Date: 201202
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Flight conditions: VMC
Weather elements / visibility: 20
Light: Night
Aircraft: PA-23-250 Aztec
Operator: Personal
Operating under: Part 91
Flight plan: VFR
Mission: Personal
Flight phase: Cruise
Airspace: Class E ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: Oil Filler Cap
Component problem: Design; Malfunctioning
Reporter organization: Personal
Reporter function: Single Pilot
Reporter qualification: Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Commercial; Flight Crew Flight Instructor
Human factors: Situational Awareness; Troubleshooting
Anomaly: Aircraft Equipment Problem Critical
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Inflight Shutdown; Flight Crew Landed in Emergency Condition; General Declared Emergency
Contributing factors: Aircraft; Equipment / Tooling
Primary problem: Aircraft

Narrative: This flight was on a direct route. All phases of flight appeared normal in every aspect during taxi; takeoff; climb; and initial cruise. There were no indications of any problems during the first 20-30 minutes of the flight. Approximately 20-30 minutes into the flight I was doing a routine 'cruise check' scanning over every instrument in the cockpit. I also look outside of the aircraft in all directions and look for any indications of problems. When inspecting the left engine nacelle it appeared to have something obscuring the paint color. I then utilized my flashlight and immediately confirmed the left engine was venting oil overboard excessively. It was trailing back from the front of the nacelle to the rear trailing edge of the wing. It may have been as many as ten minutes since I last looked over the nacelle. I advised my passengers of the problem and what I was intending on doing and asked them if they had any questions. While explaining the situation I noted that the oil pressure and temperature were still normal for cruise phase. I performed a precautionary shutdown of the left engine to prevent damage in case all the oil would eventually vent overboard. I called Approach and explained the situation and received vector assistance toward a nearby airport. They asked for fuel and souls on board. After explaining that I shutdown the engine I declared an emergency and the Controller stated he had already declared an emergency on my behalf and had emergency crews and equipment preparing for my arrival. The aircraft performed well single engine and we were able to maintain 6;500 FT MSL until we needed to descend for landing. After completing the full engine securing procedure I pumped down the gear and flaps manually and made an uneventful single engine landing. ARFF escorted me to the FBO to ensure safety and after shutdown their crew member asked if everything was under control and stabilized which I confirmed it was. Upon closer inspection the oil appeared to be streaking from the oil dipstick/filler lid. I opened it to find the oil dipstick cap unseated from its secured and sealed position. Lycoming IO-540 engines have a 'popper' like dipstick cap. They simply pop into place unlike screw type caps on many other planes. Having read that these caps have caused trouble for some pilots I routinely pay extra attention to the dipstick caps to ensure they are secured. Today was no different and I clearly remember securing both caps after adding and then checking the oil level prior to departure. In the end the situation was not immediately life threatening and all parties involved performed professionally and efficiently. I appreciate the help of the ATC controllers and ARFF crews that helped ensure the assistance I required was given and the safety of my passengers was ensured.

Callback: The oil filler cap is constructed of an 'O' ring on the dip stick which seats in a recess slightly below the top of the oil filler tube so that the stick is positively seated when the 'O' ring slips into the tube recess. Maintenance replaced the oil filler tube and dip stick. The Reporter stated that somehow the stick popped out of the seat. The Mechanic stated that positive crank case pressure; a worn; dried 'O' ring or any number of defects could cause the dip stick to be unseated or 'pop' out of the oil filler tub seat.



Synopsis: A PA23-250 oil dipstick 'popper' cap unseated in flight and permitted an oil leak severe enough that the pilot shut the engine down; declared an emergency and landed at a nearby airport.