Fourth Qantas flight turned around in 48 hours

A fourth Qantas plane has been forced to turn back mid-flight, marking 48 hours of technical misery for the national carrier.

The Boeing 717 was forced to turn back 20 minutes into its flight from Hobart to Melbourne at 6.30pm on Tuesday after an indicator light in the cockpit reportedly went off.

The Boeing 717, which holds 110 passengers, is being assessed by engineers in Hobart, the ABC reports.

It was the fourth time in two days a Qantas plane had been forced to make an unscheduled landing.

Qantas chief Alan Joyce earlier played down the recent aircraft troubles, saying such events "happen all the time".

On Monday, Qantas flight QF7, en route to Dallas from Sydney, returned four hours after take-off due to a technical issue affecting seat power, the in-flight entertainment system and some toilets.

It was the third flight to have been forced to make an unscheduled landing because of non-safety-related issues.

The A380 was the third Qantas flight to make an unscheduled landing in less than 24 hours.

A statement from Qantas said the decision to return to Australia was based on passenger comfort.

The company said customers who do not live in Sydney have been booked into hotels and travellers have been rebooked onto another flight tomorrow morning.

On Monday afternoon a flight from Perth to Karratha was turned back after a strong odour was detected.

The pilot of the 737 aircraft called for emergency landing protocols after smoke was detected in the cabin.

About 80 passengers were assessed by St John Ambulance staff over concerns about smoke inhalation.

Nobody was taken to hospital.

Earlier in the day another A380 jet, travelling from Dubai to Sydney, made an emergency landing in Perth after developing an air conditioning fault while flying over the Indian Ocean.

"As a precaution the aircraft descended to 10,000 feet and the captain requested a priority landing," Qantas said in a statement.

No-one was injured.

The grounded flight comes amid a bad 24 hour for the Australian airline as two more Qantas jets were involved in an forced grounding in the same 24 hour period.

A Boeing 737 sparked a full-scale emergency before landing safely.

Qantas flight QF904 was operating from Perth to Karratha and took off at 2pm but returned at 2.47pm.

An odour in the cabin of the passenger plane is believed to be the reason for the plane's return.

Police said the pilot notified emergency services about 2.35pm that there was an emergency on board and emergency landing protocols would need to be put in place.

St John Ambulance assessed up to 80 people for smoke inhalation at Perth Airport with nobody being taken to hospital.

The plane was carrying about 85 people.

The cause of the odour is not known.

An air-conditioning fault forced a Qantas Airbus A380 to descend quickly and make a priority landing in Perth early this morning.

Qantas flight QF2 travelling from Dubai to Sydney was diverted to Perth because of a fault in its air-conditioning system. The plane descended to 10,000 feet and the captain requested a “priority landing” at the airport.

The incident started about 1am over the Indian Ocean, around 700km north-east of Perth.

A Qantas spokesman said the jumbo, which landed safely, was being inspected.

"It was a fault with the air-conditioning so the engineers are having a look at that now,” he said. “It's taking a little bit of time to get to the bottom of it.”

The jet returned to Sydney yesterday evening empty.

Qantas flew a Boeing 747 to Perth to pick up the passengers.

Passenger Nigel Richardson tweeted pictures from the cabin showing all was calm on board.

"Full shut down of air conditioning system that keeps plane pressurised at altitude was the problem," he tweeted.

Mr Richardson praised the response of the crew to the emergency saying "#qantas first officer walking through plane giving personal situation update to everyone as best he can. Full marks to crew #QF2".

No one was injured.

The plane is being inspected by engineers.

Passengers were given accommodation and booked on the next flight to their destination.