Older Aircraft Subject To Metal Fatigue -- Shock News

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Dan Goure of the Lexington Institute has a probing question: "Where are the F-35's critics when the F-16 develops fuselage cracks?"

He does go on a bit, but the gist of his argument is that critics hold the F-16 to less-demanding standards of performance than those they apply to the F-35 program. 

Guilty as charged, I admit.

The standard is different because the affected F-16s have been in front-line service for 20 to 30 years. Aerospace has made great progress over the years in understanding what cyclic strain does to metal structures, but both predictive tools and testing lose fidelity as the years and cycles go by.

The industry has moved to damage-tolerant design, based on the principle that cracks will occur, but that the structure can be designed and managed so that cracks will be found before they cause failure. According to the USAF, this is exactly what happened: the cracks were caught in a routine inspection. 

It has happened before and it will happen again. You can mill an airplane out of solid titanium, but it won't fly very well. 

Failures in testing are different. Goure is right to point out that some failures are to be expected and that is what testing is all about. But to take the still-unsolved June 23 engine fire as an example:  it is unusual and problematic because at this stage, 13 years into engine development, major problems that are not age- and cycle-related have historically been found and fixed. 

The F-35 gets held to higher standards in this and other respects. However, given the fact that its initial operational capability date with the USAF, in a limited configuration, will be 35 years after the F-16's combat debut, and that the program has cost almost $91 billion to date, that's not unreasonable. 

As Goure correctly observes, USAF leaders have warned that the service's fleet is reaching an unprecedented average age. He does not remind his readers that, in the case of fighters, there is a reason for that. Under the plans in place when the USAF stopped buying F-16s, the USAF would have 400 fully operational F-35s by now, with 110 new aircraft arriving every year, and the F-16 force would be on its way to a well earned retirement.

Discuss this Blog Entry 4

on Aug 22, 2014

I'd think the F-18 would be a more interesting example of fatigue issues. A vortex-shedding issue led to cracking of the vertical stabilizers after a few years of service. The issue was not detected in flight test, but was noticed by one of the first export customers almost immediately. Not an old airplane...

on Aug 24, 2014

That was the F/A-18 Classic. F-16 designer Harry Hillaker used to describe vortex management and prediction as "a black art", and once place where fatigue testing can fall short is when the aero loads are not what was predicted in the tests.

on Sep 2, 2014

The guys I talked with from Canadian Forces AETE said that they noticed the vertical stabilizers oscillating immediately after they started flying Hornets. I'll buy the argument that vortex management and prediction was more difficult in the late '70s, but kind of hard to figure why they didn't react to something obvious on the full-scale airframe.

on Sep 5, 2014

Older aircraft get re-engined like the F-14D and re-winged like the A-6 then sent to AMARG. Bad deal for the taxpayer.

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