Qantas has today been forced to make further changes to its fleet of domestic aircraft due to the effects of industrial action from the Australian Licenced Aircraft Engineers Association (ALAEA).

Qantas has grounded another Boeing 767 due to ongoing concerns about the reliability of the 767 fleet and has returned a Boeing 737 to service with the reliability of B737 fleet improving.

The number of aircraft grounded remains at seven -- four Boeing 767s and three Boeing 737s -- and the cancelled flights for the four week period remains unchanged at around 500.

Qantas Group Executive Lyell Strambi said despite the temporary suspension of industrial action by the union, Qantas' fleet was being impacted by a loss of around 60,000 man hours of maintenance.

"Today's announcement is really mixed news. While our Boeing 737 fleet health is improving, the health and reliability of the B767s continues to be affected by the cumulative effects of months of union action," Mr Strambi said.

"The backlog of maintenance was caused by months and months of industrial action and doesn't just disappear overnight because the union has suspended industrial action for a few weeks," he said.

"Since August, the union has held more than 40 strikes including full day strikes at every heavy maintenance base around Australia, had overtime bans in place and had a strict work to rule in place.

"As the backlog of maintenance is being cleared and the reliability of our fleet improves we will work towards resuming our normal flying schedule. "

On time performance has slipped from 87 percent six weeks ago to 75 percent this week.

Mr Strambi said this put to rest the ridiculous suggestion from the union that these aircraft were being grounded because they were being prepared for sale.

"The sole reason that we grounded these aircraft when we did and reduced our number of daily flights was due to the backlog of maintenance which was causing reliability issues on our fleet," he said.

"The aircraft planned retirement dates are completely unrelated to the grounding of aircraft and reduction in flying. The aircraft we have grounded were due to be retired from anywhere from between mid-November and 2015.

"The Boeing 737 aircraft that is resuming flying today was grounded for 10 days. That's 10 days that it was meant to be flying and wasn't. It will now return to service until its planned retirement."

Contributed by Qantas Airways Limited