The Insidious Emergency

The Accident
So, what happened? The pilot (68) and his wife (68 and also a pilot) departed Rochester, N.Y., in a brand-new TBM 900 a little before 8:30 a.m., bound for a getaway in Naples, Fla. The airplane wasn't quite six months old, but the pilot wasn't inexperienced. In fact, he was one of the highest-time pilots in the fleet with around 5,000 hours in TBMs. It only took about 20 minutes for the plane to reach the planned cruise altitude of FL280. One hour and 16 minutes into the flight, the pilot called ATC and announced, "N900KN we need to descend to a...about 180. We have an indication that's not correct in the plane." Fifteen seconds later, ATC issued a clearance to FL250, which the pilot acknowledged, stating, "Two-five-zero and we need to get lower...900KN." ATC issued a command to turn 30 left, which the pilot acknowledged. Two minutes, 15 seconds later, the pilot announced, "Zero-zero-nine hundred kilo November!" The call sounded confused, and the mic key remained pressed for a long time. Thirty seconds later, ATC issued a clearance, "Direct Taylor," which elicited only another long key press. The pilot responded to a second call from ATC by stating, "Direct Taylor, 9-0-0-K-N." A little further along, the controller then asked the pilot if he copied the clearance to descend and maintain FL200 and direct Taylor. After two calls and more open mic noise, the pilot finally responded with, "K-N-nine-hundred-K-N." Those were his last words. After that, the aircraft stayed at FL250, and further calls went unanswered. The aircraft remained on a southerly heading and, somewhere over South Carolina, F-15s were scrambled to intercept the aircraft. The fighter pilots reported that the pilot appeared unconscious but breathing, and that the aircraft had frosted windows. The F-15s turned back short of Cuba, and the aircraft eventually crashed into the sea about 14 miles north of Jamaica.

We'll have to wait for the NTSB report for the official determination of what went wrong, but the tapes indicate that the pilot was hypoxic, and at no point does it sound like he had a mask on. Fortunately, this kind of accident is quite rare, but similar tragedies happen time and again.

Hypoxia
Airplanes love altitude. Up high, they go faster and farther, saving both time and money. But, airplanes can rapidly transport humans into an environment high above the earth that mountaineers call the "death zone." Without supplemental oxygen or pressurization, it's a place where mental function diminishes, consciousness is lost, and death can happen in short order.

We all learned in flight school about how hypoxic hypoxia works. It's caused by the inability of the lungs to transfer oxygen into the blood at low pressure. For the transfer to work, the partial pressure of oxygen in the lungs has to be greater than the partial pressure of oxygen contained in the bloodstream. Dalton's law states that the partial pressure contribution of each gas in any parcel of air will be given by the percentage of the concentration. At sea level under standard conditions, the ambient pressure is about 14.7 psi. Since air is composed of about 21% oxygen, the partial pressure of oxygen at sea level will be about 3.1 psi. Most healthy humans can easily handle altitudes up to about 12,000 feet where the atmospheric pressure falls to about 7.6 psi. So, to stay sufficiently oxygenated, the partial pressure of oxygen has to remain above about 21% of 7.6 psi or 1.6 psi. You can do that by increasing the fraction of oxygen in the air you breathe by adding supplemental oxygen or by maintaining the pressure in your airplane to keep the cabin altitude below at least 12,000 feet. Keep in mind that the amount of oxygen that you breathe through your mask will rarely be 100%. At FL400, the ambient pressure will be about 2.6 psi, so in principle, if your mask is sealed tightly, you should survive at FL400 on a strong flow of pure oxygen.

Staying Legal
The FARs require the minimum required flight crew to use supplemental oxygen anytime the cabin pressure is above 12,500 feet for more than 30 minutes or at all times when the cabin pressure is above 14,000 feet. All occupants of an aircraft above a cabin altitude of 15,000 feet must be provided with supplemental oxygen. A single pilot at the controls of any aircraft above FL350 must wear and use an oxygen mask. At or below FL410, the mask need not be worn if there's a second pilot at the controls and if each pilot has access to a quick-donning mask that can be placed on the face within five seconds with one hand.

Unfortunately, studies at Embry-Riddle University have shown that over 80% of Part 91 pilots of pressurized aircraft rarely, if ever, wear the mask when they're supposed to. It hampers cockpit communications, and most pressure masks are unbearably uncomfortable. The bet is that with a fast- donning mask, it should be possible to get it on in time. The studies don't show how often a problem is successfully averted, so it's hard to say how often pilots win this bet. However, when a crew loses the bet at high altitude, the results are often tragic. It doesn't happen very often, but remember that there's a very fine line between winning and losing this particular bet.

Taking The Ride
It's easy to talk about hypoxia, but it's hard to comprehend how serious it is without experiencing it, and there are a couple of ways to do that. The first is with a mixed- gas hypoxia-training device offered by a number of training companies. Under the care of a trained instructor, you put on a mask and breath a mixture of gas that simulates the oxygen content at high altitude. The second way to experience hypoxia is in an altitude chamber. The FAA runs a training program in Oklahoma City using such a chamber. The ride involves training, safety briefings and the chamber session, so the total time involved is typically about half a day. The advantage of the chamber ride is that you can experience almost all of the effects of a true rapid decompression, including cabin fogging and ear popping.

Everyone reacts differently to the effects of hypoxia, and either method can be used to experience and learn your own personal symptoms. Onset may be slow, but eventually, everyone gets "stupid" in one way or the other as hypoxia takes effect. It's worthwhile to experience what happens as you begin to exceed the time of useful consciousness (TUC, also called the EPT, or effective performance time), which is the time you have before you can no longer function effectively---or act to save your own life. At FL250, TUC is measured in minutes, and at FL400, it's measured in seconds. Understand that at a high enough altitude, your lungs are working in reverse to de-oxygenate your blood before it gets sent to your brain, so it's not like holding your breath. You simply won't last long without a mask on.

I've done a couple of chamber rides, and the effects of hypoxia are stunning. It's a dream-like world where you can still see and hear, but where you don't feel compelled to respond---to almost anything. Reaction time goes way down, and the effort needed to handle even a simple task becomes enormous. Asked to count backward from 100 by threes, I made it to about 94 before I just couldn't muster the effort to subtract three even one more time. Even worse, I didn't care about it. I could see everyone laughing at me, and I felt a vague urge to perform, but it just didn't seem worth the effort. I'm lucky though: My heart rate goes way up, I feel flush, and my head starts throbbing as the altitude climbs to 25,000 feet. It's very noticeable, and it feels terrible, so I've been among the first to get my mask on during a session at high altitude. Others experienced little physical effect even though they clearly couldn't function. Knowing your own symptoms can help alert you to a pressurization or oxygen flow problem.

Handling An Emergency---Any Emergency
In the world of jet operations where crews fly at very high altitudes, the standard emergency procedure is to start by putting on the oxygen mask---every single time, for any reason. That means sweeping everything off your head and into your lap with both hands, reaching around to grab the mask and, with one motion, pulling it over your head into place and checking the flow. The goal is to do it all within two to three seconds. Just remember that a rapid-donning mask has to be properly stowed in its storage cup to qualify as "rapid-donning." It can't be in your lap. If it's dark, if there's an upset, if there's a rapid decompression, or if it just falls on the floor, you don't want to be hunting for it. After the mask is on, you take care of whatever problem you might have. If you run through the checklist and there isn't any threat to the pressurization system, it's easy to take off the mask. If it's a pressurization problem, you should initiate an immediate emergency descent, check on the passengers, and when time allows, let ATC know what you're doing. Pilots of all pressurized aircraft operating in the flight levels should develop a mind-set that the very first thing to do when there's any problem is to put on the mask.

Too many pilots feel compelled to either inform ATC of their intentions, or worse, to ask for permissions to descend before they handle a crisis. Remember, it's your responsibility as PIC to do whatever you need to handle any emergency or situation that could affect the safety of the flight. You don't have to declare anything on the radio or ask anyone's permission to get it done. When you have things under control and have time, let ATC know your status. Declaring the "E" word certainly makes things clear, but it's not mandatory to handle the situation.

Other Things To Consider
If you operate an unpressurized aircraft, and you use a cannula or wear a mask to go high, remember that you have no backup. If the oxygen tank runs dry, a hose gets crimped or inadvertently disconnected, or any­thing else goes wrong with your breathing system, your only option is to immediately descend. Most unpressurized airplanes don't have any warning system to indicate when the oxygen system malfunctions, and hypoxia can creep up on you very quietly. Using a simple pulse oximeter is a good way to monitor your blood oxygen level to properly adjust oxygen flow; however, most of the over-the-counter units require a button push to get a reading. A better way to go is with an FDA-cleared unit that can make continuous measurements and has a settable alarm to indicate the onset of hypoxia. FDA-cleared units require a prescription, though they can often be easily purchased online. Using oxygen to arrive feeling fresh and to clear terrain is a great idea, but don't kid yourself---cruising in an unpressurized aircraft in the flight levels is a risky proposition.

Equipment can play another role in how willing a pilot might be to don the mask. Many turboprops, such as the early TBMs and a few pistons like some Piper Mirages, are equipped with chemical oxygen generators. The generators come in the form of canisters that contain chemicals that react to produce oxygen for some period of time (typically seven to 15 minutes.) The canisters are surprisingly expensive and must be replaced each time they're used, so many pilots hesitate to activate them until they're absolutely certain that they need them, and then it could be too late. If you have generators, consider purchasing a portable oxygen tank and keeping the mask where you can quickly access it.

In discussions with a number of pilots who operate in the high flight levels, I've come to realize that mind-set about safe altitudes can become another subtle issue. Many pilots who regularly operate above FL370 begin to think of, say, FL180 as a "low altitude," and that it might be a fine place to sort out a pressurization problem. It's not. If there's a pressurization problem, head down below 12,000 feet or, even better, 10,000 feet to get things sorted out. That's clearly something that has to be considered when planning a long-range flight over the ocean in any pressurized aircraft. A descent due to a pressurization problem will impact range and could make landfall impossible.

Finally, understand your equipment, practice using it and check it during every preflight. Make sure that oxygen supply valves are open, tanks are full, inspect the masks and practice putting them on. Simulator training sessions should include an unexpected pressurization problem accompanied by donning the mask and making an emergency descent. Lastly, make sure that you clearly understand what the warning lights and alarms mean in your airplane. Any light or alarm should be treated with the same response: Get your mask on. Remember, it's easy to take off if you don't need it.

Traveling high above the earth is a wonderful experience, but it requires training, awareness and vigilance to survive a pressurization problem at altitude---whether it's gradual or sudden. Safe flying.