Air Transport Services Group said on Monday investment firm Stonepeak was taking it private in a deal valued at $3.1 billion, including debt, sending the aircraft lessor's shares 27% higher before the bell.

Stonepeak will pay $22.5 per share for the company, representing a 29.3% upside to Air Transport's closing price on Friday.

Reuters reported on Sunday that Stonepeak was in advanced talks to take over ATSG, whose primary business involves acquiring used passenger aircraft and converting them to freighters to lease to customers.

As of June 30, ATSG had 114 freighter aircraft in service, with the significant majority being Boeing's 767 model.

It also operates freighters for Amazon.com's air cargo network. The online shopping giant holds warrants to purchase shares in the lessor.

The deal is expected to close in the first half of next year, ATSG said.

In August, it reported an 8% fall in second-quarter revenue, driven by a reduction in its aircraft leasing as well as cargo services segments. It leased fewer 767-200 freighters in the quarter from a year earlier.

However, the company expects its full-year adjusted EBITDA - earnings before interest, tax, depreciation and amortization - to be approximately $526 million, an increase of $10 million from its previous forecast, as it continues to see more interest for its newly converted freighters.

The takeover transaction has fully committed equity financing from funds affiliated with Stonepeak, along with fully committed debt financing.

The deal is not subject to a financing condition, ATSG said.