N250AD: FL410 (baro), 42,250 ft (GPS)

True (GPS) altitude of 42,250 ft is 3% higher than the baro altitude of 41,000 ft.

This means, generally, that each flight level will be 1030 ft apart in terms of true altitude.

Mike C.

N250AD: FL410 (baro), 42,250 ft (GPS)

True (GPS) altitude of 42,250 ft is 3% higher than the baro altitude of 41,000 ft.

This means, generally, that each flight level will be 1030 ft apart in terms of true altitude.

Mike C.

Correct.

Indeed, what you are looking for is that aircraft near you in both altitude and position have the SAME baro/GPS ratio. If that is the case, the two aircraft agree on what baro altitude means.

In my example, my GPS/baro ratio was 1.029375, N250AD was 1.030488. We are within 0.1% of each other so our respective altimetry systems agree very closely.

Mike C.

That 3% cannot be applied over the entire air column. And GPS undulation accounts for a lot of it, so would apply equally to both. Your actual differences were much closer to 75' (after subtracting the 1000' pressure altitude difference). Still pretty close.

That 3% cannot be applied over the entire air column. And GPS undulation accounts for a lot of it, so would apply equally to both. Your actual differences were much closer to 75' (after subtracting the 1000' pressure altitude difference). Still pretty close.

It depends on if it is Group approval or non-Group (single aircraft).

For Group the largest mean ASE should not exceed 80'. And 3 std deviations of the mean data should not exceed 200'.

For non-Group it is worst case residual SSE + expected variability in the air data equipment should not exceed 160'. Residual SSE are the errors in the SSEC correction.

These numbers are for the "Basic" RVSM envelope. For non-Group the errors cannot exceed 200' outside of the basic envelope.

I got these numbers from the latest AC. When I was doing this at Duncan 15 years ago we didn't have the AC but from memory the numbers were the same. We only did Group approvals and we used Aeromech for data collection and analysis

My TBM is under the Garmin STC. They share the Group with Daher (same group whether altimeters are Honeywell AM-250 or Garmin or a mix of both which mine are). The 2 year test limits are in the Garmin G600TXi STC, and the testing is at 3 different airspeeds in the RVSM region. There is also a physical static port inspection.

Is that a flight test or a ground test with pitot-static test equipment?

Is that a flight test or a ground test with pitot-static test equipment?

The baro versus GSP altitude difference has to be exhibited somewhere. On average, they differ by 3% over the entire air column up to 41,000 ft.


That will have an effect only for laterally separated aircraft. This aircraft passed basically under me within a few miles, so GPS undulation would be insignificant in that case.


I doubt it. We are clearly in air where pressure difference of 1000 ft clearly spans larger than 1000 ft true altitude.

Obviously, finding someone at my altitude removes the offset comparison, but they will be laterally separated, you hope!

Mike C.

The baro versus GSP altitude difference has to be exhibited somewhere. On average, they differ by 3% over the entire air column up to 41,000 ft.
...

I doubt it. We are clearly in air where pressure difference of 1000 ft clearly spans larger than 1000 ft true altitude.

This was my area of contention, because it is not distributed evenly over the entire air column. There are changes in the lapse rate below the tropopause due to humidity, and above the tropopause the lapse rate is mostly 0.

The change in lapse rate above the tropopause varies with the true temp. The change in indicated altitude vs true altitude CAN be 3%, it can also be lower or higher. If the SAT was -50C it would be about 3% (but only above the tropopause)

In Mike's crossing example, the geoid displacement is -100'.

Just a quick look at ASOSs in the area shows a baro setting of 30.01, so actual altitude was 90' lower.

So a total difference between pressure altitude and gps HAE is -190' (ignoring tropopause effects).

The rest is likely due to the true air temp at the time.

Mike, do you remember what your OAT was at the time? Preferably SAT not TAT

BTW a good reference for measurement of altimetry errors is NASA-RP-1046 "Measurement of Aircraft Speed and Altitude"

As promised, here's my letter. Unsurprisingly, it does not contain the statement "You're legal," "You're fine" or anything like it. I did get that by voice, but not in writing. The closest I got was "This RVSM performance measurement is within the + 200 feet standard of Appendix G to Part 91, Section 9 (b)."

Note: I have not flown higher than FL270 in Canada, and I advised the Canadian controller I was negative RVSM when at FL270, even though I was not in RVSM airspace.



Mr. Covington,

Regarding your question concerning operating [tail number/serial number] in RVSM airspace under the provisions of Part 91, Appendix G, Section 9 - Operations in Reduced Vertical Separation Minimum (RVSM) Airspace if the aircraft had never been formally RVSM certified.

14 CFR Part 91, Appendix G provides standards for an operator to conduct RVSM operations. Section 9 allows for authorization to conduct RVSM operations when in compliance with the applicable provisions of Appendix G, without application to the FAA. Since Section 9 is not recognized by other authorities than the U.S., RVSM operations under the provisions of Section 9 are authorized in U.S. Airspace only. It is the operator’s responsibility when conducting operations under Section 9 to ensure the operator and the aircraft meet the provisions of the Appendix G. One method for determining aircraft suitability for RVSM can be found in FAA Advisory Circular 91-85B - Authorization of Aircraft and Operators for Flight in Reduced Vertical Separation Minimum (RVSM) Airspace, Chapter 2. Aircraft Eligibility. It states that if the aircraft Airplane Flight Manual (AFM) or AFM Supplement (AFMS) indicates the aircraft has been determined to be capable of meeting the RVSM performance requirements of 14 CFR part 91 appendix G, then no further evaluation would be necessary. Other methods which include equivalent detailed engineering analysis and testing to ensure the aircraft meets the RVSM standards within the operation you intend on an ongoing basis may also be used.

Your recent operations indicate flight in the Continental United States (CONUS), Alaska, and Canada, including transit through Canadian airspace. Currently, all other countries, including Canada and Mexico, require a specific RVSM authorization, which would necessitate an Operations Specification or Letter of Authorization from the FAA prior to conducting RVSM operations in airspace outside the U.S. I have attached FAA InFO 23004 - Requirement for Reduced Vertical Separation Minimum (RVSM) Operation Specifications (OpSpec)/Management Specification (MSpec)/ Letter of Authorization (LOA) with further detail.

Please note: The North American Approvals Registry and Monitoring Organization (NAARMO) have reported the aircraft’s ASE measurements to be between 0 and 60 feet. This RVSM performance measurement is within the + 200 feet standard of Appendix G to Part 91, Section 9 (b). The aircraft is currently listed as meeting that standard on the NAARMO website for Section 9 operators here: https://www.faa.gov/air_traffic/separat ... _approvals. If the aircraft exceeds the ASE standard it would cease to be eligible for RVSM operations. FAA Advisory Circular 91-85B - Authorization of Aircraft and Operators for Flight in Reduced Vertical Separation Minimum (RVSM) Airspace, has additional detail concerning this subject, I urge you to review it.

If you would like to discuss the matter further do not hesitate to call me.

That's great, for sure. I'm curious if it was formally from the guy you mentioned previously ("John Warburton, head of the Separation Standards Analysis Branch"), or from someone at your FSDO, or... ?

Definitely the sort of letter it would be nice to get a few more examples of.

Option 1, the plane has meet prior RVSM standards and thus is known to be ready to go.


Option 2, the plane had not previously been tested for RVSM and thus it needs to now be shown to meet the performance criteria given in the rule.

Thus the FAA letter explicitly allows for planes that have NOT been RVSM certified to fly in RVSM airspace via option 2.

The FAA has not specified exactly how one demonstrates the performance requirements, but they are fairly simple. One could document by actual flight test that you meet the requirements and then use that as a basis.

There is essentially only one actual performance requirement that is of any significance and that is (a)(2), the stability of the autopilot in smooth air. A simple test is to just fly for a while and document the altitude the autopilot has held during that time. If you can show the system holds altitude smoothly within +/- 65 ft, that's a pass.

The rest of it is simple stuff: have two altimeters, have an altitude alerter that flags off altitudes, have TCAS II 7.1 (or don't have TCAS II at all), and have a qualified ADS-B out system. Pretty simple really.

Then you go fly and get your ASE report. If the ASE report passes, and you documented the rest of the criteria, that works.

Mike C.