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Case Study: Alaska Airlines Flight 261 |
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A jackscrew (or screw jack) is a type of jack you will have probably
seen stowed in your car but they are used extensively in other
mechanical devices. They operate by turning a leadscrew to produce
linear motion of a ‘load-nut’. An acme thread form is most commonly
used, as this thread is very strong and can resist the large loads
imposed on most jackscrews. Jackscrews are self-locking, which makes
them intrinsically safer than other jack technologies like hydraulic
actuators which require continual pressure to remain in a locked
position. However if not lubricated they can be dramatically weakened
through wear. One of our failure investigations concerns such a failure
of a jackscrew device. We are bound by confidentiality agreements from
publishing details of the investigation but whilst researching another
more prominent and more widely reported incident was discovered. This incident had far more devastating consequences but featured an identical failure mechanism to the one we are studying.
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What happened?
The incident concerns Alaska Airlines Flight 261, a McDonnell Douglas
MD-83 aircraft which experienced a fatal accident on January 31, 2000
in the Pacific Ocean about 2.7 miles (4.3 km) north of Anacapa Island,
California. The two pilots, three cabin crewmembers, and 83 passengers
on board were killed—the highest death toll of any aviation accident
involving a McDonnell Douglas MD-83 ever, and the aircraft was
destroyed.
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Why it happened?
The subsequent investigation by the National Transportation Safety Board
determined that inadequate maintenance led to excessive wear and
catastrophic failure of a critical flight control system during flight.
The probable cause was stated to be "a loss of airplane pitch control
resulting from the in-flight failure of the horizontal stabilizer trim
system jackscrew assembly's acme nut threads. The thread failure was
caused by excessive wear resulting from Alaska Airlines's insufficient
lubrication of the jackscrew assembly."
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ExcelCalcs Repository Calculation 1
- As is our practise we publish any calculations that might be useful
to other engineers on the ExcelCalcs site. Clearly we do not publish
any confidential information but in the course of our work we produce
generic calculations that are likely to be useful for future problems,
these we upload to ExcelCalcs. One such calculation determines the adhesive wear rate of two metals
which we can apply to our jackscrew investigation. It employs some data
published in “Wear Coefficients - Metals Section IV of 10” by E.
Rabinowicz and “Fundamentals of Machine Component Design” by Juvinall.
It considers the compatibility of various metal combinations. Materials
with increased levels of compatibility have an increased tendency
‘weld’ together and a higher wear rate. It also considers the hardness
of the material and the contact pressure between the two materials.
Reflecting on the incident it is unlikely that unsuitable materials
were selected but it is possible that pressure on minimising weight
resulted in selection of a smaller leadscrew operating under high
contact pressure and a correspondingly high wear rate.
ExcelCalcs Repository Calculation 2 - A second calculation in the repository considers the strength of jackscrew load-nuts
and how the strength of the load-nut reduces as it wears. What is
particularly interesting is the difference between the wear rates on a
lubricated jackscrew are almost 100 times lower than an un-lubricated
one. The calculation is a very useful to set an inspection intervals to
ensure that un-lubricated jackscrews will be identified before it leads
to a failure. Using the calculation you can identify the most
important parameters to decide the best way to enhance the safety of
your designs.
The calculations sadly makes it all too
clear to see how inadequate maintenance and poor lubrication led to
excessive wear and catastrophic failure aboard Alaska Airlines Flight
261. See BBC News Coverage of the incident. Please contact John Doyle if you require our assistance on your own failure investigations.
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