A U.S. military refueling plane crashed in Iraq, killing 4. Here's what to know.

"The KC-135 Stratotanker is a U.S. Air Force aircraft used to refuel other planes in midair, allowing them to travel longer distances and maintain operations longer without landing. The plane is also used to transport wounded personnel during medical evacuations or conduct surveillance missions, according to military experts."

"Based on the same design as the Boeing 707 passenger plane, the tanker has been in service for more than 60 years, supporting the U.S. Air Force, Navy and Marine Corps as well as allied aircraft, according to an Air Force description. The aging plane is set to be gradually phased out as more of the next-generation KC-46A Pegasus tankers enter service."

"Despite upgrades over the years, the KC-135s' age has fueled concern about their reliability and durability. The last of these planes were produced in the 1960s, and the transition to the KC-46A has progressed more slowly than expected."

A U.S. KC-135 Stratotanker refueling aircraft crashed in western Iraq, resulting in four fatalities out of six crew members. The crash followed an unspecified incident involving two aircraft in friendly airspace, with the other plane landing safely. The KC-135 is a long-serving tanker aircraft based on the Boeing 707 design, used for midair refueling, medical evacuations, and surveillance missions. In service for over 60 years, the aircraft supports the U.S. Air Force, Navy, and Marine Corps. The Air Force currently operates 376 KC-135s across active duty, Air National Guard, and Reserve units. Concerns about the aging fleet's reliability have prompted transition to the newer KC-46A Pegasus tanker, though this transition has progressed slower than expected.

Read at Boston.com