Changes for checking Reduced Vertical Separation Minimums

FAA proposes use of ADS-B as a path for high altitude flight level monitoring.

By Bill Gunn
Pro Pilot Regulations and
Compliance Specialist

The flightdeck of this Falcon 900B was upgraded by Duncan Aviation with the Universal Avionics EFI-890R package, which includes a dual altimeter RVSM installation.

Today, to operate at flight levels 290 to 410, aircraft must have advanced altimetry equipment, aircrews must be trained and qualified, and specific FAA approval is required.
This might be about to change. FAA issued Notice of Proposed Rule Making (NPRM) FAA-2017-0782 on August 7, 2017 to revise requirements to apply for Reduced Vertical Separation Minimum (RVSM) operations in the National Airspace System (NAS).

Comments to this NPRM closed on September 6, 2017. If enacted, appendix G to FAR Part 91 will be revised and include adding a section to address use of ADS-B that meets certain standards along with a revised definition of RVSM.

Such a change could open up RVSM flight levels to aircraft previously unable or willing to operate at those altitudes.

A study of the issue

The current ATC system is based almost exclusively on radar for position, speed and track, and pressure gradient for height. For IFR flight below RVSM altitudes, periodic pitot-static system checks and transponder Mode C accuracy verification is adequate. It is interesting that even within controlled airspace below class A, aircraft flying VFR are not required to have pitot-static system checks.

This is countered mainly by required periodic transponder Mode C verification checks for all aircraft. If a VFR aircraft is maintaining an improper altitude for any reason, ATC can advise other traffic of the VFR traffic's altitude via Mode C.

At higher altitudes, decreasing air pressure requires more sensitive and accurate pressure-measuring capability to ensure separation. Unlike altitudes where an absolute value of mean sea level elevation is used, flight levels are relative between aircraft; the absolute MSL value is not used. The key here is each aircraft must have a pressure measurement system that is accurate relative to other aircraft.

The periodic pitot-static system check is accurate enough up to pressures equal to FL280 and below. Heights at and above FL290 up to the RVSM range of FL410 require more equipment and monitoring. In addition to approved systems, training, and aircraft maintenance programs, ICAO has set up worldwide regional monitoring agencies.

They ensure all at RVSM altitudes are system compliant and are within altimetry system allowable error for individual aircraft. This is achieved by comparing actual measured height to the flight level as indicated by the aircraft's own altimetry system.

The North American Approvals Registry and Monitoring Organization (NAARMO) developed ground-based Aircraft Geometric Height Measurement Element (AGHME) as the principal means of satisfying the objective of compliance and monitoring altimetry system error within North American RVSM airspace.

AGHME units are positioned at fixed locations in the United States and Canada. They automatically produce estimates of the geometric height of suitably-equipped aircraft flying within the coverage area of an AGHME constellation. Aircraft that are RVSM-approved simply fly straight and level through any AGHME service volume between FL290 and 410. Monitoring is continuous and no prior coordination is required. AGHME sites are located at Atlantic City NJ, Wichita KS, Phoenix AZ, and Portland OR in the US, and in Canada at Ottawa Ontario and Lethbridge Alberta.

Short history of RVSM

Dassault Falcon 2000 cockpit with avionics upgraded from Rockwell Collins Pro Line 4 to Pro Line 21. This installation, performed by West Star Aviation, includes ADS-B and FANS equipment.

In 1973 the Air Transportation Association petitioned the FAA to reduce the vertical separation for high altitude routes from 2000 to 1000 feet. It was determined at that time that the technology to meet this reduced separation was either not readily available or had not been shown to meet the accuracy required. Further progress was stopped by the lack of proven altitude-holding standards in aircraft and a suitable maintenance program to ensure any obtainable standard could be maintained.

In 1981 the FAA created a vertical studies program. Findings showed that reduced vertical separation was now technically feasible with available avionics technology. The study also showed that additional flight levels would significantly increase airspace availability and subsequently economic benefits.

According to the research, an RVSM program would require "sound operational judgment supported by an assessment of system performance based on aircraft altitude-keeping capability, operational considerations, system performance monitoring, and risk assessment."

FAA added FAR 91.706 (operations within RVSM airspace) and Appendix G to Part 91 in 1997. This Appendix spells out operational, design, maintenance, and other standards for approval to operate in RVSM airspace. This initial change permitted US-registered aircraft to seek approval to operate in RVSM airspace outside of the USA.

Years later, in October 2003, a 2nd RVSM rulemaking introduced RVSM airspace in the US NAS under the same 1997 approval process. Initially, system monitoring for aircraft was based on GPS height-monitoring units carried onboard when operating in RVSM airspace. In 2005 the FAA deployed the initial 5 AGHME systems.

A review performed in 2008 showed that the requirements for RVSM standards were being met by the 15,000 US-registered aircraft approved for RVSM. National Business Aviation Association (NBAA) then asked the FAA to streamline the RVSM approval process and reduce the burden for approval and operation. An RVSM process enhancement team made final recommendations to the FAA in 2013 and processes were streamlined.

Furthermore, in 2015 the FAA eliminated the requirement for a specifically approved maintenance program for installed RVSM equipment. Maintenance occurs using an approved program but the FAA does not now specifically approve each program for each aircraft.


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