After an exciting December thus far, I thought I'd make it even more adventurous with an unpressurized flight at 25,000 ft. Prior to going back to Texas for Christmas, I had to deliver an airplane to Wichita, Kansas. Unfortunately, I did not have a large weather window to work with so I was forced to fly in some non-ideal weather. This particular airplane was not equipped with deice equipment like the plane in my last post. That meant that I either had to find some blue sky, or stay grounded until blue sky came around (because clouds in north Idaho in December = ice). I got home from Seattle on Saturday Dec. 20 at 11 pm and the forecast for the next 4 days (until Christmas) was for crummy weather. Luckily, I woke up at 6am the next morning and I saw a few breaks in the clouds. I knew I probably would not have any more opportunities so I rushed to the airport, did my checks on the plane, checked the weather, and took off around 8am. The nice weather did not last long. In my weather briefing, I was told there were pilot reports of severe turbulence, mountain waves, and high cloud tops beginning just east of the Montana boarder. I knew I was in for a bumpy ride. I did not quite realize just how "on the edge" I was going to be. The pilot reports the cloud tops were around 20,000 ft., giving me 5,000 ft to work with. (Per FAA regulations, unpressurized airplanes are only allowed to fly up to 25,000 ft. and the service ceiling of the airplane I was flying was, also, 25,000 ft.). As I approached the beginning of the bad weather, I quickly realized the clouds were much higher than 20,000 ft. I climbed to 23,000 ft and I was just skimming the tops. That altitude worked for a while, but I would eventually have to climb to 25,000 ft. At this altitude, many things could go quite wrong.
- The biggest issue at this altitude is oxygen. As we go up in altitude, there is the same number of oxygen molecules as there is at sea level, but because of reduced partial pressure, those molecules are spaced farther apart. Consequently, the partial pressure of oxygen in the bloodstream is significantly reduced; so there is not enough pressure to allow the oxygen to force its way into the blood, and you cannot breathe deeply or fast enough to compensate. (Source). This is why I had to wear the big blue pressurized oxygen mask (it is the same thing fighter pilots wear, only they also wear a helmet and theirs looks cooler). The real danger at this altitude is a slow leak or my oxygen system slowly being drained and my sensor not recognizing the O2 bottle is low. The reason this is dangerous, is that I cannot immediately tell that O2 is flowing. My flow meter tells me how much O2 is pumping, but I am not looking at that 100% of the time. If my oxygen were to stop, my EPT (effective performance time) is only 3 minutes. That means that in a little as three minutes, I would be unable to accomplish or understand remedial tasks. My TUC (time of useful consciousness) is 6 minutes, meaning I would be unconscious in 6 minutes. In essence, if my O2 flow was interrupted for 3 minutes, I would be so dumb (due to hypoxia) I would not remember how to fly, and I would be unconscious in 6 minutes meaning I would fly until I ran out of gas and crashed. So, Oxygen = IMPORTANT
- Second, at 25,000 ft., I do not have much airspeed to play with. Not to get overly complicated, but as an airplane goes up in altitude, the air is thinner and the wing produces less lift in the thinner air. Therefore, in order to continue to fly, the airplane must fly faster. BUT, as the air gets thinner, the engine produces LESS power meaning the airplane’s top speed drops as we go up in altitude. Normally, I have about 100 kts between stall speed and “top speed” at 12,000 ft. At 25,000 ft. I only have about 30 kts (meaning that my airspeed can only fluctuate 30 knots between stalling and crashing and over-speeding and the wings ripping off).
- That leads me to the mountain waves and turbulence. Mountain waves look like waves in a pool. When the wind blows fast, the mountains push it upwards and these “waves” can go all the way to 60,000+ ft. Eventually these waves also come back down. Since the waves live in the jet stream, they are blowing 80-150 knots. Wind blowing down at 100 kts creates a lot of force. Some down waves are known to push down around 1000-3000 ft./minute. That means, in order to maintain altitude, I have to climb a 1000-3000 ft./minute. Every time my airplane hit one of these “down waves,” it tried to push me down into the ice clouds below. The only choice I had was to pull up, lose airspeed, and try to maintain altitude. Mountain waves can be a very dangerous weather phenomenon. In fact, there are many instances of mountain waves pushing big, powerful, jets into the mountains below. Sometimes the wind is just so strong, even the most powerful airplanes cannot out climb them.
- On top of all that, the turbulence in mountain waves tends to be severe. This was certainly the case this day. This was the worst turbulence I have ever experienced in a GA airplane. I had a backpack in the seat next to me, I hit one pocket of rough air that caused one wing to lose lift, and we descended so fast that the backpack was pinned to the ceiling for a solid 2 seconds. To put it in perspective, later that day, a 737 flying through the same area had three passengers taken to the hospital due to turbulence.