design can possibly have a failure, and
they figured it out well before This failure occurred.
So they released an alert, an official FAA alert and it said, hey,
if you have one of these aircraft, you need to inspect the rivets and
the lap joints to make sure the lap joints haven't been disconnected and
that there's no holes forming at the rivets, right?
So Boeing does absolutely the right thing here, they changed their design.
They released an alert and they alert all of the airlines that
have this aircraft to start inspecting for this failure mode.
And so now we jump over here and we see the reason why this aircraft is the first
one across everybody to fail is because it had a high
number of cycles because it was constantly going up to altitude and
coming back down and going up and coming back down.
because it's flying several flights from relatively small islands everyday.
So its life, its N number was a lot higher than the rest of the aircrafts
released out on the field.
And so the main problem comes in here in inspection and maintenance.
So despite the alert for whatever reason,
Aloha Airlines' staff wasn't fully trained and they were not able to catch
the fact that several of their aircrafts did have these delaminated lap joints and
the cracks starting to form at the rivet hole.
So if you go through the FAA report,
it will walk you through the entire failure analysis.
But all of these factors combined for this to occur.
So if we look at the sequence of events.
So there's these cold bonded lap joints.
And they're going to get thrown.
They have a relatively thin geometry,
it's difficult to do this process repeatably so
there could be issues in the bond due to the surface.
They get thrown into this highly corrosive salt water environment and they start to
cycle a lot faster than any of the other aircraft released in the field.
And so what happens is the cracks start to form between the sheets in the lap joint.
So if this is the aircraft's skin, remember we had
like a rivet going through here and then we had these epoxy joints.
And they're going to delaminate,
which basically means that they're going to detach.
And now all of the load is coming through these rivets.
So load transfers to the rivets and
now cracks start to form, this is actually what the rivet looks like.
Cracks start to form right here, at the edge of the rivet,
because that hole had a high KF, it has a high stress concentration factor, so
the cracks form, and there's a lot of these rivets throughout the sheets.
So you would see something like this and what would happen is throughout
the entire aircraft the cracks would be forming along the rivets, growing and
then connecting along the fuselage.
So it starts out relatively small at the rivet hole and
then they connect along the fuselage.
Eventually you get rapid crack propagation and
the remaining material can no longer take the hoop stress and
then you get this catastrophic failure which looks like this.
So this is a really great example of a fatigue failure.
It shows you the importance of the stress concentration factor in geometry.
It shows you the importance of how the endurance limit can get reduced in
certain saltwater corrosive environments, and it shows you the impact of
a high number of cycles that would cause this aircraft to fail first.
So a lot of really interesting phenomenon came together and
it shows the fatigue mechanisms quite well.