Originally Posted by
cockpitvisit
Question from an SLF: Is it normal that you can trim an aircraft so far that you could not counteract the trim with the yoke, or is it a special feature of the 737?
What's the point of having a trim range exceeding the elevator authority? Are there any situations where this is needed?
That's essentially a function of pitch moment range (i.e. the extremes of nose-up and nose-down pitch moment), elevator effectiveness across the relevant speed range, and elevator force required to achieve full displacement throughout the speed range. As has been discussed ad nauseam, the pitch moment range in the 737 has expanded significantly as the engines have become more powerful and the thrust arm has lengthened (larger engines, thrust arm moved further below CG). Add to that the effect of moving the now quite impressively sized engine nacelles further forward just so they'll fit, which has the effect of reducing dynamic pitch stability in a high AoA situation, and you absolutely have the potential to find yourself in an area of the envelope where elevator control alone is not sufficient, particularly when you take into account the force required to move the elevator at high speeds.
As you've alluded to, the above is not an ideal situation, and one of the many reasons why it's not a good idea to hang on to common type certification by the skin of your teeth as you redesign an aircraft to within an inch of what's 'acceptable'. Now we'll likely see 'what's acceptable' being pulled back into line, leaving the 737 MAX sitting out there on it's own, essentially uncertifiable in it's current state. No doubt a quick fix will be rustled up, but it obviously won't fix the underlying issues.