This discussion is very intriguing to me as an engineer. Here is a photo of the Boeing 737 Classic triple slotted TE flap extended.
Triple slotted trailing edge flaps at the 40 position on a 737-400.
Now true enough, Boeing says not to extend the flaps above 20K feet as they never certified the aircraft in this configuration. I suspect they do know some things from a design analysis/stress point of view as to why this isn't a good idea, but never confirmed it in flight testing/certifying the Classic, and just said don't do it.
However, the story may be different in the Boeing 737-NG series. Here the flaps are double slotted, not triple slotted. And Boeing says this:
Although the flap placard limit speeds are different for each 737NG variant, the structural limit speed for the flaps is equal to the placard speeds (175k – F30, 162k – F40) for the heaviest variant (737-800/900). The Flap Load Relief trigger speeds (176k – F30, 163k – F40) are set to allow all variants to fly to the structural limit speed without system activation. Setting lower flap placard speeds for the –600 and –700 variants allows for greater service life of flap components due to the larger margins to the structural design speed.
For the NGs, speed is important, can you cruise and maintain level flight above 20,000 feet at less than 200k, 176k or 163k?
Going back to the Boeing Classic, Boeing says:
A flap load limiter (-3/4/500) / flaps/slats electronics unit (-NG) will automatically retract the flaps from 40 to 30 (-3/4/500) / also 30 to 25 (-NG) if the limit speed is exceeded. The flaps will extend again when speed is reduced.
At this point, I am assuming there to be an automatic sequencing of leading devises ( 2 Kruger flaps inboard of each engine) and 6 slats outboard of the engine per wing. So extending the TE flaps, these devises extend first, correct? If so, I wonder if there are stress limits determined by speed for these, more so than the TE flaps? Any thoughts, any one?