Maintenance and Certification - Torque Paint (Slippage Marks)
by Marcel Payant, Civil Aviation Safety Inspector, Standards and Procedures, Aircraft Maintenance and Manufacturing, Civil Aviation, Transport Canada
The application of torque paint (slippage marks) to fuel, air and oil lines and fittings serves more than just one purpose. The most obvious reason is to provide a visual indication to confirm that the subject line and fitting are still at the required torque value (they have not come loose). Many engine manufacturers include the application of torque paint (slippage marks) to fuel, air and oil lines for that very specific reason. It provides for an easy visual confirmation that the subject fitting has not loosened or backed off.
The application of torque paint can also be very helpful to the aircraft maintenance engineer (AME) or technician who is installing or replacing many fuel, air and oil lines during scheduled and unscheduled maintenance. During the installation of multiple fuel, air and oil lines, the application of torque paint after each correct torque value application will provide an additional confirmation that this task has been completed. Some fuel, air and oil lines are installed in a specific sequence to facilitate the installation and/or to ensure correct alignment to prevent interference with other lines. After the installation of many lines, as in the case of an engine change, it becomes very easy to forget which lines have had the correct torque value applied, and which ones have just been hand tightened to facilitate installation and alignment. The application of torque paint (slippage marks) after the completion of the correct torque value to the fitting provides this additional safeguard.
Why do we still hear and receive reports of fuel, air and oil lines becoming loose and causing serious accidents or incidents in aviation? Engine manufacturers require that all fuel, air and oil lines, be visually inspected as part of either a pre- or post-flight inspection. Without this visual aid of torque paint, the AME or technician would need to physically check the torque value of every line and fitting to ensure that they are still at the correct torque value. Visual inspection of lines and fittings for any missing torque paint provides a confirmation that the fitting is still tight. Any missing torque paint would be suspect, and is intended to alert the AME or technician to physically check the subject line or fitting to confirm whether it is at the correct torque value or not.
Any fittings found to be loose should be re-torqued and have the torque paint re-applied.
Slippage marks are also commonly used in tire assembly and build-up to reduce the possibility of tire and tube failure due to slippage. The tire is marked and indexed with the wheel rim, which provides for an easy visual indication of any tire slippage.
Not only does the application of torque paint (slippage marks) make good sense, most manufacturers require it. Operators and maintainers should ensure that torque paint (slippage marks) is applied correctly and is inspected at the required intervals to help ensure that they provide the additional safety that was intended.
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