South Korea's trainer jet made its first test flight in the United States last week after the two countries agreed to mutually recognize its airworthiness certification in September, the defense acquisition agency said Thursday.

On Saturday, the 'T-X' trainer, a modified version of the T-50 planes in service with the South Korean Air Force, showed off its flight performance as a candidate to replace the existing jets operated by the US Air Force in South Carolina, the Defense Acquisition Program Administration (DAPA) said in a statement. 

The advanced T-X trainer jet was built by the Korea Aerospace Industries Co. (KAI), the country's sole aircraft manufacturer. T-X is the US project name for the advanced trainer jet that will replace the T-38 Talons in use at present.

"The T-X's test flight in the US means that the US Department of Defense has officially accepted the airworthiness certification domestically earned for the South Korean aircraft," a DAPA spokesman said.

To obtain approval to fly in US airspace, South Korea used to have to undergo a complicated administrative process and provide extra costs.

DAPA expects the mutual recognition of airworthiness certificates will help South Korea export its trainer jets to the US in the long term.

So far, KAI's aircraft exports reached $3.4 billion helped by demand for the KT-1 basic trainers and the T-50 advanced trainer jets in emerging markets, such as Indonesia, Thailand and the Philippines.

Meanwhile, South Korea last week signed a $1.2 billion deal with Lockheed Martin Corp. to gradually upgrade its entire 134 KF-16 fighter jets during the 2017-2020 period, the spokesman said.

Under the upgrade contract, Lockheed will modernize the 134 KF-16s to a configuration similar to its advanced F-16V model.

Among the upgrades are an Active Electronically Scanned Array (AESA) radar; a modern commercial off-the-shelf-based avionics subsystem; a large-format, high-resolution center pedestal display; and a high-volume and high-speed data bus, the US aircraft company said on its webpage. 

AESA is a type of phased array radar whose transmitter and receiver functions are composed of numerous small transmit/receive (T/R) modules. AESA radars have almost instantaneous scanning rates, making them difficult to jam and allowing the aircraft employing the technology to remain stealthy. A data bus refers to a system within a computer or device.

In 2013, the British multinational defense firm BAE Systems beat Lockheed for a contract to upgrade the KF-16s for $1.7 billion. In the following year, it asked for an increase of $800 million, citing high manufacturing costs.

But Seoul didn't accept the request and scrapped the deal with BAE in 2014. It signed the upgrade deal with the US government in 2015, with Washington recommending Lockheed for the work.

This kind of deal is called a foreign military sale, or FMS, which requires the US government to oversee the selection of the contractors and guarantees the quality of the work to the foreign government.  (Yonhap)