Two Qantas planes headed for the United States and South Africa had to return to Sydney after suffering problems on Friday afternoon.
Another two flights destined for Thailand and Indonesia were also grounded.
Flight QF7 landed safely back at Kingsford Smith Airport after aborting its flight to Dallas Fort Worth due to a "technical issue".
The flaps weren't able to retract, making it difficult to fly efficiently.
"As the Dallas flight is our longest on the network, the captain made the decision to return to Sydney," Qantas said in a statement.
The flight circled off the Australian coast and dumped fuel before landing.
The second flight, QF63 bound for Johannesburg, also needed to return to Sydney due to a cracked windscreen.
Only the outer of three layers of grass was affected, and the crack "did not compromise the integrity of the aircraft", the airline said.
"The aircraft was safe to continue to Johannesburg but the captain made the decision to return as the windscreen will be replaced at Qantas' engineering base in Sydney."
Qantas said it was "working through accommodating passengers or offering them transport home before replacement services are organised".
A spokesman for Australia's Civil Aviation and Safety Authority said it would decide whether or not to launch an investigation into the incidents depending on the reports it received from Qantas.
Meanwhile flight QF23 due to depart Sydney bound for Bangkok did not take off due to the need for an engine replacement, with passengers booked on a later flight on Friday.
In a separate incident flight QF41 has been delayed departing Sydney by about five hours after a small fuel leak was detected.
Newshub.
