The delivery of the Boeing Co.’s biggest jet, its 747-8 Freighter, has been delayed by six months because of issues discovered during flight testing, the company said Thursday.

“We recognize our customers are eager to add the 747-8 Freighter to their fleets, and we understand and regret any impact this schedule change may have on their plans to begin service with the airplane,” said Pat Shanahan, general manager of airplane programs for Boeing Commercial Airplanes.

Boeing said the delay won’t affect the company’s 2010 earnings, to the relief of Wall Street. The company’s shares closed at $66.54 on Thursday, up 57 cents for the day.

Boeing had been expected to deliver the first new 747 cargo jet to customer Cargolux by the end of the year. However, Boeing officials have said in recent months that the delivery might slide into early 2011.

Boeing’s new cargo jet has encountered troubles in flight testing, including a vibration under certain conditions and an underperforming aileron actuator on the wing, which helps in turning the airplane.

The company has added a fifth flight test airplane to the program. The flight test program has accumulated 964 hours in the air in roughly 364 flights.

“We understand the issues encountered in flight test and are working through the solutions,” Shanahan said.

In August, Boeing’s Shanahan told employees he would be taking a bigger role in the delayed 747 program. Mo Yahyavi, who had led the program, was put on an undisclosed special assignment. Elizabeth Lund, who had overseen the 767 program, stepped in as deputy manager on the jumbo jet program.

Boeing’s revised 747-8 has 109 unfilled orders, mostly for the cargo version. The 747-8 passenger plane, called the Intercontinental, has just 33 orders nearly five years after the jumbo jet was launched. This larger version of Boeing’s classic 747 jumbo jet has no competition in the freighter market, given that Airbus has yet to introduce a cargo version of its A380 superjumbo jet.

This is the fourth major delay for the 747-8. The setback won’t affect the late 2011 delivery of the passenger version, said Tim Bader, spokesman for the 747 program. Initially, the jumbo jet’s debut was delayed as Boeing kept engineers on the company’s new 787 Dreamliner longer than expected. Boeing announced in August that it would push back deliveries of the 787 until mid-February of next year. The Dreamliner is more than two years behind schedule.

Also on Thursday, Boeing said it had won an order for two 777s from FedEx and an order for one 737 from an undisclosed customer. But Boeing also lost an order for three 737s this past week. The company has 288 net orders through Sept. 28.