Mike Marron wrote:

Chad Irby wrote:

I know there were a couple of cases in Vietnam where F-4s made hard
enough turns to rip the ECM pods off...

I don't know how much an F-4 ECM pod weighs, but I do know that it
would require a hellacious amount of G's to cause the bolts that
fasten the pod to the airframe to fail. Much more force than the
surrounding airframe structure itself could withstand.

For example, a standard AN6 bolt (3/8-inch diameter shank) has
a shear strength of approx. 8700 lbs. IIRC. And there is probably
more than just one of these or similiar types of bolts securing the
ECM gear to the belly of an F-4.

The only problem is that the missile well adapter isn't held in by the
full strength of 4 brand-new bolts... it's held in by the bolt
*threads*. You're not working with the shear strength of a 3/8"
diameter piece of metal - you're dealing with the actual (not
theoretical) tensile strength of the *threads* of that bolt *and* the
nut plate.

Yep - the four bolts run straight up into the fuselage, making all of
the stress rest on the four bolts, through their four nut plates. That
shouldn't be a problem, since correct installation would give you full
strength. Except...

That's the problem with thoretical and design limits. After a few
months of actual (mis)use, those numbers change. A *lot*. The spec
says that someone should replace those nut plates and bolts each time
you swap out the launcher for the MWA. Nobody did that, of course.
Took too long, cost too much.

Sure, the four bolts, when new, should have been able to hold a total of
almost 35,000 pounds. But then you add in the preload from torquing it
in (at least 600 pounds per bolt, maybe more) plus the 6000 pounds it
would have been carrying with a 600 pound pod at ten times the force of
gravity (maybe higher), and you have a load of at least 8400 pounds, on
a system that is not evenly loaded, in six kinds of vibration modes,
loading and unloading like mad.

I figure one semi-catastrophic failure over a quarter century is pretty
good, considering.

--


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