Germany has officially conducted military training programs with China in the past. Indeed, it is common for militaries to exchange technical and tactical experience. It is also not unusual for service members to take their unique skill set to the private sector after they retire.

But when it comes to China these days, these norms are getting a closer look. That was made clear recently by the strong reaction in Germany to a report shining a light on an otherwise mundane practice: A "handful" of retired German air force pilots, according to the magazine, Spiegel, and public broadcaster, ZDF, have gone to China on lucrative private training contracts.

Defense Minister Boris Pistorius, who was in Singapore attending a high-level defense summit when the story broke, could only tell his Chinese counterpart, Li Shangfu, that he expected "this practice to be stopped immediately."

This comes as the German government, at the urging of the United States, is reevaluating its economic and strategic relationship with the world's second-largest economy and growing military power.

"The [defense] ministry must now do everything possible to end this practice," Marcus Faber, a lawmaker who sits on the parliamentary defense committee, told DW in a statement. "The rules for people who, due to their work for the German state, have access to security-relevant information need to be urgently tightened."

A legal grey area

The work itself does not violate any laws. But the legal gray zone leaves the German government with limited authority to stop this kind of knowledge transfer.

Short of a clear case of sharing state secrets, however, a spokesperson for the German defense ministry told DW that retired service members and other government workers are largely free to make use of their expertise. They are subject to "'retroactive' service obligations," according to the statement. Those require them to report the work and "maintain secrecy about matters that he or she became aware of." The ministry then conducts a "conflict of interest check” and can deny the employment if it finds any.

The ministry has expressed concern that Chinese pilots are receiving not just basic flight instruction, but information on NATO tactics and operational capabilities. It is unclear, however, if that would constitute a breach of confidentiality.

In a statement to DW, the South African school, TFASA identified in the Spiegel report denied it was jeopardizing any country's national security.

"All training aspects and material are strictly unclassified, and provided either from open source or the clients themselves," a London-based communications consultancy representing TFASA said.

The employment of retired NATO pilots to train Chinese airmen may go back almost a decade, observers of the People's Liberation Army (PLA), China's armed forces, say. That was long before the European Union classified China as a "systemic rival," and the US national security strategy called China the "only competitor" with the means and desire to "reshape the international order."

But China has a long history of successfully using foreign know-how to accelerate its domestic prowess. Western advances in academia and research, industry, technology, and intellectual property have all found their way into Chinese equivalents. It is not a big leap to apply these efforts to the defense sector.

"For the PLA, working with retired Western pilots allows them to refine their doctrine, and it's essentially stealing secrets of Western countries' military exercises,'" Tzu-Yun Su, a research fellow at the Institute for National Defense and Security Research (INDSR) in Taiwan, told DW.

NATO-wide issue

The German media report concerning German air force pilots comes amid a flurry of similar coverage elsewhere. British and American pilots have also been cited for participating in Chinese training programs. Like in Germany, the British parliament is considering tightening its laws that govern the matter.

In a more extreme case, a former US Marine pilot was arrested last year in Australia, where he now lives as a citizen of that country. Daniel Duggan faces extradition to the US on charges of conspiracy, arms trafficking, and money laundering in connection to an alleged training assignment in China.

Duggan has denied any wrongdoing, saying the case against him was politically motivated, as US-China relations have deteriorated since his contract ended.

Edited by: Rina Goldenberg

DW reporter William Yang contributed to this report.

This article has been amended to remove a connection between Duggan's alleged work in China with his contract with TFASA that, according to the company, had nothing to do with China.

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