CAP→Alt for OA: where does the reading come from?

[Karon]

As per title, where does the altitude reading on the TID for Own Aircraft Altitude (from the CAP, button 4) come from? I changed both altimeters' pressure and turned off the RadAlt but they have no effect. It perfectly matches the value displayed by the bar at the bottom of the screen in the external views and it works at any altitude (I tested up to 51600ft).

 

Also, in Active Pause the GS doesn't stop. It probably is another odd interaction between avionics and active pause (same as the INS looks like). I left the F-14 go and it stops at 3600kts. See the pic below:

[Larkis]

Do you have a, DataLink established?

[Karon]

No. Initially I thought that was the source since I use that function especially in CV ops. That screenshot is from a clean mission with nothing else except myself.

[Spiceman]

Yeah, that’s a bug, they should match. The altimeter is driven by the CADC unless it’s in STBY mode, where it acts like a normal barometric altimeter like in your basic Cessna. Did you enter your own ship altitude during alignment, or was this an air start? The CADC should be driving the altimeter to read the same altitude that it’s sending digitally to the AWG-9/TID.

[QuiGon]

I would have thought the altitude shown on the TID comes from the INS, which just needs a start altitude (the one you enter during startup) to update it from there on, based on the aircraft's movement, just like it updates your horizontal posuition. I can imagine though that it also uses input from the CADC as Spiceman said, to prevent it from drifting off.

It's a really good question and what I just said is just some guesswork, so I would be interested as well how this actually works and if this is modeled correctly in the DCS Tomcat or if there is a bug.

Edited July 29, 2019 by QuiGon

[Karon]

Yeah, that’s a bug, they should match. The altimeter is driven by the CADC unless it’s in STBY mode, where it acts like a normal barometric altimeter like in your basic Cessna. Did you enter your own ship altitude during alignment, or was this an air start? The CADC should be driving the altimeter to read the same altitude that it’s sending digitally to the AWG-9/TID.

In the pic's case it's an airstart but I used this reading often during CV operations hence with standard CVA alignment (pre-aligned almost always IIRC).

I thought the reading was coming from the RadAlt initially (I used it only at low altitude) because I had the baro altimeter set to QNH and the two weren't matching. Then I noticed it was working even at higher altitude so I investigated a bit more.

[Super Grover]

If I remember correctly (I would have to dig in the documentation), the CADC doesn't use any altimeter settings (just plain 29.92). Altimeter setting (correction for non-standard pressure) is applied in the altimeter.

[Spiceman]

If I remember correctly (I would have to dig in the documentation), the CADC doesn't use any altimeter settings (just plain 29.92). Altimeter setting (correction for non-standard pressure) is applied in the altimeter.

 

It should use the altimeter setting for the synchro output that drives the altimeter in the cockpit. The digital output that goes to the IFF system to report Mode C to air traffic is pressure altitude referenced to 29.92. I guess the question is which one of those is the one that is shown on the TID? I'm not sure what good pressure altitude would be to the aircrew, other than informational to know what you're reporting to air traffic.

 

EDIT... oh wait, I see what you’re saying, that the synchro output is also pressure altitude and the altimeter makes the adjustment based on the Kollsman setting. That does make sense. So it follows that the altitude on the TID would be pressure altitude.

 

Well now I’m wondering what the F10 map shows since the OP said the TID matches F10.

Edited July 29, 2019 by Spiceman

[Karon]

Good points. I tested with mission default settings. Could it match the reference 29.92? Tomorrow I try with different parameters.

 

 

UPDATE: I changed the QNH and did some other tests. The TID reading is always the same as the F10 map (worst case ±1°).

Edited July 31, 2019 by Karon