In my times it wasn't official,but it was allowed tom enter cockpit and many people did it.
Personally, i entered ATC door via pilot door so I had idea what is going on in cockpit .
But one day Falcon 50 (YU BNA ) arrived at my airfield. don't miss it !
I tried at least 40 types of a/c ( both as PIC and SLF ) . But nothing compares with Falcon.
Outside Temperature over 40, 11 person on board and 4500 ft/min initial climb with 96 % of power.
apart from unique flying experience I have an opportunity to see how well trained and highest possible SOP works . N.b. YU BNA is state a/c for highest level of officials ( that particular day on board were lower level delegation-that explains my entrance )
forever in my mind.
