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Jul
01
2019

Maintenance Minute - All Torqued Up

Posted 81 days ago ago by jhadmin


Ever get all torqued up reading a helicopter maintenance manual because you couldn’t remember the difference between a re-torque and a torque check? I did, until I had their definitions tattooed on my — well, that’s another story.


In the meantime, let’s work out their definitions here. Because not understanding the difference between a “re-torque” and a “torque check” can lead to potentially disastrous consequences.   


When a re-torque is specified by a maintenance procedure, it requires backing off the identified fastener one to two turns, then re-torquing the fastener to a required torque value or predetermined index point. 


For example, on an Airbus AS350 there is a 30-hour re-torque requirement for the tailboom mount hardware after installation. It requires indexing the tailboom attachment nut to the tailboom ring with a marker, loosening the nuts (one at a time), re-torquing the nut to the required value, recording that torque value, and finally checking the index mark lines up between the nut and the ring. 


On the other hand, when a torque check is specified, the fastener is left in place while torque is applied in the tightening direction until the required torque value is reached. If the fastener moves before the minimum torque value is reached, a second torque check will be required at a future time.


For example, on a Bell 206 there is a 1-hour to 5-hour torque check on the main rotor mast nut after each main rotor hub and blade installation. It requires removing the mast nut lock and torquing the mast nut (in the tightening direction) to a specified torque value. If the nut moves, i.e., tightens farther, the mast nut lock is reinstalled and another torque check is entered in the aircraft logbook due at the next 1 to 5 flight hours.


Imagine if a mechanic mistakenly performs a re-torque instead of a torque check on an item that may inadvertently shift position when the original torque is released. While the 206 above is not as susceptible, loosening the mast nut prior to applying tightening torque could cause the main rotor hub to shift on the mast split cones. 


However, in the case of a Bell 407, especially with the older lower cone sets, loosening the mast nut would invariably cause a hub shift, leading to another round of main rotor track and balance procedures.      


And there can be similar problems with performing a torque check in place of a re-torque. Some installations may require the installed part to shift or resettle. For example, most wheel bearings have an initial torque that is followed by a lower re-torque. Or, aircraft fuel tank mount straps may bind after the initial installation and should be loosened to relieve any binding prior to the strap re-torque. 


So, there you have it. As long as you don’t cross your “re-“ with your “check,” life should remain grand. Tattoos optional.


About the author: After 32 years maintaining helicopters in various capacities, Scott concluded a full-time career with a major operator in 2014. When not pursuing future writing projects, he can still be seen around the flight line tinkering on aircraft for beer money. He can be contacted at tekaviationllc@gmail.com.







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