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'Pressure wave' shakes region

An RAAF pilot prepares to enter the cockpit of his F/A-18 Hornet while taking part in East Coast Air Defence Exercises off the NSW North Coast this week. A military jet is suspected of causing a sonic boom which rattled houses and woke people from their sleep across the region on Thursday night.
An RAAF pilot prepares to enter the cockpit of his F/A-18 Hornet while taking part in East Coast Air Defence Exercises off the NSW North Coast this week. A military jet is suspected of causing a sonic boom which rattled houses and woke people from their sleep across the region on Thursday night. RAAF/Sgt William Guthrie

THOUSANDS of North Coast residents had a supersonic awakening on Thursday night as a suspected ‘pressure wave’ from a military jet hit the region just before 10 o’clock.

Reports of windows rattling and walls shaking from Coffs Harbour to Tweed Heads, and as far inland as the Border Ranges, came through The Northern Star switchboard throughout the day yesterday as people tried to find out what had happened. Talkback radio and internet chatrooms also went into overdrive.

Many thought an earthquake had hit the region, but Geoscience Australia in Canberra ruled that out, having recorded no seismic activity on the North Coast.

The Department of Defence credited the East Coast Air Defence Exercises (ECADEX) off the coast of Evans Head as the ‘possible source’ of the silent pressure wave that shook North Coast homes.

A similar shock wave was reported around Newcastle on Tuesday night.

ECADEX was a major defence exercise conducted over two weeks off the coast between Evans Head and Newcastle.

It involved 28 supersonic jet fighters, plus mid-air tankers and a United States Air Force B52 flying directly out of Guam in the Pacific.

The exercises were designed to test Australia’s forward defence capabilities with ‘enemy’ Red forces operating from the RAAF Amberley base near Ipswich, and the defending Blue forces flying out of Williamtown RAAF base near Newcastle.

While all activity was conducted about 50km offshore, little effect was felt throughout the two-week exercise until it wound up with a bang on Thursday night.

Air combat commander Air Commodore Neil Hart told The Northern Star last week the exercise was being escalated as personnel familiarised themselves with the combat scenarios and that tactics were expected ‘to go supersonic’ taking up huge amounts of airspace.

Jill Harrison, of Jiggi, reported being almost shaken out of her bed in her pole house.

“It reminded me of the Sydney earth tremors in the seventies,” she said.

Greg Dougall, of The Channon, said he copped the blame from his partner for waking her up after their whole house shook.

Trish Sayers reported heavy-duty vibrations up on the Border Ranges.

“I’ve been at Woody Head during a sonic boom and it was nothing like that,” she said.

The Department of Defence said the aircraft were operating in accordance with supersonic flight rules and restrictions, but that atmospheric conditions could sometimes propel sound waves up to 100km.

Much like a boat in water, pressure waves occur when a moving aircraft compresses its own wake, which is limited to the speed of sound. As the aircraft breaks the ‘sound barrier’ the wake merges into a single shock wave. The speed of sound is also referred to as mach 1, which is about 1225km/h.

The white cloud seen on today’s front page photo is formed by condensed water droplets resulting from a drop in air pressure around the aircraft as the shock wave passes from the aircraft’s nose to its tail.

The common effect of the pressure wave was the rattling of windows and doors on people’s houses, leading many to think someone was breaking in. Police reported numerous calls from concerned residents.



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