FARNBOROUGH, England, July 10 (Reuters) - Boeing Co is well into testing of a modification to the refueling boom on its troubled KC-46A tanker program and is seeing "positive results," the head of the company's defense division said on Sunday.

Leanne Caret, chief executive of Boeing Defense, Space & Security, told reporters the company was encouraged by tests of a modified boom built by Cobham Plc, after a software fix failed to address instability issues that cropped up while delivering fuel to large aircraft.

The changes did not amount to a redesign of the boom, and would not require additional certification, she told reporters.

Caret said prospective Middle Eastern fighter jet customers were "hanging in there with us," despite delays in getting U.S. approval for several large F-15 and F/A-18E/F fighter jet sales. She said the process was clearly taking longer than the company would like, but still hoped to get the deals approved.

(Reporting by Andrea Shalal; Editing by Mark Potter)