Investigators are probing how a small "experimental" kit-built aircraft crashed killing two on board in Sydney's south-west on Monday, as one of the worst years for light plane crashes draws to a close.
The Australian Transport Safety Bureau (ATSB) Chief Commissioner Angus Mitchell said his crew had just arrived at the bushland crash site near Appin Road.
The plane came down on Monday, when emergency services responding to reports of a scrub fire confirmed the two fatalities.
The men are yet to be identified, but are believed to be aged 67 and 68.
"It's not a mass-production aircraft it was built essentially from a kit we know it came initially registered from the US," Mr Mitchell said.
"How many years it's been out here in Australia we are yet to determine.
"It's what we refer to as an experimental aircraft."
Initial inquiries suggest the aircraft took off from Temora in the state's Riverina region, according to NSW Police.
The foreign-registered Glasair Super II aircraft collided with terrain shortly before 3pm on Monday.
Safety investigators were sent to the scene to examine the wreckage of the single-engine aircraft, flight information and weather conditions.
The crash sparked a 5-hectare bushfire that has since been extinguished.
Mr Mitchell said it had been a particularly tragic Christmas for a number of families after his team were called out to another fatal small plane crash.
"This is the second fatal aircraft accident we have deployed to in as many days," he said.
"Unfortunately this year has been one of the worst years on record for light plane fatalities, we're up to 23 fatal accidents this year."
Aviation expert Geoffrey Thomas said "do-it-yourself planes" are not uncommon, and are favoured by sport enthusiast pilots.
"Are they as safe as a 747? No," Mr Thomas said.
"They don't have all the electronics or landing aides or anything that a full-blown commercial aircraft has."
There have been 1,200 light-plane kits built in Australia in the last three decades.
The aircraft was severely damaged by the blaze, limiting the amount of physical evidence and electronic data for analysis.
Using a drone, the ATSB will map the site, creating a 3D picture to assist the investigators to better understand the flight profile of the aircraft just before the collision.
Investigators hope to recover any electronic equipment, such as iPhones and iPads, to help build a picture of what had occurred.
The ATSB said the available evidence would determine the size and scope of the investigation and the timeframe for completing its final report.
A report will be published after the investigation is completed, however should a critical safety issue be identified the ATSB will immediately notify relevant parties, Mr Mitchell said.
Transport safety investigators are expected to be at the site for at least the next few days, gathering evidence and interviewing witnesses.
Anyone who may have seen the aircraft, or has relevant footage, can contact atsb.gov.au/witness or Crime Stoppers by calling 1800 333 000 or visiting nsw.crimestoppers.com.au.
