Let’s start with a real-life story
The Initial Work
An aircraft required a High Pressure Shut-Off Valve (HPSOV) replacement. The work occurred on an on-wing engine in a line maintenance environment. A Technician received the task assignment, and started the work at the end of his shift. He was able to remove the valve and prepare for the installation.
The Technician used the accepted Aircraft Maintenance Manual (AMM) procedures for the removal. This included the removal of four clamps to loosen adjoining ductwork. The two clamps not directly attached to the valve are located underneath adjacent ductwork and components. This makes them hard to see and and get access to. The ductwork and valve are a close-tolerance, rigid fit, and they remain in place without the clamps. The Technician left the work with the valve in place and the clamps uninstalled.
The Handoff Problem
The outgoing Technician provided the work status to the incoming Technician for the next shift. The incoming Technician would install the valve, and had performed the removal and installation on several occasions. However, he had learned a method from other Technicians that did not require removing the clamps not directly connected to the valve. This procedure deviated from the AMM.
At the start of the installation, the second Technician noticed that there were four clamps, two of which were in better shape than the others. He assumed someone had ordered the newer-looking clamps as replacements for the others, and that the other two were still in place. He did not notice the clamps missing from the installation. The Technician installed the two easily-accessible clamps connecting the valve to the ductwork, secured the sense line and electrical connections, and closed the cowl and signed off the log.
The Consequence
The airline dispatched the aircraft for flight. Following takeoff, the aircraft experienced a fire warning and immediately returned to the airport, declaring an emergency. The subsequent investigation revealed that the unclamped segments of ducting had separated under takeoff pressure. This allowed unrestrained, high-pressure, hot compressor bleed air inside the cowl.
When deviating from the AMM, technicians create multiple problems:
- They overlook engineering considerations
- They compromise the method to provide the partial status of a task
When the Original Equipment Manufacturer (OEM) writes maintenance procedures, they may understand the engineering requirements driving the procedure. However, they do not always understand the environment the Technician works in, or have the same level of experience in aircraft maintenance. That being said, through proper technical vetting with engineering approval, operator experience can be applied to the procedures.
Engineering and reliability can also be affected when approved procedures are not followed. In the above situation, the Technician installed the valve using the engine duct flange as a fulcrum to pry the replacement valve in place. The results of the stress on the flange or duct after a series of valve removals and replacements are unknown. However, they could show up in a future inspection as a latent failure (or as a worst-case scenario).
The Need for Standardization
To ensure consistency in the application of these procedures, the use of aircraft publications must be mandated by the operator as the standard and deviations must be approved and incorporated.
Maintenance-induced errors emanating from inconsistent procedures that deviate from the approved standard can make it difficult to determine the corrective action to a reliability problem. It is mandatory for Technicians to follow approved procedures. When they are complied with, procedural changes can be made, and reliability trends can be correlated more consistently to the revised practices introduced through the revised procedure.
