Altimeter and class Alpha

[Trip8s]

Ok, I am a low time pilot of approx 700hrs, and fly a Mooney Ovation. I rarely flyover 12 - 14k feet so I never thought much about setting the altimeter at 18k. As of late I have been fortunate enough to get a ride in the right seat of a CJ III so here is the 18k alt setting business. So now I get back in the Ovation and was wondering......if the altitude the GTX330 is displaying the same, as the Altimeter would read if I were to climb to 18k and change from local pressure to standard??

 

[Larry in TN]

Your transponder ALWAYS transmits pressure altitude. That never changes.

As you approach the Transition Altitude (18,000' in the US) during climb, change your altimeter setting to 29.92.

As you approach the Transition Level (FL180 in the US) during descent, change your altimeter setting to the local altimeter setting (QNH).

If you are not in the US, remember to check for the correct Transition Altitude and Transition Level. In the Caribbean, for example, it is frequently well below 10,000'. Also, don't confuse the Transition Altitude/Level with Class A airspace. Though they happen to be at the same altitude/level in the US, they are unrelated.

 

[Greg Bockelman]

Just to expand on what Larry said a bit.

Yes, your transponder always transmits Pressure Altitude. (And FWIW, that info comes from the encoder part of the system) What happens is those center sectors that are below the transition level (18,000 Feet) automatically convert that pressure altitude to the appropriate MSL altitude. That is why they will occasionally give you an altimeter setting.

 

[flyingron]

]
As you approach the Transition Level (FL180 in the US) during descent, change your altimeter setting to the local altimeter setting (QNH).

Actually, it's 18,000' MSL in either case. The lowest flight level usable varies with the barometric pressure.

 

[Greg Bockelman]

Actually, it's 18,000' MSL in either case. The lowest flight level usable varies with the barometric pressure.

Not following your point, here, Ron. What Larry is saying is that the transition level is different in other parts of the world.

 

[flyingron]

Not following your point, here, Ron. What Larry is saying is that the transition level is different in other parts of the world.

I was commenting about his statement about FL180 being the TRANSITION FLIGHT LEVEL in the US. It's not. It's dependent on the altimeter setting. You'll never get to FL180 if the pressure is less than 29.91.

My statement wasn't 100% right either. The lowest (therefore transition FL) is never below FL180 no matter how high the pressure gets.

(And the concept that the transition FL changes with pressure is not just a US concept either).

 

[Larry in TN]

I was commenting about his statement about FL180 being the TRANSITION FLIGHT LEVEL in the US. It's not. It's dependent on the altimeter setting.

Look at any US Jepp approach chart.

"Trans Level: FL 180 Trans alt: 18000"

In many countries the transition LEVEL and transition ALTITUDE differ by 1000' or more. It is important for a US-trained pilot to understand that so they don't assume they are both the same when they fly outside the US.

Mexico, for example, is FL195/18500'. Antigua is FL40/2500'.

To your point, when pressure is below standard the transition level is still the same. What changes is that the lowest flight level(s) become unassignable.

If you are cleared to the first usable altitude above the TA on climb, or below the TL on descent, then you're going to have to reset your altimeter early enough to make the level off.

 

[flyingron]

I disagree. Despite Jepps charting anomaly, the TRANSITION LEVEL is defined (certainly by the FAA and by ICAO, they both use the exact same wording) as

Transition Level (QNE). The lowest flight level available for
use above the transition altitude.​

It is not always FL180 in the US and docs for Europe show it varies with atmospheric pressure as well.

Here's even some eurocontrol docs on determining it...

4.10.2 Determination of the transition level
4.10.2.1 The appropriate ATS unit shall establish the transition level to be used in the vicinity of the
aerodrome(s) concerned and, when relevant, the terminal control area (TMA) concerned, for the
appropriate period of time on the basis of QNH (altimeter sub-scale setting to obtain elevation when on the
ground) reports and forecast mean sea level pressure, if required.
4.10.2.2 The transition level shall be the lowest flight level available for use above the transition altitude
established for the aerodrome(s) concerned. Where a common transition altitude has been established for
two or more aerodromes which are so closely located as to require coordinated procedures, the appropriate
ATS units shall establish a common transition level to be used at any given time in the vicinity of the
aerodrome and, when relevant, in the TMA concerned.​

 

[RotorAndWing]

(Caution: The following is sarcasm)



Greg, Just how could you or Larry (or any pilot who routinely use TA/TL) could possibly know how this really works??

 

[Greg Bockelman]

Lol. I have no idea.

 

[jordane93]

(Caution: The following is sarcasm)



Greg, Just how could you or Larry (or any pilot who routinely use TA/TL) could possibly know how this really works??

It's the same guys that are saying, "Why would you ever want to be an airline pilot. They are just glorified bus drivers"

 

[luvflyin]

Ok, I am a low time pilot of approx 700hrs, and fly a Mooney Ovation. I rarely flyover 12 - 14k feet so I never thought much about setting the altimeter at 18k. As of late I have been fortunate enough to get a ride in the right seat of a CJ III so here is the 18k alt setting business. So now I get back in the Ovation and was wondering......if the altitude the GTX330 is displaying the same, as the Altimeter would read if I were to climb to 18k and change from local pressure to standard??

Plus or minus instrument error, yes. How do you get pressure altitude if you need it to do some calculation? Set your altimeter to 29.92 and read the altitude. The transponder always reports pressure altitude.