"Matty F" wrote in message
...
On Sep 1, 8:10 am, Dave wrote:
Matty F wrote:
Grease in a torque wrench may harden and cause the readings to be
vastly wrong, causing bolts to be overtightened and threads to be
damaged.
Which fact seems to have been omitted in this report of an aircraft
windscreen that fell out:

The effect of the grease was mentioned, but it looks like it was not
detrimental to the torque loading. What was done wrong was the over
torque loading of the bolts on the assumption that at re torque after
100 flying hours, they would find a quantity that had managed to unscrew
themselves.

I gathered that the bolts were tightened to double the correct torque
because of the hardened grease in the torque wrench, but perhaps I
remember it wrong.

AIUI it stated that you couldn't tell the difference between the thread
slipping on the smaller bolt and the torque wrench operating.


What did strike me about this, was the manager didn't spot the different
bolts by either the weight, feel, or use of the fingers to run over the
screwed in bolt's countersunk head. That alone should have alerted him
to the fact that the bolts were the wrong ones. Countersunk fasteners
must comply with quite strict tolerances regarding their levelness to
the aircraft structure.

The heads of the incorrect bolts were smaller than those of the
correct bolts. That made it easy to find the incorrect bolts on many
other planes without having to unscrew them. The manager should have
noticed the head size.

http://aviation-safety.net/database/...?id=19900610-1

Made an interesting read. You would never get those sort of slip ups
that came to light in the military side of aircraft. The RAF have a very
robust system in place in the 700's books.

Test your torque wrench at least every year, or preferably every time
you use it!

Now there is a problem here, what do you cross check a torque loader
against? We have no calibrated tools to check them with.

I would check it against a simple spring gauge. Not better than 10%
accurate but a lot better than 100% wrong as was the wrench that was
used.

The report did state that the torque wrench made no difference to the
accident.