Thursday, March 10, 2011

Carbon Monoxide Poisoning

The weather is finally starting to cooperate, and many of us are eager to get back in the air.  But it's still often cold enough (especially at altitude) that cabin heat is a must. Which brings with it the risk of CO poisoning (the FAA published a safety brochure on the subject a while back) if there is a leak in the exhaust system. (Actually, even when there is no leak per se in the exhaust system, it is still possible for CO to enter the cabin as we discovered a while back in one of our aircraft.) So, many of us carry CO detectors of one kind or another.  There are many choices for CO detectors out there, which ones to use and where to put them in the cockpit for best detection? Last year, the FAA issued a Special Airworthiness Information Bulletin (DOT/FAA/AR-09/49) that sheds some light on the topic.

Thankfully, CO related GA accidents are rare.  Of the nearly 72,000 accidents between 1962 and 2007 in the NTSB accident/incident database studied by the FAA, just 62 (0.008%) was deemed to be C) related.  However, the FAA also noted that the age of the exhaust system, particularly the age of the muffler was a major contributing factor---noting that the majority of the CO-related accidents caused by cracks or leaks in the muffler was with muffler usage greater than 1,000 hours. So, if we are flying around in older aircraft (and let's face it, with the average age of the nation's single engine fleet at over 30 years, most of us are), it makes sense to take some precautions.

So, what's a dangerous level of exposure?
  • OSHA lists the max time-weighted-average exposure for an 8 hour work day at 50 ppm.
  • 200 ppm over a couple of hours will cause mild headaches and nausea
  • 400 ppm over 1-2 hours will cause serious headaches, and become life threatening after 3 hours.
  • 800 ppm will cause dizziness, nausea  in less than an hour and unconsciousness within 2 hours
  • And from there on out, the figures become ever more grim until an exposure of 12,800 ppm will cause death in a matter of minutes.
How much CO is in engine exhaust? It, no doubt, varies from engine to engine and the conditions of flight.  But suffice it to say that I've had a CO detector go off standing 20 feet behind an idling aircraft. Moreover, some of the symptoms of lower level CO exposure is similar to those of hypoxia or fatigue, and it may be difficult to tell them apart when it is happening to you, so it is probably a good idea to have a good CO detector on board.

There are a variety of CO detectors on the market from the down right cheap chemical dot types (the cards with a reactive dot that turns black when exposed to CO) to highly sensitive portable electro-chemical detectors. The good news here is that concentrations where things start to get dangerous (>50ppm) are high enough that even the cheapest chemical dot detectors should be able to detect them reliably.

The FAA SAIB is limited to testing electro-chemical type detectors, but offers some tips about preventive maintenance of aircraft, and usage and placement of CO detectors.  All around a very interesting read.

1 comment:

  1. A few more thoughts on the topic.

    - All of us have rented a plane with an chemical dot type CO detector attached to the panel. The problem with that is that you have no idea how long the detector has been there. The useful life of these types of detectors is on the order of 3~6 months, so unless you put the detector there yourself (or know for sure how old they are), best not to rely on them.

    - The biggest draw back of chemical dot type detectors is that their only mode of warning is to turn black. So, the detector needs to be on the panel where it will be a natural part of the instrument scan. Probably not an issue for IFR pilots, but for VFR pilots who spend most of the time looking out the window, this may be easy to overlook.

    - The electro-chemical detectors have the advantage that most will issue an audible alarm, so is less likely to be overlooked.

    - Electro-chemical detectors also need to have their batteries replaced, and the detector recalibrated every 12-24 months. (There are "disposable" type detectors, but they further add to the cost.) That's not exactly set-it-and-forget-it reliability, but the service cycle is certainly longer than the chemical dots.

    - The big advantage of chemical dot type detectors is that they are cheap ($5-$10) compared to electro-chemical detectors ($50-$100 for household grade, $100-$200 for portable industrial detectors). For GA applications, the extra sensitivity of industrial detectors is probably overkill. Though the peace-of-mind offered by a near-set-it-and-forget-it detector (at least for 12-24 months) and the audible warning is probably worth the extra expense.