* China to place 25 pct tariff on certain aircraft
* Some Boeing 737s could be affected
* Fate of key Boeing 737 MAX 8 model uncertain
* Boeing shares fall 6 pct in early trading
SINGAPORE/SHANGHAI April 4 (Reuters) - China on Wednesday announced plans to place a 25 percent tariff on certain U.S. aircraft, in a move expected to affect some older Boeing Co narrowbody models, according to documents from China's Ministry of Commerce and the U.S. manufacturer.
But it was not immediately clear how far they would affect Boeing's upgraded 737 MAX family, a key source of future Boeing profits, because of a lack of detail in the announcement and the fact that not all airplane characteristics are published.
Boeing shares fell as much as 6 percent in early trading after China hit back at U.S. tariffs with the announcement of duties on key U.S. imports including soybeans, cars, beef and chemicals, as well as planes.
These include aircraft with an "empty weight" of between 15,000 kilograms and 45,000 kilograms, or 15 to 45 tonnes.
Depending on how "empty weight" is defined, this places a question mark over the fate of Boeing's new big-selling 737 MAX 8 jet - though the larger MAX 9 and MAX 10 could be spared.
The stakes are high. The United States exported $15 billion of aircraft to China in 2016, ranking it equally with agricultural products like soybeans also targeted for tariffs as the biggest category of goods, according U.S. trade data.
Several definitions of "empty weight" are used in the aircraft industry and the ministry document did not contain any footnotes or further explanation.
Industry experts said the rules most likely refer to the "manufacturers' empty weight," or the core aircraft structure.
But manufacturers are usually shy about publishing this number because of its commercial sensitivity, while they do give estimates for the slightly higher "operating empty weight," which includes crew and some fluids and equipment, but not fuel.
Yet even this number can vary according to an airline's specific requirements, so it is seen as a guideline only.
Whatever category is used, the range appears comfortably to include the current-generation 737 narrowbody aircraft, which is in the midst of being replaced by the newer 737 MAX.
737 MAX 8 FATE UNCLEAR
Boeing documents available online list the "operating empty weight" of the 737-700, 737-800 and 737-900 well within the tariff zone at 37.6 to 42.9 tonnes.
The "manufacturer's empty weight" would typically be about 1 to 1.5 tonnes lower than this, a plane appraisal expert said.
In contrast, the newer 737 MAX 8 is heavier than its predecessor, with an operating empty weight of 45,070 kilograms, according to a Boeing airport document published in August 2017.
Depending on how the rules are applied, this leaves it on the edge between the tariff zone and escaping the trade spat. But the larger 737 MAX 9 and MAX 10 could both be spared.
A Beijing spokesman for Boeing declined to comment when asked about the impact of the tariffs.
Chinese airlines have been on a buying spree from Boeing and rival Airbus SE as air travel grows.
Boeing and China's Commercial Aircraft Corp of China (COMAC) are expected to open a 737 completion centre in the coastal city of Zhoushan to install interiors and paint liveries this year.
The bulk of published 737 orders from Chinese that have not yet been delivered are for the newer MAX version.
However, China Southern Airlines, Ruili Airlines and Okay Airways each have two 737-800s on order and Xiamen Airlines has four more, according to the Boeing order book.
It remains unclear whether the trade dispute between the U.S. and China could affect future jet purchases.
Asked if the airline's plane buying plans could shift away from Boeing, China Eastern Airlines Corp CEO Ma Xulun said on Wednesday "it's too early to say, we will keep an eye on the situation of the China-U.S. trade war."
Apart from some Boeing jets, the Gulfstream G650, a large corporate jet manufactured by General Dynamics Corp, also appears within the weight range targeted by the tariffs.
Gulfstream did not respond immediately to a request for comment. (Reporting by Jamie Freed in SINGAPORE and Brenda Goh in SHANGHAI; additional reporting by Tim Hepher in PARIS; Editing by Mark Potter)