Four main questions from readers about previous post on the recent crash in Nantucket:

1) Was I suggesting that JFK Jr. was somehow negligent in not using this kind of parachute-equipped airplane? No. The very first Cirrus SR-20 was delivered to the very first customer within days of Kennedy's crash in July, 1999. Before that, FAA-certified planes with parachutes not for passengers but for the entire airplane didn't exist. The initial waiting list for these airplanes was very long. I placed an order not long after, and got mine in November, 2000.

2) If this kind of plane is so great, why don't I have one any more? Sold it before moving to China. For a while I fantasized about flying here. Hah.

3) Is it really true that, if you are in the dark or in a cloud and can't see the horizon (and are not flying by instruments), you will crash? Yes. Explanation after the jump. It has nothing to do with the risk of running into something you can't see.

4) Is it really true that, as claimed in William Langewiesche's article The Turn, if you have your eyes closed you can't tell if a plane is right side up or upside down? Yes. Explanation also below. Visual proof in this famed clip showing Bob Hoover, world's greatest pilot, pouring iced tea continuously into a glass while performing a barrel roll.

Explanation, theory: . Airplanes are designed so that, when they're properly flown, the floor of the airplane always feels like "down" -- whether you are flying straight, turning left or right, or turned upside down. If this seems hard to believe, it's probably because people naturally make the connection between flying in an airplane and riding in a car.

In a car, you don't need your eyes to know whether you're turning right, turning left, or going straight. You're pushed against the side of the car whenever it makes a turn. You could close your eyes and have a very good sense of which way the car was going.

It's different in an airplane -- and the reason is that the car can operate only in a single geometric plane. Its wheels are (practically) always on the ground. If it turns, the seat stays level with the ground, and your inertia moves you against the seat and the car's walls.

Airplanes are different because the entire airframe can move freely in three dimensions. When they turn they're like a car on a banked track -- and, if flown properly (in what's called fully "coordinated" flight), it is as they are on a track whose angle of bank constantly changes to exactly match the angle of turn. Thus the seat and cabin walls are always tilting (relative to the ground) in conjunction with the forces working on your body. So during "coordinated" flight, if your eyes were closed you wouldn't feel movement to the right or left. You'd only feel a force "down," into your seat. That's what the Bob Hoover video shows: the iced tea is always pulled toward the bottom of the cup, even when that means flowing "up" relative to the ground. As in the car, you feel the familiar force "down," into your seat. But "down" is no longer related to where the ground is.

For another time is the next stage of the problem: why a pilot's inability to feel left or right means he will inevitably lose control of the plane and enter a spiral* to the ground if he is in a cloud. (Summary of the problem: an airplane is naturally stable in a front-to-back direction, so that if the nose dips slightly, normal aerodynamic forces will bring it back up. It is naturally unstable in a side-to-side direction, so without the constant corrections a pilot makes by seeing where the horizon is, it will naturally start turning in one direction or the other and then continue that turn.)

For another time too: the interesting fact that if your eyes are closed in an airplane, you simply can't tell whether an airplane is turning very steeply in one direction (which involves heavy G forces if the plane is maintaining altitude) or whether it is climbing sharply straight ahead, also increasing the G load. In each case you'll be pushed deep into your seat. Without looking, you won't know why.

Explanation, practice:. No one who has received an instrument rating in an airplane doubts any of the above, because as part of the training everyone has been through this exercise: When the plane is safely several thousand feet in the air, the instructor will tell the trainee pilot to close his eyes and keep flying the plane. He is supposed to keep it level just by feel.

Within sixty seconds or so, the instructor crisply says "my airplane" and takes control again. The trainee pilot opens his eyes and is startled to see the ground coming up toward him, since the plane is spiraling* down. . This always happens when the plane is flown blind, it usually happens very quickly, and generally the eyes-closed pilot had no physical sense that anything was wrong. This process is what tragically kills most pilots without instrument training when they unexpectedly enter a cloud; having no visual cue to what is up and down is the same as wearing a blindfold. It is also what is assumed to have happened to poor John Kennedy Jr and his passengers, in the dark and fog over the water.

* I incorrectly said "spin" rather than "spiral" in these places before. To the layman, the effects are similar, but there are important differences in flying-land. Thanks to my friend Bruce Williams, an accomplished aviator and one of the developers of Microsoft Flight Simulator, for catching this quickly.

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