The cover shown below was sold at the SPINK, Hong Kong auction on 21. January 2018 for HK$ 22.000 + costs.
A friend of mine Mr. David Lu, China is an expert on China airmails and he does not think this cover was involved in any air crash. The cover was probably damaged in other ways. David also points out that the cover was not sent from Sining (Xining) but from Siking (Xian).
In 1939 the Soviet Civil Aviation Fleet and the Ministry of Communications of the Chines Republic formed the aviation company “Hamiata”. There was one weekly flight in each direction between Hami (Kumul) and Urumchi (Ürümqi) in Western China and Alma-Ata in Kazakhstan.
There is no doubt that the cover was to go via this route (see note par Urumchi). I do not know how the cover would travel from Siking to Hami, but when it arrived in Chengtu (Chengdu) it was already damaged (black handstamp RECEIVED TORN / AT CHENGTU).