Pressure Altitude Definition / Transition Altitudes; Confusion, Help!

[tinmouse]

Pressure Altitude Definition / Transition Altitudes

Hi All,

Just want to make sure I have the correct understanding of Pressure altitude (PPL), if you can help!

My understanding of the definition is;

“The Pressure altitude is the height in ISA above the 1013 hpa pressure level, at which the pressure equals that of the aircraft or point under consideration”.

(Do please correct me if I’m wrong, any simpler definition is welcome!).

I’m assuming the reason the definition states; “above the 1013 hpa pressure level “, is to do with flight levels; i.e If the transition altitude is 5000 ft, then I change my QNH to 1013 hpa at 5000 ft and climb to my Transition Level, say 5500 ft.

But this leaves a question with the Transition layer? Because an aircraft may be flying at QNH, while I’m flying at ISA 1013 hpa, so the difference in the Transition layer may be less then 500ft separation? How do I work this out?


So, back to Pressure altitudes. If my airfield is at 100 feet. The present QNH is 1015, the pressure altitude at the airfeild is;

1015 hpa – 1013 hpa = 2 hpa

2hps * 30 ft = 60 feet.
100ft (Aerodrome Level QFE) + 60 ft = 160 ft Pressure altitude at aerodrome level.

Conversely;

If my airfield is at 100 feet. The present QNH is 1010, the pressure altitude at the airfield is;

1013 hpa – 1010 hpa = 3 hpa

3hps * 30 ft = 90 feet.
100ft (Aerodrome Level QFE) + 90 ft = 190 ft Pressure altitude at aerodrome level.

If anyone can tell me if I’m going off track here, please let me know. Thanks in advance!

Tinmouse.

:)

[airmen]

So, back to Pressure altitudes. If my airfield is at 100 feet. The present QNH is 1015, the pressure altitude at the airfeild is;

Always try to visualise, take a look at your altimeter, if the QNH is 1015 and you turn the pressure knob to 1013 you decrease your altitude so:

1015-1013=2hpa

2x30=60 or 2x27=54 check the QCM answer that match!

100feet-60=40feet or 100feet-54=46feet

Hope that helped!

[CAT3C AUTOLAND]

tinmouse,

Best thing to do with all this mate, is to draw a little picture with airfield elevations, sea level etc, it makes it easy to visualise.

With regard to the Transition Layer, this is governed by the transition altitude and the lowest possible flight level for cruising (transition level). If you look on the Jepp or Aerad approach plates you will see a box for the Transition Level, and it says ATC. This means that ATC will stipulate what is the lowest flight level that can be used depending on the sea level pressure of that particular day, therefore separation is achieved.

Hope this helps, it is all a bit of a nightmare isnt it?

[AC690]

Remember the transition layer will never be less than 1000' thick!!

The previous post are correct....if you wind on QNH you wind on altitude and opposite if you wind off QNH!!

[BillieBob]

Probably better to remember that the Transition Layer will never be more than 1000ft thick!

Also, QNE is not Pressure Altitude. QNE is the reading on the altimeter at touchdown with 1013 set on the subscale.

[AC690]

Just out of interest...transition layers are more often that not more than 1000' thick!!

Ask any ATC, the layer is not,will not ever be less than 1000' thick.

just my 2c worth.

[Engineer]

Maybe this (http://www.flightinfo.com/rulesofthumb.asp) wil help

[High Wing Drifter]

Probably better to remember that the Transition Layer will never be more than 1000ft thick!
I guess it might be if the QNH is less than 976mb. I would imagine that 1050mb is pretty much impossible.

[BillieBob]

This is a wind up....Right?

[High Wing Drifter]

Um...I have := moments. Why might it be a wind-up?

Also, QNE is not Pressure Altitude. QNE is the reading on the altimeter at touchdown with 1013 set on the subscale.
I think it is the same difference if you see what I mean.

[Tinstaafl]

Not quite. Pressure altitude is subject to temperature error in a non standard column of air so the true altitude may not be the same. QNE is at the surface so eliminates the temperature error.

[High Wing Drifter]

Tins,

Pressure altitude is the alt according to the mean ISA pressure...1013. If you are sitting on the ground then the reading with 1013.2 is the pressure altitude of the aerodrome.

The density altitude is the pressure altitude with the temprature correction applied.

True altitude is the QNH reading adjusted for temprature.

Details, details!

[Tinstaafl]

Quite right, H W D. I knew what I meant but that's not what I wrote! :oh:

[Send Clowns]

tinmouse

You are confusing yourself a little, and others here are not helping a lot, producing confusing and incorrect information.

Yes the "above the 1013 mb pressure level" is related to flight levels, in completely the opposite way to which you are thinking. Flight levels are defined as being pressure altitudes, rather than the other way round. They are defined every 500 feet pressure altitude, and denominated in hundreds of feet.

The 1013 in your definition comes from ISA, and is in fact redundant. The Pressure altitude at an aircraft or point is the altitude in ISA at which the pressure equals that of the aircraft or point under considerationwould be a simpler definition, as all altitudes are above mean sea level, and in ISA the mean sea level pressure is 1013. This does however, in practice, mean that the pressure altitude (PA) is what your altimeter reads when the subscale is set to 1013 hPa.

>> Transitiopn altitude is generally a fixed defnition for a location. Over most of the UK (I will use for later examples) it is 3000 ft. However Heathrow uses 6000 ft, and in the US transition altitude is generally 18,000 ft.

>> Transition level is the next defined flight level above the transition altitude, in the conditions of the day. In most of the UK, with transition altitude 3000 ft it can be anything from FL25 to FL45 in extremely high or low pressures respectively.

>> The transition layer is the air between the transition altitude, and the transition level. Note that (despite the garbage posted here) that means that the transition layer should never be greater than 500 feet, and can be as little as zero if 3000 feet matches a defined flight level.

For example if the QNH is 1013 then the transition level for that airfield would be FL30, which is also 3000 feet PA. It happens that if QNH is 1013 then altitude is equal to PA, so at transition altitude of 3000 feet amsl nothing happens, but the aircraft is already at FL30, so there is no transition layer.

On the other hand if QNH was 1010 then the 3000 ft transition altitude would be 3081 ft PA*. The next defined flight level above this is FL35, which is at 3500 ft PA. The transition layer is from 3081 to 3500 ft PA, which is 419 feet.

Final example is for a QNH of 1016. In this case the transition altitude of 3000 ft is a pressure altitude of 2919 ft. The next defined flight level is FL30, and the transition layer is 81 feet.

See now why the military ban instrument flying at transistion level, always use the next above as a minimum FL? It is very unwise, as the separation from traffic using altitude reference can be very small.

As a final note, since transition level is often passed to an aircraft by ATC it is possible that they are different (though never lower) than that calculated, although I don't know if it ever is. That is not your concern, if passed a high TL just use what you're given.


* To find PA turn from QNH to 1013. This turns the subscale up (subscale up, altimeter up) by 3. Each hPa gives 27 feet, 3 x 27 = 81 so the altimeter then reads 3081 ft.

[pugzi]

Tinmouse,
If you have understood any of this and not got more confussed I'd be very suprised.
Altimetry problems and pressure problems needs pictures and with all the best intentions of the above people, including myself, this is not going to make the penny drop.

Hold fire till you visit your ground school for the brush up and all will be made very clear to you I'm sure.

I'm confussed myself from all this.

Good luck

[CAT3C AUTOLAND]

Send clowns, you have probably scrambled his/her brain! I think a picture would help.

Oops, Pugzi beat me to it :D.