RVSM Aircraft Monitoring and How it Enhances Airspace Safety

With air travel and airspace usage demand increasing each year, the airspace rules are in transition to keep pace. Reduced Vertical Separation Minima (RVSM) was implemented in our airspace to accommodate increased air traffic and utilize enhanced safety features brought by modern aviation technologies. But what does RVSM do?

Separation change in RVSM Airspace Stratum

RVSM implementation meant there would be a change in vertical separation between aircraft, a decrease from 2,000 feet of separation to 1,000 feet of separation. This decrease of separation between aircraft would occur in the airspace stratum from 29,000 feet to 41,000 feet, a slice of airspace where the thinner atmosphere has fuel cost saving benefits. The decrease of separation between aircraft would also, in theory, allow Air Traffic Control to safely increase the traffic capacity of these routes potentially leading to fewer delays in the system overall.

Safety Focus of RVSM Implementation

Any change to the airspace rules must keep the same or better target level of safety. There must also be a way to confirm that safety is maintained after the change.

To confirm that aircraft were checked to be safe in RVSM airspace, RVSM Implementation included a requirement for a height-keeping performance check for those operators that wish to access the RVSM airspace stratum. Aircraft RVSM Monitoring assesses the technical risk of the aircraft operating in the reduced airspace separation environment. If the aircraft height-keeping performance is maintained within the expected error tolerance, the aircraft is considered safe for RVSM flight and should pose no additional risk to other aircraft operating in RVSM.

Once the airspace change has been determined as safe, the next challenge of maintaining a safe operating environment is continuous monitoring to maintain the level of safety.

Long-Term RVSM Monitoring

After RVSM was implemented, a recurring long-term RVSM monitoring plan was enacted to keep the airspace safe. Aircraft operating in RVSM airspace must complete an RVSM Monitoring flight test every 2 years or 1,000 flight hours (whichever period is longer) to maintain RVSM approval.

So now we have a plan to enhance safety, test for safety, and maintain it. How does it work?

The Regional Monitoring Agencies, the authorities that track and maintain RVSM airspace operators listings, determine aircraft risk with a calculation based on the performance of two aircraft systems: the Altitude-Keeping System and the Altimetry System.

Altitude-Keeping System

The Altitude-Keeping System is the feedback control system that keeps the aircraft flying a specific pressure altitude as commanded by the flight crew. This control system displays the pressure altitude flown and that pressure altitude can be observed by both the flight crew (at the altimeter reading) and the Air Traffic Control (at the secondary surveillance radar of the aircraft reported MODE C).

In RVSM, any error in the altitude-keeping system is referred to as Assigned Altitude Deviation (AAD) and is equivalent to flight technical error.

Altimetry System

The Altimetry System is regulated by a barometric pressure sensor/transducer that translates ambient static air pressure at the port opening on the aircraft. This port translates ambient static air pressure into geopotential feet (pressure altitude) by means of ICAO Standard Atmosphere.

The Altimetry System performance however cannot be observed by the flight crew nor air traffic control. There are many weather related factors that complicate performance observations, variables that can change the pressure altitude on any given day. The atmosphere pressure altitude generally operates on all the aircraft in the same environment in a similar way and aircraft are given separation for safety.

Any errors in the Altimetry System pressure altitude control system are referred to as Altimetry System Error (ASE).

So now that we know the systems that affect the safety risk, we need a way to confirm these systems are performing as expected. For RVSM implementation, there also needed to be evidence collection that the system was safe after the change was made. An aircraft RVSM monitoring program would collect the data necessary and maintain the safety of the airspace.

Monitoring the RVSM Airspace: Measuring the Aircraft Performance

The Regional Monitoring Agencies use the RVSM Monitoring tests to calculate the aircraft Altimetry System Error stability and error tolerance to provide evidence for safety. The RVSM Monitoring is a performance check on the effectiveness of Altimetry Systems installed to meet the RVSM certification and to certify as safe.

The best way to measure aircraft height-keeping performance is to check the difference between the aircraft pressure altitude flown and compare that to the aircraft geometric height flown. We should also be able to determine where the aircraft should be flying based on the pressure altitude of the flight region at the time of the flight. This is challenging, however, because it requires obtaining estimates of the geometric height of the aircraft and a geometric estimation on the constant pressure altitude to which the aircraft is assigned to fly by air traffic control.

For RVSM Aircraft Monitoring in the United States, the FAA Technical Center in Atlantic City, New Jersey, developed a process to estimate these values based on weather conditions for the air pressure altitude from meteorological data. A aircraft monitoring tool was also developed to independently collect GPS position and ellipsoidal GPS height data from inside the aircraft during RVSM flight. Later, the FAA developed a network of ground based systems (Aircraft Geometric Height Measurement Elements AGHME) that could be overflown by MODE S equipped aircraft to collect the GPS position and height data.

Using this data, the FAA and other RMAs can capture the Altimetry System Error, Total Vertical Error, and Assigned Altitude Deviation — safety values for height-keeping performance.

An RVSM Monitoring Tool independent GPS system antenna inside an aircraft

Total Vertical Error

We have already discussed the Altimetry System Error and how it covers the errors in the altimetry pressure altitude control system.

Aircraft are assigned to fly a constant pressure altitude such as “Flight Level 290 or maintain flight at an altitude of 29,000 feet.” It’s important to remember that aircraft are attempting to adhere to a constant air pressure and not a constant geometric height, they are adhering to an isobaric surface. This pressure surface can change from day to day or even hour to hour due to weather variables. This poses a problem when assessing “was the aircraft where it said it was when the air pressure conditions were this…”

Luckily we can observe some data to give us an estimate of Total Vertical Error.

Flying a pressure altitude

The Geometric Height of the assigned Flight Level can be determined by using Meteorological data from sources such as the National Oceanic & Atmospheric Administration (NOAA). This data can provide the Geopotential Height, virtual temperature, time, date, and latitude and longitude. We can match this with what we recorded on the flight using date and time. The Target Air Pressure Flight Level can be converted into a Geometric Height Estimate.

GPS Height Estimate from the Target Air Pressure Flight Level

From the aircraft operational side of things, the actual geometric height that the aircraft was flying can be captured with a portable tool inside the aircraft: an independent GPS based RVSM Monitoring Unit. This tool can record the data during the RVSM Flight and capture the geometric height flown, latitude, longitude, time, and date. Again, the time and date are crucial to compare with our Meteorological data.

We can determine how closely the aircraft was adhering to the Geometric Height of the Assigned Flight Level by comparing the heights reported by the tool and the Meteorological data.

There is one other source of data required to complete the comparison: the Altitude Flown by the aircraft from the instruments while in RVSM airspace. This is the flight record from the instruments that gives the baseline for the pressure altitude and a way to observe if the aircraft was actually where it said it was.

There are a few ways to provide records on the Altitude Flown from MODE C report data collected from the aircraft during the RVSM Flight: Air Traffic Control can provide them from surveillance, the portable tool using an Altitude Recording Device, or downloading the Digital Flight Data Recorder data from the flight.

Once the RVSM Monitoring data sources (GPS altitude, MODE C reports, Meteorological data) are collected, the aircraft height-keeping performance values can be calculated.

Error Values

If the aircraft values from the RVSM monitoring test results are withing the accepted tolerance range, the aircraft is considered in compliance for RVSM flight.

Altimetry System Error (ASE) is the difference between the instrument displayed pressure altitude and actual altitude flown. The tolerance for ASE is 75 m or 245 ft.

Total Vertical Error (TVE) is the difference between the actual altitude flown and the assigned pressure altitude. The tolerance for TVE is 90 m or 300 ft.

Assigned Altitude Deviation (AAD) is the difference between ATC assigned altitude and the altitude flown transponded or provided from the aircraft equipment. The tolerance for AAD is 90 m 300 ft.

Determining the Aircraft Height-Keeping Performance

Questions on RVSM Monitoring?

CSSI, Inc. RVSM Services are proud to support the RVSM Monitoring mission around the world. Our team has experience supporting RVSM all aircraft types and have helped more that 20,000 RVSM Monitoring Flights succeed.

We offer competitively priced RVSM Monitoring Services, sending our technicians directly to your aircraft with our monitoring tool. After the flight, we coordinate directly with your Regional Monitoring Agency to provide the monitoring data and track the results.

CSSI’s RVSM Monitoring Tool

In addition, CSSI, Inc. also sells RVSM Monitoring tools and training with our RVSM Monitoring Partnership Program, allowing our aviation industry partners to provide RVSM Monitoring Services directly to their customers anywhere in the world.

If you have any questions, please visit our website and contact our team.