Hi
BOAC,
the immutable laws of physics exist however (roughly speaking) the certification reqts can be met by good design, or alternatively by a system that guarantees that the exposure to the condition does not occur....much....
As Richard P. Feynman wrote on the Challenger disaster:
"For a successful technology, reality must take precedence over public relations, for nature cannot be fooled."
Feynman's Appendix to the Rogers Commission Report on the Space Shuttle Challenger Accident
The stall speed is defined as the minimum speed reached during the maneuver, except for those airplanes that require stall identification devices (see paragraph 29b(3)(iv), below).
(i) The pitch control reaches the aft stop and is held full aft for two seconds, or until the pitch attitude stops increasing, whichever occurs later. In the case of turning flight stalls, recovery may be initiated once the pitch control reaches the aft stop when accompanied by a rolling motion that is not immediately controllable (provided the rolling motion complies with § 25.203(c)).
(ii) An uncommanded, distinctive and easily recognizable nose down pitch that cannot be readily arrested. This nose down pitch may be accompanied by a rolling motion that is not immediately controllable, provided that the rolling motion complies with § 25.203(b) or (c), as appropriate.
(iii) The airplane demonstrates an unmistakable, inherent aerodynamic warning of a magnitude and severity that is a strong and effective deterrent to further speed reduction. This deterrent level of aerodynamic warning (i.e., buffet) must be of a much greater magnitude than the initial buffet ordinarily associated with stall warning. An example is a large transport airplane that exhibits “deterrent buffet” with flaps up and is characterized by an intensity that inhibits reading cockpit instruments and would require a strong determined effort by the pilot to increase the angle of attack any further.
(iv) The activation point of a stall identification device that is a strong and effective deterrent to further speed reduction. Paragraph 228 of this AC presents guidance material for demonstrating compliance of stall identification systems with the regulatory requirements of Part 25 of the FAR.
d. Stall Characteristics.
(1) Background. Since operational pilots may not be required, or trained, to fly to an angle of attack beyond that for stall warning, any exposure to the behavior of the airplane in an actual stall would be both unexpected and unfamiliar. Therefore, to assure a safe and expeditious recovery from an unintentional stall, it should not require any unusual piloting technique to successfully demonstrate compliance with § 25.203, nor should it require exceptional skill or repeated practice by the test pilot. The behavior of the airplane during the stall and recovery must be easily controllable using normally expected pilot reactions.
Section 25.203(b) states that “the roll occurring between the stall and the completion of the recovery may not exceed approximately 20 degrees” for level wing stalls. In level wing stalls the bank angle may exceed 20 degrees occasionally, provided that lateral control is effective during recovery.
The definition of adequate controllability is specifically described in the context of abnormal aerodynamic configurations:
“Adequate controllability” means that it is possible to produce and to correct pitch, roll, and yaw by unreversed use of the flight controls, and that there are no uncommanded airplane motions due to aerodynamic flow breakdown. If stall warning is used as the end point, then it should be demonstrated that the airplane is safely controllable and maneuverable when flown at the recommended operating speed.
Curiously, there is no testing reqt for the out of trim case to look at the condition that occurs in a stall recovery with trim/power set in an adverse configuration, nor is there an A-PC HQ evaluation required in this case. It would appear that the potential to place the A320/A310/A30-600 out of trim with large thrust couples would be worthy of evaluation, given the adverse outcomes that have occurred (B737 on occasion as well...).
FDR