When the Soviet Union joined the fighting in the Korean War in 1951, its participation was a top secret. 

Officially, the Union of the Soviet Socialist Republics (USSR) was only advising its communist allies China and the Democratic People's Republic of Korea (DPRK).

But the then Soviet leader Joseph Stalin sent air force units to support them, including Sergei Kramarenko, who had already made a name for himself as an ace in World War II. 

He was awarded with the title of Hero of the Soviet Union for his bettle service on the Korean Peninsula. 

His daughter Nadezhda remembers she came to notice that her father was awarded in 1951, seven years after the end of World War II. 

That was how she learned about the Korean conflict. 

"I remember a clear episode when a writer came to our house. My dad wasn't at home. He said he wanted to write a book about my father and his participation in the Great Patriotic War. And I said, you know, dad didn't just fight in that war, but also in Korea. And the writer said, unfortunately, that war is classified and for now we can't talk about it," she recalled.

The Soviet military's participation in the Korean war was not revealed until decades later. 

Assigned to stop American bombardments, Sergei Kramarenko and his unit were deployed to China to train the Chinese pilots. 

They had to mask their tue nationality in case of any air interceptions from their American counterparts.

"Since, officially, the Soviet Union wasn't participating in the war, they gave them Chinese uniforms," Nadezhda Kramarenko explained.

"They went out as Chinese volunteers. They had Chinese uniforms and even Chinese names. They had to give commands not in Russian, but in Chinese because if they were intercepted in Russian, it would not have been good."

Nadezhda remembers being told about one of her father's greatest achievements in the Korean War, when Soviet pilots were overwhelmed by the Americans.

"The biggest accomplishment, probably, was on April 12, 1951 – Black Thursday as it was called – when there were around 40, no more than 50 of our men against more than 200 American fighters who were carrying out bombings on Korean territory," she said. 

"Every commander that led a regiment raised the planes, and they headed towards each other. The Americans had an attitude of superiority. Nevertheless, our pilots won that battle. They shot down many (enemy) planes. As my dad would recall, the whole sky was filled with parachutes. There was a big scramble. But our guys didn't lose a single plane or a single pilot."

Sergei Kramarenko flew his last mission in 1952, when his plane was shot down by an American pilot. 

He survived and was found by a DPRK villager, returning to Moscow soon after.

He lived a long life, to the age of 97. He died on May 21, 2020, during the COVID-19 pandemic.

Moscow, where he lived, was under strict lockdown, and all of its residents had to stay at home, with almost all outdoor activities forbidden, including even a walk.

He didn't have a chance to see family members in the final months of his life. 

"I think that also killed him. He was such an active person; he was always on the move. But when you forbid someone from leaving their four walls, well, of course, it affected him terribly," his daughter said.

He contracted pneumonia twice in spring 2020, although he had tested negative for the novel coronavirus. 

Kramarenko never missed Moscow's now-annual Victory Day parade, where he was honored together with other World War II veterans. 

His death was marked by a telegram bearing condolences from Russian President Vladimir Putin, whom he met several times.