Question - Actual Altitude/FL

[Duncan White]

Hi Gergely,

 

I would like to get an understanding of how the "Actual Altitude/FL" displayed in the aircraft tag is worked out by ES.

 

Example 1 : A jet at cruise FL350 above TL with barometer set to STD.

 

Both ES & the plane think they are at FL350. (Happy)

 

Example 2 : The jet forgot to set barometer to STD.

 

Plane thinks its at FL350 & ES reports say 500 feet higher or lower. (Happy)

 

Example 3: The jet has levelled off at 3,000 with barometer set to the destination airport QNH/ALT value.

 

Why would the controller see this plane being at 2,500 feet. Is it because ES is using a barometer value of STD?

(This situation confuses me)

 

Possibly the particular plane I was flying was reporting the altitude incorrectly to VATSIM.

 

Basically I just want to understand how ES derives the "Actual Altitude/FL" above & below the transition level.

Does ES just report the value as p[Mod - Happy Thoughts]ed by the plane connected to VATSIM.

 

Regards,

 

Duncan

[Merik Nanish 1184142]

To the best of my understanding, EuroScope does NOT care about barometric pressure as far as data tags are concerned. It gets one (and only one) altitude data from the pilot's software, and that is the TRUE altitude.

 

If a pilot flies at an indicated altitude of 3,000' and sets their altimeter to the correct value (I will define "correct" in a second), then their client will determine that their TRUE altitude is also 3,000 and will report that on the network; EuroScope receives that value and shows that in the data tag.

 

If a pilot flies at an indicated altitude of 3,000' but their altimeter is incorrectly set such that their true altitude is 2,500' then:

* The pilot sees 3,000' in their virtual cockpit

* The client program, however, knows that the TRUE altitude is 2,500' and reports that on the network

* ES/VRC/other radar client will only receive that 2,500' and will show that in the data tag.

Because of this, the ONLY way you can know that the pilot is using the wrong altimeter is by asking them what altitude they are at ("say altitude") and comparing it with what you see in the data tag.

 

What happens if there is a difference between what the pilot says and what you see? You will tell the pilot to fix their altimeter setting: "(location) altimeter is 29.73, verify altitude". Then the pilot will fix their altimeter and suddenly will notice that their indicated altitude is changing! [Mod - Happy Thoughts]uming that this happens during level flight, they will end up seeing the TRUE altitude of 2,500' on their altimeter once they have corrected the altimeter setting. On VATSIM, most pilots use autopilot so as soon as they turn their altimeter setting knob, the autopilot will climb/descend them to get them back to 3,000' (or whatever altitude it was set to).

 

Now, what is the CORRECT altimeter setting? To make it very clear: it is NOT necessarily what you see in the METAR!

 

IF the client program is simulating real world weather and uses a data source to get the METAR that matches what ES uses to get METARs, then the correct altimeter setting will be the same as what you see in the METAR for that location on ES.

 

IF the client program is simulating real world weather using a non-compatible data source, then there may be differences between what you see as the barometric pressure of that location and what the client software sees. In this (rare) scenario, if you tell the pilot to set their altimeter to what you see (like 29.73) they may still end up at the wrong altitude!

 

IF the client program is not simulating real world weather at all, then most often it uses the default barometric pressure for every location (29.92). In this scenario, as long as the pilot never touches the altimeter settings (leaves it at 29.92) his indicated altitude and true altitude are always the same and you will not notice any issues on your side. Yeah, you think barometric pressure is 29.73, but pilot leaves at 29.92 and everything works!

 

For pilots flying FSX, pressing the B button at any time during the flight will automatically adjust the altimeter setting to what THE SOFTWARE thinks is the correct altimeter setting (whether it is from an external data source, or the default 29.92 value or whatever it is). Therefore, unless the pilot is simulating an altimeter malfunction, all they have to do to make sure their indicate altitude matches their true altitude is to press B.

[Duncan White]

Thanks for the explanation Merik.

 

Duncan

[Michael Pike]

To add to that, pilots should only press B (in FSX) when flying an altitude (ie below the Transition Level). For flight levels they need to have the altimeter set to STD. In Euroscope, the controller sets their Transition Altitude and (I believe) the pilot clients send the 'actual' altitude and the standard pressure altitude, calculated be reference to the air pressure in the sim. Euroscope displays whichever of these two figures is appropriate based on the controller's TA.

[Harry Sugden]

I've sometimes wondered how this works too! When aircraft are at the transition altitude I've set in EuroScope, sometimes the readout proves interesting...

 

For example, in the London TMA the TA is 6000 feet, so I set it to 060 in ES. When I've then cleared an aircraft to A60, sometimes their tag displays, for example 063 - but then, if I change the TA in EuroScope to 065, their tag will indicate their actual level as A60 (as cleared). I've told someone off for not having the correct altimeter setting before - but I make sure I double check now!

[Michael Pike]

If they are a foot below your set TA, it displays alt. If a foot above, it displays FL. I always set TA 500ft above actual.