Tuesday, September 27, 2011

Airworthiness Directive

An Airworthiness Directive, abbreviated as AD, as the link indicates contains information pertaining to the safety of operating an airplane. Transport Canada maintains a system to inform the registered owners of an aircraft when an AD that may affect an airplane registered to them has been issued. Since my airplane was manufactured in the USA the organization responsible for issuing ADs is the FAA. An AD is something like a recall issued by an automobile manufacturer, except that in the case of an AD the owner of the airplane, not the manufacturer, is usually financially responsible. As you might imagine, the arrival of an AD is a curious mixture of dread and relief. Dread because there may be a very expensive bill in the near future, relief because the AD hasn't been issued as a result of an accident investigator poking through the charred and twisted remains of one's airplane.

An AD will generally specify a part or set of parts that need to be replaced or inspected and replaced if certain conditions are found. They will also normally give a number of flight hours or calendar days in which the inspection or replacement must be completed by. This give the owner as much time as possible to arrange for the work to be done without too much interruption of normal operations. The seriousness of the defect, and the impact on flight safety will determine how much time the owner gets to comply.

I get a handful of ADs each year. Most don't apply to my airplane. They are for engine parts, navigation equipment or instrumentation that could be installed on my model of airplane, but aren't on my particular plane. A few months ago I received an AD that was applicable to my airplane, and required that I arrange an inspection and possible replacement of part of the flight control system. Yup, dread and relief came flooding in.

This AD is interesting in particular for what it tells us about how airplanes are built, and how flight safety impacts the manufacturing process. Many airplanes are controlled by the pilot manipulating a control yoke rather than the more traditional joy stick. In my airplane the control yoke shaft passes through the instrument panel and is connected to the next part in the control system by universal joint. The Piper service bulletin has some good pictures and diagrams if you are interested in how it goes together. The gist of the problem, is that during manufacture some of the shafts were not seated fully enough into the universal joint when the hole for the retaining pin was drilled. That did not leave enough metal between the hole for the pin and the end of the shaft to provide the strength needed for the control system. So, yes I wanted have my control system inspected to make sure it was safe. On the other hand, my plane has been flying around for over 40 years. If the control system had a significant weakness it probably would have failed in that time. This is probably why the service bulletin was issued in September of 2009, became an AD August of 2010 and we had a year or 100 flight hours to get the work done. This last link to the actual AD document is interesting because it also includes the submission made by interested parties as to the advisability and need to do the work, along with an estimate of the cost. It is instructive in that because two airplanes were found with this defect over 40,000 had to be inspected.

Most interesting is that at some point between the defective assembly and the discovery of the error Piper started manufacturing a witness hole in the universal joint so that the depth of the shaft penetration could be determined without disassembling. Makes one wonder what made them decide to do that. In the end though I'm glad that the inspection showed I don't have to have parts replaced so the financial hit wasn't too bad.

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