Needle, ball, airspeed, compass, and Mark I eyeballs ;-)
With the flying we do, generally in the Champ, the eyeball is more useful than the altimeter. Oh, and the seat of your pants provides a LOT of clues as well.
I love flying direct control light aircraft — you are one with the airframe.
I learned to fly in a 7-AC Champ back in 1960. It was built in 1946. Sure would love to have that plane now. It belonged to a flying club in SW Alabama. I paid $3.00 an hour, wet, for it and $6.00 an hour dual. Its fuel gauge was a piece of wire with a float in the tank sticking out through a hole in the fuel cap.
With a sufficient head wind you could practically hover the thing.
It had no radio, no electrical system and maybe five instruments on the panel. And a heater that did little more than make your left foot very hot.
Propping it off when you were along was always fun.
Yup, sure would love to have little flying machine now.
Suppose there is a specific set of conditions, a one-in-a-million confluence of factors that causes all three flight computers to reset, or stall, or simply shut down? An attitude of X, sensor input of Y and trim condition Z that uncovers a flaw in the programming? With a fly-by-wire system, you can, and often do, have a basically unstable and unbalanced flight attitude, that the computer can maintain in a "safe" manner, but that Yeager himself couldn't handle if he was directly in control, rather than one step removed.
Flying a Champ, or a 152 or a Piper Tomahawk, you can feel when the aircraft is out of balance, because the actual pressures on the control surfaces are transmitted back to the yoke and pedals by the control cables. A balanced standard turn feels good, in a Piper. If you are ham handed, the airplane will bite back. If the Air France pilot fed in a gross control correction, however, the computer wouldn't let that aircraft obey if that input was outside parameters.
A thought: I've had a couple of flight sim joysticks go bad, and it's unsettling that it could happen to the real thing...