The Pentagon's fleet of next-generation F-35 Joint Strike Fighters has been cleared to resume flying, six days after the discovery of a half-inch crack in an engine blade led to the grounding of all 50 or so training and test jets.

Engine-maker Pratt and Whitney said it is confident it has isolated the source of the crack in the F135 engine and can fix it. But the stealthy F-35's propulsion problems are anything but fixed. Finicky motors are an inevitable side effect of the plane's design.

Pratt and Whitney put the best possible spin on the turbine blade's potentially catastrophic flaw, which if undetected could have caused a crash. "Prolonged exposure to high levels of heat and other operational stressors on this specific engine were determined to be the cause of the crack," the company announced late Thursday. "No additional cracks or signs of similar engine stress were found during inspections of the remaining F135 inventory. No engine redesign is required as a result of this event."

But the F-35, meant to replace almost all of the military's existing jet fighters, could very well experience similar problems throughout its planned 50-year service life. Originally meant to be long, narrow and highly aerodynamic like most other fighters, the JSF was redesigned to be wider and more squat to accommodate the internal weapons bays that are key for radar-evading planes. "What is different is that this airplane has accelerational characteristics with a combat load that no other airplane has, because we carry a combat load internally," Lockheed exec Tom Burbage told aviation reporter Dave Majumdar last year.

But the redesign has an adverse effect on the plane's aerodynamics, making the F135 work harder than is normal for a fighter engine. Generating more than 40,000 pounds of thrust, the F135 is the most powerful fighter motor ever. Even though the Pentagon has downgraded the F-35's acceleration specs to ease the strain on the engine, the F135 runs extra hot – a problem that has concerned Lockheed and Pratt and Whitney engineers for at least seven years and likely contributed to turbine problems in 2007, 2008 and 2009.

Through an initiative called Advent, the Air Force and several aerospace companies are investigating a host of new engine technologies that could someday be installed in the F-35, potentially alleviating some of the engine problems.

According to the Air Force, Advent's advancements include "turbine section for increased thrust, third stream cooled cooling air for engine hot section thermal management system, and lightweight high temperature materials." General Electric, Pratt and Whitney's main rival, completed some major testing of Advent tech late last month. Trials of a complete Advent engine are slated for 2017. The Pentagon has said the new motor must be scaled to fit the F-35.

But Larry Burns, government program manager for Advent, said it's unlikely the full Advent motor will be installed on the F-35. Owing to the high cost, such retrofit are "few and far between," Burns told reporter Steve Trimble.

So the Pentagon's mainstay future fighter is probably stuck with its existing engine, as well as with that motor's high temperatures and fragile turbines. And that means groundings like last week's could be a distressingly common occurrence.