It appears to be standard practice for firewalls and other critical bulkheads and such to be made with lightply. Overall, not always a problem. And it's not automatically even a bad practice. But in some cases, it is a bad thing to do.

I've got a Hobbico Sukhoi 31 that is fatally flawed because of lightply where real plywood should be. The bulkhead that holds the wing on the airplane is one layer of medium light ply. The wing hold down tab that is supposed to be held securely by that one piece is simply two pieces of the same. The wing hold down started failing during the first flights of that model.

I recently helped a buddy assemble his Kanke Monocoupe. Excellent and beautiful airplane. Beautifully constructed and covered. It's firewall is two layers of lightply. We chose a 90 for it before we noticed.

I've just now discovered that my Hangar9 P47 Thunderbolt's firewall is two layers of lightply. It's such a great flying and awesome looking dude in the air that my next modeling task (after finishing the SkyBolt repairs) will be to reinforce that firewall.

So what's the bottom line?
The ARF mfg's really need to use a plywood that is appropriate for the task. In the case of the firewalls, they could have easily used decent 5-ply plywood with another layer of whatever plywood they wish. Of course, that lighter ply should be strong enough to stand the compression of T-nuts it they're used. In the case of the wing hold down bulkhead, it definitely ought to be 5-ply. The use of light ply in that model in that bulkhead illustrates brilliantly the lack of understanding of the industry that is selling that model.

Will this thread get lots of "stand up guys" testifying how their ARFs have never had any problems? You betcha. I'm surprised they haven't started already. But trust me, the ARF industry needs to raise their own bar on wood selections. And the retailers need to step up and do us a better job of QC and a better job of design when it comes down to specifying wood selections.

The ARFs we're getting now are excellent deals. We're getting awesome looking models. It's amazing what we can buy. But right now, too many of them are coming across with fatal flaws that really shouldn't be there. Hope the better ARF retailers step up on this.