Each more than two decades old, the workhorses of Qantas' international fleet were near the end of their working lives for a full-service airline.

The global financial crisis helped accelerate their retirement. It also made it harder to sell the last of the airline's 747-300-series flying machines.

Buyers were thin on the ground. The passenger planes sat in the dry desert climate until August 2010 when Qantas finally found one.

Within 20 months, one of the jumbos would be in Iran - in defiance of US trade sanctions against that country's regime, and raising questions about the dark side of the second-hand aircraft market. It reveals the lengths some will go to circumvent strict trade sanctions on Iran.

Without US authorities intervening, the two other queens of the skies, which once plied long-haul routes for Qantas, were also presumed destined for the Islamic Republic.

The trail leads from Qantas' jet base in Sydney to the Arizona desert in the US, to a small sheikhdom in the Middle East, West Africa and finally to Iran.

Qantas sold the planes to Sayegh Group Aviation, a leasing company in Sharjah, an emirate in neighbouring Dubai. Less than 300 kilometres from the sheikhdom on the other side of the Persian Gulf is Iran.

The sale to Sayegh sparked the beginning of a complex series of deals to related companies.

The case lifts the lid on the murky world of the second-hand aircraft market.

It also raises questions about the obligations of aircraft sellers such as Qantas to run the ruler over buyers to ensure their planes do not end up in the wrong hands.

A Californian aircraft leasing company, which blew the whistle on Sayegh early last year, had also been approached by the same companies from the United Arab Emirates and the West African nation of Gambia wanting to buy planes.

''[Qantas] could have easily figured it out if they wanted to. I'm pretty sure they just decided they are just going to cover it … as they are legally required to do,'' CSDS Aircraft Sales and Leasing president Benedict Sirimanne says.

''Whenever you sell an airplane, you have to do a little bit of due diligence.''

The revelations are a fresh embarrassment for Qantas. The airline is battling to turn around its fortunes after industrial disputes, the grounding of its fleet, a public stoush between chief executive Alan Joyce and his former mentor Geoff Dixon, and a dogfight with Virgin Australia in the domestic market.

The path to Tehran involved a series of deals over 16 months. The planes were shifted from one company to another - all of which were

connected.

The crucial conduit was a front company the Arab owners of the old Qantas 747 jumbos had set up in Gambia. As US investigators put it, this ''clean'' company in Gambia was created for the ''purpose of facilitating the lease of the 747s to an Iranian airline or airlines''.

After buying the planes from Qantas and registering them in the US in August 2010, Sayegh sold them to Sam Air in July 2011.

Five months later, Sam Air sold them to Aviation Legacy, the ''clean'' company in Gambia.

Then, about 10 days later, the Gambian company leased one of the jumbos - named the ''City of Tamworth'' when it was in the Qantas livery - to Iranian charter airline Aban Air. Its small fleet includes Soviet-era Ilyushin Il-76 heavy-lifting cargo planes and a Boeing 747-200 freighter.

In March last year, an Iranian plane enthusiast photographed the jumbo on the tarmac at Tehran's main airport, Mehrabad International, in basic Qantas livery featuring an ''S'' on the tail and fuselage.

In one shot, it is shown parked near an aircraft hangar emblazoned with the logo of Iran Air, sparking speculation it was the first of three former Qantas planes destined for Iran's national flag carrier.

Mehrabad is the airport featured in the final scenes of Ben Affleck's political thriller Argo, about the escape of six American hostages from Tehran in 1979 following the Iranian revolution.

In this real-life drama, Abdullah Ramadan, the managing director of both Sayegh Group and Sam Air, conceded to US authorities that the deals were done so the lease to Aban Air would appear to be through a ''clean'' company, Gambian firm Aviation Legacy.

Although the lease was signed in December 2011, it was not until three months later that the Iranian airline was to take the keys to

the 747.

Aban Air's plan was to fly the jumbo between Tehran and the Thai capital Bangkok.

But in April last year, US authorities stepped in, several months after the boss of the Californian aircraft leasing company alerted investigators at the US State Department.

With one jumbo already in Iran without approval, the acting assistant secretary of the US Commerce for Export Enforcement Donald Salo jnr concluded future violations were ''imminent''.

Ramadan had indicated to US authorities that the other two jumbos had been flown in and out of various countries in the Middle East, including Syria.

''Moreover, Aviation Legacy was created by Sam Air/Ramadan in an attempt to make the lease to Aban Air appear to be made by a clean company and … two other 747 aircraft are owned and intended for lease through Aviation Legacy,'' Salo wrote in his order last April.

''The conduct in this case is deliberate, significant and likely to occur again.''

He slapped an order on Sayegh Group and its related companies, banning them from exporting any goods from the US for six months.

The attempts to circumvent the trade embargo on Iran are not unsurprising.

After 18 years of US trade sanctions, Iran's aviation industry is desperate for planes and parts - even two-decade-old jumbo jets unwanted by airlines such as Qantas.

The embargo has had an adverse side effect: it has made air travel in Iran a risky business. More than 1000 people died in at least 15 plane crashes in Iran between 2001 and 2011, the National Iranian-American Council in Washington DC says.

After protests from Iran about the impact of the embargo, a United Nations agency recommended that experts look into the charges. They found, in a paper issued in 2005, that the US embargo had ''endangered the safety of civil aviation in Iran''. It is the last time such an investigation is known to have been conducted.

Bill Clinton first imposed sanctions against Iran in 1995 when he was president.

As part of the embargo, US companies are prevented from selling and exporting aircraft, engines and spare parts to Iran. Nor can companies in other countries resell to the Islamic Republic most equipment originating in the US, for example planes built by Boeing.

The ban on selling civilian aircraft to Iran is part of an extensive range of sanctions by Western countries. In recent years, the trade embargo has become more about preventing Iran from developing nuclear

weapons.

And it is not difficult for

companies outside the US to be caught in its net.

While they may not be based in the US or have American employees, 99 per cent of aircraft are traded in US dollars, making it nigh impossible to avoid the world's

biggest economy.

In 2007, ANZ fell foul of the US's heavy trade sanctions for facilitating about $50 million worth of US dollar transactions with countries including Iran, Cuba and Sudan over five years. While the bank did not breach Australian sanctions, it was caught out because the deals were in US dollars.

ANZ later copped a $US5.75 million fine by the US Treasury for breaching the trade bans.

It also withdrew from trade financing in about a dozen countries against which the US has sanctions, regardless of the currency used in deals, to protect itself from falling foul again. Several months before ANZ ran into trouble, the Dutch bank ABN Amro was fined $US80 million by US regulators.

The Swiss investment bank UBS also paid $US100 million in penalties in 2004 after admitting to breaching trade bans with Iran, Libya, Cuba and Serbia.

In this saga, the Californian aircraft-leasing company that alerted investigators early last year accuses Qantas of thumbing its nose at it when it showed interest in buying the jumbos on several occasions. It has also complained to the Department of Foreign Affairs and Trade.

CSDS says the ''front company'' in Gambia and another linked to Sayegh contacted it several times about buying planes. But the approaches made about three years ago quickly raised red flags, and CSDS noted the advances in a list it keeps when it has concerns.

Sirimanne says he became suspicious when people from the Gambian company and Sam Air did not reveal where the planes they were interested in buying would end up.

''Whenever that happens, we keep a really close eye on it because we don't want to walk into a huge problem ourselves,'' he says.

The Californian company had also been keen on buying Qantas' jumbos.

It had a ''solid client'' for two of them about four years ago, and Sirimanne sent his partner to Australia for meetings with Qantas.

''We were very close to buying one of the airplanes [City of Tamworth],'' Sirimanne recalls.

''Everything was going great and then all of a sudden they said, 'Too bad, the boss made a decision and two of the planes are being sold as we speak'.''

Apart from alerting the State Department and DFAT, Sirimanne emailed Qantas' Joyce last May to complain about the airline's actions.

Like other large airlines, Qantas has a team devoted to trading in aircraft.

''If you are in the trading market, that's your job: to know the demand, where things are, what kind of airplanes are trading hands,'' Sirimanne says. ''It is not rocket science.''

He says Qantas has a ''mentality where they will only deal with the people who actually write the cheque. That is where they get screwed. When you have people working and networking with you, you hear lots of things in the market. It's very hard to keep secrets in aviation.''

But Qantas refutes the claims from CSDS, which the airline describes as an ''unsuccessful bidder for the aircraft'' in question.

''It's drawing a very long bow to suggest that we're responsible for the conduct of third parties, who owned or leased aircraft several transactions after the original date,'' a Qantas spokesman says.

''Whenever we sell an aircraft, we carry out extensive due diligence on the buyer and their intended use of the aircraft and include strict, specific clauses in the sale agreement reinforcing the buyer's obligation to comply with all relevant international export controls and regulations.''

Qantas says it met all its legal obligations and took all reasonable precautions in respect of the sale of the three Boeing 747s to Sayegh Group Aviation.

It said Sayegh was not subject to any US regulatory investigation or process at the time of the sale.

At the top of CSDS' no-go list is Iran, followed by North Korea, Syria and Cuba.

Sirimanne believes plane owners have an obligation to be sure of two things before they sell: ''After 9/11, you have got to make sure you are not selling an airplane to some sort of terrorist group. Then the embargoed countries.''

Iranian airlines are eager for Western aircraft because they have better safety records than those bought from other parts of the world, including Russia.

They have been especially interested in Boeing 747s and Airbus A340s.

By some estimates, there are between 50 and 100 jumbos for sale worldwide at present. These planes have quickly been supplanted by new generations of aircraft such as A380 superjumbos, and high fuel prices are making them expensive to operate.

''There are certain airplanes that interest Iranians. I get a lot of requests for [Airbus] 340s and I believe those are coming from Iran,'' says Sirimanne, who regularly receives inquiries from federal investigators, including the State Department, about where planes have ended up.

''The brokers in the Middle East are putting the feelers out for 340s.''

In Asia, aircraft leasees say there is speculation an Iranian airline recently picked up two A340-300s previously owned by an international airline. It occurred after a company in Thailand stumped up the cash for the planes, and promptly shipped them to Iran after taking delivery.

The second-hand aircraft market has long been shadowy.

After September 11, 2001, it became what one insider describes as the ''greatest buyer's market of all time'' for second-hand planes. Demand for travel slumped, leaving airlines to park aircraft en masse.

Here, the collapse of Ansett sent the local industry into a tailspin, and put even more planes into an already depressed international market.

Qantas was also forced to scrap several 747 jumbos and short-haul 737s in the economic and political maelstrom after the US terrorist attacks because of weak demand for planes.

Its aircraft have historically been sought after by second-hand buyers because they have a reputation for being well maintained.

And they have ended up in all parts of the world, including Africa.

Every year, the aircraft leasing industry meets at a conference in Miami which, as one insider puts it, ''tells you something'' about the market. It is a place for people to build networks and reach deals.

''The second-hand aircraft market is probably dodgier than the second-hand car market,'' an executive from a large aircraft leasing company says.

''If someone said they were a used-car salesman, you'd kind of automatically set a fairly low set of expectations in terms of honesty and disclosure. What is different between used cars and used planes - it is just different vehicles and a much bigger sticker price.''

Large airlines such as Qantas have trading departments which specialise in buying and selling planes. Top of their list of items to tick off is ensuring buyers can pay.

At present, Qantas' aircraft trading department is searching for buyers for six of its Boeing 747-400s, which are between 21 and 24 years old. The airline is not advertising their sticker price.

''These are big transactions, so you want to make sure the people you are working with can meet their financial commitments,'' a source says.

For airlines such as Qantas, it is a bigger issue because they are not set up to manage situations where payment is outstanding. In contrast, large leasing companies have repossession capabilities that allow them to fly planes out of a jurisdiction if an airline goes bust.

While the top echelons of the second-hand industry are relatively respectable, the bottom tiers are notoriously murky, with planes ending up in ''funny jurisdictions'' in Africa and former Soviet republics.

''As [a plane] gets cheaper and cheaper, you get a different class of person interested in them … a potentially criminal element or these sanctioned countries,'' an aircraft leasing executive says.

''In the medium-to-older aircraft end [of the market], stuff goes into different jurisdictions. When an aircraft is full priced brand new, you have to have a fairly legitimate business to generate the cashflows for it. But as it gets cheaper and cheaper as it goes from owner to owner, you get different business models.

''Even out of Russia, you can never know whether the money is clean or not. If you start dealing with some of these guys, you just don't know where it is going to end up.''

For Qantas, the latest development is a salutary lesson in ''seller beware''.