(Reuters) — As the clock runs down to Monday’s sanctions deadline for Russian airlines to hand back more than 400 leased planes worth almost $10 billion, foreign lessors are fast losing hope that they will get their aircraft back.

Most of the planes are still flying Russian domestic routes, according to consultancy IBA, although Bermuda and Ireland —where most are registered — have suspended airworthiness certificates, which usually means they should be grounded.

Aviation was an early business casualty of Moscow's invasion of Ukraine, as the West and Russia imposed tit-for-tat airspace bans. Now, lessors face big write-downs or a long insurance battle as the March 28 deadline looms for terminating plane leases under European Union sanctions.

“I'm afraid that we are going to witness the largest sort of theft of aircraft in the history of commercial civil aviation,” said Volodymyr Bilotkach, an associate professor of air transport management at the Singapore Institute of Technology.

Dual registration is not allowed under international rules, but Russia has already moved more than half of the foreign-owned aircraft to its own registry after passing a law permitting this, Russia's government said Wednesday.

The government also said 78 planes leased to Russian carriers had been seized while abroad and would not fly back to Russia, Interfax news agency said Tuesday.

Although the planes are insured, the unprecedented nature and scale of the potential losses will likely mean years of litigation between lessors and insurers before any decisions on payouts are taken, analysts say. 

And even if the planes are repossessed, the recoverable value would be in question because the aircraft must have accurate maintenance records to ensure they have been fitted with genuine, traceable components — another area targeted by Western sanctions on Russia. 

Although the total value of the planes is huge, the impact on individual leasing companies may still be manageable even if write-downs are required as aircraft leased to Russian airlines account for less than 10% of most leasing company portfolios.

“It's not going to cripple these businesses,” said Brad Dailey, a director at Alton Aviation Consultancy who previously worked at Ireland-based leasing giant AerCap Holdings. “What it does do in my view is it changes the future market potential of Russia,” he said.

Some private airlines have indicated their willingness to hand back planes to the lessors, although it is unclear whether the Russian government will approve the transactions.

Russia's UTair Airlines said on March 14 it would withdraw from service all nine of its leased Boeing 737 NGs, citing owner requirements, a move that could preserve longer-term relations after sanctions end.

Those planes have not flown since that announcement but remain in Russia, data from flight-tracking website FlightRadar24 shows.

In some cases, lessors have security deposits that could be forfeited and could help cover a portion of any losses — but still only a fraction of a plane's value.

Ratings agency KBRA said security deposits generally range from one to four months of rent, depending on the credit assessment of the airline leasing the plane.

Russian national carrier Aeroflot had been viewed as the best credit risk before the invasion, industry sources said, although that assessment no longer applies after Moscow's move to register its planes in Russia.