Passenger jets at Melbourne Airport were at risk of colliding during take-off on two separate occasions in 10 months, safety investigators have found.
The two incidents mirrored one another: a slow climbing airliner was effectively tailgated into the air by a faster second passenger jet, with air traffic controllers failing to alert the respective air crews that the safe gap between them was rapidly disappearing.
Safety investigators found in both cases, in 2010 and 2011, the minimum safe distance between the two aircraft had been breached, leading to a "loss of assurance" the planes would not collide.
That such "separation" breaches occurred highlighted a fundamental oversight in departure procedures from Melbourne Airport: the procedures did not take into account differences in airspeed between climbing aircraft.
Only now are the procedures being revised, the Australian Transport Safety Bureau reports.
In the first incident, on December 5, 2010, a Virgin Australia 737 took-off from Melbourne Airport bound for Brisbane.
But it was heavily laden, with pilots reporting to air traffic control that they were unlikely to reach the expected airspeed, and were on a slow ascent.
But under "auto release" Melbourne departure procedures, a minute later the next aircraft, a Qantas 767, was cleared for take-off.
It took off bound for Sydney, and rapidly closed on the lumbering 737. Both planes had been assigned to head for the same altitude, 24,000 feet, on similar flight paths.
The minimum safe distance between the two aircraft was supposed to be 5.6 kilometres horizontally and 1000 feet (304.8 metres) vertically — but the speed difference meant that gap rapidly vanished.
Within minutes the minimum safe gap between the two was breached; the planes, closing at the rate of 60 knots, came within 3.7 kilometres and 300 feet of one another, then 3.5km and 500ft.
"...At no stage did the controller issue a traffic alert to the crew of either aircraft," investigators said. The controller "expected the 737 and 767 would climb at similar speeds".
In the second incident, on October 12 last year, the first aircraft, a Jetstar A320 took-off from Melbourne bound for Auckland, followed little more than a minute later by a Virgin Australia 737 heading for the Gold Coast.
In a similar fashion, the Jetstar plane was heavily laden and climbed slower than the Virgin jet coming up from behind.
The safe gap between them was breached, coming as close as 3.9 kilometres and 800 feet, closing at a rate of 50 knots and again controllers did not alert either set of pilots.
The two incidents exposed a flaw in Melbourne's departure procedures "that had the potential to lead to higher-risk situations", investigators said.
The Melbourne departures procedures were based on those used at Sydney Airport. But the Sydney Airport procedures took into account the respective airspeeds of the planes — the Melbourne procedures did not.
"The procedures at Melbourne required no minimum speeds for departing aircraft ...," investigators said, "... with no documented requirements to ensure jet aircraft would maintain a set climb speed" — a procedural flaw considered to be a "significant safety issue".
While the Sydney air traffic control procedures were workshopped by Airservices Australia before adopting them in Melbourne, staff didn't identify airspeed differences as "considered hazards" in departures procedures.
Airservices told investigators "it was not an issue specific to auto release [departures procedures] but applied to all phases of flight".
Airservices has now rethought its position: it applied to the Civil Aviation Safety Authority on March 23 to revise its Melbourne departure procedures to take into account differences in airspeed between climbing aircraft.