Russia is reportedly close to finalizing a major sale of advanced fighter aircraft to China worth $4 billion. Such a sale, newsworthy in itself, would have been hard to imagine as recently as a few years ago considering the history of previous agreements.

The anticipated deal, first reported by Kommersant, sees the two sides “practically agreed on the delivery of 48 Su-35 multirole fighters, worth $4 billion, to China.” Some reports suggest that Chinese officials have dismissed talk of such a deal. But if true, an agreement like this would certainly make sense for Beijing, giving China access to an advanced fighter. The Su-35 or “NATO Flanker-E +” is described as a “4++ generation (fighter) using fifth-generation technology.” The fighter is “armed with 30-millimeter cannons and has 12 points of suspension for suspension of arms, including missiles and bombs.”

Russia, having lost out in India’s MMRCA fighter competition to French competitor Rafale, would for its part gain an important new military contract. But it seems like Moscow also has much to lose through the sale. After all, purely business considerations aside, Russia is said to have been stung before after selling China advanced aircraft.

And there’s another question raised by the reported deal – why would China want to procure advanced 4th generation fighters when it’s developing a 5th generation plane, the J-20, which is believed to have stealth capabilities?

A look into the recent past offers some possible answers.

The last major aircraft deal between Russia and China involved the SU-27 flanker in the 1990s. Moscow hadn’t sold major weapons systems to Beijing since the so-called Sino-Soviet split, when a rise in tensions sparked border clashes in 1969.

When the Soviet Union collapsed, the Russian arms industry was struggling to survive. Brimming with modern weaponry that could help the Chinese leap generations in equipment and technology, a partnership was seen as benefitting both nations.  In 1992, China placed an order for the Su-27, paying $1 billion for 24 turn-key aircraft.

For China, gaining access to better military technology was of vital importance. Chinese military planners were shocked at the speed at which the United States was able to defeat Iraqi armed forces in the first Gulf War. They realized much of their equipment was now antiquated in the face of U.S. precision munitions, stealth weapons, and advanced fighter aircraft. For the Chinese, as military analyst Andrew Scobell put it, “the primary lesson of the Gulf War in the eyes of many PLA leaders is the primacy of airpower, particularly the importance of controlling airspace or at least denying it to a hostile power.”

In 1996, Beijing paid $2.5 billion for a license to build an additional 200 Su-27s at the Shenyang Aircraft Company in China. The agreement had a very important stipulation. The Chinese version of the SU-27, renamed the J-11, would include imported and highly-advanced Russian avionics, radars and engines.  The aircraft couldn’t be exported, presumably protecting Russia from competition in the international arms market from their own technology.

The deal would not, however, see completion. After the building of 100 or so jets, China canceled the contract in 2004. As The Diplomat security analyst Richard Weitz has noted, Beijing claimed the aircraft no longer met its specifications. Three years later, China seemed to have fully broken the agreement when it would demonstrate the aircraft, the J-11B, on state TV. The plane looks extremely similar to the Su-27. The Chinese deny allegations of copying the craft, claiming that it used 90 percent indigenous parts and that it utilizes better Chinese avionics and radar equipment.

So what would be China’s motivation to purchase new planes from Russia now? Some have argued that maybe all is not well with China’s cloned craft. Vasily Kashin, an expert at the Center for Analysis of Strategies and Technologies speaking with Kommersant, felt  the “willingness to buy such a large batch of fighter jets indicates that the Chinese are faced with serious technical problems during the work on the modification of its aircraft that are based on the Su-27.”

One must also consider China’s highly touted and much-debated J-20 fifth generation fighter might suffer from one major problem – the Chinese have struggled with the domestic production of strong jet engines.  Reports have surfaced that at least one of the J-20 prototypes uses a Russian borrowed engine. Access to Russian engine technology may therefore be one of the driving factors behind Beijing’s interest in the Su-35. As regular Diplomat contributors Andrew Erickson and Gabe Collins have noted, “China’s inability to domestically mass-produce modern high-performance jet engines at a consistently high-quality standard is an enduring Achilles heel of the Chinese military aerospace sector and is likely a headwind that has slowed development and production of the J-15, J-20, and other late-generation tactical aircraft.”

If Russia is to go forward with such a sale, it must ensure the strictest safeguards are in place to protect highly sought after military technology.  Indeed, even with legal guarantees in place, what would stop China from learning all of the technological secrets of such an advanced fighter and incorporating new technologies in its current fleet of aircraft in upgrades or future weapons platforms?  Considering the rapid rise of China’s military over the last decade, fuelled in part by the sale of Russian military equipment and Beijing’s alleged copying of Russia defense technology, Russia would be wise to be cautious in any sale.

While relations between the two are currently stable, Moscow and Beijing may find their interests diverging. The new deal looks good for China, but Russia would do well to be wary when the future of ties is still uncertain.