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Why does Boeing use a trim wheel in the 737 and not any other products?

Is the trimmed state for the 737 just more important than in the 777? Or even 747?

It gives the impression that a secondary control for the trim (If the power to the pitch trim motors have been cutout) is extremely vital, seeing it features two fold out handles that are located 90 degrees apart, so a pilot has good leverage at any point in the rotation to their desired trimmed state.

I take it that this style of a trim system is implemented in the 737 for a good reason, so I’m curious what makes this “style” the perfect fit.

On the 747, and 777 it almost seems that once power to the pitch trim motors have been cut-out, there isn’t a way to manually move the stabilizer? I’m very aware that I could also be 100% wrong, but why not have a trim wheel in all products? Or vice versa?

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    $\begingroup$ The B707 and B727 also had trim wheels, so the question should be "later products" rather than "other products". $\endgroup$
    – StephenS
    Apr 1, 2020 at 18:40
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    $\begingroup$ The B747-100/200/300 had trim wheels too, though it appears to have been removed for the -400 and later. $\endgroup$
    – StephenS
    Apr 1, 2020 at 18:55

3 Answers 3

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Trim pitch "wheels" as you describe date back to the time when turning that wheel actually pulled on steel cables that were connected to the hinge mechanism for the control surface itself. This mechanical/manual system was a simple and robust method of manually trimming the aircraft and was very widely used before electronic/fly-by-wire control systems were invented.

Since the 737 is the oldest basic design platform in the Boeing product line, its original design dates back to a time when trim wheels were mechanical and pilots were transitioning out of still older planes into Boeing's new offerings- and those pilots expected manual trim wheels.

It is likely that trim wheels in newer planes are a "legacy" feature, allowing pilots familiar with an older revision of the cockpit layout to transition to another newer plane in the design series.

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    $\begingroup$ Keeping the "legacy" flight systems and characteristics is very important to Boeing since it means that pilots need little to no additional training to fly a newer version of an older aircraft. It's what led them into the whole MCAS debacle with the 737-MAX series. +1 from me. $\endgroup$
    – FreeMan
    Apr 1, 2020 at 17:42
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    $\begingroup$ @FreeMan but the problem lies within the decision to use the same flight system, that looks at one sensor at a time, which is the side the flying pilot is on. Had they opened up the computer to read both at the same time, we would be talking about how successful the Max is currently. But it’s not like the failure would be nonexistent. And it’s not like the failure would appear to be something abnormal, or something that a crew hasn’t seen. A 5 decade old procedure was abandoned by the airlines. $\endgroup$
    – George Clooney In a Mooney
    Apr 1, 2020 at 17:52
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    $\begingroup$ @Firefighter1 that's a whole different discussion that's been well beaten to death here and elsewhere on the web - not worth pursuing in comments to an unrelated question. I simply used it as an example of their desire to keep functionality the same in the "same" named aircraft. As you note, other Boeing models do not use the trim wheel because there is no (or not nearly as much) type crossover between the 737 and any others, so one expects to learn totally different systems. $\endgroup$
    – FreeMan
    Apr 1, 2020 at 18:11
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    $\begingroup$ The 737 pitch wheels ARE mechanical. There is an electric motor to spin them with a push of a button on the yoke, and turning them by hand gets really difficult when the aircraft is far out of trim, but they are still mechanical. The larger designs have the electric motor inside the tail directly turning a jackscrew and no mechanical link to cockpit. $\endgroup$
    – Jan Hudec
    Apr 2, 2020 at 7:28
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    $\begingroup$ @JanHudec That is what I have read as well. One of the supposed problems with the recently crashed 737s was that starting with the 737 NG the trim wheels are smaller to make space for a new, larger flight management computer. Because of the smaller diameter the pilots could not exert enough torque/power when they tried to manually move the horizontal stabilizer, severely out of trim at high speeds. $\endgroup$
    – Peter - Reinstate Monica
    Apr 2, 2020 at 18:03
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While the other answers are correct they are missing a key point:

Because it was certified that way

When an aircraft is certified with particular equipment, changing that can be difficult and may require the filing of a MAJOR REPAIR AND ALTERATION form. Its likely cheaper for Boeing to keep the trim wheels the way they are. The common 737 type certificate notes that the airframe was certified against (and must be rigged in accordance with) a particular set of Boeing drawing.

To insure proper operation of the airplane, the movements of the various control surfaces must be carefully controlled by proper rigging of the flight control systems. The airplanes must, therefore, be rigged according to the following FAA Approved data:Boeing Drawings No.

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65-45104 Control Installation, Stabilizer Trim

I cant seem to find a copy of that drawing but it likely indicates the trim wheel as part of the system.

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    $\begingroup$ His question is really at the conceptual level. As in, why not do that on newer designs. $\endgroup$
    – John K
    Apr 2, 2020 at 0:33
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The leading edge of the horizontal stab on jets is driven up and down for trim using a very large acme thread style screw jack. The Cessna 180/185 family and the Piper Cub/Super Cub/Pawnee family also do nearly the exact same thing, just with much smaller cable operated screw jacks. The early jets using jackscrew driven stabs included electric motors but maintained the manual cable-drum drive out of design conservatism, up through the 737.

The cable system is a lot of weight and maintenance, and the loads on the jack can be high enough in some out-of-trim flight conditions where you can't actually move the surface manually until you temporarily unload the stab with a large elevator input. In the case of the Ethiopian crash, they were boxed in by this problem by being too fast and too close to the ground - the elevator control inputs required to unload the screw jack so they could turn it would have made them dive even steeper (Piper Cubs also suffer from this problem where the little cable drum slips and you have to unload the surface to get it to move - on the 737 you just can't move it with the little wheel).

Then when you get to ever larger aircraft, to have a manually operated screw jack would require a cockpit wheel the size of a ship's helm, so in the late 60s and 70s, the designers developed multi-channel electrical control with sufficient redundancy to meet the single-point-of-failure risk standards without a mechanical system (they weren't "fly-by-wire" actually, more like "control-by-wire"; in CBW, the output is the same as the input but it's just done electronically, in FBW, the output is what a computer decides it should be after receiving the input command and may vary significantly from the actual input).

Anyway, bottom line is multi-channel control by wire stab systems (and FBW systems on FBW aircraft) meet the risk requirements to deal with trim failures (quits working, or runs away). The risk requirement for stab failures is not that high, usually categorized as "Major" in the Minor/Major/Hazardous/Catastrophic risk hierarchy, which determines minimum redundancy levels in design, and a dual channel CBW system with the appropriate controls and indicators meets that without old style cables (Boeing used that logic to justify the MCAS's sketchy architecture in the risk analysis - that's really where the ball was dropped).

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  • $\begingroup$ Ethiopian was like 7000' AGL. I keep hearing this meme that they were close to the ground, but go look at the data. $\endgroup$
    – Harper - Reinstate Monica
    Apr 2, 2020 at 4:42
  • $\begingroup$ The problem with the Ethiopian flight was the high speed, not so much the altitude. They were flying with the clickers (overspeed alert) active, so the aerodynamic forces on the vertical stabilizer were much larger than usual. $\endgroup$
    – Bianfable
    Apr 2, 2020 at 6:46
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    $\begingroup$ @Harper, 7,000' is not that much if when you need to do a push over for significant time. Of course you can do a pull up first, but they were struggling just to keep it level. $\endgroup$
    – Jan Hudec
    Apr 2, 2020 at 7:37
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    $\begingroup$ It's the high speed + the altitude. Push over at 300 kt+ in a jet and the ground 7000 ft below you looks like it's only1000. In a jet, time and space are compressed. In a C-172, you operate in a mental perceptual box maybe 5 miles across and a coupe thousand feet deep. In a jet that same mental space is 25 miles and 10000 ft. It's what is meant by "staying ahead" of the aircraft. $\endgroup$
    – John K
    Apr 2, 2020 at 12:37

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