Flight Crew Flight Instructor; Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 20; Flight Crew Total 7000; Flight Crew Type 500
Situational Awareness; Training / Qualification
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Other / Unknown
Flight Crew Landed As Precaution; Flight Crew Became Reoriented
Human Factors
CFI applicant was instructing me on proper preflight and systems. During the course of the lesson we took off the oil filler cap and inserted a funnel intending to put in 1 qt of oil. We determined that the oil level was sufficient for the flight (8 qts) and it would be unnecessary to add the additional quart. We removed the funnel and took the quart of oil back to the supply cabinet; returned to the aircraft and closed the access door neglecting to put the filler lid back on. During climb out I noticed oil streaming out from the access panel and realized that we forgot to put the cap on. We [advised ATC] and landed without incident. Upon removing the cowl at [the] ramp we discovered that we did leave the cap off. Less than 1 quart was lost according to the dipstick. Maintenance was called and the aircraft was taxied to them to have the engine cleaned. Without having a clear understanding of who was doing the actual preflight and who was instructing; we both missed that the cap was left unsecured.
C182RG Instructor pilot reported taking off with the oil filler cap not installed.