Descend towards the Barrington Tops, northwest of Newcastle, and the creases and folds of the topography come into focus like a screwed-up ball of tissue paper laid flat on a table.

Only at ground level is it possible to truly appreciate those jagged striations: cliffs and ridges climbing towards the clouds, ravines and valleys cloaked in the kind of vegetation that hides the sun almost fully and hides secrets even better.

Convicted murderer Malcolm Naden managed to evade capture for years in these ranges, using the inhospitable terrain to his advantage.

And here, one of Australia’s most enduring air crash mysteries continues to confound investigators.

Somewhere in all that green lies the wreckage of VH-MDX, a single-engine Cessna 210M Centurion, and the remains of the five people on board when it went down more than 35 years ago.

Hopes are rising it won’t stay lost forever.

The final transmissions from that doomed flight on August 9 1981, last only about two minutes when edited together, the creeping urgency clearly audible in pilot Michael Hutchins’s voice.

Even 35 years on, it is chilling to listen to — particularly his final, panicked radio call indicating the plane’s perilously low altitude, 1000ft below the minimum safe flying height in the Barrington Tops area.

Each man on board must have known at that moment he was in mortal danger.

The plane would crash perhaps seconds later ... certainly no more than a few minutes.

Over the coming days and weeks, police and other authorities would undertake one of the biggest air and land searches in NSW history.

Yet not a trace of the crash has ever been found.

Investigators know it is a long shot, a real needle-in-a-haystack scenario, but they’re working towards changing that outcome.

Using thousands of hours of exhaustive research, modelling of the plane’s most likely trajectory after its last transmission and aerial drone photography of possible crash sites, teams from the Bushwalkers Wilderness Rescue Squad (BWRS) will head into the Barrington Tops this weekend, chipping away at the monumental task of solving the mystery.

Glenn Strkalj has been wrestling the VH-MDX “conundrum”, as he calls it, for two years.

Strkalj is a former RAAF fighter controller, an aircraft maintenance engineer and a commercial pilot who has logged hundreds of hours of flying in the same kind of Cessna 210 as VH-MDX.

He joined the Bushwalkers Wilderness Rescue Squad two years ago, aiming to establish and lead a team to compile, analyse and publish information about the mystery, and will co-ordinate this weekend’s search.

We can’t lie to ourselves to make it more rosy as others have done in the past, but this is a conundrum that can be solved

“The original tasking of the squad in 1981 was to find the aircraft and that’s still what we want to do,” he said.

Strkalj believes the plane’s wreckage can be found, but only with “critical and continual” research, coupled with boots on the ground in the possible crash zone.

BWRS has narrowed that zone based on VH-MDX’s “radar fade area” — the point where the plane dropped too low for Sydney air traffic control to pick it up on radar.

It spreads about 22sq km into the Barrington wilderness, the search of which represents a huge challenge, given the rugged terrain.

Factor in the possibility the plane descended gradually rather than plummeted in a spin and the potential crash zone balloons to about 80sq km.

“We can’t lie to ourselves to make it more rosy as others have done in the past, but this is a conundrum that can be solved,” Mr Strkalj said.

VH-MDX took off into clear sky from Proserpine on Queensland’s Whitsunday coast on Sunday, August 9 1981.

Hutchins, a pilot with more than 3400 hours of flying experience and 4400 hours logged as a navigator, had been tasked to fly four men —Ken Price, 54, Noel Wildash, 40, Rhett Bosler, 33, and Philip Pembroke, 43 — to Bankstown in Sydney for a routine flight.

The small group was homeward bound after sailing together up the east coast to deliver a yacht to its new home.

The plane made a refuelling stop at Coolangatta, and as the sun set over the Great Dividing Range west of Yamba, spirits aboard the cosy Centurion must have been jovial.

These were men’s men — happy in the company of other blokes but looking forward to getting home to family.

Back then, like now, planes flying the north-south coastal route sometimes had to obtain air traffic control clearance before entering a circle of military airspace radiating from Williamtown RAAF base just north of Newcastle.

The pilot opted to fly around the restricted zone instead, turning inland just south of Taree towards the eastern foothills of the Barrington Ranges.

Conditions that night were relatively clear, but as the Cessna cruised above the mountains, turbulence and local cloud churned up by the landscape’s peaks and troughs started to play havoc.

VH-MDX ended up tracking 45km northwest of the intended route, and entered unexpected cloud along the way.

Then came the failure of two crucial pieces of equipment on-board: the artificial horizon, indicating a plane’s level flight and the direction indicator, showing the plane’s compass heading.

Adding to the trouble, ice began to accumulate on the plane’s wings as it traversed the Barrington Ranges, severely reducing the its aerodynamic capability

The pilot radioed in at 7.36pm to report that the plane was “up and down like a yo-yo” and that his backup compass was swinging around “like blazes”.

Two minutes later, VH-MDX started losing 1100ft of altitude a minute, but even then Hutchins appeared calm and in control of the situation as he radioed Sydney flight service.

At 7500ft, the plane was now just 2300ft from the highest point of Barrington Tops.

But within a minute it had dropped to 6500ft, with the pilot’s concern about the situation palpable.

At 7.39pm came the final, haunting transmission: “Mike Delta X-Ray ... 5000!”.

Then, silence.

After virtually circumnavigating the Barrington Tops, VH-MDX ended up in some of the worst conditions possible for aircraft icing.

“When an aircraft has accumulated significant ice, the lowest speed it can fly - the stall speed - is no longer valid,” Mr Strkalj explained.

“The aircraft will stall at some higher speed. Sometimes much higher.

“With VH-MDX it was a case of attempting to hold out long enough to clear the ranges whilst heading for the coast.”

“But despite all the theories out there, it can’t be known until the aircraft is located whether VH-MDX did stall and spin or if it simply flew into terrain.”

VH-MDX was fitted with a locator beacon designed to be triggered by the G-forces of a crash, but it had to be switched to automatic activation mode.

Teams involved in the latest search are focusing on an area about 16km east of the traditional search zone.

How the search will unfold

Mr Strkalj said new analysis techniques looking at the minimum altitude the plane could fade from radar view, and cross-referencing that data with the plane’s course, the pilot’s reported altitude and interviews with “key players”, had prompted the shift.

Conventional line searchers — participants spread shoulder-to-shoulder, scanning the ground ahead — will form part of the work, but so too will more mobile squads probing more difficult territory.

These units will cover any high-probability areas, looking for wreckage scatter.

Drones will also be used to obtain an aerial view.

After 35 years, it is possible the wreckage will be buried beneath soil and vegetation.

But Mr Strkalj said wreckage found at other small aircraft crash sites around the world, even those that had gone undiscovered for years, showed there was usually something at ground level to indicate what had happened.

There are also other, hi-tech options.

A project pioneered by BWRS members involves using an airborne laser to penetrate the tree canopy in search of “hot-spots” where VH-MDX might be.

Mr Strkalj said such a device had the potential to detect metal on the forest floor invisible to the naked eye.

“There are going to be a lot of false positives,” he said.

“But it will be found. The time frame is the question and unfortunately no-one can answer that.”