High Altitude Flying in an Unpressurised Aeroplane

Hypoxia

Hypoxia

 

Gday Crew,

Bellow is an email from one of our LOBO Oz members who has a beautiful twin turbo Lancair IV. As we know our aircraft are very capable of cruising in the fligh levels.

I did ask Peter what oxygen system he uses and if they were using a canular or mask at the time so not sure as yet.

Please have a read of his experience below and comment if you have had similar experience.

 

hx-in-action2

 

I have provided Peter with some very good information on high altitude flight directly from our LOBO Lancair IVP/IV manuals.

 

hx-in-action3

Dear Gary and other Lancair owners.

I would like to ask the group about the bends at high altitude. As a scuba diver, I know all about this with diving, but have little knowledge with aviaiton.

I have read up a bit lately.

The bends seems to occur above 18 000. It is more likely for longer duration flights, and is more common in passengers with high body fat content..e.g. women. It is much more likely in rapid ascent.

As my Lancair IV is not pressurised, it seems that the risk of the bends is my altitude limiter, rather than my oxygen blood level which I can monitor with pulse oximetry.

I flew back from Ayres Rock the other day, chugging along at 335 nts at 22 000. A passenger with psoriasis started experiencing tingling in their skin lesions, a sign of the bends. I descended to 21 000 and the tingling went away.

Does anyone have personal experience or advanced knowledge of incidents of the bends in aviation? Is there an accepted safe maximum altitude for non-pressurised flight?

Finally, has anyone heard the theory that the bends explains many of the unexplained high-altitude crashes in Lancairs?

Interested in the knowledge bank.

Regards,
Peter
VH NEH

 

hx-in-action4

 

3 thoughts on “High Altitude Flying in an Unpressurised Aeroplane

  1. Thanks for the thought provoking post.
    I am also a keen diver and owner of a Lancair propjet, I had considered the issue of bends and barotrauma when building my aircraft.
    The good news as experienced by the author of the previous post is that heading back to the ground facilitates re-compression alleviating symptoms (although perhaps not all issues) whereas surfacing from a dive does the opposite, leaving less pleasant options for recovery.
    I have taken to ensuring my dive computer travels in the cockpit with me if I have or will dive in any reasonable time of flying as the computer will factor in altitude in future dive profiles and no fly times.
    But back to aircraft, I met a PC9 formation landed at YFRT refuelling on one of my trips home from QLD to Perth and suggested we race back to perth.
    After one of the pilots offered to swap planes he explained they where limited to 18,000ft due to concerns with bends, the limit had only recently been introduced at time.(around 2004) I imagine the 18,000 limit has a margin in it, but the pilots were on pressure oxygen systems not cannula.
    I had not heard the theory of bends causing the loss of Lancairs at high altitude nor have I heard many reports of “bent pilots” (although I’ve met some).
    The recent loss of an Evolution in California followed closely by a Mooney some weeks later, perhaps indicates hypoxia as the likely culprit, although there is always a range of other ways to be debilitated at the controls.
    I would be interested, in any further information members have on the subject, particularly from any IV-PT pilots in Tewantin,QLD.

    P.S. In the last 2 weeks I have become the proud owner of an “Alt Alert” personal aircraft cabin pressure monitor. (That’s air pressure not pilot pressure)

    Cheers

    Laurie Fitzgerald

  2. Hi all, An important topic that should not be taken lightly. The first rule is don’t use cannulas above 18,000 feet and an Oxy meter should be mandatory!
    When Alex Schenk and I flew around the world in 1998, we chose cruising levels between FL210 and FL290 in order to achieve a high TAS. Our longest sector was from Muscat to Phuket, 2482 nautical miles at FL250 with a 40 knot headwind. This took 10 hours with substantial weather diversions after passing abeam the Andaman Islands.
    Neither of us got the bends, but we both found the Mountain High system was not providing enough oxygen (above 21,000 feet), and we had to press the continuous flow button regularly to stop getting hypoxia. We flew Brisbane to Pago Pago to Hilo then Redmond Oregon on the one large Kevlar bottle.
    As an aside, these bottles have a 15 year life, then are time-expired (throw away item).
    It is a shame the RAAF are no longer allowing civilians to experience the baro chamber, as it demonstrates the many hazards associated with high altitude flying!
    For those pilots who are divers, a safe and conservative rule of thumb is 24 hours between a deep dive and then going to altitude.
    Remember to check on your passenger’s health, as although you may feel ok and not have symptoms, others may (as in Peter’s flight).
    The RAAF lost an FA-18 in the Gulf of Carpentaria due to hypoxia, so even after all the training, this pilot was still caught out!

  3. Hi all

    In regards to Peter’s initial question, I’ve written a short article on Evolved Gasses and Decompression Illness (DCI) which I’ll send to Weeksy and he can post or just send to Peter as he sees fit. But briefly, as a guide the probability of getting DCI (bends, creeps, chokes or staggers) depends on a number of factors, the key factor being cabin altitude. As seen in the table below, there is an increasing risk of DCI from 18 000 to 25 000 feet; and above 25 000 feet, the risk increases significantly.

    CABIN ALT TIME PROBABILITY % DCI
    FL300 >4 hrs up to 100% (ALMOST CERTAIN)
    FL250 60 min 25% (PROBABLE)
    FL225 60 min 10%
    FL200 60 min <5% (IMPROBABLE)
    FL180 60 min <5% (CRITICAL RATIO)

    Also, for multiple flights above a cabin altitude of FL180, the risk multiplies (due to bubbles in the body not going back into solution) and therefore it is recommended:

    a. Only one sortie above FL180 is allowed in any 24-hour period.

    b. After a sortie exceeding FL250, there should be no flying for four hours, then not above 12 000 feet for a further eight hours.

    c. Pre-breathing 100% O2 is required prior to take-off and for the duration of the flight as follows:
    30 minutes before going above a cabin altitude of FL220,
    45 minutes before going above a cabin altitude of FL250 (up to FL3000.

    Hope this is useful.

    Regards

    Pat Bridge
    LIV602 (51% to go)

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