This Koskisen 1mm Birch plywood stamped as AB/B was sold with an inference as GL2 by a supplier back when I purchased this. The website was recently changed showing that GL2 was available for order. The AB/B has areas that may meet the GL2 criteria, but I still must determine what an AB/B quality actually is relative to a Germanischer Lloyd graded plywood.
I have cut and discarded large areas of the AB/B plywood. A GL2 product is shown as available for order from Koskisen, but this is not what I have. The stamp on the wood is attached. I just like learning and someone must have an idea regarding this! Come on EAA tell me I'm not having to learn this on my own, now that my plans building is an oddity! By the way I am not trying to single out any supplier discussing this. I understand one mill closed last year when I was talking to a distributor regarding quality and grading a few months back. The cost to send an inspector to grade plywood as aircraft quality is no small cost (I have the actual numbers!).
Aircraft Spruce posts a Germanisher Lloyd criteria booklet for the GL graded ply (which this is not). According to Table I in that information a 0.1mm deviation (about the thickness of a laser print paper) is acceptable in a ply layer of 1mm metric plywood. That means that some, but not all, of the planer tearing on the AB/B plywood would fall within a GL specification for aircraft grade plywood.
Very General information sheet on the Koskisen Birch plywood
Photo information sheet with photos roughly showing the visual qualities of the Koskisen grade.
The Germanischer Lloyd booklet “Rules for Surveying and Testing of Plywood for Aircraft”
Files Attachment(s):Kos Ply.jpg