Originally Posted by
zzuf
FDR you have totally misinterpreted the reason for the requirement contained in FAR25.255.
This standard was developed to address the "jet upset" problems experienced in the early days of jet transports.
The problem was seen as either insufficient elevator power, or stabilizer jack stalling as a result of deliberately or inadvertently applied longitudinal trim.
The regulation required that a defined level of stability and control was available if the mis-trim was:
1. 3 seconds at the no load trim rate;
2. 30 lbf stick force for manual trim:
3. The maximum trim which could be applied by the autopilot before it disconnected due to overload.
Each FAR standard is written as a stand alone requirement, you cannot try to apply a requirement written about mis-trimming to an event which is clearly about a control runaway.
The appropriate paragraphs for B737 stabilizer malfunctions are 25.671, 672 and 677.
Unless you are experienced in interpreting these requirements you may have difficulty working out what is really required.
Please don't forget to read the appropriate AC's as well.
It may also be helpful to read the equivalent paras of FAR 23 and AC 23-8.
I know these are for light aircraft but the guidance material about runaways is more comprehensive, it may give you an idea of how the FARs may be interpreted.
They will also give you some information about control open loop/closed loop recognition delay times used for malfunctions/runaways
You have next to no chance of working out what was required for certification or what tests were made without, copies of all certification meeting minutes, the agreed means of demonstrating compliance and the compliance reports for each FAR standard.
Unfortunately the FAR certification standards, as written, can be just the beginning of a long and complex path before all parties actually agree to the test required and the results expected.
AC25.07D covers the background for 25.255, however the B737 is an aircraft with a powered trim system ergo
"it must be possible from an overspeed condition at VDF/MDF to produce at least 1.5 g for recovery by applying not more than 125 pounds of longitudinal control force using either the primary longitudinal control alone or the primary longitudinal control and the longitudinal trim system". The 30lb load is applicable to an aircraft that did not have a powered trim system. This saga has highlighted that the aircraft can be placed in an condition that exceeds the 3 seconds out of trim motion of the trim system. In that respect it is similar to the awareness that grew out of AA587 of the limitations of the term Va in the protection of the structure. The industry learnt from that lack of comprehension and moved on without tails falling off around the circuit. Having an long forgotten technique that exceeds the criteria of 25.255 is hardly comforting, and suggests that remediation is needed, the simplest being by training.