To fly! Aviation is a miraculous, dangerous industry. There isn’t a lot of room for error, whether you’re a fighter pilot, a helicopter repairman, or a manufacturer producing aircraft products or articles. Small mistakes can have large repercussions. That’s why aviation is closely regulated by federal and state authorities, principally the Federal Aviation Administration (FAA).
Strict regulations ensure that small mistakes in production, maintenance, or flight don’t become catastrophic failures. For example, pilots must undergo a specified amount of training and receive FAA certification before they can fly. For production and maintenance managers, this means making certain each fastener in the aircraft is torqued to specification, inspected, and documented before being certified by the FAA.
The Importance of Torque Control
It’s fairly obvious how pilot error can cause an aviation incident or accident—if someone takes off without enough fuel or flies too low and gets tangled in transmission lines, the error is visible to even an untrained observer. Pilot error is only one cause of aircraft accidents, however. A significant percentage of aircraft incidents and accidents occur through no fault of the pilot, but rather due to production or maintenance errors, such as the improper application of torque.
According to the National Transportation Safety Board (NTSB), between 2009 and 2015, more than 45 accidents and incidents occurred due to improper torque applied to aircraft engine fasteners by maintenance personnel. Fasteners that were too tight or too loose deformed or worked free during flight, damaging and destroying engine components, necessitating forced landings, and resulting in serious injuries to passengers and crew.
While hair-raising by themselves, these 45 are only a selected subsection of the accidents and incidents that occurred due to improper torque during this time. More accidents resulted from improper torque applied to non-engine fasteners or improper torque applied during production. To review more accident information, consult the NTSB’s Aviation Accident Database and Synopses.
The severity of the potential outcomes of poor torque control is such that for both aviation production and maintenance workers, the FAA has developed stringent guidelines to make certain proper torque is achieved and maintained on every fastener.
When producing aircraft articles or products, there are three steps you must take to ensure your fasteners are torqued correctly:
Determine torque value
Apply torque value
Certify torque value
Not only are these steps important to keep your business from running afoul of the FAA, they can also be the difference between life and death for the passengers and crew who will eventually fly your aircraft.
Determining Proper Torque for Production
The first step towards ensuring proper torque is determining the appropriate levels of torque for each of your fasteners. For manufacturers, this means consulting the design data for the article or product you’re producing and disseminating this information to your technicians and assembly workers.
For designers, it means consulting reference tables supplying torque values for the fasteners and materials you intend to use, such as this circular prepared by the FAA. Below is a sample chart of standard torque values from the circular:
While charts provide guidance, it’s not enough for designers to simply copy and paste torque values from reference tables into production manuals. Rather, designers must test a range of torque values of critical applications in order to determine the actual, appropriate level. When doing so, keep in mind mitigating influences, such as the effects of high and low temperatures and the addition of friction drag torque when fastening bolts with self-locking nuts.
Achieving Proper Torque in Aircraft Production
Once the appropriate torque values for your fastener have been determined, it’s a matter of applying them in a consistent and fail-safe manner. To accomplish this, it’s necessary to equip your technicians and assembly workers with the right tools. Proper torque tools will generate the force necessary to achieve the correct levels of torque and will prevent the operator from applying too much torque.
The tools you’ll need vary by application. It’s important not to mix them up; inch-pounds tools won’t generate the torque necessary for larger bolts, while foot-pound tools lack the accuracy to appropriately torque fasteners with inch-pound torque values. Here’s the breakdown:
For small articles, boxes, or other areas with small fasteners, you’ll need hand torque tools which produce and measure inch-pounds of force.
For larger fasteners within articles, interfaces, or products, you’ll require hand tools which produce and measure foot-pounds of force.
For the largest, highest torque value fasteners, power assembly torque tools are often necessary.
Depending on the criticality of the fastener, different mechanisms for controlling torque can be effective. For less critical fasteners, tools such as click wrenches can be used. These wrenches inform their operator with an audible click when the correct torque has been reached. While these can effectively apply torque, operators must be trained to use them correctly and can potentially make mistakes.
For more critical fasteners, choose a torque tool that mechanically prevents the operator from over-torquing. Wrenches with a “cam-over” mechanism do this by slipping when the desired torque value is reached to stop the operator from applying more pressure. In assembly situations, it’s common to find internally adjustable, or preset, cam-over wrenches. These wrenches are preset to a certain torque value, preventing the operator from accidentally setting their tool to the wrong torque. Power and robotic assembly tools also have mechanisms to physically prevent improper torque values from being reached.
Finally, it is essential—and required by the FAA—to test your torque tools regularly to ensure they are calibrated correctly. If improperly calibrated, they will apply the wrong amount of torque to every fastener they’re used on. The manufacturer of your tools will provide a recommended calibration schedule. Torque testers, analyzers, sensors, and calibration equipment are available to assess and correct the calibration of your torque tool. The Federal Aviation Regulations, regarding equipment testing and calibration, state that as part of a producer’s inspection and testing requirements, they establish:
“Procedures to ensure calibration and control of all inspection, measuring, and test equipment used in determining conformity of each product and article to its approved design. Each calibration standard must be traceable to a standard acceptable to the FAA.”
Inspecting Torque Values During Production
Even with the right tools and well-trained operators, mistakes can still happen. As such, it’s not enough just to apply torque to your fasteners, then call the process complete. Your fasteners must be inspected as well. Using diagnostic tools such as a dial torque or electronic torque tools, quality control technicians can test the torque values of fasteners after they have been applied and document their findings.
Whether displaying results on an analog dial or electronic display, these tools present the torque value of the fastener to the operator. As with any other torque tool, these instruments are only as good as their calibration and must be tested and calibrated regularly to ensure they provide accurate readings.
Determining and Achieving Proper Torque During Maintenance and Repair
Many of the same procedures used in aircraft production are also necessary during maintenance and repair. Maintenance and repair operations must follow the same basic steps when adjusting or replacing fasteners:
Determine torque value
Apply torque value
Certify torque value
Fortunately, maintenance workers should be able to determine necessary torque values with relative ease. To find the torque value of any fastener on the aircraft, they must consult the aircraft’s manual. These manuals are required by the FAA for every aircraft and will be produced by their aircraft’s designers. That said, sometimes design issues or flaws only become visible after aircraft have left the factory. To ensure the information is current, maintenance and repair operations must keep abreast of FAA produced advisory circulars and airworthiness directives relating to the aircraft models they’re working with. Airworthiness directives are regulations to correct unsafe conditions in aircraft products, while advisory circulars are updated guides to compliance.
Once the torque value is determined, the next step is applying it. Like assembly workers, maintenance workers and repairmen must use tools that can provide the appropriate level of torque for their fasteners and prevent operators from over-torquing. Technicians should use tools measured in inch-pounds for low-torque applications, foot-pounds for applications with higher torque, and power tools as necessary.
While click wrenches and preset wrenches can be found in maintenance and repair operations, externally adjustable cam-over tools are ideally suited for the job. Like preset wrenches, these tools slip to mechanically prevent over-torquing. However, these tools are easily adjustable from the outside, allowing maintenance workers to use the same tool to efficiently tighten many fasteners with different torque values. The greatest potential for error when using an adjustable torque tool is accidentally using the tool is when it’s on an incorrect setting. Operators must pay careful attention to the settings and double-check their work with inspection tools like dial wrenches. As with any other application, it’s also important for maintenance and repair workers to ensure their tools are calibrated according to the schedule provided by their manufacturer.
Choosing Torque Tools You Can Trust
Whether you’re involved in the production or repair side of the aviation industry, it’s equally essential to ensure your torque control procedures are accurate and complete. You need to know your torque values, apply your torque values, inspect them, and document them. It’s only through this rigorous, exacting process that pilots, crew, and passengers are able to safely fly.
Choosing the right tool manufacturer is a critical aspect in ensuring proper torque control. As you shop around for torque tools, pay attention to each manufacturer’s record and reputation for dependability, as well as their willingness to help you find the best tools for your application. After all, pilots, crew, and passengers place their trust in the quality of your work. In turn, you need to be able to trust the tools you’re working with to apply the right torque, every time.
Mountz Torque has been a trusted provider of torque tools for the aviation industry for over half a century. We know you put a lot of faith in the tools you use, so we work hard to make ours as accurate and dependable as possible. To find the right tool for your application, ask our representatives a question or request an appointment.