Turbine Engine Maintenance Back to Basics
Paying attention to details enhances quality and safety
By Parker A. Grant and James F. Mazeski
You have been working on Pratt & Whitney gas turbine engines for years, and have become quite familiar with performing the maintenance tasks necessary to maintain these engines. Routine inspections on engine components such as fuel nozzles, ignitors, fuel and oil filters, and compressor bleed valves have been performed. Time limited components on the engine have been inspected and repaired. You have become proficient in rigging the engine system and have even performed hot section inspections. Troubleshooting principles have become second nature, and engine trend analyses are being performed to detect deterioration of performance.
So, what can be done to improve the safety and reliability when working on these popular workhorses? Believe it or not, a return to the basics is key to increasing the quality of work and improving safety.
Jim Mazeski, Sr. Technical Instructor for the Pratt & Whitney Customer Training Center, states that the major maintenance errors that occur when working on these engines are in the areas of incorrect torques, O-Ring installations, and clamp installations.
"These are the kinds of things we all learned about in A&P school," says Dick Wellman, General Manager of Pratt & Whitney's Customer Training organization. "We've conducted informal surveys with our customers and also researched our engine reliability database and found that some of the top causes of maintenance errors are found in these areas."
Parker Grant, Pratt & Whitney's Manager of Training Operations, adds, "Refresher training in these specific areas is helping improve reliability statistics for the operators, when properly followed. It sounds rather fundamental, but not everyone understands the reasons why torque wrenches should be used and why O-rings should be properly installed for safe operation of the fleet. Oftentimes, we are rushed to complete the
maintenance so we can get the airplane back in the sky and as a result, correct procedures are not followed."
Standard practices for maintenance can help improve the safety and reliability of engines and aircraft. The following is a discussion of some standard practice tips to consider.
How incorrect torques can lead to failures
Be sure to follow specified torque values.
Many times, screws, bolts, tie-rods, or B-nuts are tightened without a torque wrench, hence the traditional gage being the "elbow." Whether you are a veteran maintenance technician or a rookie recently graduated from an A&P school, common sense says that elbows are still not the best measure of torques. There is a reason that torque limits are specified in the maintenance manuals. These limits should never be ignored. A basic understanding of this reason will hopefully help the maintenance technicians be reminded of the consequences of incorrect torques.
Torque is measured by inch-pounds, where a force is applied to turn a bolt [pounds] at a certain radial distance from the axis of the bolt head center [inches]. The torque is equal to the force times the radial distance. The force is perpendicular to the radial direction from the axis of the bolt head center to the hand. For example, if a maintenance technician applies a 10-pound force on a torque wrench with his hand 8.5 inches away from the axis of the bolt head center, he would be applying 10 X 8.5 = 85 inch-pounds (or in.-lbs.). This torque should always be greater than the load or stress, which the part will receive during service. If the torque is below the minimum required (under torque), the fastener can become loose or fail from mechanical fatigue. On the contrary, if the torque is above the maximum limit (over torque), it can reduce the fastener's resistance to shear stress and its elastic safety limit. Hence, a fastener can fail from stripped threads or fracture. Furthermore, the part being held by the fastener can be damaged (e.g. a crack in a flange).
"One of my favorite exercises I do in the Maintenance Standard Practices course is to have my students torque a few bolts without the aid of a torque wrench, as close to the assigned torque values as they can," says Mazeski. "Many experienced technicians are surprised to see that their elbows are really not as calibrated as they think."
This exercise is important because it emphasizes the need for torque wrenches. To keep the torque wrenches in proper working order, check the torque indicating devices and calibrate them on a regular basis. DO NOT check one torque wrench against another. Handle torque wrenches with care and use them according to the manufacturer's instructions for reliable and accurate results. Another tip is to choose the correct size torque wrench for the torque value being applied.
Another important area in the lesson of torques is proper lubrication. About 60 percent to 90 percent of the torque is used to overcome the friction of the thread and/or the seating surfaces. Where required, lubricants help reduce and stabilize friction on the threads and other bearing surfaces. Without proper lubrication, bolted connections may not receive the correct torque at assembly and that could cause the separation of parts during engine operation. The final torque values depend on whether or not a lubricant is used. Lubricant should be applied to fasteners whenever they are torqued unless otherwise specified in the maintenance manual.
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