Dec 13 (Reuters) - Boeing Co :
* The second aircraft upgrade is already under way, with the entire fleet slated for completion in 2018.
* Boeing begins digital flight deck upgrades to NATO fleet
* Thirteen additional NATO AWACS will receive installation of the upgrade as part of a $257 million modification effort
* The modernization has also been installed on a U.S. Air Force AWACS, which is completing extensive flight tests and will deliver in 2017 Further company coverage: