One important factor could act as a brake on China's rapid progress while another has almost certainly assisted it, to an unknown extent. The limiting factor is engine technology: so far, all China's modern, operational fighters fly with Russian-supplied engines. The positive factor is China's intense and apparently effective campaign of cyber-espionage against Western industries and governments.

The degree to which China's progress has been aided by cyber-espionage is not easy to determine, but that is not surprising. What is known euphemistically as the Advanced Persistent Threat (APT)—a pattern of sophisticated and continuously evolving penetration attempts aimed at computer networks, predominantly originating in China—was not identified in the West until 2006, and Chinese aircraft and weapons that have been disclosed in any detail mostly date from before that.

Details of newer products, such as the low-observable systems—materials, edge treatments, door and aperture design and electronic apertures—on the J-20 and J-31, and the J-10B's active, electronically scanned array (AESA) radar, will be a more telling indicator of the value of the APT.

What is certain, however, is that cyber-espionage is potentially the most valuable addition to spycraft since the advent of signals intelligence. The intake can include large volumes of detailed technical information that can be disseminated with relative freedom to end-users—people designing and engineering systems. The relative freedom comes from the fact that no agents are at risk and the techniques and software used for network penetration are not designed for a long life: The presumption is that they will be detected, countered and replaced with something new.

In fact, there is no fundamental reason why a defense engineering organization should not have its own network-penetration unit or engage a contractor for the mission, so that project managers can actually ask for the specific data that they want. And some benefits of this kind of intelligence may be invisible: As well as knowing how another team solved a problem, it is useful to know when and how your rivals hit a dead end.

But given that, why does engine technology represent such a stumbling block? The underlying reason is that the design and manufacture of high-performance, reliable combat engines in the West rely on a sophisticated, large, but tightly controlled base of suppliers, feeding only four primes with the demonstrated ability to deliver such engines.

The supply chain incorporates advanced technologies and a knowledge base that is mostly unique to turbine engines, focusing on high-temperature metal alloys and processes for designing and fabricating critical components such as blades, disks and shafts. An important point, however, is that the supply chain has not been dominated by military-engine business for decades, and today, the market for commercial engines—which drives the same critical technologies to deliver performance and durability—dwarfs the defense business.

China's engine developers cannot tap into a supply chain of that competence and efficiency, and no matter how good the nation's access may be to Western technical data, that remains a physical and economic disadvantage that is hard to overcome.

Nor, so far, has the need to import Russian engines turned out to be an actual limiting factor, rather than a potential one. The number of AL-31s ordered by China explains why Sukhoi's complaints about the development of the J-11B and J-15 have resulted in no action, with engine deliveries continuing unabated. China has been buying fully half of Russia's output of that engine. For the time being, Russia's engine industry is hooked on Chinese cash.