Date: 201407
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Weather elements / visibility: Fog; 5
Light: Daylight
Aircraft: Small Aircraft; Low Wing; 1 Eng; Fixed Gear
Operator: FBO
Operating under: Part 91
Flight plan: VFR
Mission: Training
Flight phase: Cruise
Airspace: Class D ZZZ
ATC / advisory: Tower ZZZ
Aircraft component: Exterior Pax/Crew Door
Component problem: Failed
Reporter organization: FBO
Reporter function: Instructor; Pilot Flying
Reporter qualification: Flight Crew Commercial; Flight Crew Flight Instructor; Flight Crew Multiengine; Flight Crew Instrument
Human factors: Physiological - Other
Anomaly: Aircraft Equipment Problem Less Severe; Flight Deck / Cabin / Aircraft Event Illness / Injury; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Landed As Precaution; Flight Crew Returned To Departure Airport; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: I was instructing a student simulating instrument conditions under a flight visor. An annunciator indicating 'CO LVL WARNING' illuminated and held. I took the flight controls from my student and instructed him to take off his visor. I made the decision to fly direct via GPS to ZZZ airport and executed an initial descent to 2;000 feet. I also asked my student to check if his fresh air vents were open to insure we were receiving airflow to the cockpit. When 10 miles out; I was feeling hot and had a tingling sensation in my fingers. Whether due to the stress of the situation or to CO levels; I was also having trouble taking full breaths. It is possible adrenaline caused these effects. When I asked my student; who was unaware of what carbon monoxide was or did; how he was feeling; he said he was okay. I called the Tower 10 miles north of the airport and reported my situation. When the Tower asked if I [had] an emergency; I did. When asked if I wanted emergency services; I said yes; due to the symptoms I was feeling. Enroute I had my student run the Descent/ Approach Checklist and Landing Checklist in preparation for landing. I also asked him to monitor his symptoms and to be prepared to take the controls if I were to lose consciousness. Upon reaching 4 miles north of the airport we reported our position and were given priority to land. Somewhere around that time the 'CO LVL WARNING' annunciator extinguished and I reported it to Tower. After landing we opened the doors and taxied back to the ramp. After being checked out by emergency services both my student and I were allowed to return home and were asked to call if we felt bad later in the day. The only way to get tested for CO poisoning was a blood test; but after getting on the ground and vacating the airplane; we decided to forego going to the hospital. I canceled all of my following dual flights that day and returned home to rest. The CO annunciator was tripped due to exhaust entering the cockpit through a loose seal in the bottom latch of the door. I noticed a whistling on takeoff which typically indicates an open door latch. However; when I pushed to test the latch it had appeared to be fully latched. After a few seconds; the whistling stopped. The amount of exhaust was likely just enough to trip the CO warning annunciator; which I learned after the flight; has a long reset period. I will do a much more thorough inspection of my door latch before takeoff and will land immediately upon having any door latch problems in the future to avoid repeating this situation. Given the situation; I believe my decision to [advise ATC of] an emergency was warranted and in the best interest of the safety of the flight.




Synopsis: This CFI reporter; during flight and while giving instruction to a student in a modern high performance single engine aircraft; describes an enunciated indication and encounter with carbon monoxide in the cabin and the steps taken to bring this event to a safe conclusion.