Date: 201911
Local time of day: 0601-1200
Location: ZZZ.ARTCC, US
Flight conditions: Mixed
Light: Daylight
Aircraft: Regional Jet 200 ER/LR (CRJ200)
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Ferry / Re-Positioning
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Powerplant Lubrication System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Troubleshooting; Situational Awareness
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Flight Crew Diverted; Flight Crew Landed As Precaution; Flight Crew Overcame Equipment Problem
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: I was the PM (Pilot Monitoring); and the FO (First Officer) was the PF (Pilot Flying). In cruise flight at 22;000 feet; experienced loss of oil pressure on the left engine. Received L ENG OIL PRESS warning message and aural warning. Left engine oil pressure was fluctuating between 29-36 PSI. Left engine oil temperature was steady at 117 Celsius. The left engine oil pressure warnings were intermittent with the fluctuations in oil pressure. Ran the QRH (Quick Reference Handbook) procedure for L ENG OIL PRESS. Per the QRH; we reduced the left engine thrust lever to idle. This prevented the oil pressure readings from going below 30 PSI and the intermittent warnings no longer persisted. The left engine oil temperature readings were holding in the normal range and the oil pressure readings were now holding in the green (normal) range; yet still fluctuating between 31-36 PSI. We elected to leave the left engine operating at idle. We completed the QRH procedure; advised Dispatch and Maintenance Control of our situation.We discussed alternate landing airport options versus continuing to ZZZ1. Due to adverse weather conditions in ZZZ1; we elected to divert to ZZZ as suggested by our Dispatcher.During our diversion preparation; we talked about the possibility of having to shut down the left engine and conducting a flaps 20 landing. However; since we never shut down the left engine; we did not actually run that QRH procedure. The only guidance in the QRH procedure for the L ENG OIL PRESS warning regarding usage of the affected engine states; 'as required.' This is very vague and unclear as to whether we can use the engine above idle thrust. My thinking was that we needed to keep the engine at idle and not advance thrust on that engine as it would risk another warning and further complications which could lead us to an inflight engine shutdown; or the engine could fail altogether. Our APU was deferred; as well; and I was not in favor of shutting down the left engine so long as it operated within limits at idle.On final for XXR visual into ZZZ; we began configuring the aircraft for landing. In my mind; we were expecting a normal flaps 45 landing. We also briefed a flaps 45 landing. This seemed right as the left engine was still operating; but at idle. Approximately 4 miles from the FAF (Final Approach Fix); the PF called for 'gear down; flaps 30.' When the flaps were at 30; I noticed it took a lot of thrust on the right engine (approximately 85% N1) to maintain altitude and the speed continued to decrease below 160 kts. I referenced our speed cards to determine our flap 30 minimum speed to be 131 kts. I became concerned with our ability to maintain airspeed and altitude if we went flaps to 45. It never occurred to me to use the left engine thrust for this landing so that we could then maintain the necessary speed for a flap 45 landing. Since we had already talked about the possibility of a flap 20 landing earlier; we briefly discussed that option again. I referenced the Vref (Final Approach) speed for flaps 20 from the speed card (which we were still above). In my mind I knew that we had more than double the actual landing distance available on a dry runway for landing. However; I failed to state this out loud. But after a brief discussion about conducting a flap 20 landing; we both agreed this may be the safest course of action at that time. So; we reconfigured the aircraft for a flap 20 landing (just outside of the FAF). We landed visually.Post flight debrief: Looking back; there are things that should have happened differently. One being; the QRH states; 'Affected thrust lever...as required.' I should have realized this meant we could have advanced the thrust lever out of idle for the short time we would have been configured at flaps 45. Secondly; since we elected to land at flaps 20; I should have requested cancelling the approach clearance and requested time to properly configure the aircraft for a flaps 20 landingper the QRH instead of hastily configuring at a 6 mile final. Admittedly; I was conflicted with thinking the QRH Single Engine Approach and Landing procedure did not apply to our situation (since the engine was operating) and nothing in the QRH procedure for L ENG OIL PRESS with stabilized engine parameters (green oil temp and oil press indications) drove us to the Single Engine Approach and Landing procedure. Nor did the L ENG OIL PRESS QRH procedure clearly state actually using the left engine above idle. I failed to understand what the QRH was implying regarding the affected thrust lever use.We are unsure of what caused this reduction in oil pressure. With regards to the decision to land flaps 20; my lack of understanding of what the ENG OIL PRESS QRH procedure was implying and being of the opinion that the Single Engine Approach and Landing did not apply to our situation led to the decision.It would be helpful to have a note in the QRH procedure for L(R) ENG OIL PRESS to the effect of 'consider the use of the affected thrust lever during a flap 45 landing or consider the necessity to land flaps 20 if the thrust lever is to remain at idle'. In the heat of the moment and under stressful conditions; a note like this could help crews determine the best course of action. Also; I highly recommend exposing flight crews to a scenario like this in the simulator. Allowing us to experience what the airplane can actually do with one engine at idle and fully configured for landing would be helpful.




Synopsis: CRJ-200 Captain reported a loss of oil pressure in left engine which resulted in a diversion.