# How does altimeter reading vary when departing from ISA conditions?

I am a soon-to-be student-pilot and am struggling with a basic concept that is altitude. After watching multitude of online videos, I feel that most of the people do not have good teaching skills.

Introduction

The altimeter is a barometer, it reads atmospheric pressure. At 0' altitude, it should measure "this certain pressure that we find at 0'" and indicate 0'. This is obtained by setting the reference pressure in the Kollsman window measured somewhere near the airport. One day it might be 1013 hPa, others it will be 998 hPa or 1036 hPa.

The Scenarios

We are in the aeroplane, at the airport which is at MSL (0' altitude).

SCENARIO 1 (International Standard Atmosphere):

• Elevation: 0', temperature: 15 C, pressure: 1013.25 hPa
• METAR reads QNH 1013

1. we set our Kollsman window to 1013 => it correctly indicates 0' altitude
2. we set it to less than 1013 => it will indicate more than 0' (say 230') LESS than 0'!
3. we set it to more than 1013 => it will indicate less than 0' (impossible but bear with me) MORE than 0'

SCENARIO 2 (low pressure):

• Elevation: 0', temperature: 15 C, pressure: 998 hPa
• METAR reads  QNH 998

1. set our Kollsman window to 998 => it correctly indicates 0' altitude
2. set it to less than 998 => it will indicate more than 0' (say 230')
3. set it to more than 998 => it will indicate less than 0'

If I got the above correctly than the following should apply too:

SCENARIO 3 (International Standard Atmosphere):

• Elevation: 5000', temperature: 5 C (as per lapse rate, -2C for every 1000'), pressure: 847 hPa (as per lapse rate, -1hPa every 30')

• METAR reads  QNH 1013 (actually our pressure of 847 hPa, adjusted down to MSL)

1. set our Kollsman window to 1013 => it correctly indicates 5000' altitude
2. set it to less than 1013 => it will indicate more than 5000' (say 6200')
3. set it to more than 1013 => it will indicate less than 5000'

My Questions

What happens at this altitude of 5000' if we move out of the ISA?

• if the temperature drops further to, say -2 C?
• if the pressure goes up to 900 hPa?

My premise of "...we set it to less than 1013 => it will indicate more than 0' (say 230')" was wrong all the time! By setting the pressure in the Kollsman window we are effectively "telling" the altimeter at which pressure the 0' elevation is (or what is the pressure at 0' elevation).

If we "tell it" that the 0' is at 1000 hPa, and it is still sensing 1013 hPa as I initially stated, it will indicate only possible altitude - LESS than 0', not more.

• Where is the question? – Eugene Styer Feb 14 '17 at 18:56
• @EugeneStyer Last three lines. – Steve Feb 14 '17 at 18:57
• Eugene Styer - Well, the question is basically - is this all correct? :) If it is, then, there are two sub-questions at the very end of my post. If not, then it's back to the drawing board for me. Thanks! – Alex Starbuck Feb 14 '17 at 18:58
• Forgive me but TLDR. Could you please edit to make it digestible? As it is, I have no idea what your doubt is. – Simon Feb 14 '17 at 19:01
• If you're just setting it, then it's QNH or QFE as directed by ATC, if controlled, or according to jurisdiction which you haven't specified. Sorry, it's probably me being dense but I still don't get what you're asking. – Simon Feb 14 '17 at 19:20

We set it to less than 1013 → it will indicate more than 0'

Above statement is incorrect for all scenarios.

(Source)

Remember, the anticlockwise rotation of the knob to set lower than 1013 also corresponds to anticlockwise rotation of the altimeter hands.

(Source)

Above image is what you're after.

Above the transition altitude (varies by location) you set standard setting regardless of the ISA deviation.

Below the transition altitude (TA) or when descending through the transition level (TL), you set whatever the ATC advises. From a pilot's perspective, it's that simple.

Unless you will be performing an approach in a cold area (usually below 0°C/32°F), then special altimetry equipment may be required. Requirements will be noted on the approach plate.

Related:

• You might also add that in extremely cold temperatures you have to make an additional adjustment on top of the altimeter reading. See aviation.stackexchange.com/q/22478/8730 – TomMcW Feb 14 '17 at 20:54
• Got it! If I am at 0', the measured pressure is 1013 and I set the altimeter to 1000: altimeter will "think" (sense, measure) that 0' is at 1000 hPa pressure level but at the same time it will sense higher pressure of 1013. Therefore, it will indicate LOWER than 0'. Vice versa is also true. Many thanks! – Alex Starbuck Feb 14 '17 at 20:59

What happens at this altitude of 5000' if we move out of the ISA?

if the temperature drops further to, say -2 C?

if the pressure goes up to 900 hPa?

If we summarize this as saying simply, what happens if the pressure changes en route (as I think that is what you are asking). The answer is simply, change your altimeter setting! As a matter of fact generally when flying if you are getting flight following or talking to a controller in any kind they will give you the local altimeter setting when you approach the area. Your read back of this setting is required and important for this very reason. If you are not on with control, you should be picking up local weather from airports along your route to ensure a proper local altimeter setting.

You are correct that all altimeters are calibrated for ISA and variance in these conditions will lead to an incorrect reading. In some cases you may need to apply a correction. You can find a comprehensive guide on how to do so here.

When To Apply Corrections

When the aerodrome temperature is 0°C or colder, the temperature error correction must be added to:

• DH/DA or MDH/MDA and step-down fixes inside the final approach fix (FAF).

• All low altitude approach procedure altitudes in mountainous regions (terrain of 3000 ft AMSL or higher)

If you are at an airport you should set the altimeter to the current field altimeter setting as per the ATIS. The ATIS generally is updated frequently or if a condition change warrants it. In some cases at a towered field the tower may update you in real time if the current altimeter varies from the ATIS. If none of these tools are present to you, and there is no local weather to draw from you should set your altimeter to the known local field elevation (as per your chart or AF/D) prior to departure. This should be done before every flight. If the conditions change, so should your altimeter setting.

• Dave, thanks for the comment but actually this is a "step after" my question. I am still not flying from area to area, but rather waiting at an airport, trying to set my instrument correctly. :) – Alex Starbuck Feb 14 '17 at 19:11