I was flying as a co pilot on a citation from biggin hill to the Balearic isles. We departed at night and climbing through FL240 over the channel the cockpit started filling with dense acrid smoke. We asked for an immediate return to biggin hill and were given a direct with a clear descent. The pax were terrified and on landing the whole aircraft and our clothing stank.
The fire was caused by a fan motor on the air ducting system.With any smoke situation if you carry oxygen put the masks on. Try and identify if the fire is electrical and shut down all that you can landing as quickly as possible. Do inform Atc as they will point you in the right direction and clear airspace on route. Unless you have experienced it you can not possibly imagine how acrid and choking this smoke can be as well as terrifying for the PAX who will think that they are about to die so keep calm yourself.
Pace