After a year and a half of flight tests, Boeing's largest airplane ever, the 747-8 jumbo, received the Federal Aviation Administration's approval to enter service as a cargo jet, Boeing announced Friday.

The 747-8 freighter also received certification from the European Aviation Safety Agency.

Cargolux of Luxembourg is expected to take the first delivery next month.

"This is such a great day for everyone on the 747 team," Jim Albaugh, president and CEO of Boeing Commercial Airplanes, said in a statement.

The plane will enter service two years later than originally planned when the program was launched in fall 2005, mostly because of a late design decision and a cascade of resultant aerodynamic issues.

In 2008 and 2009, Boeing recorded a total $2 billion in accounting charges because of cost overruns on the 747-8. Joe Campbell, an analyst with Barclays Capital, estimates Boeing has spent about $4 billion to develop the airplane.

So far, Boeing has orders for 78 of these jumbo freighter jets.

The 467-seat passenger version of the 747-8, which has 36 orders, is on a separate development track. The first of those, a VIP private version thought to be for the Kuwaiti government, is scheduled to be certified and delivered by the end of the year.

Production to increase

Boeing will increase 747-8 production from the current 1.5 to two planes per month next May.

To finally win certification for the freighter, Boeing engineers had to resolve a series of vibration problems that dogged the program in flight tests.

The 250-footlong jet was initially conceived as a simple stretch of the existing 747-400 jumbo jet, with two extensions to the fuselage but minimal structural changes otherwise.

However, design engineers discovered the new 787-style engines that would make the jet so much more efficient wouldn't perform as promised without major changes to the wing.

In the end, they essentially designed a new wing. The 747-8's total wingspan is almost 225 feet — 13 feet wider than the previous model.

The new design required balancing changes to the tail and strengthening of the fuselage, adding tremendous cost and time to the jet's development.

The modifications, designed with the latest digital tools, did not always fit precisely with the parts built to the older design. And Boeing had a limited capacity to work through those issues because most of its design engineers were still busy on the delayed 787 Dreamliner.

Then, after the maiden flight in February 2010, additional issues showed up that required more design work.

At a media briefing in June before the Paris Air Show, 747-8 program chief Elizabeth Lund outlined the jet's troubled flight-test history.

Early on, pilots found that when they extended the landing flaps on the trailing edge of the wing — which allows the plane to fly more slowly — the air flowing around the flaps caused buffeting.

Because the airplane's low-speed performance proved better than expected, this problem was solved by changing the flap settings, without structural modifications.

"We solved the problem and moved on," Lund said.

Then, under certain flight conditions the test pilots discovered a more serious phenomenon: flutter, which occurs when natural vibration is amplified to violent levels by wind or airflow. Flutter can potentially bring down an airplane.

Small vibration

In this case, the vibration was small, deflecting the wing only about an inch at its tip. And it remained steady at 2.3 cycles per second, rather than getting worse as it continued.

Boeing considered the problem a "ride-quality issue" that was just perceptible at cruise speed, not a safety issue, said Michael Teal, 747-8 chief project engineer.

Still, Lund said, "technically, it was flutter," and FAA regulations required that to be addressed.

After several months of work using simulators, a crack engineering team at Boeing devised a solution that again avoided structural changes.

They installed an electronic suppression system that senses any vibratory movement of the wing in real time and immediately counters it by popping the movable trailing edge surfaces near the wingtip, called ailerons.

The aileron movement gives a tiny push to the wing in the opposite direction to the vibration and cancels it out.

A similar system is used to counter movement from wind gusts on airplanes with long fuselages. But this was the first time it had been used to suppress flutter.

Boeing flight tests showed the system worked. The test crews also had to fly the plane with the suppression system switched off and show that even with the vibration, the jet was still safe to fly.

The FAA was satisfied and ruled the issue closed.

Another problem discovered in flight was vibration of an actuator that operated the inboard aileron. Boeing solved that by replacing the actuator with a new one and making various system changes.

When those three in-flight issues were finally "resolved, certified, done," as Lund put it, the way was cleared for completion of the test flights and the gathering of all data needed for certification.

Lund said in June that the complex flight-management computer system, built by Honeywell, was not functioning to the level Boeing would like.

She said the jet will initially have all the functionality of the 747s currently in service, and that a software upgrade around year-end will provide the additional capability.

Pilot training

The final product handles just like the older 747s in service today, even though it's much bigger, said 747-8 chief test pilot Mark Feuerstein.

Pilots now flying 747-400s will need only three days of ground training, with no simulator time, to qualify to fly the new jet.

The final test flight for the 747-8 freighter model, a 17-hour marathon, took off about 7 a.m. Aug. 2 from Paine Field.

To mark the occasion, the test crew flew a distinctive route, tracing 747 in giant digits across the continental U.S. — skywriting that stretched invisibly from Everett to Duluth, Minn., in the north; and from San Luis Obispo, Calif., to the Texas border near Wichita Falls in the south.

On Friday, Boeing stock closed down $1.39, or 2.4 percent, at $57.54 in a generally declining stock market.

Dominic Gates: 206-464-2963 or dgates@seattletimes.com