoff: You're welcome!

Robin, actually, the faster you get, more nose up tendency.

???? Huh? There is a reason it is called "Mach tuck". Tuck, as in under, down, etc.. :) I know of no aircraft that pitches UP as a result of high Mach number, due to the physics involved. I think you are confusing the effects of high speed flight, with the effect of an increase in speed on the trimmed condition of an aircraft. Most will start to pitch up as they accelerate from a trimmed level-flight condition. This is not Mach tuck, but is the effect countered by the speed trim. Best regards,Robin.