MSL altitude from available instruments onboard?

[IrLED]

Hi

 

Is there the way to determine true (MSL) altitude of my plane? not F10 map way.

 

Possible sources of such information are

 

1.Barometric: Altimeter (DCS A10C Flight Manual EN.pdf, p.124)- reading depends on pressure level set

 

2. CDU SYSTEM/CADC page (p.216)

Pressure Altitude (P ALT), L4 - MSL altitude for uniform pressure 29.92 inHg

Barometric Altitude (B ALT), L5 - digital value of the altimeter (see #1)

 

3.CDU POSINFO page (p.178) GPS Altitude (G ALT), L9 - might be reliable source but the value is somehow offsetted (it is zeroed to airfield you start at, i didn't check its value if you spawn airborne)

 

The only reliable source of the altitude i see is G ALT (if corresponding subsystems function). But that G ALT offset IS the problem. Can i affect it? Or can i get my MSL altitude the other way?

 

Thanks

 

P.S. What it's all for you might ask. I'm testing myself with navigational challenges. The next one in mind is landing in difficult weather conditions (HUD turned off, night, fog, uneven pressure) at one of the abandoned airfields (GPS 3D-NAV glideslope) - i.e. masochistic fun :joystick:. There is no ILS, PAPI, ACT/QFE value for that field to set the altimeter with. Resulting barometric altitude and waypoint elevation (CDU) affect ADI horisontal needle and if values don't correspond - fail.

Edited June 27, 2014 by IrLED

[PFunk1606688187]

Well considering its extremely unlikely for an aircraft to be landing somewhere where they don't at least have an ATIS to give them local pressure, onboard instruments to determine local Baro pressure directly aren't usually available (in fact I don't know if any plane would have such capability).

 

That said, as long as you know the elevation of at least one spot on the ground you can use the Radar altimeter to determine the offset from mean sea level pressure and get local Baro from that, ie. set 2992, overfly the airfield you intend on landing at, observe your Baro altimeter relative to your radar altimeter, and change pressure until your Baro alt matches known ground elevation + radar altitude.

[IrLED]

...

That said, as long as you know the elevation of at least one spot on the ground you can use the Radar altimeter to determine the offset from mean sea level pressure and get local Baro from that, ie. set 2992, overfly the airfield you intend on landing at, observe your Baro altimeter relative to your radar altimeter, and change pressure until your Baro alt matches known ground elevation + radar altitude.

 

That "probing" pass might have worked if i had HUD turned on (or maybe there are other ways to read radar alt value from CDU/MFCD).

 

Still most precise would be POSINFO G ALT value with suppressed vertical offset.

[PFunk1606688187]

Another other option is just to go nap of the earth and use that to calibrate your altimeter, with something like a 100ft error.

 

It worth noting that the radar altimeter is built into the aircraft while the GPS sources are external, so not using the former but using the latter would seem... backwards with respect to trying for a stripped down landing experiment, but to each his own.

[IrLED]

After some investigation - looks like a bug

 

Pressing UFC ENT key should give true MSL in XXXXG section according to Flight Manual, section Delta Radar/EGI GPS Altitude Numerics (p. 395)

The second value is the true MSL altitude computed by either adding radar altimeter altitude to steerpoint elevation or using GPS altitude. It is displayed below the delta value as a signed 5-digit numeric followed by an “R” or “G.” Valid range is -1000 to 32767 in 1-foot increments.

 

http://forums.eagle.ru/showthread.php?t=126257

[PFunk1606688187]

Lots of those forgotten CDU pages contain bugs. There is a page that allows you to actually change the time in the CDU, but for whatever reason every interger is multiplied by 2, so changing it forward one hour makes for 2 and 4 hours to 8.

 

I'm fairly certain it will never be fixed. :P