Up Time Magazine - February/March 2009 - (Page 36) Figure 8 - Avionics maintenance being performed on a Cessna Citation Excel. As a desperate measure, the control unit was replaced again. From that point on, the system operated normally throughout all operational demands. Root Cause Analysis The root cause of the initial system malfunction (when the valve would stop during operation) was a faulty vent valve. The control unit that was first installed was a rogue component, which caused the valve to intermittently modulate during high operational demands. However, this rogue failure would not manifest itself until a serviceable valve was installed, since the faulty valve would lock up during operation, preventing the modulation from occurring. This type of compound problem does not happen all the time. Usually a rogue component causes the original problem to continue until it is resolved after multiple replacements of the same part, when a “good” part is finally pulled from the spare inventory. To illustrate an effect on Asset Management, the following describes a real-life event: each having an autopilot system comprised of a control panel, pitch computer, roll computer, air data sensor and a number of servo motors and sensors. In order to minimize aircraft down time, it was determined that 6 pitch computers were needed as on-hand spare inventory. Over time, the fleet began to experience an increasing number of pitch related complaints and the spare pitch computer inventory was reduced to zero on a number of occasions. More computers were procured to accommodate the increasing demand. This chain of events repeated over a number of years until there were ultimately 28 spare computers to support the 40 aircraft that were in service. Root Cause Analysis The root cause of the inordinate amount of spare inventory that had accumulated was that a substantial amount of rogue components had developed. After a thorough analysis of the pitch computer population, it was discovered that 20 of the 28 spare computers were rogue. This caused the high replacement activity that would quickly decimate the inventory when an aircraft experienced a pitch related autopilot complaint. “Real Life” Case in Point: An aircraft operator had a fleet of 40 aircraft, 36 february/march 2009 http://www.bakerinst.com http://www.bakerinst.com
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