B744 gps alt
 

I was reading an incident report the other day involving a series of system failures, resulting in severe degradation of the pitot static system on a B744, the problems affected all three ADC’s and the standby ALT. Not a good day out! However after a few busy minutes, and some great work by the crew the aircraft was landed safely.
My question is this, does anyone, cockpit or engineer, know how to access the altitude computed by the GPS system on the B744? I flew the bus for a few years and it is available on that FMC, but not, as far as I have been able to find, on the Boeing. I have also tried, unsuccessfully to find it in the CMC/ACMS, large amounts of info are available if you know how to ask for it, SAT,TAT,TAS etc so I though GPS ALT may also be hiding in the system somewhere too?
Before anyone starts jumping up and down, I’m fully aware of the limitations on the accuracy of GPS computed ALT. However, given that it’s completely independent of the pitot/static system, it could provide useful info to keep you out of terrain/sea.
Anyone got any ideas?

Cheers Buggs

I think you access it through the FMC. Try the Pos Ref page, it may be on page 2 or 3 but I've definitely seen it without going into CMC/ACMS.

I had asked a question along these lines a couple of months back. The best I can determine is the data is not accessable in the B777. As I was told, that's why we call it VNAV baro. I'll be watching to see if you come up with something better!

We had an indicated GPS altitiude displayed on the Honeywell/Trimbel 9100T GPS which was equipped on our 727s. The altitude read out was "INFORMATION ONLY", not accurate for altitude reference since there was no baro input to our GPSs. According to the Techies at Honeywell, the altitude error/accuracy was equal to that of any lateral error(s).

Greetings
GPS altitude is equal to your true altitude corrected for geodic. :ok:

On our -400s there is no way of accessing this info via the FMC but I think it may be possible via the CMC/ACMS. I seem to remember getting the GPS time from there a while back (Hrs:min:sec, not the time shown on the POS REF page). Maybe the GPS altitude is in the same group of pages somewhere.

My next flight is on the 5th Jan so I'll have a play and let you know what I find.

Happy new year to all my readers (all 3 of them)!

Eck