Why would you ever have a TL that is below a TA? That can put two airplanes, if one was near the floor of the level and one at top of the altitude, potentially at the same altitude while thinking they were at different altitudes.

No it wouldn't, it would still retain the required safety separation. See Manual of Air Traffic Services Appendix 6

No it wouldn't, it would still retain the required safety separation. See Manual of Air Traffic Services Appendix 6

Well, I'm just on my phone. so I'm not going go pick through some humongous pdf like that to look for the one sentence you are thinking of. I would expect that they outline a series of procedures, dependent on the current altimeter setting, that are supposed to keep uncontrolled aircraft below the TA separated from aircraft that above the TL while allowing the use of a TL below the TA. I am sure. My question is why? When all you have to do iis keep the TL the same, or just raise it on a low altimetry day, and you can very easily keep planes separated with a minimum of fuss.

The FAA also requires adjustments to Flight Levels - perhaps you should ask them?

The FAA also requires adjustments to Flight Levels - perhaps you should ask them?

Only an upward adjustment of the lowest useable flight level during low altimetry, which makes easy sense. They don't make downward adjustments of FL. That's ok you don't have to answer because I am sure you don't know the answer, mgh. You just know how to google the topic. Thanks for the link.

Edited January 13, 201214 yr by KevinAu

you don't have to answer because I am sure you don't know the answer

Let's not forget that this sub-thread began because you didn't know the difference between TA and TL, and that TL can be numercally below TA.

Let's not forget that this sub-thread began because you didn't know the difference between TA and TL, and that TL can be numercally below TA.

It was jbinner who did not know the difference. My post was below his correcting him. That's right, I don't know that you drop TL below TA over there. And that's interesting because there are inherent problems with that.. That's why I asked why. It was'nt a flame, it wasn't meant to provoke. It was just an honest f-ing question the same way I gave an honest f-ing answer to you when you asked why we would choose 18000. I hope you are not like this in person. Because it's got to be a sad, angry and lonely life if it was.

1. Transition altitude is 18,000 feet. Transition level is FL180.

That is not generally true. Under standard conditions vertical separation would not be maintained if ATC allocated FL180 as the TL.Also there are no inherent problems in ATC allocating a TL either above or below the TA. Vertical separation is maintained in all cases

Edited January 14, 201214 yr by mgh

That is not generally true. Under standard conditions vertical separation would not be maintained if ATC allocated FL180 as the TL.Also there are no inherent problems in ATC allocating a TL either above or below the TA. Vertical separation is maintained in all cases

I was making no pronouncement of any specific transition altitude or transition level for any specific locality in that statement. The use of the numbers 1, 8, and 0 were merely for random example purpose. That sentence was to correct jbinner's "Transition altitude in US is FL180. NOT 18,000" statement. You do see the problem of what he wrote, mgh, don't you? A transition altitude is given in feet, not levels. A transition level is given in levels. Jbinner declared that the transition "altitude" is "flight level" so and so. That is the oxymoron I was correcting.Having said that, it is generally true in the US that FL180 is the transition level. And it is an assignable flight level as long as the local altimeter is 29.92 or higher. Only when the local altimeter is less than 29.92 is a change made. And that change is to the lowest assignable flight level, which becomes FL190, or higher as the conditions warrant. A flip to a random approach chart in my Jeppeson book brings up Chicago, IL, ILS or LOC Rwy 9L dated 1 Apr 11 21-3. The briefing bar tells us that "Trans level:FL180 Trans alt:18000' " Even if the altimeter was well below 29.92, we would still use FL180 as our transition level. We do not use a variable TL in the United States as far as us pilots are concerned.There are inherent problems in ATC allocating a TL below a TA. Let's say you have an aircraft at FL25 and another aircaft at 3,000'. The local altimeter setting is 30.42". When the aircraft at FL25 dials the altimeter from 29.92 up to 30.42, he finds that he is actually flying at 3,000'. You have two aircraft at the same altitude thinking they were at different altitudes. A moment later, the two collide. That is the inherent problem of shifting a TL below a TA. Shifting a TL above a TA inherently makes things safer. In conditions of low altimetry, having a TL higher away from the TA is safer because once you dial in the local altimeter setting, you find that you are at a lower altitude. The shift higher gives you more room away from people below. In conditions of high altimetry, having a TL higher than the TA is just as safe since when you dial in the higher altimeter, you find that you are actually at a higher altitude away from the traffic below the flight levels.

Edited January 14, 201214 yr by KevinAu

It's soooooo much easier in "The Rest of the World!" e.g an a/c cleared to take-off "Climb to FL60" an a/c descending "Descend to FL60" thereafter "Descend to 4500 QNH 1016" what could be simpler!!!!vololiberista

Edited January 14, 201214 yr by vololiberista

It's soooooo much easier in "The Rest of the World!" e.g an a/c cleared to take-off "Climb to FL60" an a/c descending "Descend to FL60" thereafter "Descend to 4500 QNH 1016" what could be simpler!!!!vololiberista

Somebody would nab you for reckless flying if you did that here. Buzzing all those rooftops, you daredevil.http://www.denvergov.org/

Edited January 14, 201214 yr by KevinAu

Minimum height above GL over populated areas here is 1,500ft. you certainly wouldbe nabbed if you got caught below that height.vololiberista

Minimum height above GL over populated areas here is 1,500ft. you certainly wouldbe nabbed if you got caught below that height.vololiberista

FL060 wouldn't work so well here, then, would it?

There are inherent problems in ATC allocating a TL below a TA.

As I said previously,

Transition Level is the lowest Flight Level available for use above the Transition Altitude.

The TL always ensures that there is a minimum vertical separation of 500 ft between an aircraft flying at the TL and one flying at the TA regardless of the numerical value of the TL.The risk you describe in your example, therefore, doesn't exist - ATC would not allocate FL25 in the that situation

As I said previously,The TL always ensures that there is a minimum vertical separation of 500 ft between an aircraft flying at the TL and one flying at the TA regardless of the numerical value of the TL.The risk you describe in your example, therefore, doesn't exist - ATC would not allocate FL25 in the that situation

What ATC is doing by not using FL25 in that situation is using a procedure to overcome an inherent problem.If ATC is not going to use a TL below a TA in that situation, what situation would they use it in? When the altimeter is below standard? You can't do it in that case. That would be a case for raising the TL above the TA.So again I ask the question, what purpose does having a TL that can be moved below a TA serve? And an honest question it is. I really would like to see you explain that to me in a non-condescending, forthright way. I am not trying to flame you. Thank you.

The required vertical separation between an aircraft flying at TA (ie below the transition) and one flying at the TL (ie above the transition) must be 500 ft.With a TA of 3000 ft amsl and the standard QNH (1013 hPa), FL35 corresponds to 3500 ft amsl. so the TL is FL35.As QNH increases the aircraft's altitude amsl increases at fixed indicated FL. For example, at a QNH of 1050 hPa, FL35 would correspond to 4500 ft amsl However, FL25 would correspond to 3500 ft amsl so the TL is FL25.If FL35 were used as the TL, then there would be unused airspace because level flight at 3500 ft amsl (FL25) and 4000 ft (FL30) isn't allowed. Level flight is not permitted between the TA and the TL. This would have adverse effects on planning ATC procedures in busy airspace, specially approaches.

Edited January 15, 201214 yr by mgh