Possible Bug? Altitude Discrepency Between HUD and Game Engine

[Sabre-TLA]

In our F-15C Basic Flight Training Qualification Campaign - mission BFT10 - Instrument Flight Plan Navigation, a user reported that they continually got an Altitude Failure at the 5th WP when flying at the correct altitude (12,000').

 

We have determined that there is a discrepancy between the HUD and Altitude indicator displayed values and what the game engine is determining as the aircrafts altitude. It's off by a few hundred feet but it's enough to cause our trigger to fail.

 

For example our mission triggers are detecting +/- 300' from 12,000' and if you fly with your HUD or Altitude indicator at exactly 12,000' the game engine is actually registering your aircraft higher than 12,300' and that puts you outside the success trigger zone resulting in an Altitude Failure call.

 

HUD Altitude Value:

 

 

Game Altitude Value:

 

The short term fix is to fly to WP 5 at about 11,800' altitude instead of exactly at 12,000'.

 

We don't know if this is effecting other campaigns (A-10A, A-10C or MiG-29) but we will be testing those to find out.

 

In the mean time, just fly slightly below 12,000' and you should be able to pass the mission.

 

The other WP's in the mission don't exhibit the same issue.

 

The failure triggers for this WP are testing UNIT ALTITUDE HIGHER THAN 3796 OR UNIT ALTITUDE LOWER THAN 3611 which should equate to higher than 12454' or lower than 11847'.

 

ED is this a known issue? Can it be fixed?

Edited June 28, 2015 by Sabre-TLA

[DarkFire]

Given how professionally designed these missions are, I'm sure you probably thought if it, but could that be the difference between MSL and AGL altitude? Or possibly a difference between default & pressure calibrated altitude?

[Sabre-TLA]

That's a good suggestion. I'll try changing the setting in the ME for that particular WP and see if it makes a difference.

 

I'd like to keep the trigger range the same if possible but the other option is to just make the trigger range bigger to account for the difference.

 

In the A-10C you can use the Pressure Set Knob to set a barometric altitude manually but in the FC3 Aircraft I believe they are all defaulted to a standard setting.

 

For mission designers the trick is in understanding what reading the Pilot is seeing on their instruments and what reading the game engine is actually using. It's much easier if they align. :thumbup:

 

Edited - I just checked and the WP is actually at 6,000' MSL as we want the players to fail if they don't follow the Flight Examiner instructions and change altitude to 12,000'. :doh:

Edited June 28, 2015 by Sabre-TLA

[DarkFire]

The Su-25T and Su-27 allow the default pressure setting to be changed in-cockpit. Not sure if the F-15 provides the ability to do this but I think that the plan is that they all will eventually. That being said, I'd be surprised if it made that much difference, unless your mission uses conditions that are very different from a 'standard' DCS day...

[Beagle One]

The Su-25T and Su-27 allow the default pressure setting to be changed in-cockpit. Not sure if the F-15 provides the ability to do this but I think that the plan is that they all will eventually. That being said, I'd be surprised if it made that much difference, unless your mission uses conditions that are very different from a 'standard' DCS day...

In mission that use dynamic wether you can easily have 100-300 meter missalignment during a single flight if you never set the right presure values you get from ATC at take off and before landing. Yes the F-15 has a working pressure dial like the Su-25 Su-25T and Su-27S The differdet atmospgeric pressure and temperature zones on a map with dynamic weather also affect general aircraft aerodynamics and engine performance.
Edited June 28, 2015 by Beagle One

[Sabre-TLA]

Thanks for the replies.

 

Unfortunately the F-15 doesn't allow for any pressure setting changes (at least nothing I could find listed in the controls menu). Looking forward to the day that it does (and a clickable cockpit too!).

 

The mission is using standard summer weather so there shouldn't be any weather effects in play either.

 

I know the devs are busy with DCS 2.0 so wouldn't want to take them away from that but I wanted to report it anyway so they and other users would be aware.

 

I think I'll adjust the triggers to accept a bigger range and retest it again when DCS 2.0 comes out. :thumbup:

[DarkFire]

Thanks for the replies.

 

Unfortunately the F-15 doesn't allow for any pressure setting changes (at least nothing I could find listed in the controls menu). Looking forward to the day that it does (and a clickable cockpit too!).

 

The mission is using standard summer weather so there shouldn't be any weather effects in play either.

 

I know the devs are busy with DCS 2.0 so wouldn't want to take them away from that but I wanted to report it anyway so they and other users would be aware.

 

I think I'll adjust the triggers to accept a bigger range and retest it again when DCS 2.0 comes out. :thumbup:

 

If your missions are using a 'standard' DCS day then the QFE differential probably isn't the problem. Long story cut short I did a ~600Km flight recently whilst trying for the Su-27 altitude & speed personal best thread and over the course of the entire ground track measured a difference of only 0.25% in the QFE between the origin & destination airfields.

 

Unless the altimeter in the F-15 is set to some weird default value. To be honest I haven't logged enough hours (minutes really) in it to notice.

 

This is strange. I'll have to take my 27 for a spin to see if I can replicate the problem.

[Ironhand]

It's not in just the F-15 that there's a discrepancy between the cockpit and info bar baro altitude readings. It's probably there for all aircraft.

 

Using the ME's default weather settings:

 

For example, on the runway at Beslan in the Su-27 your baro altimeter reads 540, while the info bar reads 542. At 4020m in the cockpit you're seeing 4092 on the info bar. 7000m cockpit, 7124m info bar; 14,000m cockpit, 14420 info bar. So the difference is more noticeable the higher you fly. And, FWIW, the ratio between the two gets larger as well. At 540m, the difference is 0.99630, while at 14000m, the difference is 0.97087.

 

Don't know if this is a bug or if the two are actually reporting different things.

Edited June 30, 2015 by Ironhand

[Svend_Dellepude]

There is something going on with the altimeter.

When you transition through around 37.000 ft the altimeter makes a little jump.

Could be related.

[DarkFire]

As Ironhand noted, the Su-27 does exactly the same thing, and the error percentage appears to increase with increasing altitude. Very strange indeed.

[Izaiak]

Hello,

 

Sorry to answer late,

 

1st of all i tough :

 

In the F15 you alitmer setting is 29.92 (=1013 Hpa).

On your HUD you are AT FL120.

If you were using a QNH you would read 12500 feet ( or 11500 ft, depend on the weather condition of the day ).

 

But after testing it, i wounder if it is just that like in reality the altimeter is less accurate with altitude. But here it is a real bad altimer !!

 

 

 

I think the altitude in F2 view is the QNH altitude. But in the f15 i don't know which setting we are using.

[=4c=Nikola]

Per my test, F15 altitude is always QNH. I'm not sure how well atmosphere is modeled and how well altimeter is modeled so cannot comment about difference between F2 and altimeter.

[DarkFire]

For information, Tacview reports the same altitudes as the F2 view does, which I think are the true altitudes ASL as opposed to the cockpit which displays pressure-calibrated altitude i.e. QNH.

[winz]

I think this is not a bug, barometric altimeter are only accure in ICAO standard atmosphere. If you're not flying in standard atmosphere, then there will be deviation.

The standard weather in DCS doesn't match the ICAO standard atmosphere, i.e. the starting temperature is 20 degrees, that will cause the altimeter read less, then is the actuall atlitude.

Flying in cold condition will cause the altimeter to read more than is the actual altitude, this is an issue in real aviation and is something the pilot must be aware of when he's flying an approach in very cold weather.

Edited October 17, 2015 by winz

[Acidictadpole]

I think this is not a bug, barometric altimeter are only accure in ICAO standard atmosphere. If you're not flying in standard atmosphere, then there will be deviation.

The standard weather in DCS doesn't match the ICAO standard atmosphere, i.e. the starting temperature is 20 degrees, that will cause the altimeter read less, then is the actuall atlitude.

Flying in cold condition will cause the altimeter to read more than is the actual altitude, this is an issue in real aviation and is something the pilot must be aware of when he's flying an approach in very cold weather.

 

Kind of interesting, but makes sense. Does this mean that the variation in altitude we're seeing might be affected by what season the flight is in? If not, then it might still be a different problem.

[Spt]

Same thing here : http://forums.eagle.ru/showthread.php?t=134272

[SinusoidDelta]

I think there is something unintended going on here. The cockpit altimeter error gets much, much larger with altitude. At extreme altitudes the difference between the F10 map and altimter reading can be over 20,000 ft at the end of a zoom climb.

[Spt]

I think we need a new input command to increase or decrease the pressure ( altimeter setting ) ;)

[DarkFire]

I think this is not a bug, barometric altimeter are only accure in ICAO standard atmosphere. If you're not flying in standard atmosphere, then there will be deviation.

The standard weather in DCS doesn't match the ICAO standard atmosphere, i.e. the starting temperature is 20 degrees, that will cause the altimeter read less, then is the actuall atlitude.

Flying in cold condition will cause the altimeter to read more than is the actual altitude, this is an issue in real aviation and is something the pilot must be aware of when he's flying an approach in very cold weather.

 

This is quite possible, but do we know if DCS models its atmosphere on the standard ICAO atmosphere, or a modified one that includes humidity for example? I noticed that aircraft ram air speed instruments are calibrated for sea level in an ICAO atmosphere, but I wonder if the Russian ones conform to this standard? Apologies, I know this is an F-15 thread but it's interesting to consider the differences there might be.

 

As an aside, anyone who uses the QFE calibration capability in the Su-27 or Su-25T will know exactly how useful this is for all sorts of things such as more accurate IAS values and more accurate barometric altitude values so in that respect I would totally support it's inclusion in the DCS F-15C. Surely the real F-15 has this capability?