Why did we pick TL 18.000ft?

[johnnyb]

In the old altimeter days of the round dial, that is the mechanical makeup of the instrument itself (the bellows and comparison of ram and static air), the errors were too great between individual altimeters above 18,000ft.

It's called hystorisis ( stiffening due to repeated expansion and retraction ) of the aneroid wafers contained in a barometric altimeter : why this useless bit of information has stuck with me I'll never know ...

 

[Donsa320]

It's called hystorisis ( stiffening due to repeated expansion and retraction ) of the aneroid wafers contained in a barometric altimeter : why this useless bit of information has stuck with me I'll never know ...

Again, that was the reason for 2 thousand foot intervals above 290 but not the reason for a standard altimeter setting.

BTW the original Flight Level rule began in the "Golden Triangle" which had Chicago, Atlanta, and New York as its corners and started with a floor of FL240. Reason; that's all the FAA had for complete center enroute radar coverage and remote radio facilties. We did not have the ability to talk directly to center controllers all over the lower 48 until the 1960's.

 

[Mercy98]

Thanks for the re-education guys. You can teach (semi) old guys new tricks.

 

[Donsa320]

Thanks for the re-education guys. You can teach (semi) old guys new tricks.

I've had my instrument rating since 1953. History is a lot easier to remember when you live it first hand. <grin>

 

[westwind driver]

Here is something to ponder. On Jepp Plates there is the following: Transition Level: FL180
Transition Altitude 18,000.

Well as some us know from ops such as in Europe, a transition LEVEL is where we switch from local barometric to 29.92 (climbing). And transition ALTITUDE is changing from standard 29.92 to local barometric.

Well what happens here in the U.S. if ATC descends an aircraft to FL180? 18,000 is a flight level, not an altitude. So would a pilot be wrong for leveling at FL180 and setting the local baro?

And I guess that is why most other countries have a split in the transition level / altitude?

Would it be that because ATC assigned a flight level ("Descend and maintain FL180") the pilot is to remain on 29.92? Seems either FAA of JEPP is confused about this thing.

Like I said, 180 is not a proper altitude and you will never hear it assigned as one-eight thousand.

Curious to what others think.

Level at 18,000 on the descent do you set local baro or remain STD, and what about climbing? Or is it the ATC terminology a pilot follows to determine that 180 is a Flight Level at to fly that level at 29.92?

 

[Donsa320]

Here is something to ponder. On Jepp Plates there is the following: Transition Level: FL180
Transition Altitude 18,000.

Well as some us know from ops such as in Europe, a transition LEVEL is where we switch from local barometric to 29.92 (climbing). And transition ALTITUDE is changing from standard 29.92 to local barometric.

Well what happens here in the U.S. if ATC descends an aircraft to FL180? 18,000 is a flight level, not an altitude. So would a pilot be wrong for leveling at FL180 and setting the local baro?

And I guess that is why most other countries have a split in the transition level / altitude?

Would it be that because ATC assigned a flight level ("Descend and maintain FL180") the pilot is to remain on 29.92? Seems either FAA of JEPP is confused about this thing.

Like I said, 180 is not a proper altitude and you will never hear it assigned as one-eight thousand.

Curious to what others think.

Level at 18,000 on the descent do you set local baro or remain STD, and what about climbing? Or is it the ATC terminology a pilot follows to determine that 180 is a Flight Level at to fly that level at 29.92?

I think you answered your own question.... if there was one in there. <grin>

 

[westwind driver]

Well, the question comes from whether a foreign pilot flying in the US would read the JEPP chart and might accidentally dial in local baro if given a descent to FL180. If he/she did that, it could cause an altitude bust with the FAA. So I think more so the problem is with how JEPP shows the TRANS ATL, TRANS LEVEL. I'm pretty sure if given a descent to FL180 that's exactly that, stay on 29.92. Plus you'll never hear ATC here say one-eight thousand. But, a foreign pilot not so fimiliar with english and might become confused between the plate and ATC instructions, and may dial local baro while leveling off at 180.

I think you answered your
own question.... if there was one in there. <grin>

 

[Donsa320]

Well, the question comes from whether a foreign pilot flying in the US would read the JEPP chart and might accidentally dial in local baro if given a descent to FL180. If he/she did that, it could cause an altitude bust with the FAA. So I think more so the problem is with how JEPP shows the TRANS ATL, TRANS LEVEL. I'm pretty sure if given a descent to FL180 that's exactly that, stay on 29.92. Plus you'll never hear ATC here say one-eight thousand. But, a foreign pilot not so fimiliar with english and might become confused between the plate and ATC instructions, and may dial local baro while leveling off at 180.

I would say that is a problem that ATC would have to anticipate if it is a problem.

A more likely problem is a pilot setting the local altimeter in early on a descent from the flight levels to a lower altitude and then getting an interrmediate level-off in the flight levels and not resetting to 29.92. Same thing in the climb, setting 29.92 early and then getting a level-off while still down below the transition level and not resetting to local baro.
Don't ask me how I know. <grin>

 

[727gm]

My understanding ATC will only assign FL180 if regional area pressure/altimeter setting allows it's use clear of potential traffic at 17000 or 17500 that are using local altimeter. Much the way IFR flight has some restrictions in Alaska during periods of extremely high pressure.

 

[Donsa320]

My understanding ATC will only assign FL180 if regional area pressure/altimeter setting allows it's use clear of potential traffic at 17000 or 17500 that are using local altimeter. Much the way IFR flight has some restrictions in Alaska during periods of extremely high pressure.

Well,yes. If the local altimeter setting is 29.91 or less then FL180 is unusable. At 28.91 FL190 is unusable but the hurricane you would be in would probably be a bigger problem.<grin>