13D Filing: Red Mountain Capital Inches Up Stake In Air Transport Services Group, Inc. (ATSG)

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

SCHEDULE 13D/A

Under the Securities Exchange Act of 1934

(Amendment No. 11)

AIR TRANSPORT SERVICES GROUP, INC.

(Name of Issuer)

Common Stock, par value $0.01 per share

(Title of Class of Securities)

00922R105

(CUSIP Number)

Red Mountain Capital Partners LLC

Attn: Willem Mesdag

10100 Santa Monica Boulevard, Suite 925

Los Angeles, California 90067

Telephone (310) 432-0200

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

March 8, 2016

(Date of Event Which Requires Filing of this Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of Rule 13d-1(e), 13d-1(f) or 13d-1(g), check the following box. ¨

Note. Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See Rule 13d-7 for other parties to whom copies are to be sent.

* See Instructions

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* See Instructions

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This Amendment No. 11 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, and Amendment No. 10 thereto, filed with the SEC on August 14, 2012 (together, this Schedule 13D), by (i) Red Mountain Capital Partners LLC, a Delaware limited liability company (RMCP LLC),
(ii) Red Mountain Capital Partners II, L.P., a Delaware limited partnership (RMCP II), (iii) Red Mountain Capital Partners III, L.P., a Delaware limited partnership (RMCP III), (iv) RMCP GP LLC, a Delaware
limited liability company (RMCP GP), (v) Red Mountain Capital Management, Inc., a Delaware corporation (RMCM), and (vi) 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. Through this Amendment No. 11,
Red Mountain Partners, L.P., a Delaware limited partnership (RMP), joins as a filer of this Schedule 13D. In addition, this Amendment No. 11 will constitute an exit filing for RMCP II and RMCP III. RMCP LLC, RMCP II, RMCP III,
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. 11) 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.

Item 2 of
this Schedule 13D is hereby amended to include the following information:

Red Mountain Partners, L.P., a Delaware limited
partnership (RMP), joins as a filer of this Schedule 13D. RMCP GP is the general partner of RMP and thus may be deemed to control RMP. The principal executive offices of RMP are located at 10100 Santa Monica Boulevard, Suite 925, Los
Angeles, California 90067. The principal business of RMP is investment. RMP was not formed for the specific purpose of investing in the securities of ATSG.

During the last five years, RMP has not been convicted in a criminal proceeding (excluding traffic violations or similar misdemeanors) or been a party to a civil proceeding of a judicial or administrative
body of competent jurisdiction as a result of which RMP was or is subject to a judgment, decree or final order enjoining future violations of, or prohibiting or mandating activities subject to, federal or state securities laws or finding any
violation with respect to such laws.

Item 3 of this Schedule 13D is hereby amended to include the following information:

Amendment No. 10 to this Schedule 13D, which was filed with the SEC on August 14, 2012, reported that RMCP II and RMCP III held in the aggregate 10,550,425 shares of Common Stock. Between August 14,
2012 and August 16, 2012, RMCP II acquired an additional 602,000 shares of Common Stock using its own working capital increasing the aggregate Common Stock held by RMCP II and RMCP III to 11,152,425 shares.

Prior to December 31, 2012, RMP was the sole limited partner of each of RMCP II and RMCP III, and RMCP GP was the sole general partner of
each of RMP, RMCP II and RMCP III. On December 31, 2012 as part of an internal restructuring, RMCP II and RMCP III distributed all Common Stock held by them to RMP in a transaction that resulted in a change in the form of ownership of such Common
Stock by RMP but not in a change in pecuniary interest in such Common Stock by RMP or a change in the control over such Common Stock by RMCP GP and RMCP LLC. As a result of such internal restructuring, RMP became the direct holder of these shares of
Common Stock, and RMCP II and RMCP III ceased to beneficially own any Common Stock. Each of RMCP II and RMCP III was subsequently dissolved.

In connection with Mr. Teets service on the Board of ATSG since 2009, ATSG has issued Mr. Teets, in the aggregate, 77,929 restricted stock units for a zero purchase price (the
RSUs) as part of his director compensation, all of which RSUs are fully vested. Mr. Teets has transferred beneficial ownership of all such RSUs to RMCP LLC.

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

RMP beneficially owns,
in the aggregate, 11,152,425 shares of Common Stock, which represent approximately 17.3% 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 11,152,425 shares of Common Stock.

Due to its ownership of the RSUs, RMCP
LLC may be deemed to beneficially own directly 77,929 shares of Common Stock underlying the RSUs (the RSU Shares), representing approximately 0.1% of the outstanding Common Stock (after giving effect to the RSU Shares). RMCP LLC has the
sole power to vote or direct the vote, and the sole power to dispose or direct the disposition, of the RSU Shares.

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 LLC may be deemed to beneficially own, in the aggregate, 11,230,354 shares of Common Stock, representing 17.4% of the outstanding Common Stock after giving effect
to the RSU Shares. In addition, because each of RMCM and Mr. Mesdag may be deemed to control 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 RSU Shares beneficially owned by RMCP LLC, as well as all of the shares of Common Stock beneficially owned by RMP. Therefore, each of Mr. Mesdag and RMCM may be deemed to beneficially own, in the aggregate, 11,230,354
shares of Common Stock, representing 17.4% of the outstanding Common Stock after giving effect to the RSU Shares.

Other than
shares of Common Stock beneficially owned by RMP and RMCP LLC, none of the Reporting Persons or Mr. Teets may be deemed to beneficially own any shares of Common Stock.

Each of RMCP LLC, RMP and RMCP GP affirms membership in a group with each other but disclaims membership in a group with RMCM or Mr. Mesdag. Each of RMCM and Mr. Mesdag disclaims membership in a
group with any person.

The filing of this Schedule 13D shall not be construed as an admission that any Reporting Person is
the beneficial owner of any of the shares of Common Stock that such Reporting Person may be deemed to beneficially own. Without limiting the foregoing sentence, each of RMCM and Mr. Mesdag disclaims beneficial ownership of all shares of Common
Stock reported in this Schedule 13D. In addition, the filing of this Schedule 13D shall not be construed as an admission that any partner, member, director, officer or affiliate of any Reporting Person is the beneficial owner of any of the shares of
Common Stock that such partner, member, director, officer or affiliate may be deemed to beneficially own. Without limiting the foregoing sentence, Mr. Teets disclaims beneficial ownership of all shares of Common Stock reported in this Schedule
13D.

Item 5(e) of this Schedule 13D is hereby amended to include the following information:

Item 6 of this Schedule 13D is hereby amended to include the following information:

On March 8, 2016, RMP entered into a voting agreement with Amazon.com, Inc. (Amazon) and ATSG (the Voting
Agreement), pursuant to which RMP has agreed to vote all of its shares of Common Stock in favor of, among other things, an amendment to ATSGs certificate of incorporation that would authorize a sufficient number of shares to permit the
exercise in full of certain warrants granted to Amazon in connection with commercial agreements whereby ATSG agreed to operate an air cargo network to serve Amazon customers in the United States. The Voting Agreement also contains a restriction on
RMPs ability to sell its shares of Common Stock until immediately following ATSGs annual meeting of stockholders to be held in the 2016 calendar year.

The Voting Agreement will terminate upon the earliest to occur of several events, but no later than immediately following ATSGs annual meeting of stockholders to be held in the 2017 calendar year;
provided that the restrictions on RMPs ability to sell its shares of Common Stock shall terminate immediately after ATSGs annual meeting of stockholders to be held in the 2016 calendar year.

The foregoing references to and description of the Voting Agreement do not purport to be complete and are subject, and are qualified in
their entirety by reference, to the full text of the Voting Agreement, which is incorporated by reference in this Item 6.

Item 7 of this Schedule 13D is hereby amended to include the following information:

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true,
complete and correct.

Dated: March 14, 2016

EXHIBIT INDEX