Passengers in the first class section of QF32 from London were stunned at the pilot’s amorous antics with the woman.
He was seen sitting on her lap during the flight in the luxurious premium section of the Qantas A380 jet before things became quite steamy, sources told the Herald Sun.
The seat has privacy walls and reclines to become a bed. The pilot was off-duty and not in uniform at the time of the incident.
He was scolded twice by crew as passengers became annoyed at the public display of affection, sources said.
After the flood of complaints, the pilot shifted to economy.
Qantas said an investigation was under way that could cost the pilot his $240,000-a-year job.
The female passenger left the flight in Singapore and the pilot returned to the first-class cabin.
It was unclear whether the pilot knew the woman before boarding the flight, or if he was intoxicated.
First-class passengers are entitled to a range of wines, spirits and beers.
The pilot stayed on the flight, which landed in Sydney at 8.10pm on Friday.
A Qantas spokeswoman confirmed there was an incident.
“Qantas is aware of an incident involving a travelling crew member on board QF32. The matter is being investigated internally in line with Qantas policy,” the spokeswoman said.
“Interaction of that nature is something that wouldn’t be sanctioned.”
The Australian and International Pilots Association has vowed to defend the pilot.
AIPA vice-president Captain Richard Woodward said he was aware of the incident and was gathering more detail.
“The rights of this member, like any other, will be defended by AIPA should the need arise,” he said.
It is not the first time that amorous behaviour on a Qantas flight has hit the headlines.
Actor Ralph Fiennes was sprung in a tryst with former Qantas hostess Lisa Robertson in January 2007.