The A2A Simulations Community

When I read the AOPA Safety Alert and saw they recommended a Pulse Oximeter (Bottom line: If you fly regularly above 10,000 feet msl, a pulse oximeter should be part of your equipment)... curiosity piqued, I did a search for "pulse oximeter" and found an AVweb article by By Mike Busch and Brent Blue - Nonin Onyx Pulse Oximeter - where Mike relates a personal encounter with hypoxia. I am now convinced it needs to be something in my flight bag.

This has also fired up my interest in doing that "high altitude training" offered by the FAA's Civil Aerospace Medical Institute which includes the use of a hypobaric chamber. I think this has been mentioned before here in the forums... and maybe... Oracle has done this? Anyone 18 & up can sign up for this (pilot or not) but you need a third class medical to participate in the altitude chamber training.

pedwards wrote: I recently watched a program on Youtube that contained information on Hypoxia. https://www.youtube.com/watch?v=DiEJKvbpOF0
In the clip Micheal Portillo thought he was performing very well and answering all questions correctly. Hypoxia limits the chances of us recognizing that we are suffering from Hypoxia.

That is my understanding as well... why a pulse oximeter could save one's life.

No I haven't done training in a hypobaric chamber, but if I ever get into higher performance aircraft I definitely intend to.

The P O2 is going to show you your O2 level dropping well before any effects take hold...so with safety in mind, a level should predetermined that you would need to get to lower altitude....Yes I have seen people function quite well at a pulse ox of 93 to 94 room air, but that same reading at altitude, in a normal healthy pilot, would indicate a fairly dramatic decrease in oxygen intake and should be rectified quickly....even O2 applied at 2 lpm via a nasal cannula would tremendously help that pilot to keep functioning

I just picked this up at Walgreens Pharmacy for $50:
http://www.walgreens.com/store/c/walgre ... 51-product

I'll get some readings a bit later at the next high altitude flight and report back.

Scott.

Great video! Although I had to search it on YouTube because it was blocked here in the States, I nevertheless found it. So fascinating how he thought nothing was wrong. And I dare say he performed loads better than I would have. I wish there was a reliable way to detect hypoxia in pilots and incorporate that into a safety feature. Chris Trott, my reliable source of all things aviation - how does an airliner's "sudden loss of cabin pressure" detection circuit work where the masks drop down automatically? Is there a percent difference formula or some delta between settings that they pop down at?

Mig_Driver wrote:how does an airliner's "sudden loss of cabin pressure" detection circuit work where the masks drop down automatically? Is there a percent difference formula or some delta between settings that they pop down at?

At least in 737, the mask release circuit works just by a pressure switch (located in EE bay) that closes when the cabin altitude is higher than 14000 ft or 14650 ft, depending on the airplane's mod status. The switch activates relay which in turn gives power from DC BAT bus to the solenoids that unlatch the individual mask doors. As far as I understand it doesn't have any intentional sensing of cabin rate but is triggered by exceeding that given cabin altitude. In A32S the system is similar.

-Esa

AKar wrote:

Mig_Driver wrote:how does an airliner's "sudden loss of cabin pressure" detection circuit work where the masks drop down automatically? Is there a percent difference formula or some delta between settings that they pop down at?

At least in 737, the mask release circuit works just by a pressure switch (located in EE bay) that closes when the cabin altitude is higher than 14000 ft or 14650 ft, depending on the airplane's mod status. The switch activates relay which in turn gives power from DC BAT bus to the solenoids that unlatch the individual mask doors. As far as I understand it doesn't have any intentional sensing of cabin rate but is triggered by exceeding that given cabin altitude. In A32S the system is similar.

-Esa

That makes sense, thanks, Esa! I was wondering how and at which altitude or pressure it tripped. I've been digging pretty deep into the 777 lately but I couldn't remember if they gave any specifics.

Most airliners use a system like what Esa said. Light aircraft, however are usually very simple. On the Cessna light aircraft series, they use a simple barometric pressure switch that closes when the cabin pressure drops below an equivalent of approx 12,500 ft. The switch resets when the cabin altitude drops below approx. 11,500 ft (or thereabouts depending on exact model). I believe there is an STC out there which also adds a warning horn to this switch loop as well. On the TBM 900, the pressurization system has 2 modes from what I can find, "Auto", and "Max Differential". As it uses the G1000 avionics, there are no "hard" warning lights for the cabin pressurization or readouts of its status, it is all displayed on the center MFD. As TBM does not publically release their manuals, I cannot see what it says as for what warnings are generated, but I suspect that a "Master Caution" light and specific message would be displayed if there was a fault detected (like the cabin altitude rising in level flight) and a "Master Warning" light and horn activated with specific message if the cabin pressure or differential exceeded the allowed limits.

One thing to remember though is that above approx 18,000 feet, most people have mere seconds to respond to a rapid depressurization prior to loss of consciousness so that is a possibility as well.

Mig_Driver wrote:That makes sense, thanks, Esa! I was wondering how and at which altitude or pressure it tripped. I've been digging pretty deep into the 777 lately but I couldn't remember if they gave any specifics.

No problem. I might as well add some further points.

Using the 737 as an example, there is a similar relay functionally parallel to the one that is signaled by the cabin pressure switch I mentioned. It is not actuated by the pressure switch but by a guarded overhead switch in the cockpit to manually deploy the masks. That way there are two independent ways to energize the mask door solenoids in case the pressure switch or the auto deploy relay malfunctions.

Moreover, it is good to notice that oxygen mask system is separate from the cabin pressurization system. The two cabin pressure controllers in the EE compartment have their own pressure sensors which have nothing to do with the pressure switch that operates the mask system. And further there is a yet separate pressure switch, or several switches (again, depending on the individual airplane), that make the cabin altitude warning horn to sound and in most cases the red CABIN ALTITUDE indicator lights come on (in some airplanes they are inoperative). Hopefully that warning prompts the crew to take action before the automatic mask deployment occurs. Note that the cockpit has no automatic masks nor oxygen generators, but the emergency oxygen is provided to the manually operated masks from a pressurized bottle under the floor behind the cockpit.

So both the warning and the emergency oxygen systems use their own switches and are independent from each other and from the cabin pressurization system.

CAPFlyer wrote:Light aircraft, however are usually very simple.

Yes indeed. To use late series Cessna P210 as an example (as I happen to have some stuff about it still on my own handy collections), there is, as you say, a pressure switch that closes when the cabin altitude exceeds 12400 ft ± 100 ft, as it is quoted, and resets at about 11700 ft. All it does is that it illuminates the red light labelled 'CABIN ALTITUDE'. An STC of adding a horn to the system would make much sense for those regular high-flyers.

The pressurization system controls are altitude selector and ON/OFF switch for the system (the airplane could as well be flown unpressurized). The altitude selector sets the altitude at which the pressurization begins and at which the cabin is maintained until maximum differential is reached. This is done by controlling the outflow valve at the rear wall of the cabin. Adjacent to it is a safety dump valve, which is similar but normally held close during pressurized flight and is controlled by a solenoid that is activated by setting the ON/OFF switch to OFF, thereby releasing the cabin pressure and preventing cabin from pressurizing. Both of the valves have a set safety differential at which they open to protect the structure from overloading. There is also a mechanical dump valve operated by a T-handle next to the master switch that needs to be pushed in for the cabin to pressurize.

As these planes are often flown unpressurized, I wonder how easy it is, in practice, to mismanage the system - especially if not routinely flown similar airplane pressurized.

-Esa

I did some flight testing the other day at 7,.500 feet, not that high, but interesting. I found my oxy level went down to the high 80's (from 99), but as soon as it did, my body had the urge to take a few deep breathes. Then it came back up to mid / upper 90's, then back down, etc. There is clearly a natural response. However, if I make a point to breath slow and deep, and it really wasn't difficult at all to do, I was able to maintain high 90's at 7,500 feet.

I'll be flying Westbound next week and, weather permitting, will pop up to higher altitudes to get more info.

Ideally, this would come with some kind of settable alarm.

Scott.

Yes, that natural urge to try and get more oxygen by the body.....that alone should be an indicator to someone flying a pressurized airplane at high altitude that something is wrong.....but how many pilots would clue into that indication? In a pressurized airplane? Flying an aircraft as yours at high altitude, you know you only have x amount of time without supplemental oxygen. I think complacency is the biggest threat with hypoxia, indications are there, but they are subtle. Question is.....would pilots with thousands of hours in say a TBM850, question the validity of that new fangled thing stuck to their finger telling them their oxygen is low, while not questioning the probability of a slow pressurization problem in a plane they have never had issues with?

Scott - A2A wrote:I did some flight testing the other day at 7,.500 feet, not that high, but interesting.

That is very interesting, Scott. Thanks for reporting back btw. Yeah, my thoughts exactly with respect to an alarm when oxygen saturation drops below a set point.

I looked at the Nonin's and didn't see anything with an alarm until you got to the tabletop models.

One other thing I can see is the hassle of having to check every couple minutes when you get into the FLs and are pressurized. I'll say this, every time I see a Part 135 reg (like 135.89 - Pilot requirements: Use of oxygen) I have to wonder why I wouldn't want to comply with those regs i.e. follow 135 over 91.

If you can find an article from IFR Magazine The 91-Percent Solution - Oxygen and Hypoxemia - that appears to fit with Scott's experience (worth a read).

taildraggin68 wrote:Question is.....would pilots with thousands of hours in say a TBM850, question the validity of that new fangled thing stuck to their finger telling them their oxygen is low, while not questioning the probability of a slow pressurization problem in a plane they have never had issues with?

I'm sure, especially for those who fly in the FLs, that the Payne Stewart accident is in the back of one's mind. And if you go to the trouble to buy a pulse oximeter and monitor your O2 saturation, that it would be like any other "system" you monitor.

I think there would be more concern over getting a low O2 saturation from an oximeter when you are outside that 'time of useful consciousness' zone - and responding correctly. Or, depending on altitude, getting a low O2 saturation, and responding within sufficient time to avoid an accident.

From this Flying Magazine Doomed TBM 900 Pilot Twice Asked for Lower Altitude it sounds like the pilot knew he had a problem, took corrective action... but his corrective response was just not quick enough to prevent the accident.

Good little article Rob, But that only goes to demonstrate lacking on ATC's part in this incident.....I only hope that this incident triggers conscious thought on "what would you do?" discussions such as we have here......I do hope they are able to recover the aircraft soon so the families can get closure but as well to be able to determine what happened.

I suppose the pilot didn't get his emergency oxygen on and working in time - perhaps he never even attempted that before he got too impaired from lack of oxygen. The oxygen 'should' be the number one priority but obviously for some reasons he got his priorities in other order.

-Esa

Having accusim aircraft that mimic hypoxia is a great training aid as well. Just thinking that ok approaching 10000 ft better put oxygen on keeps hypoxia awareness fresh on the mind, especially if you are flying several times a week.