13D Filing: Red Mountain Capital and Air Transport Services Group Inc. (ATSG)

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CUSIP No. 00922R105 SCHEDULE 13D/A PAGE 7 OF 10 PAGES

This Amendment No. 15 amends and supplements the Schedule 13D filed with the Securities and Exchange
Commission (the SEC) on November 20, 2006, as amended by Amendment No. 1 thereto, filed with the SEC on September 24, 2007, Amendment No. 2 thereto, filed with the SEC on February 7, 2008, Amendment No. 3
thereto, filed with the SEC on December 3, 2008, Amendment No. 4 thereto, filed with the SEC on January 6, 2009, Amendment No. 5 thereto, filed with the SEC on February 3, 2009, Amendment No. 6 thereto, filed with the
SEC on May 26, 2009, Amendment No. 7 thereto, filed with the SEC on June 1, 2009, Amendment No. 8 thereto, filed with the SEC on August 29, 2011, Amendment No. 9 thereto, filed with the SEC on June 18, 2012,
Amendment No. 10 thereto, filed with the SEC on August 14, 2012, Amendment No. 11 thereto, filed with the SEC on March 14, 2016, Amendment No. 12 thereto, filed with the SEC on June 23, 2016, Amendment No. 13
thereto, filed with the SEC on July 15, 2016, and Amendment No. 14 thereto, filed with the SEC on June 2, 2017 (together, this Schedule 13D), by (i) Red Mountain Capital Partners LLC, a Delaware limited liability
company (RMCP LLC), (ii) Red Mountain Partners, L.P., a Delaware limited partnership (RMP), (iii) RMCP GP LLC, a Delaware limited liability company (RMCP GP), (iv) Red Mountain Capital Management,
Inc., a Delaware corporation (RMCM), and (v) Willem Mesdag, a natural person and citizen of the United States of America, with respect to the common stock, par value $0.01 per share (the Common Stock), of Air Transport
Services Group, Inc., a Delaware corporation (ATSG), formerly known as ABX Holdings, Inc. and ABX Air, Inc.

RMCP LLC, RMP and
RMCP GP are sometimes collectively referred to herein as Red Mountain. Red Mountain, RMCM and Mr. Mesdag are sometimes collectively referred to herein as the Reporting Persons. The filing of any amendment to this
Schedule 13D (including the filing of this Amendment No. 15) shall not be construed to be an admission by the Reporting Persons that a material change has occurred in the facts set forth in this Schedule 13D or that such amendment is
required under Rule 13d-2 of the Securities Exchange Act of 1934, as amended.

This Amendment
No. 15 will constitute an exit filing with respect to this Schedule 13D filed by the Reporting Persons.

ITEM 5. INTEREST IN SECURITIES OF THE ISSUER.

Items 5(a) and 5(b) of this Schedule 13D are hereby amended and
restated as follows:

(a)-(b) RMP beneficially owns, in the aggregate, 2,326,108 shares of Common Stock, which represent approximately 3.9% of the outstanding Common Stock.(1) RMP has the sole power to vote or direct the vote, and the sole power to dispose or direct the disposition, of all such 2,326,108 shares of Common Stock.

Because each of RMCP GP, RMCP LLC, RMCM and Mr. Mesdag may be deemed to control RMP, each of RMCP GP, RMCP LLC, RMCM and Mr. Mesdag
may be deemed to beneficially own, and to have the power to vote or direct the vote, or dispose or direct the disposition, of all of the Common Stock beneficially owned by RMP. As a result, RMCP GP, RMCP LLC, RMCM and Mr. Mesdag may be deemed
to beneficially own, in the aggregate, 2,326,108 shares of Common Stock, representing 3.9% of the outstanding Common Stock.(2)

(1) All calculations of percentage ownership in this Schedule 13D are based on approximately 59,503,749 shares of Common Stock estimated to be issued and outstanding as of May 30, 2017, and giving effect to
the retirement of 380,637 shares of Common Stock purchased by ATSG from the Underwriter pursuant to the Underwriting Agreement, in each case as reported in the prospectus supplement filed by ATSG with the SEC on May 31, 2017.
(2) Excludes 87,626 shares of Common Stock underlying restricted stock units previously granted to J. Christopher Teets, a Partner of RMCP LLC, in connection with his service on the board of directors of ATSG that
may be transferable to RMCP LLC upon cessation of Mr. Teets service thereon.

Follow Air Transport Services Group Inc. (NASDAQ:ATSG)

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