IMO It’s pretty much as **Richard Pearse **already said, at least for airliners.
I’ve flown the 727, 757, and 767. I’ve ridden in the cockpit of the 707 and 737. I’ve been inside the cockpit of various parked or shut down or simulated 747s, 777s, and 787s, but have never ridden in their cockpits nor watched a crew prepare to fly one.
With a few minutes puzzling I could get any of them started even if it was totally dead / cold when I got there. And fly it adequately on a sunny day to get around the traffic pattern or even from familiar city A to familiar city B with some help and indulgence from ATC. It wouldn’t be pretty or professional; in fact it’d probably look a bit like that airshow act with the supposedly-drunken pilot in a Piper Cub. But I wouldn’t be in fear of my life nor the folks below me. The landing touchdown would suck, but I’ve probably already made worse.
IOW, they’re really all variations on a theme. The tech progresses, and the older airplanes would be harder just because I’m used to all the newfangled stuff making life easier.
OTOH, I’d be much less confident I could do the same thing in an Airbus. It’s different enough that there might be traps I don’t know about. Likewise Richard’s Avros / BAEs. Those are definitely different. If somebody else started those I’d be OK to fly it around the traffic pattern but there’d be a few junctures of sucked-in breath followed by “Well, here goes nothing. I *hope *this works like I think it does.” Which it probably will.
I’d be far more worried about a pilot who’d gone from lightplanes to Airbus without ever passing through Boeing, McDonnell Douglas, or similar aircraft. An Airbus-only pilot trying a non-fly-by-wire big jet alone for the first time without training has a tough row to hoe.
Conversely, all Airbus are designed to be operated almost identically. An A320 pilot will feel pretty much immediately at home in an A380. Which is decidedly not true for Boeings.
As to fighters: There’s a lot less family resemblance since until recently there were many manufacturers and lots of years & tech progress between any two by the same company. And the latest ones are so cosmic it’d be hard for somebody even with my now 30 years out of date F-16A experience to make heads nor tails of the instruments and systems.
But for similar levels of tech and a current pilot it wouldn’t be too bad. e.g. A current F-15C pilot could eventually figure out how to start an F-16C or F/A-18C or Panavia Tornadoe. And fly it around the pattern OK. But there’s no way in hell they could fight with it. That’s waay too deep in the machine-specific details of weapons control.
They could do unarmed dogfighting: i.e. zooming about the sky here and there, but to no actual effect. And compared to somebody who knew the aircraft they’d be a sitting duck in that simulated combat. The real edge in fighting comes from knowing how to exploit the special 10% of the envelope uniquely suited to your type. A newbie wouldn’t have that.
I recall in the Olde Dayes sitting in captured Soviet equipment and thinking that there’s no way in hell I could start or fly this thing cold. If somebody else started it I might get it off the ground, but I’d be ejecting rather than landing. Assuming I could figure out how to even do that. It was so foreign it was more like alien.
Which gave me a heightened appreciation for just how alien truly alien (non-human) tech might be, even if it was within a century or so of progress versus our current practice. But that’s another thread.