I don't see why. As a rule, engineers don't build things, they *design* them. Once the design is fully complete and you have a production model fully built and tested, I suspect you need far fewer engineers to build the next hundred. If you have lots of orders for existing models, but few orders for new or heavily customized designs, then you need lots of assembly technicians and may well be hitting the limits of your production facilities, even while you have a bunch of much more expensive engineers sitting around with nothing much to do.
Now, if you expect custom orders to return in the near term, maybe even the mid term, then it probably makes sense to set those engineers working on "side projects" that don't directly effect the bottom line, just to retain their expertise - long term technology overhauls like speculative next-gen plane designs and other "busy work" that might prove valuable eventually, but whose primary purpose is just to retain talent. At some point though it becomes more cost effective to just fire them and hire new engineers when business picks up again. Especially if you have a list of poor performers, "problem individuals", and those whose retirement package is about to vest. Layoffs can be an excellent way to cull such especially costly individuals without the same level of legal scrutiny risked by individual firings.