The Importance of Using Approved Procedures


Let’s start with a real life story:

An aircraft required a High Pressure Shut-Off Valve (HPSOV) replacement on an on-wing engine in a line maintenance environment. The Technician assigned to the task started the work at the end of his shift and was able to remove the valve and prepare for the installation. The Technician used the accepted Aircraft Maintenance Manual (AMM) procedures for the removal, which had included the removal of four clamps to loosen adjoining ductwork to facilitate the removal. The two clamps not directly attached to the valve are located underneath adjacent ductwork, and components and are hard to get access to and were not easily seen. The ductwork and valve are a close tolerance, rigid fit and the valve and ducts remain in place without the clamps. The Technician left the work with the valve in place and the clamps uninstalled.

The status of the work was provided to the Technician on the next shift. The Technician assigned for the installation of the valve had performed the removal and installation on several occasions and used a method he learned from other Technicians that did not require the removal of two of the clamps not directly connected to the valve. This procedure deviated from the AMM.

At the start of the installation, the second Technician noticed there were four clamps, two which were in better shape than the others and made the assumption that the newer looking clamps were ordered as replacements for the others and that the other two were left installed. The second Technician did not notice the clamps missing from the installation. The Technician installed the two easily-accessible clamps connecting the valve to the ductwork, secured sense line, electrical, closed the cowl and signed off the log.

The aircraft was dispatched 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 separated under takeoff pressure, allowing unrestrained high pressure, hot compressor bleed air inside the cowl.

When deviating from the AMM, not only are the engineering considerations overlooked, the method to provide the partial status of a task has also been compromised.

When the Original Equipment Manufacturer (OEM) writes maintenance procedures they may understand the engineering requirements driving the procedure the procedure; but do not always understand the environment the Technician works in and may not have the same experience level in maintaining an aircraft. 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 valve was installed 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 possibly as a worst scenario.

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. Following approved procedures is mandatory and 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.