Standard altitudes

[flyingron]

The chance of a head on would be pretty small I think. I would prefer not ever getting hit. A guy going the direction 179 toward the guy going 003 is pretty close to headon- but now we try to bunch them all at the same altitude

Har, I've been there several times. Yes, you have degenerate cases at the edges, but it works well in most directions. I like to avail myself of all the options available: Maintaining a proper visual lookout, flying at the prescribed altitudes, electronic traffic avoidance, and ATC radar services.

 

[Hank S]

Slackly, it's just like having all cars in the right side of the road, except there's no centerline to cross.

VFR traffic, who can see each other, have two altitudes to fly at, one for eastbound and one for westbound. Instrument traffic, who may or may not be able to see, also have two altitudes to fly at, and these are different than the VFR guys. IFR traffic uses the IFR altitudes regardless of visibility, it makes it easy for ATC to tell them apart.

The dividing line isn't painted in the road, it's right there in your compass / DG, are you flying to the East side or the West side of the 180-360° line? Easy peasy. This reduces the chance of midair collisions by letting everyone know where to look. IFR flight is permitted up,to 200' variation in altitude, but I try to never get that far off. Nevertheless, IFR flight is permitted as long as my altimeter reads within 75' of departure field elevation when set to the current altimeter setting. That's an automatic 150' spread.

If you have every plane choose an altitude within a 400' spread, spotting other traffic will be much more difficult as you'll need to look up and down as well as in front and to both sides, and little planes a few miles away are hard to spot below you when flying over developed urban areas. So the rules make it easier to spot traffic, while providing separation between airplanes flying in opposite directions.

 

[Slackyhacky]

Hank,

Thanks for the answer. Makes sense to me. I think significant digits in the altimeter is probably 3, so like you mentioned, there is already built in a '150 spread just on the nature of non-precise instrument.

 

[MAKG1]

I see a few errors in this thread.

The hemispheric rule is magnetic COURSE. You do not read course off your DG. You read it off your chart or your GPS. In a spam can, I've seen wind correction angles as high as 30 deg. It's not always a small effect.

And, yeah, the head on configuration happens and can require rather fast reactions. I once found a Mooney leaving SAC at 178 while I was flying toward it at 002, and both of us were at 3500. Not much warning; there had to be over 300 knots relative.

 

[Rykymus]

Hank explained it pretty well. I know that planes don't seem like they take up that much vertical space. But If I'm allowed to be 200 ft off, and my Altimeter is allowed to be 75 ft off, that's 275 ft off my altitude. And if you're the same amount off? BAM.

It will only take you one near miss to see that 500 ft isn't all that much, and you don't want it less.

 

[MAKG1]

Hank explained it pretty well. I know that planes don't seem like they take up that much vertical space. But If I'm allowed to be 200 ft off, and my Altimeter is allowed to be 75 ft off, that's 275 ft off my altitude. And if you're the same amount off? BAM.

It will only take you one near miss to see that 500 ft isn't all that much, and you don't want it less.

If you've ever sat in the left seat of a 747 (on the ground), you'll know they certainly can take up a lot of vertical space.

The bottom of the #2/#3 engine cowlings to the top of the vertical stabilizer is about 60 feet. That's a 7 story building. The static ports are near the bottom of the fuselage up front.

Don't forget that there are additional altimeter errors if alternate static is being used, or if you cross into another airmass (I've seen more than 100 feet crossing the Diablo Range on a hot day). The opposing aircraft isn't necessarily in the same sector and may not have the same altimeter setting.

 

[Sundancer]

Big sky, small planes, even the heavies, who aren't down there with you much outside Class B. And outside terminal areas, you could probably shut down ATC for traffic below 18,000. Someone said he got a descent call, then saw the traffic a bit later, else they would have collided. But, probably not; even if the altimetry was perfect, like as not a minor lateral offset would have existed.

It might feel creepy, for sure, but even VFR without FF, collisions in cruise are very rare. The chatter with ATC makes us feel safer, but is more theatre than substance. Nearer to airports, things are very diffrent, and aluminum does merge on occasion. . .