Editors' pick: Originally published March 8.

United (UAL) - Get Report said Tuesday it is "deeply disappointed" that activist investors are trying to make board appointments at the exact time as the carrier seems to be turning around without them.

Investors Altimeter Capital Management and PAR Capital Management, which own a combined 7.1 % stake in the company, said in a letter released Tuesday that they wanted the airline to add six new directors including Gordon Bethune, former CEO of Continental, which merged with United in 2010.

The firms went public after United expanded its board by three seats over the weekend. New directors include former Delta executive Jim Whitehurst.  In their letter, Altimeter and PAR called that move "a cynical attempt to preserve power."

Also over the weekend, change agent CEO Oscar Munoz returned to work after recovering from a heart transplant. Since taking office in September, Munoz has been credited with initiatives to improve labor relations, upgrade the passenger flight experience, and return cash to shareholders.

On Tuesday, United said it will accelerate its fleet modernization with an order for 25 additional Boeing 737-700s.

Responding to Atimeter and PAR, Henry Meyer, non-executive chairman of the United board, declared,  "We are deeply disappointed that after United attempted to engage in a constructive, good-faith dialogue with PAR and Altimeter, repeatedly communicated our willingness to make meaningful changes in our board, publicly announced our intention to name four new independent directors with deep relevant experience and named three of them yesterday.

"PAR and Altimeter have unilaterally taken this hostile action with no concern that a proxy fight could distract the  company from executing on Oscar's strategic plan," Meyer said in a prepared statement.

United said that in negotiations with PAR and Altimeter it "clearly communicated its interest in negotiating a reasonable settlement" and even offered to extend its March 12 director nomination deadline.

"Not only were PAR and Altimeter uninterested in extending the deadline, but they also refused to cooperate in making their nominees available to be interviewed," the carrier said.

United shares fell 3% in early trading Tuesday.

This article is commentary by an independent contributor. At the time of publication, the author held no positions in the stocks mentioned.