The fourth Boeing 787, ZA004, is now undergoing tests at the Phoenix-Mesa Gateway airport.
ZA004 is performing a short series of tests requiring a combination of hot weather and low-altitude conditions.
Boeing says that a crew of nearly 75 is supporting testing in Arizona, which is expected to last for only a few days.
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The four test aircraft have logged more than 820h of flying during the last six months, says Boeing.
Two more aircraft are expected to join the flight test fleet soon.
Boeing recently opted to freeze 787 shipments from suppliers until early June in order to fix or replace shear ties, which affix fuselage frames to the skin of the aircraft.
The airframer in December 2009 discovered that sheer ties in the 787's tail cone pulled away from the fuselage after repeated cooling and warming of unpressurised section 48 of the aircraft.