A Jetstar plane coming into land at Adelaide Airport came within 30 seconds of colliding with a second plane on the tarmac after that flight crew misheard instructions, a report reveals.

The Australian Safety Transport Bureau (ASTB) said the Alliance Airlines flight had just landed from Olympic Dam with 53 people on board during August, when its pilot taxied onto the tarmac, into the path of an incoming Jetstar Airbus transporting 150 passengers.

The Jetstar pilot spotted the smaller plane from the air and abandoned the landing.

The ASTB reported that the start of transmissions from the airport’s surface movement controller (SMC) were clipped, or “over-transmitted”, so that words at the start of a sentence were not heard by its intended recipient.

The SMC told the flight crew of the smaller plane: “Unity 3201 hold short of runway 23, I’ve got you going to 50 golf”. But what the flight crew heard was: “Runway 23, I’ve got you going to 50 golf”.

Because the instructions included the parking bay, the plane’s check captain believed the SMC had instructed them to cross runway 23, and responded by saying, “cross runway 23 to 50 golf, Unity 3201”.

The SMC thought the check captain read back “short runway 23” and assumed that the word “hold” had been clipped.

The aerodrome controller (ADC) sighted the plane on the taxiway and heard the SMC’s instructions for it to “hold short”, but did not hear how its flight crew responded. The ADC then handed their position over to another controller.

Jetstar plane takes evasive action

Both flight crew members of the smaller plane believed they had been given clearance to taxi across the runway.

The captain did a visual check to the left and stated the runway clear to cross. The check captain looked to their right and sighted the approaching Jetstar flight, commenting that it looked a lot closer than expected.

The captain continued to taxi across the runway and did not sight the Jetstar Airbus at any time.

The SMC, who had looked down at the screen, looked up to see the smaller plane crossing the holding point.

The SMC called “hold short”, but then realised it was incorrect terminology, and said: “Unity expedite expedite”.

The Jetstar flight was 30 seconds from touchdown when its crewmembers spotted the smaller plane crossing their runway.

The SMC looked up to see it about 30 metres above the runway and already taking evasive action. A few seconds later the ADC instructed it to go around.

Clipping issue blamed on radio technique

Reporting to the ASTB, the ADC said the clipping of transmissions was a known fault that had been reported via the Airways Systems Issues Database (ASIC) and controllers had become “desensitised” to it.

The SMC reported that the first second of a most read backs transmitted from pilots at Adelaide Airport were clipped.

They said it was difficult to tell when a controller’s push-to-talk (PTT) button was released, and whether a frequency was open or closed, and suggested radio controllers wait two heart beats after pushing the PTT button before talking.

Airservices Australia provided ASTB with a report into the transmission issue.

It said in August 2014 a copy of the ASID report was updated to incorporate the clipping issue into a voice system training manual.

In 2015 the report was updated to say the issue would “be addressed in the next software release, which is currently scheduled for delivery in June 2016”.

But in 2016, a comment was added to the report suggesting that the issue was not a system defect, but was instead attributed to poor radio technique by pilots or air traffic control. It also said controllers should release the PTT as soon as possible to make sure the receiver was unmuted.

Pilots commended for their actions

The ATSB commended the Jetstar pilots for taking evasive action.

It said the smaller plane’s pilots could have queried their instructions to cross the runway when they spotted the larger plane approaching for landing.

In air traffic control, it concluded that the SMC did not question the pilots about missing words because clipping was an ongoing issue.

It also said the ADC was in the process of a handover and was not watching the landing of the larger plane, or the runway, because they assumed the smaller plane would hold short.

It recommended that controllers and pilots used correct phraseology at all times.

It also said they needed to challenge instructions when they were not heard in full or were possibly misheard.

– ABC