VIENNA, Austria  – The United States may be the mightiest military and economic power in the world but when it comes to shuttling its top diplomat around the globe, it’s beginning to look like a poor orphan.

For the fourth time this year  – and the second time in three months, Secretary of State John Kerry was forced to fly home commercially when his aging Air Force Boeing 757, known in military parlance as a C-32, was grounded on Thursday with a mechanical problem in Vienna.

Inconvenient? Undoubtedly. Kerry, heading back to Washington from nuclear talks with senior European and Iranian officials, made light of the situation, telling aides: “If the hardest thing that happens in a given day is that you have to fly commercial, your life is pretty good.”

Embarrassing? Perhaps. Iranian Foreign Minister Mohammad Javad Zarif chuckled when he learned of Kerry’s predicament. “So it is not just our planes,” Zarif said. Iran hasn’t been able to refurbish its pre-1979 fleet of Boeing aircraft because of heavy U.S. sanctions.

But trivial? Not so much. Officials say far more than appearance is at stake, particularly in the midst of multiple world crises like Ebola, the military campaign against IS militants, the crisis in Ukraine, Israeli-Palestinian relations and yes, the Iran nuclear talks.

Without access to the secure phone links and classified data on his own plane, Kerry was effectively out of the loop during the nine-hour flight from Vienna to Washington. Aides said he had to cancel or reschedule several calls with world leaders and other members of President Barack Obama’s national security team.

“In the world we live in, we do high-stakes diplomacy via phone and secure phone,” said State Department spokeswoman Jen Psaki. “None of that is possible when any secretary of state is flying on a commercial plane without secure communications with hundreds of people.”

“Every minute of their day is scheduled,” she said. “There is not a single flight where Secretary Kerry isn’t calling in via secure phone to an interagency meeting or receiving sensitive national security information, or reading classified information or briefings.”

And he flies a lot.

Before his current trip, Kerry had logged more than 566,000 plane miles this year alone, according to the State Department. That’s nearly 1,220 hours or more than 50 days in the air.

Problems with the 1990s-era Air Force fleet that ferry America’s top officials aren’t new, and secretaries of state have long complained about the aircraft they must fly for official business. Some have joked about being envious of colleagues with newer, more efficient and more luxurious planes.

Secretary of State Hillary Rodham Clinton encountered several aviation breakdowns when she was in office, including a tire that burst on landing in the United Arab Emirates, leading to an unscheduled overnight stay in Dubai. But she never had to resort to flying commercially.

Yet the problems seem to be becoming more frequent and more serious.

Thursday’s incident was the fourth with one of Kerry’s planes this year. The previous two _ in Switzerland in January and in Britain in March – were resolved with only minor delays to his schedule.

But in August, an electronics issue forced Kerry to return to Washington from Hawaii on a commercial flight at the end of a round-the-world diplomatic mission.

On Thursday, the crew discovered that an auxiliary fuel tank was leaking. An Associated Press reporter who visited the aircraft found the cabin full of fumes. And, one technician involved in trying to patch up the leak complained of feeling ill.