Sub-standard maintenance is the most likely reason for the crash of a military transport aircraft in south-east Iran which killed 302 passengers and crew, experts say.
The Il-76 was first designed in the late 1960s
Two decades of sanctions by the West against Tehran have left the country with a fleet of mainly ageing planes, often fitted with unofficial spare parts.
However snowy and fog-bound weather in the mountainous region near the border with Afghanistan, where the Russian-built jet came down, could also have contributed to the accident, aviation analysts say.
The crash, which killed 284 members of Iran's elite Republican Guard and 18 crew, is the worst air disaster in Iranian history.
Reports have identified the crashed jet as a Russian-built Ilyushin Il-76 transporter plane, a four-engined model originally designed in the late 1960s and which undertook its maiden flight in 1971.
"The safety of such models is not particularly good, especially if they have not been properly maintained," said Chris Yates, Aviation Safety and Security Editor for Jane's Transport.
The aircraft came down in mountains
"The first point is that for some of these Soviet-era planes, the spare parts are simply not available or originate from some other part of the world," he told BBC News Online.
"The majority of air crashes are caused by three major things: firstly, poor aircraft maintenance; secondly - and this is the most common reason - poor weather; and lastly pilot error.
"In this case I would look to maintenance and spare parts in the first instance."
Since Iran's Islamic revolution of 1979, trade embargoes by western nations have forced Iran to buy mainly Russian-built planes to supplement an existing fleet of Boeings and other American and European models.
According to aviation analyst Jim Eckes of Indo-Swiss Aviation, there is one main problem with maintaining the Russian-built planes.
"It is the supply of spare engines and spare parts from an approved factory source - and there are many unapproved sources where you get the bogus equipment that are available," he told the BBC World Service's World Today.
Another issue was pilot training, he added.
"There is not a simulator available for it (the model of plane) in Iran, so the pilots cannot train under the most extreme conditions, which a simulator provides."
After another Russian-made plane, a civilian Tupolev-154, crashed in south-western Iran in February 2002 with the loss of all 119 on board, Iranian authorities declared all its Russian-built aircraft grounded, the AFP news agency reported.
However this appears to have been widely ignored.
Iran has in recent years attempted to circumvent such problems by building its own planes, but this too has been marred by tragedy.
In December last year, 44 people died when a Ukrainian-built Antonov An-140 aircraft flew into a mountainside while preparing to land near the city of Isfahan.
Those killed were mainly Ukrainian and Russian aerospace experts and executives who were in Iran to oversee the maiden flight of a joint Ukrainian-Iranian aircraft based on the An-140.
Iranian investigators blamed pilot error for the crash.
Almost exactly a year ago, 230 people on board an Iranian Air Force Il-76 - also reportedly Republican Guards - had a narrow escape when a fire reportedly broke out in mid-air after it took off from the north-eastern city of Mashad.
The pilot was able to make an emergency landing, according to reports.