I thought this spelled it out but I guess not.
“There are pilots out there that think with the new RVSM rules, ADS-B, they can just go up there and ATC will tell them if they are off altitude. These are airplanes that are not approved under the old rules. I am wondering if ATC has the ability to see if the airplane is actually off altitude or only have mode C data. Several of us have said that the new rules only do away with the LOA requirements. Some say that they can just go up there and ATC will tell them if their altimeter system is off from the ADS-B data. Just telling them the new rules only do away with the LOA does not work. They just quote the new rules, which is poorly written about how meet the requirements.”
If ATC does not have real time access to the airplanes actual RVSM altitude no one will know if that airplane is actually on altitude until later. Since these airplanes have never been RVSM certified who knows how large their altimeter system error is up there.
With the RVSM AGHME altitude monitoring system it could be quite a while until someone knew their ASE was too large. As I understand the new ADS-B RVSM monitoring the operator will find out will find out a lot sooner. My question is how soon, does ATC have this data real time?
I mean, that shouldn't be too much of a problems.
91.217 Data correspondence between automatically reported pressure altitude data and the pilot's altitude reference.
(a) No person may operate any automatic pressure altitude reporting equipment associated with a radar beacon transponder—
(1) When deactivation of that equipment is directed by ATC;
(2) Unless, as installed, that equipment was tested and calibrated to transmit altitude data corresponding within 125 feet (on a 95 percent probability basis) of the indicated or calibrated datum of the altimeter normally used to maintain flight altitude, with that altimeter referenced to 29.92 inches of mercury for altitudes from sea level to the maximum operating altitude of the aircraft; or
(3) Unless the altimeters and digitizers in that equipment meet the standards of TSO-C10b and TSO-C88, respectively.
(b) No person may operate any automatic pressure altitude reporting equipment associated with a radar beacon transponder or with ADS-B Out equipment unless the pressure altitude reported for ADS-B Out and Mode C/S is derived from the same source for aircraft equipped with both a transponder and ADS-B Out.
I'd say this is actually about the same level of accuracy. The RVSM altimeter check requires 120' of accuracy at least and that at 3 standard deviations 245' of accuracy.
The data out requirements above list 125' of accuracy at all altitudes and flight levels the aircraft can operate at with 95% probability.
Assuming altimetry error is normally distributed (and I'd be surprised if it wasn't) 95% probability essentially means it's accurate to 125' to 2-standard deviations. So sigma is 62.5. at 3-standard deviations, which sets the maximum permissible error for RVSM compliant airplanes the maximum error should be around 188'.
This is actually pretty close to what we have now. It's a little sloppier at each altitude, but a little tighter at worst. Old airplanes need to have ASE show no more than 245' at 3 standard deviations and newer airplanes need to show ASE to 200'.
For the final question about AGHME, they're seeing your geometric altitude with every ping of the ADS-B, they're also seeing your baro alt, looking at everyone flying in a particular region and figuring out who looks weird would be pretty straight forward. I'd imagine they'll just look for statistical mismatches between you and all the other airplanes. This isn't appreciably different than what AGHME actually does.
I have never been in RVSM altitudes without an LOA! Just trying to stop other pilots from being up there with us and being off altitude.
