* Better interim evaluation from Air Force

* Cut time to reassemble helicopter after transport

* More Chinook variants fielded, lowering risk

By Andrea Shalal-Esa

WASHINGTON (Reuters) - Boeing Co (BA.N: Quote, Profile, Research) has cut the price and shortened the time it takes to reassemble its HH-47 helicopter as part of its bid to win a $15 billion Air Force competition for a second time, a Boeing executive told Reuters.

The Air Force surprised analysts and even some top defense officials when it picked the twin-rotor HH-47, a variant of Boeing's popular Chinook helicopter, as its new combat search-and-rescue helicopter in late 2006.

But the Air Force had to relaunch the contract competition after congressional auditors twice upheld protests filed by the losing bidders -- Lockheed Martin (LMT.N: Quote, Profile, Research) and Sikorsky Aircraft, a unit of United Technologies Corp (UTX.N: Quote, Profile, Research).

The Air Force initially hoped to announce a new contract winner this spring, but that date has now slipped to November at the earliest, industry sources say.

Boeing remains convinced that its proposal was and remains the best one for the Air Force, but it has also been making minor changes to make its bid more attractive, Boeing HH-47 program manager Rick Lemaster said in an interview.