Re: Elevator Trim Springy Thingy Really Needed?

Rick Hole

The elevator spring and sparrow strainer work together to establish a default for elevator deflection which is somewhere within the range of sustainable flight should either fail or elevator linkage fail. The builder should consider failure modes in understanding what each does.

Relying on spring pressure alone, and the spring system fails can, and has, caused catastrophic failure of the aircraft. Sparrow strainers could potentially become attached. Elevator linkages have broken. In all cases we want the elevator to stay in a flyable state. In this event altitude can be controlled with the throttle if necessary to make an emergency landing.
Velocities have the same basic issue. I have seen builder omit the sparrow strainer or even (!) install it upside down and flown successfully that way and never seen a failure mode arise.
But it only takes once...

As for depending on autopilot to hold the elevator, remember they are not designed to continuously provide pressure, only to maintain altitude given a properly trimmed condition. I have seen this principle ignored and pilots go to unusual attitude when the autopilot is disengaged either manually or unexpectedly. It is at best embarrassing in front of passengers and at worse...

Rick Hole

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