Going to FL180 in a few hours

[Lead Sled]

Interesting. Being an ignorant one who has not yet taken an altitude chamber ride, I am curious about Time of useful conciousness... Why the heck is it much less than 7 seconds? If I exhale with all my might here at sea level - and hold it - without inhaling first - I have no trouble typing a sentence or two here... takes quite a bit longer than 7 seconds. Does the ambient pressure at 470 really 'suck' the O2 out of you that quickly?

Absorption of oxygen depends on what is called partial pressure. Partial pressure is the pressure of the air at that altitude, and the percent of oxygen in the atmosphere. So if pressure decreases, you can either add more pressure (only to a certain point), increase the percentage of oxygen, or both. The quick don masks in the cockpit provide both.

The reason that useful consciousness goes down so sharply is that when there is less oxygen outside of the body than inside, the oxygen will flow out of the body because of the law of gaseous diffusion. The less the pressure or oxygen outside, the quicker it will flow out. In my experience in the altitude chamber at 25,000 feet, I lasted about 3 minutes.

Hypoxia isn't the only problem you have with a rapid decompression. The possibility of Decompression Sickness (DCS) also exists. A high percentage of the atmosphere is made out of Nitrogen. Nitrogen is not used at all by the body, but it still gets taken in by the lungs. When the pressure on the body decreases, this nitrogen often comes out of solution and forms in a number of places. The more common places are in the joints (called the bends), in the lungs, in the central nervous system, and under the skin. All of them are quite serious. The way we avoid this while going into the altitude chamber is by breathing 100% oxygen for 30 mins. While breathing oxygen, the amount of nitrogen outside our body is less than inside, so the nitrogen flows outwards thus reducing the risk of DCS.

We routinely operate at FL410 and higher in our bizjets. The highest for me personally is FL450 and I've been there many times. There are many bizjets certified to FL510 and they are able to operate there. The big problem with those kinds of altitudes is if you suddenly loose the cabin you're dead - the time of useful consciousness is just to freaking short to allow any type of meaningful response.

My, highest flight in a glider was 17,500 feet - I had the O2, but ATC wouldn't open the window for me to climb any higher. In a normally aspirated aircraft I've "soared" a 65 hp Aeronca 7AC Champ to 14000' a couple of times.

Personally, I feel that there are far too many pilots out there that approach high-altitude flight with a careless and cavalier attitude. In reality, there has never been (that I am aware of) a catastrophic (total, sudden, & complete) pressurization failure with any aircraft certified to operate at those attitudes. Have there been failures? Of course, but they are very manageable. Does that make the real dangers of high altitude flight any less significant? In my opinion the problem is one of inadequate training. One trip to an Altitude Chamber is a very eye opening experience and they only take civilians to 25000'.

We use the Scott Eros Mask System which uses the inflatable straps. At altitudes above FL430, as far as I'm concerned, it's mandatory to have one, if not both pilots, on 100% O2. Assuming the worst failure, any pressurization problems at FL450+ would probably have to be considered fatal - if not for the crew then for the passengers. Like I said the TUC is just too short to have any type of meaningful response.

As far as what flight crews actually do when it comes to wearing masks...

Let's just say that pilots probably take more liberty with the O2 regulations than any other.

This is probably a good time to put in a plug for altitude chamber training. All of you guys who are flying high performance aircraft (or hope to someday) should get yourselves schedule for a ride in an altitude chamber. The cost is VERY reasonable (read: cheap) and you'll walk away with a new found respect for your physical limitations. There are several chambers located around the country, I'd just give your local FSDO a call and get them to point you in the right direction.

‘Sled

 

[152SIC]

yeah, today at FL450 i checked the O2 masks finally (in between the USA Today Life section).

an EXPLOSIVE decompression at FL450??

yeah, let me know if you have the guts to tell me about that story...

 

[rumpletumbler]

This is fascinating, I'm going to 8,000 tonight. Wish me luck.

lol.....the suspense is killing me. Do tell!

 

[navigator72]

Hmmmm

Excuse my lack of knowledge of high altitude flying but didn't Payne Stewart, the golfer run into a rapid decompression sort of thing in his Lear?

 

[navigator72]

Payne Stewart

Here's a small news article about Payne Stewart

U.S. Air Force pilots reported that the cockpit windows were obscured by frost. The conditions of the windows are consistent with a loss of pressurization and a subsequent rapid drop of temperature. It is likely that the pilots and occupants may have lost consciousness due to hypoxia, or a lack of oxygen. Between the last communication between the aircraft and air traffic control and the aircraft's final descent, the aircraft was reportedly flying as high as 45,000 feet.

 

[rumpletumbler]

Excuse my lack of knowledge of high altitude flying but didn't Payne Stewart, the golfer run into a rapid decompression sort of thing in his Lear?

http://www.ntsb.gov/Publictn/2000/AAB0001.htm

 

[Diesel]

GVflyer will probably correct me if i'm wrong but I believe the GIV and GV along with our X has an auto descent feature.

This is how it works in the X. With the autopilot on if the cabin rises above 14.5 or around there. (also depends on the presurization controller where it's set could be higher or lower) if the cabin rises above 14.5 the aircraft goes into an auto descent feature.

Since we don't have autothrottles like the other guys we have to pull the power levers to idle and extend the speed brakes which is really easy because they are in the way of everything. The plane regardless of power or speedbrakes will automatically turn 90 degrees of the course (to get off track) and then descent at MMO. If the powers at idle and the speedbrakes extended it will do 10,000 feet per minute.

At around 12.5 or so it levels off and waits for you to wake up. A nice feature to have if you're going high like most buisness jets do today.

 

[Lead Sled]

Payne Stewart

I'm typed in the Lear series and have nearly 3,000 hours PIC in the Lear 35 although most of that time is in later production models with the 51000' pressurization system - not the earlier system that was on the Payne Stewart aircraft. The official cause of the accident is that the crew failed to receive supplimential oxygen "for undetermined reasons". A couple of years ago, during one of our 6-month FlightSafety recurrents, we were having the obligatory discussion on the Payne Stewart accident. The systems instructor brought up some new information that may possibly shed new light on what happened.

The thing that always puzzled me about the accident was that the Lear's pressurization system has automatic cabin altitude limiters designed into it that make it practically impossible for the cabin altitude to go above 14,000' or so in a structurally sound airplane. The F-16 chase planes saw no evidence of airframe damage - only frosted up windows. 14,000' is hardly an altitude that will bring on terminal hypoxia.

The fact that the windows were frosted means nothing - only that the cabin temperature control hadn't been turned up. At low altitudes, the cabin gets pretty warm because of the pressurized air. It's normal to have the heat turned way down and or the airconditioner turned on to compensate. If you've ever spent much time in a Lear at altitude you'll know that they do have a tendency to frost the windows - especially if the heat's off and you've got a bunch of people in the cabin. No big surprise there.

Nothing about this accident has made much sense to me until the FlightSafety instructor mentioned that they have recently found witnesses who mentioned that the passengers had loaded some frozen fish in the baggage compartment in the cabin behind the rear bench seat. There are also witnesses who say that there were several pounds of dry ice packed with the fish.

If these reports turn out to be true, then this will change everything. Dry ice is a VERY dangerous commodity to carry in the passenger compartment. It will readily displace the oxygen in the space and easily explains the accident. It wouldn't be the first fatal aircraft accident involving dry ice. It remains to be seen if this new information is merely another aviation urban legend or if it can be confirmed. It does, however, put forth a plausible explaination.

'Sled

 

[Immelman]

Sled, thank you for the informative write-up. I think I need to get myself to an altitude chamber one of these days and quit putting it off.

 

[EagleRJ]

Your TOC/TUC tends to level out at around seven seconds as you get up to around 45,000'. At that altitude, the partial pressure is no longer sufficient to provide much of a transfer of O2 molecules across the membranes in the lungs. Seven seconds is the time it takes blood to travel from the aveoli in the lungs to the brain. When that oxygen-poor blood hits the brain, it's lights out!
Most people can hold their breath for a few minutes and remain conscious, but in that case, you have sea-level pressure air stored in your lungs, and O2 transfer will continue to take place from the trapped air at a gradually decreasing rate. In the case of a decompression where the partial pressure in your lungs goes from 8000' to 47,000' in seconds, O2 transfer ceases instantly, and all you have left is the oxygen already in the bloodstream. This is the reason for the FAA regulation that quick-donning O2 masks must be able to be put on in five seconds.

There's no question that a lot of pilots treat O2 regulations with a cavalier attitude. How many GA pilots have climbed to 16,500' without oxygen to take the short route over terrain, reasoning that they felt fine, and the FAR's were just a 'technicality' to cover pilots in poor physical condtion? How many times has a 121 or corporate pilot gotten up to go to the can, and the other pilot either glances at the O2 mask compartment before continuing with the USA Today, or takes the mask out and lays it in his lap? Hypoxia can be either slow and insideous or quick and dramatic, but pilots need to respect the fact that the rules are there for a reason and they can't just trust on their reaction speed to protect themselves.
Think about it for a second: You're in cruise at 45,000' doing a crossword or updating your Jepps or whatever, and suddenly there's a muffled bang from the back of the airplane. You involuntarily exhale forcefully, as the temperature drops by 50 degrees instantly, and the air is filled with a cold fog, as well as loose paper, dust, pretzel crumbs, and paperclips from the floor. The cabin altitude alarm is going off, and as you shake off the shock and try to figure out what just happened, you remember that the oxygen mask box is under your Jepps binder you were just working on. Five seconds have already elapsed!

Every pilot who flies at high altitudes absolutely needs to go take an altitude chamber ride. Every person's hypoxia symptoms are different, and a chamber ride will let you identify your own symptoms, as well as remove a lot of the unknowns about high-altitude physiology (most chambers will let you experience an actual rapid decompression). Pilots should also review their oxygen practices. Is your mask/ cannula in good shape, and are you at an altitude appropriate for a diluter/demand system? Is your mask's hose untangled and the quick-release device working properly? Do you fly with all kinds of stuff sitting on your mask's storage box? Have you actually put your mask on, learning how sunglasses and headset will interfere, and can you put your hand on your mask without looking? It's sobering to watch pilots fighting with the mask in the simulator, because the last time they put it on was their last simulator!

At altitudes above FL430, as far as I'm concerned, it's mandatory to have one, if not both pilots, on 100% O2.

And actually, it's already required above FL410. FAR 91.211(b)(ii).

This is how it works in the X. With the autopilot on if the cabin rises above 14.5 or around there. (also depends on the presurization controller where it's set could be higher or lower) if the cabin rises above 14.5 the aircraft goes into an auto descent feature.

Since we don't have autothrottles like the other guys we have to pull the power levers to idle and extend the speed brakes which is really easy because they are in the way of everything. The plane regardless of power or speedbrakes will automatically turn 90 degrees of the course (to get off track) and then descent at MMO. If the powers at idle and the speedbrakes extended it will do 10,000 feet per minute.

I've heard about this system, and wondered how it worked. I knew the autothrottles were tied into the sequence, but I didn't know it was also available without autothrottles. The descent rate won't be very high at Mmo at cruise thrust, would it? I guess if you do manage to pull the thrust levers back before blacking out, that makes it paramount to wake up from your nap as soon as possible when the plane levels off- before you stall!


We've got some serious thread drift going on here! At least it's educational.