Temperature Effect on Altitude

[Jugs08]

Hi Guys, Im doing my ATPL's and it says that as the temperature drops you have to add a temperature correction e.g at -10 the decision altitude is 2050ft when the airfield elevation is 1500ft and you require a 500ft decision altitude.

If the air is colder is it surly more dense and that means the altitude will under read eg. 800ft indicated is actually 1000ft. But it thinks its lower due to the temperature. Cant get my head round this so if anyone can simplify it it would be great!

[paco]

It is only really significant when it gets down to -16C (in Canada, anyway).

For example, at -20°C at 500 feet, your altimeter should be reading 570 ft to ensure a height above ground of 500 ft. Just remember that, when temperatures are less than ISA, you will be lower than the altimeter reading, so you must add a correction.

[FlyingStone]

You are thinking in the right direction. Since colder air is also denser, pressure layers within the atmosphere tend to be closer to each other, which basically means that instead of 30ft/hPa (near MSL), you could get 20ft/hPa (very emphasized, I'm not in the mood to do calculations, but I think it's too much for a real case). This basically means that 850 hPa pressure layer is located lower in cold air than in ISA conditions. And since your altimeter is just a pressure sensor, which has a calibrated scale in feet (or meters some times), it senses the correct pressure, but since it's calibrated for ISA temperature, it displays the wrong (higher) indicated altitude. The same thing goes for warmer-than-ISA air, which has a tendency to increase the distance between pressure layers and you get let's say 40ft/hPa instead of 30ft/hPa, which means that 850 hPa pressure layer is actually higher in warmer air than in ISA conditions. If you're looking for an easy way to remember this, just think of piece of metal or anything similar: if you heat it, it's length increases and if you put it in a freezer, it shrinks :)

The practical issue is, we never correct altitudes for temperatures greater than ISA, but just for lower than ISA, since by flying higher than our altimeter says, we are on the safe side (obstacle clearance increases), while we have to correct for temperatures lower than ISA, since if it's very cold, our obstacle clearance could converge to zero and we would have a CFIT with altimeter indicating "safe" altitude.

[Jugs08]

thanks paco and flying stone.
that's really helped get my head round it rather than just accept that colder the altimeter will over read and warmer the altimeter under reads. Links in nicely with the High to low down you go aswell explanation of the pressure layers. That was driving me crazy.

Ty for your time.

[keith williams]

You could also visualise the situation by imagining a column of air extending upwards from the surface of the earth. There is a certain mass of air in the column and your aircraft is in the comumn at some fixed height above the earth.

The static pressure around your aircraft is determined by the mass of air that is in the comun above it. Your altimeter indication is determined by that static pressure.

If the air suddenly gets colder in will contract. Gravity is pulling it down, so more air will sink down into the part of the column that is below you. This means that there will be less air above you, so your static pressure will decrease. This will cause your altimeter to indicate a higher altitude.

If the air gets hotter it expands. It cannot expand downwards because the earth is in the way. It cannot expand sideways because other air is in the way. So it expands upwards. You now have a greater mass of air above you. So your static pressure increases and your indicated altitude decreases.

[kaptn]

does this apply when you have a tempereature inversion?

[MarkerInbound]

Depends how high the inversion is. You are really just concerned about the last couple thousand feet. The normal guidance is to apply the correction to altitudes inside the final approach fix.

Here's a short powerpoint from the US Air Force.

http://44rf.com/misc/USAF_AIS_Cold_WX_Altimeter.ppt

[bfisk]

The normal guidance is to apply the correction to altitudes inside the final approach fix.



That might put you in some real uncomfortable situations in some parts of the world. Temp -25C (that's 40 below ISA = 16% addition) and MSA of say 7100ft (in a place I go to alot) gives you a lot to think about.


Remember: correcting for low temps, low pressure and strong winds is the responsibility of the flight crew. These procedures were not made up to make your life harder.