I've often wondered why these a/c seem to have the "primary" instruments (the AI and DG) powered by air, and the T&B is electric (not allowed to be powered by the same source).
Why not the other way around. I have had more failed vac. pumps than I care to remember, but only ever two failed T&B. And while I'm at it, why these pin head sized suction gauges (Cessna), hidden behind the co-pilots control column.
And if the "electric" instrument fails, it is usually the instrument itself at fault, not a failure of electric power supply. When you drop a vac pump, you now (typically) have
two perfectly good instruments deprived of vacuum, thus rendered useless. Both of them.
Does it go back to the days of venturi derived suction (almost impossible to fail) when electrics also probably weren't what they are today?
I once had a vac pump replaced, then taxied away, took off, got no more than 20 miles away and had to turn around - the "new" pump had just failed. Less than one hour. But wait - I told the LAME when I pulled up that it had just crapped itself, and his remark? He had one he'd just fitted fail in the runup bay yesterday...
Also once saw the vac gauge the second it dropped to zero. Day VMC, so I flew along casually watching the AH and DG. After a few minutes they went ABSOLUTELY HAYWIRE BEZERK. Spinning and toppling and carrying on. To fly in IMC with that happening in front of you would have to be impossible. That aircraft was due to be dispatched the next day on night freight... Thank God it didn't last one more day or someone would've had an interesting time.
CR.