Air-Minded: Old Bones

Boy, does this make me feel old:

In early November 2007, the nose and cockpit section of a Missouri Air National Guard F-15C broke away from the rest of the aircraft during a training flight.  The pilot ejected and survived.  Here’s a simulation of what happened:

Following the mishap, the USAF grounded its entire fleet of F-15s, including the newer air-to-ground F-15E models. The F-15Es have all been returned to flight status, I believe, but of the 300-plus air-to-air F-15s (the A, B, C, and D models) still in service, 163 remain grounded, possibly for keeps.

The problem?  Faulty longerons, support structures connecting the cockpit area to the fuselage.

The Missouri ANG jet was about 25 years old at the time of the mishap.  Some of the oldest F-15s in service are 30 years old.  Any F-15 pilot will tell you that virtually every air-to-air F-15 has been overstressed during its career; most of the older jets have been overstressed again and again.  Overstress incidents are tracked, of course, and the aircraft involved are inspected afterward.  But what about over-Gs that occured before airframe stress recording devices were retrofitted in the mid-1980s?  Most of those were probably never reported.

Overstressed longerons can’t simply be replaced; they’re part of the basic skeleton of the aircraft.  About the only way to fix the problem is to rebuild the aircraft from scratch, and the F-15 production line has long been closed.

In 1981 I investigated an F-15 airshow crash in The Netherlands.  The pilot had smacked into the ground in a flat attitude, and the jet didn’t look all that bad afterward.  True, the cockpit had broken away from the fuselage (in exactly the same place as Missouri ANG jet), but it looked like it would be a simple matter to bolt it together and return it to flight status.

We called in a structural engineer, who turned out to be a young, smallish woman.  She crawled inside the wreckage, looked around, and crawled back out.  “This airplane will never fly again,” she said. “The longerons are bent.”  I’m ashamed to say the colonel in charge of the investigation sent her packing and told the USAF to send us an older male engineer.  The male engineer came two days later and made the same report.  Bent longerons = no fixee.

I’m not so naive as to think the USAF isn’t exaggerating the scope of the problem in order to get more money for additional F-22s.  Of course it is.  But I do think bad longerons will be a showstopper for air-to-air F-15s, and eventually air-to-ground F-15Es too.  You can only over-G an airplane so many times.

I started flying F-15s in 1978 and flew them into the mid-1990s.  They were new when I started, and still felt new when I quit.  Now all of a sudden they’re old and breaking apart.

My own longerons aren’t feeling so hot, reading about all this.

Credit where credit’s due: an excellent ongoing discussion of the F-15 grounding saga is available at the Defense Tech blog – look for individual articles under the “fast movers” category.

© 2008 – 2015, Paul Woodford. All rights reserved.

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