The way I read this section is that it is defining the power and capability that a trim system must have. To my reading, it's determining how much capability the system needs to have so that the pilot can achieve a fast enough response from the system. In other words, the 3 seconds is defining what the system needs to be capable of, not what the pilot's response time needs to be. This section also defines how the forces should not behave erratically as the system moves through its trim degrees.
I will post here the opening explanation for the section you are quoting. Not anywhere in this whole section, and certainly not in the explanation, is there mention of the actions of an automatic trim system, or its expected characteristics as regards pilot response time. I think it's important to quote the section's opening explanation, as it gives context to everything else contained in the section.
33. Out-of-Trim Characteristics - § 25.255.
a. Explanation. Certain early, trimmable stabilizer equipped jet transports experienced “jet upsets” that resulted in high speed dives. When the airplane was mistrimmed in the nose-down direction and allowed to accelerate to a high airspeed, it was found that there was insufficient elevator power to recover. Also, the stabilizer could not be trimmed in the nose-up direction, because the stabilizer motor stalled due to excessive airloads imposed on the horizontal stabilizer. As a result, a special condition was developed and applied to most part 25 airplanes with trimmablestabilizers. With certain substantive changes, it was adopted as § 25.255, effective with amendment 25-2. While these earlier problems seem to be generally associated with airplanes having trimmable stabilizers, it is clear from the preamble discussions to amendment 25-42 that § 25.255 applies “regardless of the type of trim system used in the airplane.” Section 25.255 is structured to give protection against the following unsatisfactory characteristics during mistrimmed flight in the higher speed regimes:
(1) Changes in maneuvering stability leading to overcontrolling in pitch.
(2) Inability to achieve at least l.5g for recovery from upset due to excessive control forces.
(3) Inability of the flightcrew to apply the control forces necessary to achieve recovery.
(4) Inability of the pitch-trim system to provide necessary control force relief when high control force inputs are present.
The FAA's language above is talking about the pilot's ability to recover from a runaway of the trim system, in either the nose up or the nose down direction. I'm not sure of the sense of your quoted sentence below
"The way I read this section is that it is defining the power and capability that a trim system must have."
The FAR 25.255 requires that the pilot be able to overcome the impact of a runaway stabilizer and the state it will leave the airplane in after a 3 second runaway. For instance, if the airplane is at Vmo/Mmo at the moment that the stabilizer runaway begins and the pilot takes no action for 3 sec and then begins to control the airplane via the elevators, the airplane must be able to recover from the overspeed and not exhibit any unusual flight control behaviors.
To make sure the airplane can meet this requirement, trim speed might need to be set at a low rate to make sure the airplane isn't badly out of trim after 3 sec. Is that what you mean?
By the way, here's the exact FAR language:Sec. 25.255
[(a) From an initial condition with the airplane trimmed at cruise speeds up to VMO/MMO, the airplane must have satisfactory maneuvering stability and controllability with the degree of out-of-trim in both the airplane nose-up and nose-down directions, which results from the greater of--
(1) A three-second movement of the longitudinal trim system at its normal rate for the particular flight condition with no aerodynamic load (or an equivalent degree of trim for airplanes that do not have a power-operated trim system), except as limited by stops in the trim system, including those required by Sec. 25.655(b) for adjustable stabilizers; or
(2) The maximum mistrim that can be sustained by the autopilot while maintaining level flight in the high speed cruising condition.
The 3 sec is in (a)(1)
I now understand what is actually happening in this discussion. The original document shared is this one, which is what I was reading: AC 25-7C Flight Test Guide For Certification Of Transport Category Airplanes. https://www.faa.gov/documentLibrary/med ... -7C%20.pdf
I now realize that you are quoting the PART 25—AIRWORTHINESS STANDARDS: TRANSPORT CATEGORY AIRPLANES,
as found here: https://www.ecfr.gov/cgi-bin/text-idx?n ... 18.104.22.168.11
However, I do maintain that a pilot reaction time of 3 seconds is still just an extrapolation from this text. It's rather a causation vs correlation situation. One could write a guideline such as this, based on a 3-second reaction time. However, there is no definite path leading the other direction, from reading this guideline to assuming a 3-second reaction time.
Until someone can show me something in official certification documentation which actually says that pilots are expected to respond to a runaway stabilizer within 3 seconds, then I am going to assume this 3-second concept was Boeing's. I feel that it was a very poor choice to design MCAS around the standard of a 3-second reaction.
As with my TCAS example, I feel that a 3-second reaction is reasonable, in response to the direct instruction of a visual and aural command. As to runaway stabilizer, a situation which first requires diagnosis, especially if it comes with a host of other alarms, I think 3 seconds is inappropriate. It's somewhat mind-boggling to me that Boeing would design around a 3-second reaction time.