More years than i care to remember i flew DHC2 Beavers which had a P tube attached on the back of the pilots seat.If required you had to trim the A/C ,unstrap and kneel facing rearwards,no A/P,no co pilot.the ground crews favorite trick was to turn the exhaust tube to face into the airflow, which was behind the left main wheel as they removed the chocks.On one flight i had a couple of senior officers in the back and one of our G/crew in the front left seat,about half way i looked over to see said G/crew laughing,he pointed over my shoulder.One of the officers was trying to use the P tube to comunicate with me,must have seen to many Biggles films