Altimeter - Effect of Temperature on Pressure

[IronHog]

Hi,

My questions concerns implementation of the altimeter in the simulation.

For sure the deviation of the pressure from ISA is modeled, but how about temperature?

 

As far as I know the rule of thumb is that for each 10 degress that the outside air temperature is warmer than ISA standard the indicated altitude should be increased by 4% to get the true altitude.

 

So for example:

airfield elevation: 1000ft

temperature: 43 degrees (30 more than ISA at this altitude)

pressure 29.92"Hg (standard see level pressure - I am assuming such a pressure is still possible even having such high tempereture?)

 

Applying the 4% rule of thumb the altimeter at this airfield should read 12% less which gives about 880ft.

 

I am just very curios if this effect is implemented in the simulation altimeter ( I would check it myself but for next 7 days I am thousands miles away from my rig and not knowing this is killing my vacations ;) )

[effte]

This is where it gets tricky. :)

 

If the airfield QNH is 29.92 inHg, then your altimeter will read 1000' at the airfield, regardless of temperature - that's the way QNH works.

 

However, at 20 below ISA, climb to 3000 feet indicated. On an ISA day, that would mean 2000 feet above the airfield. As it is 20 below, you subtract 4 percent of the altitude above your datum for each ten degrees.

 

Your true altitude above the field will be (3000-1000)*0.92, or 1840'. If 3000 was your minimum safe altitude, guaranteeing 1000' of clearance above terrain, you just lost 150' of your safety margin.

 

Normally you never bother when it's warmer. All that does is increase your terrain clearance. As the temperature drops, however, you need to start adding to all altitudes to make sure that you do not impact cumulugranitus.

 

Cheers,

Fred

[y2kiah]

real altimeters don't work this way so why would the altimeter in the sim?

A real altimeter does not correct for non standard temperature or humidity. You are asking for density altitude, but an altimeter reads indicated altitude (height above a datum plane which is set in the kollsman window)

[effte]

A real altimeter is affected by the temperature, and will overread when it is cold. That is why you need to correct for non-standard temperatures.

[Frostiken]

That is why you need to correct for non-standard temperatures.

 

No you don't. The A-10 has temperature probes connected directly to the ADC for this reason. Did you think the OAT reading was just so the pilot can decide if it's nice enough outside to turn the jet into a convertible and let the wind through his hair?

 

Things might change if you're using pure barometer on the analog instruments, but as far as typical flight goes, no, there's no arithmetic you have to do in your head.

Edited March 11, 2012 by Frostiken

[IronHog]

This is where it gets tricky. :)

 

If the airfield QNH is 29.92 inHg, then your altimeter will read 1000' at the airfield, regardless of temperature - that's the way QNH works.

 

However, at 20 below ISA, climb to 3000 feet indicated. On an ISA day, that would mean 2000 feet above the airfield. As it is 20 below, you subtract 4 percent of the altitude above your datum for each ten degrees.

 

Your true altitude above the field will be (3000-1000)*0.92, or 1840'. If 3000 was your minimum safe altitude, guaranteeing 1000' of clearance above terrain, you just lost 150' of your safety margin.

 

Normally you never bother when it's warmer. All that does is increase your terrain clearance. As the temperature drops, however, you need to start adding to all altitudes to make sure that you do not impact cumulugranitus.

 

Cheers,

Fred

 

Great - thanks for the answer - now I am sure I understand it correctly :)

[IronHog]

real altimeters don't work this way so why would the altimeter in the sim?

A real altimeter does not correct for non standard temperature or humidity. You are asking for density altitude, but an altimeter reads indicated altitude (height above a datum plane which is set in the kollsman window)

 

First I need to say that I am a noob in those subjects so my thinking may be wrong here.

 

From my understanding I am definitely not asking about density altitude which is a pressure altitude corrected for non standard temperature (it shows how temperature affects density of the air and by that aircraft performance)

 

What I am asking about is indicated altitude corrected for non standard temperature (true altitude modulo instruments errors) - it shows how temperature affects pressure of the air and

as the pressure is the input value to the altimeter which calculates altitude based on the given pressure and applying standard pressure lap rates to it, the not standard temeprature may cause wrong readings on the gauge

 

What is more if we may have a standard 29.92"Hg despite very hight temperatures it looks like under non standard temperatures the lap rates of pressure are different from ISA ones and that is why the altimeter shows wrong altitude.

[IronHog]

No you don't. The A-10 has temperature probes connected directly to the ADC for this reason. Did you think the OAT reading was just so the pilot can decide if it's nice enough outside to turn the jet into a convertible and let the wind through his hair?

 

Things might change if you're using pure barometer on the analog instruments, but as far as typical flight goes, no, there's no arithmetic you have to do in your head.

 

And this answers my question - it looks like the simulation does not have to implement it as far as concerning the A10 altimeter :D

 

I was just about to ask for the purpose of the temperature reading :)

ADC? OAT? - what they stand for ?

 

So as I understand A-10 altimeter will show true altitude despite non standard temperature because of being supplied with the current free air temperature value at given altitude and then as I assume applying it to ISA temperature lap rates?

[effte]

No you don't. The A-10 has temperature probes connected directly to the ADC for this reason....

 

Now, ponder this: Every aircraft has an altimeter which conforms to the specifications for altimeters, meaning they read the same and are all affected by non-standard atmosphere conditions.

 

This ensures that aircraft flying at 4000 feet on their altimeters are 1000 feet apart from aircraft flying at 5000 feet on their altimeters.

 

It's not exact altitudes above ground under non-standard atmospheric conditions, but the errors are the same for all aircraft so separation is ensured anyway.

 

Now you are suggesting that aircraft with ADCs will have those errors taken out and fly more correct altitudes, as far as height above ground goes.

 

Non-ADC aircraft maintaining their assigned 10,000 feet on a cold day would be at 9000 feet AGL. Then ATC gets an A-10 through the airspace and assigns it to 9000 feet to make sure there's 1000 feet of separation to the other traffic. Only the A-10 has an OAT probe and an ADC so it swiftly corrects and flies 9000 feet AGL...

 

I think you need to try thinking that one through again. It would appear you were unlucky on the first attempt. Must be all that riding in convertibles with the wind through the hair... :music_whistling:

 

If you dig into the literature, you'll find that the OAT isn't exactly without other uses. For one, it enables you to correct your altimeter reading for non-ISA temperatures.

 

There's INS height to use, which would indeed be unaffected. That's not what you will see on the altimeter though.

 

Edit: Last but not least, even with OAT, the ADC don't know how to correct unless it has the reference elevation for the QNH source. And where do you tell the ADC whether you are flying QNH, QFE or QNE?

Edited March 11, 2012 by effte

[Headspace]

The altimeter gauge measures barometric altitude, just like any altimeter. The only difference is that it has a selectable source. Normal operation would mean getting its source from the CADC.

 

You can use the EGI to deconflict pressure altitude from true altitude MSL.

[Kaiza]

Have to agree with effte here, altlmeters are not corrected for OAT, even in digital ADCs. Doing so would cause an air traffic nightmare.

 

Some systems do have the capability to compensate for temperature, however that is only used to pass on to aircraft sensors rather than the altimeter. Some aircraft also have the ability to compensate for temperature on an instrument approach so that the aircraft does not fly below minimum descent altitudes, however this is generally just modern airliners. Most aircraft have a table in their -1 with temperature compensation for OATs below 0. If you look at any BARO-VNAV approach it will have a minimum temperature that the approach can be flown, this is so you do not bust altitudes due to the affect of temperature.

 

As effte said the ADC uses OAT for many other uses, primarily the calculation of TAS.

Edited March 11, 2012 by Kaiza

[IronHog]

I haven't thought about this earlier - but certainly having different altimeter readings on different aircrafts would be a terrible idea so thanks to effete and Kaiza for the clarification :)

 

My question then is still to be answered - is the impact of non-standard temperature correctly modeled in simulation in regards to altimeter readings?

 

Cheers

Luk

[Frostiken]

Why would all errors be the same when temperature varies greatly with altitude? Hell, it varies simply depending on where you are.

Edited March 11, 2012 by Frostiken

[BlueRidgeDx]

No you don't. The A-10 has temperature probes connected directly to the ADC for this reason. Did you think the OAT reading was just so the pilot can decide if it's nice enough outside to turn the jet into a convertible and let the wind through his hair?

 

Things might change if you're using pure barometer on the analog instruments, but as far as typical flight goes, no, there's no arithmetic you have to do in your head.

 

At the risk of piling on, that's just not right.

 

As effte said, seperation is ensured because the altimetry errors in a given airmass are the same for everyone. You can't have some airplanes correcting for errors while others don't.

 

As kaiza said, there most certainly are instances where you need to manually account for altimetry error at low ambient temperatures. For example, approaches using baro-VNAV.

 

The A-10's altimeter is uncorrected. It is normally servo driven by the CADC, but the CADC is feeding it raw Baro altitude sensed by the pitot/static system.

 

The CADC will use the BARO reference set in the altimeter's kollsman window to correct for temperature and other factors, but this data is displayed only on the HUD, and only when in a weapon delivery HUD mode (not NAV or Air-to-Air).

[BlueRidgeDx]

Why would all errors be the same when temperature varies greatly with altitude? Hell, it varies simply depending on where you are.

 

Any two airplanes in danger of running into one another will be experiencing the same local atmospheric conditions and will observe similar errors.

[Yskonyn]

A real altimeter is affected by the temperature, and will overread when it is cold. That is why you need to correct for non-standard temperatures.

 

Indeed as was nicely pointed out in our last proficiency check in the sim. We were doing extreme (cold) weather operations.

Hardly anyone thought about this while flying. ;)

[BluesmanSF]

@BlueRidgeDx

 

Simple, yet thorough answer, thanks for clearing this up. =)

[effte]

Allright, this had to be verified. I set up two missions, one on a very hot summer's day at ISA+30 (45°C on the ground) and one on a cold winter day at ISA-50 (-35°C on the ground). Otherwise, the missions were identical.

 

As I wanted to verify that nothing is awry with the GPS height, I monitored the GPS height and flew right over the top of a significant peak. QNH for Batumi, so essentially no difference between QNH datum and sea level.

 

[TABLE=head]|Hot day|Cold day

GPS height|11160|1150

Altimeter reading|10280|13100

OAT|14°C|-42°C

[/TABLE]

 

Running the hot day figures through the CR-3 gives a true altitude of 11,100'.

 

The cold day figures give 11,500.

 

Very much in the ballpark, I'd say. Done right enough is my verdict. I suspect it will be even better using more exact methods than the CR-3*, which is probably stretched to its limits by those extremes. Well done, ED!

 

It is also very satisfying to see the performance difference. On the hot day, with a rather heavy plane, the far end of the runway at Batumi was approaching at quite a rate by the time I reached lift-off speed. At 3500 feet/200 KIAS I got around 2000 fpm.

 

On the cold day, I had plenty of runway to spare and screamed through 3500 at 5000 fpm. :pilotfly:

 

Cheers,

Fred

 

*) Got that software on the laptop, but didn't feel like fetching it tonight.