BPF
I also was involved with a smoke filled Citation flying out of Biggin Hill when I was an FO!
I had an elderly German Captain on the night flight to Majorca
We had a full cabin and passing FL240 I thought something was wrong with my eyes
Looking at the cabin in the dark cockpit his head was shimmering!
I blinked thinking my eyes had gone funny then came that burning acrid smell!
I looked at the air ducts and could see smoke pouring out!
Looking back at the PAX they were hidden in a fog and becoming highly alarmed
We were given a direct descent back to Biggin to be met by the fire engines on landing
Until you have experienced an electrical fire you don't realise how acrid and toxic the smoke is! My clothes stank of that smoke which turned out to have been caused by a fan in the ducting system