A 2002 FAA report titled General Aviation Maintenance-Related Accidents: A Review of Ten Years of NTSB Data notes that of the 1,474 NTSB maintenance-related GA accident reports analyzed, 295 (20.0 percent) cited installation as a primary cause or factor in the accident. The next three most frequently occurring maintenance activities included maintenance (217 or 14.7 percent), maintenance inspections (202 or 13.7 percent), and annual inspections (124 or 8.4 percent). Based on that information, installation and maintenance account for more than 34 percent of the accidents. In this article, we will go back to the basics to help prevent a maintenance-caused engine malfunction from occurring.
So why go back to basics? It boils down to a big problem in maintenance — complacency. Webster’s dictionary defines complacency as satisfaction accompanied by unawareness of actual dangers or deficiencies. In aviation, complacency can be catastrophic. Any of us is in danger of complacency, but especially those of us who have been working in the field for a while. So let’s step back and take a look at some of the basics we learned in A&P school to refresh our memory and help avoid complacency when working on turbine engines.
So what basic items should we focus on? Well, Pratt & Whitney Canada’s Customer Training Center has conducted informal surveys with its customers and also researched its engine reliability database and found that some of the top causes of maintenance errors are in the areas of incorrect torques, O-ring installations, and clamp installations. In the rush to get an aircraft out in a timely manner, it could be easy to overlook these items. So let’s look at these topics.
An old joke is that a while back, an aeronautical engineer caught his wife cheating with an aircraft mechanic. From that point on, engineers vowed to make mechanics’ lives as difficult as possible. We may be tempted to think that all the torque values we are expected to follow are a result of that vow. But following proper torquing procedures is critical to aircraft safety.
Many of us have fallen into the trap of using improper torquing techniques. Instead of going all the way to the tool room to check out that torque wrench, we just use our “elbow torque” to estimate how much torque we are applying. Don’t fall into this trap! Torques are specified for a reason, and not following proper techniques can be harmful.
When engineers design the aircraft, thorough analysis is done on the type of stresses that will affect each part. When the engineer designs a particular assembly, he takes into account all of these stresses that will be encountered as well as other factors like temperature changes, fatigue, and corrosion possibility. Fasteners are then chosen that will best fit the application and provide the necessary preload.
In order for a fastener to perform properly, it must be tightened accurately. Under-torqued hardware will result in inadequate preload. This can lead to unnecessary wear of nuts and bolts as well as the assemblies that they secure. Over-torqued hardware can exceed the design limitations of the structure or hardware and lead to a failure. Failure of the bolt or nut can result from overstressing in the threaded areas.
When choosing which torque wrench to use, try to select one that has the required torque in the second or third quarter of the wrench’s torque scale. The torque settings at the first and last quarter of a torque wrenches scale are not as accurate as those in the middle quarters.