How planes stay away from each other in the air


Have you ever had the chance to see an airplane in flight? If so, have you ever wondered how far this plane was from yours? High-flying aircraft, like most airliners we have today, follow a rule called Reduced Vertical Separation Minimum (RVSM) to maintain vertical separation. RVSM came into effect in the late 1990s and requires aircraft to maintain an altitude separation of 1000 feet when flying between 29,000 feet and 41,000 feet.


In the 1940s, the vertical separation between aircraft was set at 1,000 feet, regardless of aircraft altitude. However, in the late 1950s, this separation had to be reviewed with the introduction of turbojet aircraft. These new aircraft flew higher than their piston-powered counterparts, and with increasing altitude the accuracy of the altimeter, which depends on barometric pressure, decreased.


So, in June 1954, ICAO formed a group of experts on vertical separation to develop a new set of rules for the altitude separation of aircraft. The panel successfully identified the factors most likely to cause the greatest loss of aircraft separation based on the technology available in the aircraft at the time. He concluded that aircraft flying above 29,000 feet must maintain a separation of at least 2,000 feet.

Things changed in the 1970s when fuel prices rose and the volume of air traffic increased. Airlines were eager to fly at more fuel-efficient altitudes. Unfortunately, this was not possible with a separation of 2,000 feet above 29,000 feet. ICAO was once again put to work. This time to reduce the vertical separation above 29,000 feet to 1,000 feet.

As before, ICAO formed a panel with its core member states. This panel, known as the General Concept of Separation Review Panel (RGCSP), began in 1971, beginning a 19-year long project. If it took so long, it’s because altimeters are inherently inaccurate. They overread when the temperature decreases and underread when the temperature increases. In technical terms, we call this Altimetry System Error (ASE). Thus, the panel had to find a way to implement RVSM while safely mitigating this factor.

The RGCP first recommended the implementation of RVSM in the North Atlantic (NAT) region. This made sense as aircraft flying in this region were well equipped as they flew away from navigational aids for long periods of time. So in 1997, RVSM was implemented in NAT ocean routes. And by 2005, most parts of the world had adopted the RVSM concept.

The implementation of RVSM reduced the vertical separation of the aircraft to 1,000 feet from 2,000 feet. Photo: Cirrus Aviation

Benefits of RVSM

The benefits of flying at reduced vertical separation minima include:

  • It reduces fuel consumption because airlines can fly at more optimal altitudes. It also reduces carbon emissions.
  • It increases airspace capacity. The current volume of air traffic would not have been possible without the implementation of RVSM.

RVSM now

Even to this day, RVSM is highly regulated. It is part of a type of regulation known as the SPA part (Specific Approvals). There are strict aircraft and operator requirements that must be met before an operator is allowed to fly RVSM. The airline or operator is required to update its operations manual specifying RVSM procedures. This includes procedures for monitoring and reporting altitude hold errors during flight.

An RVSM training program should also be established by the training department for pilots involved in such operations. This includes initial RVSM training and periodic refresher training to maintain RVSM recency. This is what is required from the flight operations side of the airline.

Then there’s the engineering side of things. The airline must train its engineers to perform RVSM-related maintenance tasks. This includes items such as processes for monitoring aircraft changes that may affect the RVSM status of the aircraft. Airframe geometry also plays a role as bumps can affect airflow over the airframe, which can, in turn, affect airflow over the sensors used to measure altitude. Thus, engineers must also learn the techniques of geometric evaluation of aircraft.

Once the airline proves to its national regulator that it can meet the above requirements, an RVSM operational approval is given to the airline.

If an aircraft is RVSM capable, it is mentioned in the Aircraft Flight Manual (AFM) by the manufacturer. According to the regulations, an aircraft must have the following systems as a minimum requirement to fly in RVSM airspace:

  • Two independent altitude measurement systems
  • An altitude alert system
  • An automatic altitude control system
  • A secondary surveillance radar (SSR) transponder with an altitude reporting system that can be connected to the altitude measurement system used for altitude control

Most modern airliners are RVSM capable. Photo: Airbus

Flying with RVSM

As pilots, we must constantly monitor our altimeters when flying in RVSM airspace. The two altimeters (the captain’s and the first officer’s) must be within plus or minus 200 feet. This check is scheduled once per hour. Different airlines use different methods to keep this record.

In many airlines, pilots record this in the operational flight plan (OFP), and this document is retained by the airline in post-flight documentation. This data can then be used by the operator to find any errors in their aircraft’s altimetry system that may prevent them from flying in RVSM airspace. This data should also be made available to the competent authorities during their periodic audits of the airline.

One of the most interesting facts about flying in RVSM airspace is that an autopilot capable of altitude hold is mandatory for such operations. So once you are above 29,000 feet, the autopilot should be engaged until the aircraft descends below 29,000 feet and exits RVSM airspace.

Also, in RVSM airspace, if the altitude deviates by plus or minus 300 ft from the authorized level, the pilot must inform Air Traffic Control as soon as possible. Such deviation can be caused either by equipment failure or even by turbulence. Control may then increase the separation minima, or you may need to descend below 29,000 feet to exit RVSM airspace.

Did you know that planes stay safely separated?


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