Date: 201905
Local time of day: 1201-1800
Location: CRQ.Airport, CA
Flight conditions: Mixed
Light: Daylight
Aircraft: Skyhawk 172/Cutlass 172
Operator: FBO
Operating under: Part 91
Flight plan: IFR
Mission: Personal
Flight phase: Final Approach
Airspace: Class D CRQ
ATC / advisory: Tower CRQ
Aircraft component: ILS/VOR
Component problem: Malfunctioning
Reporter organization: FBO
Reporter function: Pilot Flying; Single Pilot
Reporter qualification: Flight Crew Instrument; Flight Crew Commercial
Human factors: Situational Awareness; Workload
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Weather / Turbulence; Inflight Event / Encounter CFTT / CFIT
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Became Reoriented
Contributing factors: Aircraft; Human Factors
Primary problem: Human Factors

Narrative: Picking up ATIS returning from a VFR flight in a rental plane; I found that the ceiling has gone from clear to 900 overcast. I was on flight following so informed ATC that I would need the ILS but would remain VFR for the time being. Our prevailing weather often generates lower stratus above the airport; with clear skies along the ILS; and it was my assumption this would likely be the case again.While establishing on the localizer outside of the final approach point; I eventually noticed a flag on the glide slope for the #1 CDI (Course Deviation Indicator); and no movement on the needle. I noticed that the #2 CDI had a glide slope display as well; and since I was still in VMC I tuned the ILS in #2 and saw no flag; so I made the poor choice to continue the approach utilizing #2; still assuming I would likely remain clear of clouds.Before reaching the final approach point I did enter the clouds. Although the altitude at the final approach point gave a reasonable cross check; I was noticing that the stability of the glide slope almost looked too good. Soon I noticed that I was continuing to hold a higher than normal descent rate to hold the needle centered; and quickly realized that the needle was not truly responding to the glide slope but was simply centered and that I'd been blindly following faulty vertical guidance.While task load being single pilot in IMC I struggled to juggle decision making about the best course of action (which should have been ingrained to just go missed); and soon I realized I was breaking out into VMC again below the deck. The PAPIs confirmed that I was indeed below the glide slope. However at this point I could continue with visible contact with the ground and airport.The chain of bad decisions here started when I first noticed an issue with the glide slope of the #1 CDI. I was in VMC and could easily have told the controller that I had an issue and would like to come back to do another approach; with time to troubleshoot; set up a non-precision approach; etc. I suppose get-there-it-is; my preconception that I'd likely never enter IMC on this local approach where I knew the local terrain; and the thought that I could fix the situation on the fly egged me on.The second set of bad decisions came when I ignored by feelings that something just didn't feel right about the #2 glide slope and continued the approach rather than confessing that there was an issue and taking the relatively quick climb back into VMC above to figure out the issue there. By the time that really hit me I was pretty task saturated and my delay in acting resulted in me breaking out into VMC.I'd like to think that had the ceilings not been so high; or had it been an unfamiliar approach to an unfamiliar airport; I would have immediately made the right call and canceled the approach. But apparently the trap I let myself get into was being complacent with the familiar approach to my home airport; which could have ended up just as bad.




Synopsis: C172 pilot reported descending on approach without a reliable glide slope in IFR conditions.