Learjet flight crew observed engine anomalies during descent.

Narrative:

Observed a 15% split between engines at flight idle during descent. Same problem as reported by previous pilot to [director of maintenance]. I was advised by chief pilot of this pilot's report and informed me he had a discussion with [director of maintenance] and would correct [the] problem prior to dispatch. Upon returning from mission; I noted the problem still existed and the [director of maintenance] advised discovering a piece of the fin that connects the N1 (low pressure compressor) fan blades on the #2 engine had broken off and impacted the intake wall and nicked some blades in the secondary fan before being ejected ostensibly through the bypass. This was determined to be the cause of slight vibrations and unusual tone in engine during power changes.I was then advised by the [director of maintenance] and chief pilot they believed the damage occurred during a flight preceding mine and were closely monitoring the N1 fans on both engines because of strange tones and vibrations reported by several pilots who flew the plane.my question is why the damage wasn't discovered by the mechanics inspection preflight and post flight inspection before being released; especially since the mechanic was performing adjustments due to the recently reported discrepancy. (Damage to N1 was not clearly obvious to a pilot peering in the intake; it required one to look behind the blades.)

Original NASA ASRS Text

Title: Learjet flight crew observed engine anomalies during descent.

Narrative: Observed a 15% split between engines at flight idle during descent. Same problem as reported by previous pilot to [Director of Maintenance]. I was advised by Chief Pilot of this pilot's report and informed me he had a discussion with [Director of Maintenance] and would correct [the] problem prior to dispatch. Upon returning from mission; I noted the problem still existed and the [Director of Maintenance] advised discovering a piece of the fin that connects the N1 (Low Pressure Compressor) fan blades on the #2 engine had broken off and impacted the intake wall and nicked some blades in the secondary fan before being ejected ostensibly through the bypass. This was determined to be the cause of slight vibrations and unusual tone in engine during power changes.I was then advised by the [Director of Maintenance] and Chief Pilot they believed the damage occurred during a flight preceding mine and were closely monitoring the N1 fans on both engines because of strange tones and vibrations reported by several pilots who flew the plane.My question is why the damage wasn't discovered by the mechanics inspection preflight and post flight inspection before being released; especially since the mechanic was performing adjustments due to the recently reported discrepancy. (Damage to N1 was not clearly obvious to a pilot peering in the intake; it required one to look behind the blades.)

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1636099
Time
Date	201904
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Bombardier Learjet Undifferentiated or Other Model
Operating Under FAR Part	Part 135
Flight Phase	Descent
Route In Use	Direct
Flight Plan	IFR
Person 1
Function	Captain
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 120 Flight Crew Total 12000
Events
Anomaly	Aircraft Equipment Problem Critical Deviation - Procedural Published Material / Policy