Ground School

In my last post, I mentioned a near-incident I had, involving some power lines, but I never really went into details. I've decided to go into details now, but to fully appreciate what I'm telling you, I have to start with a little bit of private pilot ground school (plus I'm eager to try out my explanation of density). I'll try to keep it from getting overly technical. Here we go.

Air density is, essentially, how many air molecules you'll find in a given parcel of air. For instance, if you could jump up to 10,000 feet MSL (mean seal level) and snatch up a one-inch by one-inch by one-inch cube of air, it would have a certain number of air molecules. That's the air's density. The air becomes progressively less dense the higher you go, because there's progressively less pressure pushing those molecules together. Think of a really thick book laying on its back on a table. The last page of the book is under more pressure than the first page of the book, because it has all the other pages pressing down on it, whereas the first page of the book has no other pages pressing down on it. Same thing with the air: the air down low is more dense because it's under more pressure due to all the air above it pushing down on it. However, other factors can also affect the density of the air. High temperatures, for instance, cause the air molecules to expand, causing air to become less dense. Your 1x1x1 cube of air now has fewer molecules in it because they expanded and some were pushed outside of your box. When we have a really hot day (and we have plenty!), the air at ground level expands to such a degree that it ends up having the density of air we'd normally find, say, 3,000 or 3,500 feet above the ground. We call this "density altitude." It means, not how high the air is, but how high the air is acting. The ground here at the school is about 4,000 feet MSL so, in the situation I just described, the density altitude would be 7,500 feet MSL. The air at 4,000' is acting like the air would at 7,500'. This is not usually a good thing. In fact, I struggle to think of a scenario where high density altitude would be beneficial to anyone. I digress.

Air density, and thus density altitude, directly affects the airplane's performance. The propeller works, essentially, by grabbing and pushing off against those air molecules, so if there are fewer air molecules for the propeller to push against (like if we're at a high altitude, or if we have a high density altitude), then the propeller's not as effective. If the propeller's not as effective, then you can't climb as fast or as high (or, sometimes, at all) as you would be able to in denser air.

All that said, here's what happened on my flight Monday. There are power lines a pretty good distance off the departure end of runway 5. They're far enough away from the runway that they aren't typically a factor for us when we take off. However, by 3:30 in the afternoon, it's good and hot here and, accordingly, the density altitude is nice and high, which means it takes us longer and farther to take off, and longer and farther to climb up to a safe altitude. On my very first landing, I came in a bit fast, which meant I didn't actually touch the ground until I was close to halfway down the runway. So when I added power to take back off, I didn't have much runway to work with. Had I been alert to how much runway I'd already used up, I could have done a short-field takeoff, which would have made everything okay. But I didn't. I did a normal takeoff, and didn't get very far off the ground before I ran out of runway, which meant I was sort of skimming the desert floor as I struggled to climb out in the distressingly un-dense air. I felt like I was doing okay, because there were no close mountains or anything else for me to collide with, but then I saw the power lines in front of me. They were still a fair distance ahead, but I wasn't making much progress in climbing, so I started to wonder a little. Then I started to worry a little. Then I asked Erin if we were going to hit the power lines. After a moment of tense (for me, at least) silence, she finally answered that no, we should be okay. And we were, but I sweated until we were physically over and clear of the lines. The experience did its job, though; I learned my lesson and did mostly short-field takeoffs for the rest of the lesson.

And that's the story of the near-incident involving power lines. It should be stated, though, for the sake of establishing the severity of the situation, that Erin's opinion is that 1)hitting the power lines would probably have killed us, and 2)there's really nothing we can do in a situation like that to avoid hitting them, if we're pointed at them and unable to climb any faster. Thanks to everyone who prays for my safety!

P.S.- I would appreciate any feedback regarding the clarity and/or understandability of my explanation of air density. Thanks!