Wash basin drains in the toilets and galleys go overboard through a heated drain mast.
Flying over the Med one day, a B737 ahead and above us had a faint contrail fron the APU. Transmitted over the radio "Forgot to turn off your APU?". About 10 seconds later the contrail stopped.
Of course there are times when the APU maybe required on a twin, ie. one engine generator inop.
Last edited by dixi188; 7th Apr 2020 at 11:26.
Reason: more