Mir
Topic Author
Posts: 19108
Joined: Mon Jan 05, 2004 3:55 am

### Question About The Hemispherical Altitude Rule

I've got a question that may sound pretty stupid, but what the heck:

I know that when heading between north and south via east, you fly at odd thousands, and that when heading between south and north via west, you fly at even thousands (in the US and not above FL410).

What happens when you start off heading southwest, but over the course of your route you start heading southeast? Do you have to either climb or descend 1,000 feet? Or does it not really matter?

-Mir
7 billion, one nation, imagination...it's a beautiful day

TripleDelta
Crew
Posts: 1039
Joined: Sat Jul 17, 2004 4:13 pm

### RE: Question About The Hemispherical Altitude Rule

 Quoting Mir (Thread starter): What happens when you start off heading southwest, but over the course of your route you start heading southeast? Do you have to either climb or descend 1,000 feet?

Yes, the a/c has to climb or descend. Otherwise it'll pose a threat to other traffic on the same level, but coming from the opposite direction, possibly leading to some nasty head-to-head situations.

However, operationally, this depends on a lot of factors, traffic included. For separation and safety reasons you can have an a/c remain on its level until sufficient separation between other traffic has been achieved (given that there's no "threatening" traffic on that level), with a level change afterward.

[Edited 2006-09-14 08:57:56]
No plane, no gain.

3DPlanes
Posts: 167
Joined: Thu Apr 27, 2006 11:12 pm

### RE: Question About The Hemispherical Altitude Rule

Just to elaborate, the hemisphere rule starts at 3000 feet. Below that is open (outside of controlled airspace).

IFR aircraft fly on the 1000s, VFR aircraft fly on the same 1000s + 500. And, the altitude is based on your -track- not your heading, to account for crosswinds leaving your nose pointed the wrong way for your altitude...

In real life, as suggested above, both IFR and VFR aircraft can be given altitudes that don't conform, assuming they are being controlled.

And, the altitude you use is for the major lengths of the routes. A diversion for traffic or weather (cloud clearance if you're VFR) won't necessitate an altitude change, assuming its short and you return to your original track..
"Simplicate and add lightness." - Ed Heinemann

deltamike172
Posts: 62
Joined: Sun Mar 04, 2001 2:52 pm

### RE: Question About The Hemispherical Altitude Rule

The official rule is 000-179 (i believe it is) you should file an eastbound odd alt. 180-359 you should file even. Now, when its a north south routing, it can be difficult to figure out which one works better. Occasionally, certain routes like this will have some sort of SOP that states that all aircraft on such a route should be at either odd or even all the time. Other times the altitudes are determined in relation to other traffic. Anything procedurally related will be relayed to the pilots when their altitude is amended, and air carriers quickly catch on to what to file the next time they fly that route.

And of course, the final solution is simply make sure traffic won't hit, using whatever rule you need to use.

There is plenty of traffic at the appropriate altitude for direction of flight that are in conflict with each other. If one plane is eastbound on a 090 heading at FL210, and another plane is eastbound on a 070 at FL210....they're gonna be in conflict eventually. Thats what ATC is for.

DM

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