"As a previous poster put "you'll know"!"
Trying to sleep down the back on an empty night ferry flight to Jeddah; flash - almighty bang - attention getting vibration. Oh yes I knew alright. Couple of flight attendants came looking for me with an urgent message from the Captain to join him on the flight deck. Back in Algiers, big surprise (not) nothing but Taconite pellets in the tailpipe of Number 4.