Be-90 captain reported diverting to an alternate airport after inflight shutdown of the left engine;...

Narrative:

[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.

Original NASA ASRS Text

Title: 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.

Narrative: [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.

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	1756020
Time
Date	201809
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Flight Conditions	VMC
Aircraft 1
Make Model Name	King Air C90 E90
Flight Phase	Cruise
Component
Aircraft Component	Engine
Person 1
Function	Pilot Flying Single Pilot
Qualification	Flight Crew Commercial Flight Crew Instrument
Events
Anomaly	Aircraft Equipment Problem Critical Deviation - Procedural Published Material / Policy

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.