Altimeter and temperature

[Speedbird777]

I'm making my way through the private pilot syllabus and would appreciate a some help with altimeter error and temperature. Here's what is confusing me:

1) If I am on the ground at an airport and the temperature is higher than standard conditions, the air will be less dense and so the altimeter will read higher than the true altitude of the airport ie my plane's true altitude will be LOWER than the indicated altitude.

BUT then according to the books..

2) If I am flying along at an indicated altitude of say 1000 ft and fly to an area that has an even higher temperature, but I hold the 1000ft indicated altitude, I will be HIGHER than the indicated altitude (seemingly the opposite of #1).

This confuses me because if I fly to hotter LESS dense air, will I not need to descend to maintain the pressure reading which corresponds to 1000ft and so would actually be LOWER in terms of true altitude?

Thanks in advance......

[Radix]

............

[Speedbird777]

Thanks, I'm just having trouble reconciling the two or should I just consider two separate but related concepts i.e.

1) is the effect of temperature on air density

and

2) is the effect of temperature on change in pressure of x change in altitude

[Mariner9]

With regards to your point 1) Speedbird, an altimeter reads pressure, not density. The air pressure will increase ( and hence altimeter reads lower, ie understating true altitude once temp >ISA) for a given altitude as temperature increases.

With regards to your observed error on the ground Radix, that suggests to me that the given QNH was incorrect

[A and C]

Temp correction

When flying IFR and the temp is zero C or below I always make corrections for decision altitude, MSA and engine out acceleration.

Typically at -10 C the altitude correction will be about 30 ft for an airfield close to sea level.

[Pontius]

Speedbird,

Have a look at this and all will become clear:


Altimetry

[Speedbird777]

Appreciate the replies but still not completely resolved my conundrum. Take this example for instance.

If I take the example one step further and say that I have two airports at exactly the same elevation. Airport A is under standard conditions and is 100ft MSL. Airport B is at 100ft MSL but temp is 25 degrees hotter. I take off from airfield A where my altimeter reads 100ft MSL. I fly at 1000ft to the location of airfield B where the temperature is hotter. I do not change my altimeter setting. According to the theory, at location B, I think I am flying at 1000ft but my altimeter is understating my height and so my plane is HIGHER than the indicated reading on my altimeter. However, if I land at airfield B (100 ft MSL but hotter than airfield A), my altimeter will read higher than 100ft because the air pressure is lower in hotter air (density altitude increases with higher temp).

Thanks again smart people.......

[India Four Two]

Everything you need to know about temperature corrections, from a country where they know what cold means

RAC - 9.0 INSTRUMENT ARRIVAL FLIGHT RULES (IFR) ? ARRIVAL PROCEDURES - Transport Canada

See 9.17

Edited to add:

Going back to the OP's question, this section is particularly well written:
AIR - 1.0 GENERAL INFORMATION - Transport Canada

See 1.5.4.

1.5.4b reads

When the current altimeter setting of an airport is set on the subscale of an altimeter, the only time a pilot can be certain that the altimeter indicates the “true” altitude is when the aircraft is on the ground at that airport.

Note that "altimeter setting" is the North American equivalent of QNH.

[24Carrot]

I'n not sure I understand the difficulty with your two airport scenario.

I'm assuming A and B have the same QNH, so the surface pressure is the same at each place.

With an acccurate QNH set at A, you take off at 100' amsl, climb 900 ft, and now you are at 1000' amsl. Your altimeter reads that too. It has gone from 100 ft standard pressure to 1000 ft standard pressure.

Now you fly to B, using the altimeter to maintain height, i.e. maintaining constant air pressure. As the air beneath you becomes less dense, you need more of it to maintain that pressure, so you slowly climb to, say 1080' amsl. Your altimeter still reads 1000.

Now you land at B, so you descend 980 ft (from 1080' amsl to 100' amsl). Your altimeter measures the increase from 1000 ft standard pressure to 100 ft standard pressure, so it shows 900 ft less, not 980 ft.

The pressure decrease at A equals the pressure increase at B, so the altimeter winds up and down the same amount at each place.

In standard air at A you needed 900 ft to do it, in less dense hot air at B you needed 980ft to do it.

[Lightning Mate]

[Pontius]

Speedbird,

Your fundamental error is assuming the QNH at airfields A & B changes. In the real world of course it's going to but for your theoretical question to work you have to assume they remain the same. Look at Lightning Mate's diagram and just picture those columns of hot and cold air as columns of changing pressure. The rate of change is higher in the cold, dense air but the pressure brought to bear by those columns of air is the same at sea level because the hot air, while less dense, has a taller column (with the same altimeter 'pressure' reading) for all the molecules to whizz around in and exert pressure at the Earth's surface.

[Mariner9]

However, if I land at airfield B (100 ft MSL but hotter than airfield A), my altimeter will read higher than 100ft because the air pressure is lower in hotter air (density altitude increases with higher temp).

That is the bit you are getting wrong Speedbird. An altimeter does not measure density altitude, it measures pressure. Pressure at a given altitude will be higher at a hotter temperature, not lower as you suggest.

If you hadn't changed your altimeter setting from airfield A your altimeter would show less than 100' on the ground at B. The difference however would be tiny with such a low airfield elevation, as the temperature correction is applied to the altitude. In fact your example (rather extreme) 25degC increase would equate to a correction around 10% of the altitude, and thus for your 100 foot AMSL airfield, the correction would only be around 10 feet meaning your altimeter would read 90' on the ground at B.

[Speedbird777]

Mariner,

Thanks, I understand that altimeters read pressure. Pressure, air temperature and density are related though no?

That is the bit you are getting wrong Speedbird. An altimeter does not measure density altitude, it measures pressure. Pressure at a given altitude will be higher at a hotter temperature, not lower as you suggest.

Increased temperature results in lower pressure air which would result in a higher altimeter reading (hence takeoff performance etc is worse on hotter days - less dense air, lower pressure - if I am wrong here then sorry for wasting everyone's time). So if you are sat on the ground and the temperature rises by 25 degrees (for the sake of example) and you do not change your altimeter setting, your altimeter will give you a HIGHER reading i.e. your plane will be LOWER than your altimeter reading. Is this not correct?

Pontius - thanks. I understand that I am somewhat overthinking these and it would be more useful to think about them in a vacuum. I would just like to reconcile the two. From looking a little deeper, it would seem that the impact of temperature on rate of change is significantly greater than it is on altimeter reading at ground level.

[Radu Poenaru]

High temperature does not imply low pressure (actually quite the opposite)
Yes, temp and pressure are related via the ideal gas law, but for most meteorological purposes you have to start to view them separately:

1) High pressure -> High density (good for performance)
2) High temperature -> Low density (not so good for performance)

And leave the temp<->pressure dependencies alone. They will get you into trouble in meteo classes and altimetry problems

[bravobravo74]

So if you are sat on the ground and the temperature rises by 25 degrees (for the sake of example) and you do not change your altimeter setting, your altimeter will give you a HIGHER reading i.e. your plane will be LOWER than your altimeter reading.

If the airfield's elevation is zero and sea-level pressure is set in the altimeter sub-scale, my understanding is that the altimeter will indicate true altitude regardless of the temperature. Temperature errors occur when an aircraft is above mean sea-level.

If an aircraft is above msl and the temperature is higher than standard then the altimeter will under-read (indicate an altitude which is lower than its true altitude). This is essentially a safe error. If however the temperature is lower than standard the altimeter will over-read. This is potentially hazardous for obvious reasons.

[Radu Poenaru]

If an aircraft is above msl and the temperature is higher than standard then the altimeter will under-read

Just being pedantic here, but it might help with the OP's confusion:

"If an aircraft if above REFERENCE ALTITUDE for computed QNH and temperature is higher than standard, then the altimeter will under-read"

The reason for that is pressure error is always corrected in QNH, and when they compute the QNH (as supposed to QFF) they compute is so the temperature error is exactelly zero at aerodrome reference altitude. If you are above/below that altitude, then +/-ISA temps will incur a small error in indicated vs true.

Ok, maybe it won't help much. Try this:

There is a pressure error (air has a diff pressure than ISA), and there is a temperature error (air has a diff temp than ISA). Note that there is NO "density" error in an altimeter (ok, there is, but that's not what we call it).

Pressure error is always corrected by using the correct QNH (and is about 27-30 feet per mb at low altitudes).
Temperature error is only corrected (by using the correct QNH) while at aerodrome elevation (and is about 4% for each 1000 feet above/below for each 10deg above/below ISA).

Still doesn't help?

[phiggsbroadband]

air bubbles dont count?

Hi Guys, I can't see how this works... In all the discussions, 'columns of air' are mentioned. I don't often encounter 'columns of air'.
The most usual thing to meet is a bubble of warm air, such as a thermal. and also another bubble of cold air.. a cloud.

So say I am flying along Straight and Level at 3000ft.(GPS.) If I then enter a warm bubble, then a little later I go through a cold bubble, what would have happened to my altimeter reading?

I don't think it would have changed; The theory only concerns 'columns of air' reaching the ground, and out into space, not air bubbles.

[Radu Poenaru]

What is the size of your "hot air bubble" ?

Is it a (small) thermal column of rising air? Then, there, static pressure will remain the same, temperature increases, density is smaller (why it's rising in the first place). Result is altimeter stays constant, and TAS increases if you are to maintain constant IAS.

If your hot bubble is actually the size of an air mass (ie. passing through a warm/cold front), then it's a different answer altogether.

[Lightning Mate]

What are you guys talking about?

Temperature error cannot be corrected.

[Radu Poenaru]

Originally Posted by Speedbird777

1) If I am on the ground at an airport and the temperature is higher than standard conditions, the air will be less dense and so the altimeter will read higher

Altimeter does not measure density!!, it measures pressure! Static pressure.
And while there is a general connection between temp, pressure and density, you need to get your notions straight first in order to understand these problems

Temperature error cannot be corrected.

Sure it can, they do it all the time when they release a QNH: It is so computed that if you set your altimeter to QNH it will indicate the airport elevation while sitting on the ground. The temp error is 0 at airport elevation and not-zero everywhere else.