Interesting thread. Having no ASI is not a huge problem providing you're not trying an ultra short field landing, just fly attitude and normal power. Of course, if any one instrument fails then you become correspondingly more reliant on he others and you lose redundancy.
I did think about this recently when the MP gauge failed on the Yak, it was before take-off so I didn't fly. Had it been in the air I would have had no real problem but it then makes you think about what if the ASI subsequently fails, e.g. wasp down the pitot on the takeoff roll.
In an aeroplane with relatively low pitch stability and a CS prop, if you had no reliable indication of either airspeed or power setting it would be a bit harder again to judge the speed, but (though I haven't tried this in the Yak) still do-able using nose attitude and the VSI. Something like the Cub with strong pitch stability and a fixed pitch prop, no problem flying in nice weather with nothing but an oil pressure gauge and a wet compass
Well it's a good thing to try out dual on my next revalidation flight!
Whirly, I know nothing about helicopters. Would it be possble to fly one of those without certain instruments, or does the lack of inherent stability make it a no-win situation?