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On a beautiful morning, October 2, 2025, I headed to the airport (KUGN) for a morning self-training flight. In the weeks before, I had several longer flights including an Angel Flight, breakfast flights, and at least one long cruise flight. All good, and my 2000 Mooney M20S Eagle was running about perfectly. It should be a great day. Sunny, high pressure, light winds, minimal turbulence – perfect to work on those basic skills: slow flight, stalls, steep turns, chandelles, lazy eights.
My 2000 Mooney M20S Eagle had been running about perfectly.
Pre-flighted and ready to go, except for one issue. I have a battery-powered Sensorcon Inspector AV8 Carbon Monoxide detector velcroed onto my panel, and turning it on is part of my checklist. It’s toward the very end of my checklist, well after engine start, because I added it a few years ago to a checklist that is much older.
In warm months, my engine start happens with the door and side window open, which results in a CO reading of 40 ppm or less while going through my pre-taxi checklist. Not a toxic level; it simply tells me the detector is working correctly. Today, however, it read 80 ppm, but I expected it would drop to zero once I took off and air began to flow – a highly optimistic assumption. Plus, red lights appeared on the detector, which I had never seen before. (A blinking red light really ought to tell me something!)
Everything else was perfect. Noise levels, vibration, power, and every instrument, except the CO detector, were exactly where they should be. So my optimistic brain said, “Let’s go – it’ll be cured in flight.” Always worked in the past.
Still reading 80 ppm as I taxied, I departed RW23 (6,000’ long), went full power, gauges live, totally focused on the takeoff, rotate and climb, gear up, flaps up. I glanced at the CO reading (mounted off to my lower right), and it was now 160 ppm. Uh-oh! Not what I expected. Past the point of landing straight ahead on RW23, I calmly called the tower to request a lap in the pattern, without explanation. The controllers were excellent, and my request was approved immediately. Make left traffic – report midfield left downwind, 23.
As I reached pattern altitude, I glanced at the CO detector again: 300 ppm – not good. Vents open, side window open, heat off, throttle back, gear down, partial flaps. I called at midfield and got immediate clearance to land. Fortunately, there was no one else in the pattern, so I could make an efficient approach. Had there been traffic, I would have declared an emergency for priority landing. Glancing again, the reading was higher; my brain was fogging, a headache building, and I don’t recall the peak, but it was over 300 ppm.
I did an unusually gentle full-flap landing, exited to the taxiway, opened the door to increase airflow, and breathed. Muscle memory was working fine, even as my brain fog threatened to override it. Then my habit almost got me: the tower cleared me to taxi back to RW23 for departure, and I started heading that way. A little fog cleared, and I requested taxi back to my hangar, citing a high CO reading.
Taxied, shut down, opened the door completely, and relaxed. One of the shorter flights of my 2,000+ hour career was also one of the most dangerous. Fatigue and a mild headache had set in, leaving me non-productive for the rest of the day.
The Cause
My A&P is methodical and detail-oriented. Opening the engine compartment, he traced the issue through air hoses, the muffler, and exhaust manifold route. The likely culprit emerged: surface discoloration on the left exhaust manifold indicated blow-by at the manifold connection, and the air intake hose feeding the cockpit had a hole worn by nearly 2,000 hours of vibration, strategically near the leaky exhaust manifold. Both issues will be fixed, the CO detector recalibrated, and a second mounted as backup before I fly again.
I now fly with a primary CO detector and a backup.
Reflecting
Without a detector, I would have had no warning. Any significant inbound traffic or pattern aircraft could have delayed my landing, and I likely would have ended up as an NTSB case study. I survived thanks to a chain of fortunate events: having a CO detector, Peyton Metzel sharing a tip from Mooney Summit speaker Dan Bass (a rare CO crash survivor), and paying attention to details.
I’ve updated my checklist: “Turn on CO detector” is now before “engine start,” and the detector is mounted in my primary field of view, with a second as backup. My grandchildren, in flight training, will soon receive their own CO detectors as Christmas gifts – hopefully used religiously. Grampa may be a bit old, but he got here for a reason, and paying attention to details is one of them.
Bob serves as the Treasurer for the Waukegan Post District, which governs Waukegan National airport, where he is based.
He is a Civil Engineer, retiring as Chairman of Gewalt Hamilton Associates, Inc.
Now married 53 years, he has 4 children and 8 grandchildren, 2 of whom are headed for an aviation career.
- Be Ready! Carbon Monoxide Could Happen to You - December 17, 2025
- I Am UNSAFE Checklist—Lessons Learned on a Fateful Night - February 28, 2025
- Multiple Cessna 172RGs Made Me a Better Pilot - September 27, 2024
Between the Mooney and CO2, I immediately thought of the ASI video. And then you named dropped Dan Bass at the end. Well done! (and I’m glad everything worked out obviously)
I heard Dan Bass speak about his accident, and immediately bought a Sensorcon CO detector. It is velcroed to my panel right beside the main air vent in my Mooney.
Glad to hear that you made it back safe, Bob!
Bob, thanks! Up until now I’ve operated with my binary (ok/not ok) CO2 detector on my Sentry. No more, I’m getting one with actual level readings and will have it mounted where I can see it easily.
I had a similar issue due to a cracked muffler – luckily I could smell the exhaust and ended the flight! And today you have inspired me to upgrade to an electronic sensor.
You mentioned that you were going to purchase CO detectors for your pilot trainee kids. Be sure to get a hard wired, no battery, no requirement to turn it on/off, panel mounted unit, such as the Guardian. You have the perfect open spot for it in your panel. What use is the parts-per-million readout? You don’t adhere to it anyway. This is simple: The alert go/no-go item. When it goes off, you take immediate action and land as soon as possible. Your failed cockpit air hose was probably a long-standing preflight item that you should have seen months ago; the failed muffler connection allowed the holes in the Swiss cheese to line-up and tried to ruin an otherwise perfectly good airplane.
The parts per million readout is indeed useful in identifying a change from normal. A higher than normal reading indicates a developing problem you can identify and address before it becomes a big problem. Some CO leaks start insidiously.
How would he see a failed air hose during preflight? The Mooney is too tightly cowled for such visual attention during preflight.
The bigger question: when did the mechanic last service the plane and why didn’t they see the developing problem?
This is yet another justification for investing in a portable CO2 detector. I bought one, and no matter what airplane I’m in (I’m a CFII), I make sure either the plane has a C02 sensor, or I throw mine in and turn it on. It makes me tingle to think that at one point we relied on those little CO2 cards with the brown-turning button to tell us we had a problem! Yikes!
Thanks for posting this Bob. I’m really happy everything worked out well.
Your story may be an example of “normalization of deviance”, where you were used to seeing an elevated CO indication and the higher numbers didn’t cue you to suspect something was up. Or, as you stated, allowed you to assume everything was essentially normal and would resolve in flight as it normally does. Your combination of leaks were probably worsening over time and finally got to a failure point. It’s a good lesson for us all. Thanks for that.
I’m also a big fan of the Sensorcon detectors, and there are a number of other brands that are very good. Mine saved me in a situation very similar to yours, only mine was during an IMC departure. A story for another time. I’ve gifted a Sensorcon to each of the instructors I’ve worked with over the last 10 years and carry one with me when I’m instructing or doing flight reviews in client’s airplanes. I put a remote Guardian in with the new panel in my airplane, with the PPM displayed on the PFD. I periodically cross check it with my Sensorcon and will have it recalibrated per the manufacture’s recommended schedule. It’s a good piece of gear.
Tell Peyton I said hello.
Thank you all for reading and commenting on this article.
The most important thought is that, absent the red blinking light on the CO detector, I would have gone flying, as intended, and had no indication of a CO problem. The really frightening issue, is that a few miles from my home airport, I could easily have gotten really fuzzy and not realized the problem, taken a nap like Dan Bass, and become an NTSB case study – actually, way too close to that happening!
Fly Safe! And, live to be an Old Pilot.
Firstly glad it at all turned out ok. In response to a prior comment, the reason we ll survived in the era of little brown dots or not detector at all is that these actually are rather rare events. Good inspection by mechanics and preflights do catch most of these potential problems.
Nonetheless, given the low cost of the portable electronic detectors, it is cheap insurance. The over-priced certified in panel ones – not so sure.
Bob Hamilton told me this story first hand and we are all very grateful that the flight ended safely. I finally realized that the “Save A Life” standard carbon dioxide detector, that changes color when CO is present, would never be in my scan, especially during takeoff, like in Bob’s case. I have since purchased a Sensorcon AV8 Inspector Portable Carbon Monoxide Monitor, and feel much better about recognizing this potentially deadly scenario.
Very well done Bob!! A very professional handling of what could have been a fatal accident. And this from a retired airline pilot. Kudos to you.
Bob,
Well done, and thanks for this important contribution. I’ve been using a portable CO detector that I got from Sporty’s years ago. Peyton just sent me the link for the much more capable AV8 model, which I’ve ordered.
See you at the next Buzzard’s breakfast!
Steve