wrong altitude callouts ?

[Dbarnea 10]

Hey guys,

A weird issue, I fly the PMDG 737 NX and the altitude calls are not always precise. I hit the runway on the 10 call, sometimes even on the 20. Doesn't happen on all runways and for those that supports CAT III, it autolands the aircraft smoothly so no problem there.

Any idea ? I have the latest Navigrpah. Where does PMDG take the height from ?

Doron.

[Chock 17,769]

No idea where PMDG are taking the data from for the radar altimeter, but what I can tell you is that the real radar altimeter is very accurate for landing, and by accurate, I mean in the sense that it will give a correct height reading, however, this does not guarantee that a touchdown will be perfect under every circumstance, particularly with ILS approaches, and there's a good reason for that...

The radar altimeter pulses out a conical beam signal underneath the aeroplane; the signal and its return is sent out from an antenna array located near the landing gear on most Boeings, and near the tail in most Airbuses. In both cases, it is calibrated to give the correct height of the main gear above the runway surface when the aircraft is in the proper landing attitude, and it will indeed do that providing it is properly calibrated. Now this might make you think that such a system would mean it would make an autoland perfectly every time, but if you are using the autopilot for an approach, it's worth noting that on many airliner types, the Flight Director and the Autopilot are not updated with information from the radar altimeter, they go off the info they receive from the localiser and glideslope signal.

This might be why you are experiencing what you describe, assuming that is, that the PMDG simulation does replicate the way the real thing operates and the potential issues it can have.

Edited September 30, 2018 by Chock

[FDEdev 955]

5 hours ago, Chock said:

...if you are using the autopilot for an approach, it's worth noting that on many airliner types, the Flight Director and the Autopilot are not updated with information from the radar altimeter, they go off the info they receive from the localiser and glideslope signal.

Not sure which aircraft you are referring to, but the Boeing and Airbus systems I know are all phasing out the GS signal at very low altitude.

The phase out usually begins at around 150-200ft AFAIR and below approx. 30-50ft the GS signal is being completely disregarded and the AP/AT system relies only on the RA.

Since automatic thrust reduction, align, and flare are tied to the RA it's essential that this instrument is very precise. You definitely shouldn't hear a 10 or even a 20 callout at touchdown!   

[eslader 1,784]

Interesting conversation. Something I've noticed, Air Hauler 2 has altitude callouts and because I keep forgetting to turn them off, I get double-callouts in the PMDG 747 and 737 -- one from the plane, and one from Air Hauler.

The interesting bit is that the callouts are slightly out of sync, so I'll hear "20..20" instead of them both calling it out at the same time.  That tells me that they're figuring altitude differently. I've been assuming AH is going off of the altimeter while PMDG is going off of the RA since I have that toggled, but it could also indicate that one (or both) of them is simply inaccurate.

 

[Chock 17,769]

14 minutes ago, eslader said:

The interesting bit is that the callouts are slightly out of sync, so I'll hear "20..20" instead of them both calling it out at the same time.  That tells me that they're figuring altitude differently.

Not necessarily, it's far more likely that the sim can't load and play both sound files at the same time, so it loads one first, then plays it, then loads the next one and plays that.

[eslader 1,784]

Maybe, although in other planes with altitude callouts they're simultaneous.