Teal Group analyst Richard Aboulafia said he thinks that's doable despite the delays, given that the 1980s-era 767 is such a well-proven machine.

"There's no reason why they can't get through those problems," he said. "They have the domain knowledge of the jet and the technology."

Richard Aboulafia, an analyst at the Teal Group, said: "Delta's established a rich pattern for buying older planes. The only surprise here is that they actually went ahead with their order. They were the least enthusiastic about new-generation airplanes."

Delta declined to say how much it would pay for the planes, which have a list price of more than $13 billion. But a single bulk order effectively guarantees it will receive a steep discount.

"You get half off just for showing up," Mr. Aboulafia said.

"This tells us three things: One, this new export campaign is an export of an export; two, they don't have the technology themselves; and three, they're relying on Russian engines, which are no great prize," says Richard Aboulafia, VP of Analysis at the Teal Group Corporation.

He also has numerous questions about what's inside the FC-31. For example, how advanced are the jet's active electronically scanned array (AESA) radars, electronic warfare systems, and sensor fusion? "That's a big capability—fusing all the sensor inputs together into an air combat management picture for the pilot," Aboulafia says. "That's huge. It's one of the key enablers in fighter technology."

He doubts there's much of a foreign market for the FC-31, especially not in China-leery East Asia. And while the Chinese could begin serial production of their own J-31 fighter in five years, "it's not really clear what they get out of that," he says. By the end of the decade, the United States would have already rolled out hundreds of Joint Strike Fighters, which, development problems aside, will be superior planes.

"They cut costs in anticipation of this downturn," said Richard Aboulafia, an aerospace consultant with Fairfax, Va.-based Teal Group. "Given the shifting political winds, it's likely that the downturn isn't even going to be as severe as feared."

"I would just guess it's for radar signature testing," Richard Aboulafia, an analyst with the Teal Group, said when asked what the F-117 could be doing. "It could conceivably be testing aerodynamics, also. It was one of the earlier generations of planes that shouldn't have been flying but did, thanks to the magic of fly by wire and computers. It could also be fatigue testing for materials that were used on the plane, to see how well those are holding up over time."

Another option is they are test beds of a different kind — retrofitted into unmanned systems, perhaps to check on the capability of optionally-manned systems for stealth aircraft. And of course it's possible they are being kept warm in case of military need, but it's hard to see what gap they would be fitting specifically in the military network.

All of which is to say, as Aboulafia put it, the planes could be used to test "any number of things."