Altimeter Question

[tcasdescend]

I understand that altitude is dependent on temperature.
But isnt temperature localised?
I mean wouldnt the temperature change due to the weather (e.g. sunny or cloudy or depending on the season?)
Wont the temperature change when you start and stop an engine for instance?
In that case how will a pilot know how to calibrate his or her altimeter?

[Uplinker]

Altitude is primarily found by measuring (static) air pressure, which reduces the higher you go. You set the current air pressure on the ground, (from the airfield weather report) on your altimeter, which will then display zero altitude on the ground and altitude above the airfield in feet after you have taken off. Temperature can affect altimeter readings to a lessor extent, and the siting of the static port(s) on the aircraft is carefully chosen to avoid local distortions as much as possible.

But there is an awful lot more to it than that - including different definitions and reference points for altitude - and rather than risk confusing you, I strongly advise buying the Trevor Thom series of Air Pilot's Manuals for PPL training. Or perhaps just Volume 4, which explains the aircraft technical systems, including instrumentation and their faults and compromises.

I am sure other text books are available.

[A320LGW]

Originally Posted by tcasdescend

I understand that altitude is dependent on temperature.
But isnt temperature localised?
I mean wouldnt the temperature change due to the weather (e.g. sunny or cloudy or depending on the season?)
Wont the temperature change when you start and stop an engine for instance?
In that case how will a pilot know how to calibrate his or her altimeter?

Aircraft in lower areas and in terminal areas use the local QNH (local pressure setting) to indicate their altitude, this ensures all aircraft in the area are working off of the same reference. When higher up, aircraft all switch to a standard datum called (a permanent QNH of 1013 hPa), and use this pressure setting as the reference.

The actual true altitude with QNH 1013 at flight level 250 when it's -20C at that flight level compared to when it's -40C are of course not the same, but these differences are ignored because they don't usually have any significance, unless you're flying around the Himalayas.

The only time a pilot would really adjust their temperature is at a lower altitude or when around high terrain and the temperature is lower than 0C, it's done using a procedure called temperature compensation. If you are going off an RNP approach chart for example it will have a lower temperature limit, if the OAT goes below this temperature you must use the temperature compensation procedure to still be able to use the chart.

[pilotmike]

Originally Posted by tcasdescend

I understand that altitude is dependent on temperature.
But isnt temperature localised?

No and NO!

TROLL ALERT - TROLL ALERT = TROLL ALERT!!!!!!

Oddly, you may think, aircraft are freely able to climb and descend from sea level to their service ceiling. This means they can change their altitude pretty much at will, almost irrespective of temperature. No more, please!

Do you remember, 'tcasdescend', that you also asked all about how instruments worked and also asked about learning to fly in a simulator, 10 years ago, back in 2012.

Now a question from me, for once - How did that go? Ten whole years ago? And why exactly are you back to ask all the same questions all over again?

Are you flying a 737 yet with just 16 hours training? When do you think you'll complete the course, so we can have the honour of you being our captain on our next flight?

[tcasdescend]

Went to the simulator once. Did nothing much except a TOGA.

Now that I am older and have some financial resources, I am thinking about my next move.

[tcasdescend]

Originally Posted by A320LGW

Aircraft in lower areas and in terminal areas use the local QNH (local pressure setting) to indicate their altitude, this ensures all aircraft in the area are working off of the same reference. When higher up, aircraft all switch to a standard datum called QNE (a permanent QNH of 1013 hPa), and use this pressure setting as the reference.

The actual true altitude with QNH 1013 at flight level 250 when it's -20C at that flight level compared to when it's -40C are of course not the same, but these differences are ignored because they don't usually have any significance, unless you're flying around the Himalayas.

The only time a pilot would really adjust their temperature is at a lower altitude or when around high terrain and the temperature is lower than 0C, it's done using a procedure called temperature compensation. If you are going off an RNP approach chart for example it will have a lower temperature limit, if the OAT goes below this temperature you must use the temperature compensation procedure to still be able to use the chart.

Thanks for answering.
Guess I have to stop relying on YouTube

[pilotmike]

Originally Posted by tcasdescend

Went to the simulator once. Did nothing much except a TOGA.

'Doing' a TOGA. Priceless! Is that a cross between a Tango and some Yoga?

Originally Posted by tcasdescend

Now that I am older and have some financial resources, I am thinking about my next move.

Obviously, which self respecting Aviator wouldn't be?.

May I suggest a FOGA? Very similar to T/Off and G/Around, but rather more like a F/Off and G/Away. It shouldn't cost you much, but it would be a big saving for everyone else.

[Hew Jampton]

QNE is not an altimeter sub-scale setting of 1013 nor any other setting. QNE is the altitude (amsl) of the airfield when the altimeter is set to 1013, or in other words the Flight Level of the airfield. It is only used in very high or low atmospheric pressures that cannot be set on the altimeter sub-scale, and is even less used with modern altimeters that can take very low or high pressure settings.
Ralf D. Kloth DL4TA - List of Q-codes refers.

[A320LGW]

Originally Posted by Hew Jampton

QNE is not an altimeter sub-scale setting of 1013 nor any other setting. QNE is the altitude (amsl) of the airfield when the altimeter is set to 1013, or in other words the Flight Level of the airfield. It is only used in very high or low atmospheric pressures that cannot be set on the altimeter sub-scale, and is even less used with modern altimeters that can take very low or high pressure settings.
Ralf D. Kloth DL4TA - List of Q-codes refers.

Interesting, I could have sworn i was told QNE was a another name for 'standard setting'. I have removed the reference to it, not relevant ..

[Hew Jampton]

Originally Posted by A320LGW

Interesting, I could have sworn I was told QNE was a another name for 'standard setting'. I have removed the reference to it, not relevant ..

You almost certainly were told it; at least one ground studies instructor seems to have been peddling this misinformation.

[PFD]

I'm a ground school TKI. I wasn't aware of QNE, so thanks for this. Not an ICAO code anyway, but I can see how someone might think it's SPS. Of course, when you look at the definition, it's evident that it is not the setting but the altitude at that setting on landing, or FL, as someone said. Very interesting. Every day's a school day, literally in my case

[tcasdescend]

Originally Posted by A320LGW

Aircraft in lower areas and in terminal areas use the local QNH (local pressure setting) to indicate their altitude, this ensures all aircraft in the area are working off of the same reference. When higher up, aircraft all switch to a standard datum called (a permanent QNH of 1013 hPa), and use this pressure setting as the reference.

The actual true altitude with QNH 1013 at flight level 250 when it's -20C at that flight level compared to when it's -40C are of course not the same, but these differences are ignored because they don't usually have any significance, unless you're flying around the Himalayas.

The only time a pilot would really adjust their temperature is at a lower altitude or when around high terrain and the temperature is lower than 0C, it's done using a procedure called temperature compensation. If you are going off an RNP approach chart for example it will have a lower temperature limit, if the OAT goes below this temperature you must use the temperature compensation procedure to still be able to use the chart.

Why cannot all aircrafts just use QNE so that they do not need to change to QNH in different parts of the world?

[iggy]

Originally Posted by tcasdescend

Why cannot all aircrafts just use QNE so that they do not need to change to QNH in different parts of the world?

I really hope that you never get to fly solo.

[gerpols]

Hahaahhaha, so do I iggy, what a jerk !

[tcasdescend]

Originally Posted by iggy

I really hope that you never get to fly solo.

Why only qnh at lower altitude?
If it is possible why not qne standard throughout the world?

[tcasdescend]

Originally Posted by gerpols

Hahaahhaha, so do I iggy, what a jerk !

Why I do wrong
I know it is a rule to use QNE at high altitude and qnh at low altitude but I not know why.

Why not standardize everything at 1013hPa?

[Hew Jampton]

Originally Posted by tcasdescend

Why I do wrong
I know it is a rule to use QNE at high altitude and qnh at low altitude but I not know why.

Why not standardize everything at 1013hPa?

QNE is not 1013 nor any other altimeter pressure setting, as explained to you in post #8. Using 1013 does not give altitude above mean sea level (except when the QNH just happens to be 1013) and therefore is no good for terrain clearance purposes, nor for take-off and landing, except when the QNH/QFE is extremely high or low and cannot be set on the altimeter sub-scale, in which event the QNE procedure can be used. Many pilots and air traffic controllers do not know about the QNE procedure, nor how the value is calculated (for UK readers, it's in MATS Part 1, Appendix A Manual of Air Traffic Services - Part 1 (caa.co.uk)).

[Pyotr]

Originally Posted by tcasdescend

Why only qnh at lower altitude?
If it is possible why not qne standard throughout the world?

Because at low altitude it is more important to avoid flying into terrain/mountains, but at higher altitude the primary concern is to avoid hitting other aircraft. We could use QNH at every altitude, but then we would have to adjust the altimeter all the time since the pressure is always changing. Therefore when climbing through the "transition altitude" we set the altimeter to standard pressure I.e. 1013hPa, and maintain that setting until the descent. It's just more practical.

Regarding altitude and temp. Yes, density and thus the readout on the aircraft altimeter is affected by temperature. But thing is: how could you(or the altimeter) know to what extent the temperature is affecting the air between the aircraft and the ground. Is the aircraft just flying in a especially cold little pocket of air or is actually the entire column of air between the aircraft and ground cold? This is very difficult to know precisely so instead all aircraft altimeter are calibrated to measure the air according to the "International Standard Atmosphere". It assumes the temperature at sea-level is 15C and decreases by 2C every 1000ft of altitude.

In other words: the altimeter does actually not take into account the temperature and most of the time pilots do not either take it into account. Thus the altitude readout is never exactly accurate, but close enough anyway. As mentioned in another comment: unless it is really cold we don't compensate for the temperature while flying.