The plumb bob would only give erroneous information whilst an external input is continuously applied, turning, accelerating, climbing or decending. Once that input is removed, the plumb bob would give accurate vertical information.
It would be in error any time you aren't in straight unaccelerated flight.
A sensible power setting, along with removal of inputs to the side stick/control column (not a natural response, I know) would soon indicate the correct recovery action required.
Removing control inputs will not make the aircraft flight path suddenly become fixed. Its still be turning and or accelerating unless you happened to be in the correct attitude for straight flight at whatever vertical speed you happen to have and also have the correct power set. In otherwords, if you're already recovered. So your technique is actually not valid until the recovery is already complete, because thats the only time your plumb bob is reliable.
Loud rushing sound getting louder, pull up
Better hope the bank angle is not too large, or you'll be pulling the wings off before the airspeed trends the correct way. Better hope its not >90.
It worked on gliders before I flew powered aircraft, along with a bit of wool on a stick in front of my nose. Admittedly it was in the airflow outside but the basic principle holds good.
In cloud at night? Because that was your original condition in the previous post.
I'm sorry, but the idea that a plumb bob can be used as an alternate to a standby attitude indicator is complete rubbish.
If nothing else, look at it this way.... if it was a simple, foolproof, reliable method how come it isn't standard fit?
pb