Why are these altitudes different?

Can someone explain why the altitudes on the 2 altimeters are different on this image? Both are QFE 1012.

The qfe setting is not in steps, but like a slider. The left one might be at 1012,5 and the right one at 1012,2 for example

Gesendet von meinem SM-G960F mit Tapatalk

Also tried to find some answers to this a while back in this thread.

https://forums.eagle.ru/showthread.php?t=199856

I have also tried to calculate the QFE and QNH. Start at one airport and land on another with a different altitude from sea level.

When using different aircraft calculators online I can never get the same result as the aircraft says when I tuchdown on the other airport.

The qfe setting is not in steps, but like a slider. The left one might be at 1012,5 and the right one at 1012,2 for example

 

Gesendet von meinem SM-G960F mit Tapatalk

Thanks, but I'm sure it's not that. See another image attached. By making the heights the same, the QFEs are hugely different, i.e. 1049 compared to 1012.

There must be an explanation for this.

Aren't you supposed to tune them both to zero meters when on the ground? Did you do that?

Aren't you supposed to tune them both to zero meters when on the ground? Did you do that?

That is what I did in my post.

It doesn´t work. They still starts to differ at altitude.

Aren't you supposed to tune them both to zero meters when on the ground? Did you do that?

Yep. See attached. First on ground zeroed, second at altitude. Very different readings on the 2 altimeters.

Could it be a feature? Both gauges use different sensors, they could, at that altitude, show different readings. This could be modelled, but I do not know if it is in fact so in the Viggen module. And even if it is modelled, is the difference representative?

Could it be a feature? Both gauges use different sensors, they could, at that altitude, show different readings. This could be modelled, but I do not know if it is in fact so in the Viggen module. And even if it is modelled, is the difference representative?

It could be, but like a lot of questions here, it would be nice to get an answer from the devs.

What about when you land, are they both back to 0?

What about when you land, are they both back to 0?

Good question. Yes, they are both back at zero.

Is it possible the backup is just less sensitive and gives a bigger error the higher you go?

Is it possible the backup is just less sensitive and gives a bigger error the higher you go?

Yes, that’s possible. There are lots of possibilities. It would be useful to get the definitive answer from the devs.

I think this is because the main altimeter is controlled by the CK37 computer. It calculates the altitude above ground, at the airfield, or target height. The standby altimeter is just a dumb air pressure gauge.

I think this is because the main altimeter is controlled by the CK37 computer. It calculates the altitude above ground, at the airfield, or target height. The standby altimeter is just a dumb air pressure gauge.

Really? Afaik the altimeter always shows barometric altitude. The hud is a different story though.

Really? Afaik the altimeter always shows barometric altitude. The hud is a different story though.

Like I said, I’m not sure. But going by the different info about the main vs. stby altimeters in the manual...

Isn't this explained in the user manual?

Isn't this explained in the user manual?

Can you point us to where?

That was a question. I have the Viggen Module, but since I didn't come yet to really flying it (Despite some just flying around), I haven't read the manuals yet.

But from reading the other manuals --> Those gauges are explained usually?

Like I said, I’m not sure. But going by the different info about the main vs. stby altimeters in the manual...

Maybe I’m missing something, but I can only see the manual stating that both are barometric pressure. No other differences stated afaics.

I may have misinterpreted this sentence «The aircraft uses QFE (altimeter pressure setting calibrated to the airfield or ground elevation), which is used both for instrument landing and aiming calculations for the weapon system.» as that the indicated altitude is the same that is used for TILS and weapons calculations.

Also, the real aircraft manual makes a distinction in branding of the main altimeter as ”Höjd Indikator” (Altitude Indicator) and ”Reservhöjdmätare” (Reserve Altitude meter, or altimeter). The difference being that an indicator usually displays a calibrated or calculated signal, and a meter just displays the measured signal. But, I can find no evidence that the main altimeter receives any signal, so I’m probably wrong here.

Ony guessing here, but could it be that the static ports for the two are located at different places with the main one at the most beneficial place and the secondary at a more protected plase (more protected from weather/icing and hostile fire)?

I guess the reserve gauge is basically only for getting a sufficiently good indication to bring the bird down safely at an landing site (airdrome or road base) with known QFE? Then if it shows same reading as the main one at low altitudes/ground it would be sufficient.

Another thought regarding the possibility the main indicator being calibrated by the CK37. If so, it will give different readings than other aircrafts without the calibration, so to keep safe vertical separation, especially during peace time (Sweden have not been in combat on own soil for over 200 years), the pilot needs a dumb, purely pressure based indicator.

Edit:

Stupid me used "pitot probe" instead of "static port". Corrected.

Also, to add to my post above, many aircrafts has their alternate static port inside the cockpit if not pressurised. The cockpit in most fighters, Viggen included, doesn't have pressurised cockpits.

With this configuration the pressure altimeter will read a higher altitude at the same QFE/QNH compared to when using the main static port due to suction effects when up in speed. If the backup altimeter in the Viggen has its static source from inside the cockpit (or elsewhere inside the shell), that would fit the observation done here, with a higher reading on the backup altimeter due to speed (i.e. not realy due to altitude, even if the altitude might have an effect on size of the difference at the same speed).

So I have opened the AJS37 Manual located in my aircraft folder (Install directory --> \DCSWorld\Mods\aircraft\AJS37\Doc) - It states the following regarding the first altimeter on Page 29:

"Indicates barometric altitude in metres. A pressure setting dial is located on the bottom with a corresponding scale. The pressure is for hectopascal (hPa)..."

Further on it tells you to set the standard pressure, that the aircraft uses QFE (pressure calibrated to the airfield or ground elevation).

On Page 35 (Backup flight instruments) it says basically the same thing.

Here you can also set the pressure in hPa seperatly for the 2nd altimeter (Maybe useful for missions - main is for your home airport, 2nd is for the bombing place).

For the 2nd Altimeter there isnt anything mentioned about special technical specifications, how it measures them - So either it does not the same as the first, or maybe it does, it just isn't mentioned a 2nd time?

The screenshot in your first post shows clearly, that you have set the same pressure for the main and backup altimeter... :huh:

Aren't you supposed to tune them both to zero meters when on the ground? Did you do that?

Just air bus drivers do that...

But the issue reminds me of the different altitudes shown in the Hornet.