A number of years ago I was flying as a FO on a Citation out of Biggin.
We departed at night en route to Majorca and passing FL240 in the climb working London I noticed a shimmer around the Captains head which made me think my eyesight had gone funny.
I could then smell burning.
The smoke got worse and became very acrid and dense. We went through the normal procedures for smoke and London Control were brilliant giving us an emergency descent and vectors straight down through Gatwick in a direct line to Biggin.
We had by then evacuated the smoke which was now diminishing and landed at Biggin to be met by the Fire engines who removed the petrified passengers who all though they were going to die.
That smoke was so acrid it ruined all my pilot clothes which reeked.
It is a horrible experience and the passengers could not believe how laid back and casual I was about the whole thing (Show for their benefit
Obviously in a piston single if you have an oxygen mask put it on a cheap pair of swimming Goggles will keep eye watering smoke from your eyes.
If not open the side window which will help to remove smoke as well as giving you access to fresh air. Even think of carrying a short piece of tube you can breathe through?
Yes if you suspect an electrical fault switch off the electrics then switch off all the avionics bar one radio. Switch back on to make a call and appraise the situation.
If the smoke continues to build with the master on and one radio go sans radio but ATC can be of help if for no other reason than giving you vectors.
How else will you navigate with maybe diminished visibility and all the electrics off?
But usually get on the ground as quick as possible and ATC can help there!
BTW in our case it was a fan motor in the ducting system.
Pace