King Air C90 E90
Flight Crew Commercial; Flight Crew Instrument
Situational Awareness
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Landed in Emergency Condition; Flight Crew Landed As Precaution; Flight Crew Inflight Shutdown; Flight Crew Diverted; General Maintenance Action
Aircraft; Human Factors
[While in cruise flight] at 10;500 ft.; [I] noticed [that the] torque gauge was fluctuating on the left engine. I then noticed oil pressure was dropping. [I] made a 180 turn back for ZZZ was talking to the Chief Pilot. We agreed that I needed to shut down the left engine and make an emergency landing at ZZZ. Once on the ground; safe; [the] aircraft was towed over to Maintenance. [They] found that the dip stick was laying on top of the engine. [They] filled [the] engine with oil; test ran [it] and Maintenance returned [the] aircraft back in service. [The cause of the incident was an] inadequate preflight check.
BE-90 Captain reported diverting to an alternate airport after inflight shutdown of the left engine; where a ground inspection revealed the oil dipstick was not returned to its receptacle after preflight inspection.