FL180

[BoilerPilot2007]

Anyone know why the transition level is 18,000 ft (FL180) as opposed to 5, 6, 7, 8,000 ft etc other places?

Not a quiz... just wondering if there was a reason for it..

Cheers,

 

[flyover]

Good question. I'm sure there was a good reason decades a go when they did it, or maybe they just threw a dart at a board.

But it is different in different couthry's airspace. Something you have to keep track of as you leave the US border.

 

[flyover]

It just struck me that a determining factor would be terrain. Within the US 180 is well clear of all terrain. You would not want to be on standard altimeter if working close to the Rocky Mtn MOCAs.

 

[Tim]

I will go with Dave's suggestion. Then again who knows how drunk there were when the created some of the rules.

 

[mrivc211]

Very good question. Dont know the answer!

 

[Kingairer]

Ive always heard the terrain answer.

 

[TallBoy85]

I'm a firm supporter of the dart board theory.

 

[Guest]

Well this is how it happened. A group of people at the FAA got a hat and each one put an altittude inside. FL180 was the winner and they never bothered to change it.


I seriously don't know.

 

[Alchemy]

I believe they did it because 18,000 (500mb) is about halfway through the atmosphere density-wise.

At 18,000, you have about as many air molecules above you as you do below you.

 

[BoilerPilot2007]

Thanks for your responses everyone..

Seems like the two most likely answers are the 500mb level (pressure), and terrain...

Any others?

 

[Aussie]

The Transition layer in Oz is 10000ft to FL110

Due terrain, dont know about the USA

 

[jergar999]

It's terrain. FL180, assuming barometric pressure is within the parameters, assures legal ground separation. I have seen this published somewhere but I can't remember where. I will see if I can find it.

 

[H46Bubba]

FL180 is a transition from the lower altitudes to standard 29.92 pressure flight levels. Its purely to simplify atc separation on in Class A airspace. It keeps pilots from changing altimeters every couple of minutes as they fly over stations. If it were that way, aircradft would be going up and down constantly as the pressure changes.

 

[ex208driver]

I am also curious as to why in Mexico then trans level is at 18500' and trans alt is 19500'. I may have these backwards, but they are definately different alts.