What is a mayday 7700 call? Distress signal from Qantas Flight 144 that startled Australia explained

A Qantas flight has safely touched down in Sydney after the pilots issued a distress signal when an engine malfunctioned – with the nation taking a collective sigh of relief.

Dozens of emergency service vehicles dashed to Sydney International Airport on Wednesday afternoon after QF144 from Auckland issued a mayday 7700 emergency call to air traffic control. 

Air traffic controllers use squawk codes, which are transponder codes used to identify planes in the airspace. 

The code can be changed from the specific one assigned to that flight in certain circumstances, with the code changed to 7700 in incidents when an aircraft is experiencing an emergency event.  

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