It's not the cannula that's "only rated" by Mountain High to 18,000 feet; that is the limitation imposed on cannula use by the US FAR 23.1447 regulations. In fact MH EDS systems with cannula have been chamber tested to over 35,000 feet and maintained 95% blood oxygen saturation. The mask would fulfill the same duty. However, I would suggest this test was conducted with a fit, young and healthy user. The same limit for cannulas was adopted by CASA and was promulgated in Civil Aviation Order 108 para 5.1.3. The issue is not related to the ability of the equipment to deliver the appropriate oxygen flow (because they almost all will do that) but in officialdom insisting on an unbelievably large safety margin. The safety margin is designed to minimise the hypoxia risks associated with individual pilot oxygen response variability, breathing patterns, incorrect installation, incorrect operation etc, etc, etc. It is a huge (about 100%) overkill and takes no account of the current ability to monitor blood oxygen saturation and adjust one's oxy supply as appropriate for the prevailing conditions. Note that blood oxy saturation is not the ultimate determinant of hypoxic risk - it is not, as it only gauges the arterial blood supply, not the ultimate delivery of oxygen to the cells of the recipient organs (brain, eyes, muscles etc.) but it is the best bit of equipment a glider pilot can use to monitor the most commonly encountered forms of hypoxia that he/she can control.
The EDS "N" setting can be used to supply oxygen from the ground up and is not limited to cannula use. It is true that it finds extensive use for night flying in powered aircraft where one of the early symptoms of impending hypoxia is diminished night vision.
The next range of settings vary depending on the model EDS you use. In the case of my D1 model (with 9 volt power supply) the D5, D10 and D12 settings allow the oxygen flow to commence at 5,000, 10,000 and 12,000 feet respectively with the pulse length automatically regulated to supply the appropriate oxygen volume as the height changes. Note that this may not be appropriate (i.e. may be insufficient) for you as an individual and the recommended way of verifying this is by using a digital pulse oximeter to monitor your blood oxygen saturation and adjust the flow to maintain the BOS value in the (approx.) 90-95% range. The MH systems allow you to this in steps by using the "F" settings that supply an additional oxygen flow equivalent to 5000 ft , 10,000 ft and 20,000 ft of altitude.
It is very clear to me from Ashley Boyle's comments on his 24,000 ft Stirlings climb that he was inadequately briefed on how the MH system operates or maybe failed to implement what he was told. Either way he ended up in a situation where he was at significant risk and has obviously learned from the experience. Some might have been less fortunate in those circumstances.
I have explored cannula use on countless wave flights over the past 5 years and I'm satisfied that I personally can reliably maintain adequate BOS to at least 27,000 feet. The risks with the cannula at high altitudes are these: Failure to breath consistently through the nose; partial/total blockage of the nasal passages; potential to dislodge the cannula sideways by one nozzle. Below 15,000-17,000 feet you have time to recover the situation (assuming you identify the problem) but not at 25,000 feet, unless you are closely monitoring BOS. I change from cannula to mask around 20,000 ft to avoid the above risks and always use the oximeter to monitor BOS. For extreme altitude flights (over 30,000 ft) I plan to use the MH cannula system with a totally independent constant flow system mask over the top. The oximeters (duty unit plus a backup) will monitor BOS and that will determine the ultimate altitude to be flown.
Food for thought and good flying,
Geoff V
At 11:34 AM 23/01/2008 +1030, Anthony Smith wrote:
>
>Just goes to show that people will need to read the
>instructions a little bit more. A few pointers:
>
>The canula is only rated to 18,000 ft according to MH.
>
>The mask is rated to 24,000 ft according to MH.
>
>The N setting is for night time with the canula. It is not
>for the mask.
>
>The mask settings are after the N setting (labelled M
>followed by a number I think). You need to find the right
>setting for your mask though. I haven't gotten that far as
>I doubt I will get the Bergy beyond 18,000 ft without
>Justine getting frostbite.
>
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