Originally Posted by tomgoodman

That's what a lot of asked when we started flying the MD-88. The good news was that you could shut off the new stuff and still have a DC-9 underneath, with some components even predating that airplane. For example, the old Douglas instructor at the LGB factory school asked: "Why does the right hydraulic aux pump have a spring-loaded 'override' position?" After getting the "book" answer about bypassing an overheat shutoff, he said: "Actually, we had a lot of those switches left over from the DC-8 program, so we had to come up with a function for that position on the DC-9."
The slat actuators came from the DC-10, and were therefore so large that they cavitated both hydraulic systems until a restrictor valve was retrofitted. And the sliding windows? Looked like DC-6 vintage!

Great airplane though.