A United Airlines flight carrying 239 passengers has landed at Sydney Airport after an emergency mayday call.
The worrying signal triggered a full emergency response and all major roads around the airport were closed as the UA839 service landed safety just after 6.30am.
There were reports the mayday call was made after dry ice began leaking on the plane which was en route from Los Angeles — however a Airservices Australia spokeswoman told news.com.au this was not the case.
"There is no risk, the plane has landed safely and everybody is OK," a spokeswoman for the corporation said.
She said the mayday call was sent automatically because the plane's fuel had dipped below a certain level.
However, she said there was "heaps of fuel" left in the tank and the signal was merely a precautionary measure.
A police traffic control plan was activated at 6.36am, with some major roads surrounding the airport closed as a precaution.
However, NSW Police said the roads were reopened just minutes later after the plane had landed safely.
Passengers from other flights said they barely noticed the incident.
"This is the first time I'm hearing about it on the way out the front door," a passenger from another flight told Nine News.
"Everyone was calm, no panic, no announcement. I can't explain it."
A spokesman for United Airlines said there was a "mechanical issue" with the plane.
"United flight 839 from Los Angeles to Sydney landed safely in Sydney following a mechanical issue," he said. "The aircraft taxied to the gate and all customers disembarked normally."
A spokesman for Sydney Airport told news.com.au the there have been no knock-on effects from the incident and no delays.
Airport Firefighters have declared the plane safe.
There are three different types of emergency alerts from planes, and according to the Airservices Australia website:
• A 'mayday' call indicates an aircraft is in grave and imminent danger and requires immediate assistance. Controllers immediately alert aviation rescue fire fighters (ARFF) services if available, as well as local emergency services, with details on the incident to enable them to respond appropriately and provide information and assistance to pilots throughout the emergency.
• A 'pan' call is used to describe a less urgent situation but one that still requires attention from air traffic control. In this case, controllers provide information and assistance to pilots as well as informing ARFF or local emergency services as necessary. Examples of 'pan' situations include a passenger medical emergency or instrument malfunction.
• A priority landing occurs when controllers reschedule aircraft departures and arrivals to allow an aircraft that might have a problem to land as soon as possible. Medical emergencies may lead to priority landings but priority landings are not 'emergency landings' or evidence of any actual problem with an aircraft.