How does the altimeter work?

[dudeman17]

Hello all. When flying recently in a mission I created I frequently ran into an issue where I said to a friend to meet at 30,000 ft only to find that he (not in a hornet btw) would always be way above me. After a minute and a few sanity checks. I determined that there indeed was an issue. according to my hud, I was almost 2000 ft off of what the f10 map said I was at. for the sake of the mission included in this post here are facts regarding the mission.

 

Caucasus

OAT -12C

Pressure setting 30.22

On Baro A/P with an altimeter setting of 29.92 leveled off at 25,660 according to hud on f10 shows 23,393. when I input the mission created altimeter setting. (i know, above fl180 it should be standard but I figured id try it.) so with the altimeter set to 30.22 at f10 23,393 Hud indicates 25,940.

Persists in both mp and sp and other hornet pilots on the mission have reported the same issue.

 

Let me know if any more information is requested. happy to help solve any issues I can.

altitude bug.trk

Supercoolrealthingbackup .miz

dcs.txt

[JNelson]

This may or may not be a bug. It may be to do with how real altimeters guess altitude based on ISA. Whatever it is it has been in the game for a while on all modules.

[dudeman17]

That's the thing though. Shouldn't the guy next to me also be reporting the same altitude when in say an f-15 or perhaps wouldn't the tanker also report an altitude for rejoin that isn't 2000 off of what I have indicated?

[Ramsay]

Shouldn't the guy next to me also be reporting the same altitude when in say an f-15

No, FC3 aircraft have different system modelling, so just like their (magnetic) compasses read True bearings (F10 map), their altimeters are also simplified and won't match full modules that take more factors into account.

... or perhaps wouldn't the tanker also report an altitude for rejoin that isn't 2000 off of what I have indicated?

IIRC the AI will fly to the height set in the mission editor, rather than standard ISO (760mmHg, 29.92inHg, 1013mb) or other settings.

 

So with OAT -12C and 30.22 inHg at sea level, your altimeter will read differently to the F10 map, if you use 'standard' pressure.

Edited October 22, 2018 by Ramsay

[mvsgas]

I do not have the F/A-18 module, does the pitot heat come on by itself or do you have to turn it on?

[Chuck_Henry]

I do not have the F/A-18 module, does the pitot heat come on by itself or do you have to turn it on?

 

The pitot heat has two settings - ON and AUTO. You shouldn't have to touch it if the automatic function is doing its job, but if you get an INLET ICE caution, then you should set both Engine Anti-Ice and Pitot Anti-Ice to ON.

[Frederf]

Analysis.

Editor settings

SLT -12.00 C

QFF 767 mmHg (30.20'')

 

Vaziani (original mission)

1531'

-15.04C

QFE 28.395'' (ATC 28.41'')

QNH 30.01''

PA 1446.3'

 

Vaziani (editor set to +15C standard)

+11.96C

PA 1286.9'

 

 

First let's note some features straight away: The temperature set in the editor is the sea level temperature (SLT), not the one locally at Vaziani. For both SLTs Vaziani's local temperature is -3.04C compared to sea level which is simply due to the elevation of Vaziani. This is a normal amount of ~2C/1000'.

 

We also note that the pressure altitude for the two situations is not the same despite the fact that the only change was temperature. The reason is that cold air is more compact as an air column, changing pressure more rapidly per vertical distance than the standard model. The air pressure at sea level was 30.20 inHg in both situations but the actual local air pressure at Vaziani was not.

 

Take four pilots at sea level, Batumi, and Vaziani, and 30,000' and see what altimeter setting they need for their altimeters to read their actual altitude.

Sea level - QNH (QFF) of 30.20''

Batumi - QNH of 30.19''

Vaziani - QNH of 30.01''

30,000' - ~26.5'' (can't crank the instrument low enough, estimated)

 

The proper thing to ask if someone says "This is the QNH" is to ask "Where is that the QNH?" Every elevation has a different QNH even if the atmosphere is uniform. Setting QNH for your elevation only guarantees that the altimeter will show true altitude at that elevation. Above or below that elevation it's not true. The difference grows proportional to the difference in elevation from the calibrated level, how non-standard the pressure is, and how non-standard the temperature is. This isn't a DCS issue but simply how altimeters are.

 

So by the time you get up to 30,000' the pressure is much less than if you had climbed through a standard air column. Remember the cold air column is more compact so you went through more pressure difference climbing to 30,000' than usual.

 

The F/A-18 in particular gets its barometric altitude from the ADC which applies some minor corrections but otherwise is not significantly in principle to the typical altimeter. While the ADC may have a calculated altitude which compensates for such errors, this is not displayed on the HUD (at least in navigation mode) as it would be dangerous to fly around using a different method than other airplanes.

 

For comparison here is what the altimeter reads in various scenarios:

SLT standard

SLP 29.92'' (760 Torr is 29.9213 so not exactly 29.92)

30,009' true = 30098' indicated

+89'

 

SLT standard

SLP 30.20''

30,000 = 30,086

+86'

 

SLT -12C

SLP standard

30,001' = 32,934'

+2933'

 

SLT -12C

SLP 30.20''

29,999 = 32939' (+9.8%)

+2940'

 

The pressure change is pretty much linear with altimeter setting (0.28'' being 200-240' but the temperature effect is huge. It's real to have a ~10% misreading for -27C temperature anomaly. Try here: http://www.luizmonteiro.com/Learning_Alt_Errors_Sim.aspx

 

That's the thing though. Shouldn't the guy next to me also be reporting the same altitude when in say an f-15 or perhaps wouldn't the tanker also report an altitude for rejoin that isn't 2000 off of what I have indicated?

AI do not understand altimetery and have no instruments. They always follow and tell their "F2/F10" number which is their perfect true altitude.

[dudeman17]

So it appears you all are correct. I loaded up the mission and proceded to check three other modules. (AV-8B, F-5, A-10) I was able to observe this deviation on both the f-5 and the hog. however, the harrier does not have it. I can't believe after the thousands of hours it isn't something I've noticed before.

 

So then I guess what I would hope for is that there is an effort to make sure this is consistent across all playable DCS aircraft fc3 included. and ideally, the tankers and everything could be included in that in order to add just that extra bit of realism.

 

Thanks for the help and explaining that to me.