Back in September 2018, I was working on the trim tab hinge where you countersink the tab hinge and tab front spar. The plans call for a countersink that goes all the way through one part and partially into the the next one giving you a knife edge. Van's own advice is to not do this (see page 05-08 Machine Countersinking in the plans), yet here in the plans they have you do this exact thing.
My career is in aircraft structural analysis. I'm intimately familiar with crack growth analysis and this type of countersink is asking for trouble. The knife edge is a crack starter, which is why you don't want to have one. The famous Aloha Airlines accident in 1988 had this condition on the lap splices which heavily contributed to the structural failure of the upper half of a large portion of the fuselage.
I went so far as to draw up the trim tab spar in Solidworks to thicken it so that it could be countersunk without leaving a knife edge. I got a metal bending shop quote to make me a couple of spars, but ultimately I decided on a simpler route. If you bond the two pieces together and then countersink, the knife edge condition is mitigated and I can breath easier. No more built-in crack starter.
I did talk to Van's tech support about this, but I was disappointed when they told me "it hasn't been a problem up to now". It's never a problem until suddenly it's a problem. The only way to inspect for a crack like this is with a low frequency eddy current probe and those things are very expensive (about the cost of a full blown experimental avionics panel with all the bells and whistles).
I'm not suggesting that anyone else worry about this because it's probably OK as is. However, I'm cursed with too much knowledge on this subject and this issue hit a nerve with me so for my own peace of mind I'm deviating slightly from the plans by bonding and fastening the hinge.
Chapter 5, Section 5.5 of the RV-14A plans. Van's says don't do it like example 1.
