This is an excerpt from a report made to the Aviation Safety Reporting System. The narrative is written by the pilot, rather than FAA or NTSB officials. To maintain anonymity, many details, such as aircraft model or airport, are often scrubbed from the reports.
On a training flight we went out to a practice area. We were at about 5,000 feet MSL heading north when a carbon monoxide (CO) level warning came on.
My student quickly canceled the message, but I was able to catch a quick glimpse of the message. At this point I decided to turn back to the airport and conclude the lesson.
I opened the storm window and shut off the heat/defrost. I took flight controls and started a descent down to 2,500.
The descent was steep but nothing out of the usual (taking into consideration the rate of descent during a simulated emergency descent). However, on this descent my student bent over and grabbed his eye and started complaining of severe pain behind his eye.
I contacted approach and informed them about the high CO level warning and that my student was in considerable pain. I requested priority for landing and a straight in approach for the RNAV XXR. We leveled off at 2,800 and proceeded towards the Initial Approach fix.
During this time period I was in constant communication with my student. He remained responsive but continued to experience severe pain. I told him I would keep talking for the remainder of the approach discussing items related to the approach. I told him to listen to me to make sure I was making sense when I was talking just in case I started succumbing to the “CO” without knowing I was.
During the flight inbound we both noticed a chemical smell in the cockpit.
We landed the aircraft and were escorted by the firetruck and ambulance. I asked to taxi on XYL to taxiway X onto ramp. Once we were there I shut down and had my student go into the ambulance to get checked over. They said he should go to the doctor the following day.
I believe there was a leak from the exhaust into the cabin. I did have the heat/defrost on, which utilizes the heat from the exhaust. I assume there might have been a crack in the exhaust system that resulted in CO coming into the cabin.
Our aircraft are usually kept to a very high maintenance standard so I would not recommend anything on their part.
However, I would recommend a possible redesign of the heating system/exhaust systems from the manufacturers that would make it harder for CO to get into the cabin.
Primary Problem: Aircraft
ACN: 1856213
The student’s pain was probably from a blocked sinus cavity behind the eye due to the high descent rate. It happened to me in a altitude chamber exercise during rapid decompression.
“Back in the day” mechanics often worked in unventilated car repair shops with the doors closed and vehicles running…. but not burning ethanol.
The student either has an extremely low tolerance for CO, which is good to know about, or another medical/dental issue irritated by the dive.
Telling somebody with a potential carbon monoxide exposure to go see their doctor the next day is terrible medical advice.
The student was told to go to the doctor not because of the carbon monoxide but because of the severe pain when descending at a higher rate. I have had severe pain in an ear from a normal descent from 7500.