Passenger accounts varied: some felt the plane had plunged, others that it had climbed sharply.
One passenger, Deborah Sylvester, 34, said it felt like the steep run on a rollercoaster.
"It was very sudden. Both going down and back up were steep. You got that feeling where you lost your stomach.
Doesn't sound like a TCAS avoidance manoeuvre. Only a see and avoid scenario would result in that type of flying, and even then only when you were really close.