Reduced Vertical Separation Minimum (RVSM)

Reduced vertical separation minimum (RVSM) is the use of 1,000 ft vertical separation, instead of 2,000 ft, between FL290 and FL410 inclusive. It adds six flight levels, FL300, 320, 340, 360, 380 and 400, to the band where jet airplanes cruise, so controllers have more levels to work with and more aircraft can fly near their optimum altitude. Step climbs are typically made in 2,000 ft increments, which keeps an airplane within the ±2,000 ft of optimum altitude where its specific air range stays at 99 % of the maximum or more.

Halving the separation removes most of the margin for height-keeping error. RVSM is therefore open only to aircraft whose altimetry and automatic altitude control meet a demanding standard, flown by operators approved for it, and the height-keeping of the fleet is measured in service. It began over the North Atlantic on 27 March 1997, came to 41 European states on 24 January 2002 and to the domestic United States on 20 January 2005, and now covers most of the world.

What RVSM is and where it applies

The vertical separation minimum is 1,000 ft below FL290 everywhere. Between FL290 and FL410 it is 1,000 ft in RVSM airspace and 2,000 ft where RVSM is not applied; above FL410 it is 2,000 ft. RVSM is applied in airspace designated by regional air navigation agreement, and the regional procedures are published in ICAO Doc 7030, the Regional Supplementary Procedures. ICAO's guidance for states and operators is Doc 9574, which contains the worldwide altimetry standard aircraft must meet, the Minimum Aircraft System Performance Specification (MASPS). In the United States, 14 CFR 91.180 governs operations in RVSM airspace, also FL290 to FL410.

The semicircular rule still applies, with 2,000 ft between levels in each direction:

The North Atlantic organized tracks are one-way, so each track is given both odd and even RVSM levels. The NAT High Level Airspace, FL285 to FL420 inclusive, contains the whole RVSM band (see oceanic and North Atlantic operations).

RVSM is for IFR traffic. Under SERA.5005, VFR flight above FL285 is not authorized where a 1,000 ft vertical separation minimum is applied above FL290 (ICAO Annex 2 states FL290). ATC also tightens its check on altitude reports: the tolerance for verifying SSR Mode C level information is ±200 ft in RVSM airspace, against ±300 ft elsewhere (see separation standards and cruising levels and minimum heights).

Aircraft requirements and altimetry system error

In Europe, SPA.RVSM.110 requires:

• two independent altitude measurement systems;
• an altitude alerting system;
• an automatic altitude control system;
• an SSR transponder with altitude reporting that can be connected to the altitude measurement system in use for altitude control.

Part 91 Appendix G, in the United States, lists the first three for aircraft approval.

The altitude alert must warn the crew at a deviation from the selected level of no more than ±300 ft (90 m), as European exam material gives it. Appendix G sets ±300 ft for types whose type certification application was made on or before 9 April 1997 and ±200 ft for later ones, and expects the automatic altitude control to hold the aircraft within ±65 ft of the acquired altitude in straight and level flight in smooth air (±130 ft for some older types whose altitude control takes flight management inputs). The transponder link matters in practice: on the ATR 42/72, the transponder and the autopilot and flight director must use the same air data computer for RVSM. ACAS is required by aircraft category rather than by airspace: in EU commercial air transport, every turbine-powered airplane above 5,700 kg MCTOM or with more than 19 passenger seats carries ACAS II (CAT.IDE.A.155), and European exam questions describe its use in RVSM airspace as mandatory. In the United States, a TCAS II flown in RVSM airspace must be version 7.0 or later.

Altimetry system error (ASE) is the difference between the altitude the aircraft's system displays, with the correct barometric setting, and the pressure altitude of the undisturbed air around it. Part of it, the position error, starts at the static source, where the airframe disturbs the flow by an amount that depends mainly on Mach number and angle of attack; air data computers remove it with a static source error correction. What remains must be small, because it eats directly into the 1,000 ft (see pressure altimeter and air data computer and air temperature).

View full schematic

Height-keeping as a whole is described by total vertical error (TVE), the difference between the height the aircraft actually flies and that of its assigned flight level. TVE is the sum of ASE and the assigned altitude deviation (AAD), the difference between the altitude the aircraft's altimetry shows and the assigned level. Appendix G allows aircraft to be approved as a group, sharing a type design, the same static system installation and the same static source error correction, or individually as non-group aircraft.

Only the primary systems count. The Boeing 737's limitations state that the standby altimeters do not meet RVSM accuracy requirements, and the Embraer E190-E2's standby instrument, with its own air data computer, is not for RVSM use either. The 737 limitations also set the altitude display checks:

Operator approval and the flight plan

Approval covers the aircraft and the operator together: the equipment, crew training and procedures, and an MEL that reflects RVSM. In Europe it is a specific approval under Part-SPA (SPA.RVSM); in the United States it is given under 91.180 and Appendix G. Since 2019, Section 9 of Appendix G lets an operator fly in US RVSM airspace without a specific authorization if its aircraft has the required equipment, qualifying ADS-B Out and monitored height-keeping; to fly RVSM outside the United States it still needs operations specifications, management specifications or a letter of authorization.

The MEL decides which failures end RVSM dispatch. An inoperative yaw damper typically brings an altitude limit and may prohibit RVSM; on the Boeing 787 the altitude alerting system may be inoperative only if, among other conditions, the autopilot's altitude hold and capture work normally and RVSM does not require it (see technical log, MEL and deferred defects).

RVSM approval is shown by the letter W in Item 10a of the ICAO flight plan. SW means standard equipment plus RVSM approval; an aircraft also approved for the NAT High Level Airspace adds X (see ATC flight plan).

Operators must also report height-keeping errors. Under SPA.RVSM.115, any recorded or communicated error caused by an equipment malfunction or of an operational nature that equals or exceeds a TVE of ±300 ft (90 m), an ASE of ±245 ft (75 m) or an AAD of ±300 ft (90 m) is reported to the authority within 72 hours, with an initial analysis of the causes and the measures taken, and the operator acts at once to correct the conditions that caused it. Appendix G uses the same thresholds for the reports US operators make to the FAA: a TVE of 300 ft or more, an ASE of 245 ft or more or an AAD of 300 ft or more.

Height monitoring

Height monitoring checks that approved aircraft really hold their level. Under ICAO Annex 6, at least two airplanes of each of an operator's aircraft type groupings must have their height-keeping monitored at least once every two years. Measurements come from ground-based height monitoring units (HMU), from GPS monitoring units (GMU) carried on board and, over the North Atlantic, from analysis of ADS-B data. Regional monitoring agencies, such as the North Atlantic Central Monitoring Agency (NAT CMA), compare each aircraft's measured geometric height with that of its assigned flight level and use its own altitude report to isolate the altimetry system error.

Crew procedures before and in RVSM airspace

Before flight the crew confirms that the required equipment is serviceable, that the flight plan shows W, and that the altimeters pass the ground checks: each must read the known elevation within the PANS-OPS tolerance of ±60 ft for an instrument with a test range up to 30,000 ft, and the pilots' displays must agree within the flight manual limits. Before entering RVSM airspace the two primary altimeters are compared; over the North Atlantic this is done before the oceanic area, and a difference of more than 200 ft is reported.

The margins are tighter in the air as well. Under the definition used by EUROCONTROL and SKYbrary, an unauthorized vertical deviation of more than 200 ft from the cleared level in RVSM airspace is a level bust, against 300 ft elsewhere.

A non-approved aircraft says so with the phrase NEGATIVE RVSM on the initial call on each frequency in RVSM airspace, in requests for level changes and in readbacks of level clearances within or through the band. An approved aircraft that loses the capability reports UNABLE RVSM DUE EQUIPMENT or UNABLE RVSM DUE TURBULENCE.

Wake turbulence needs watching in RVSM airspace: with 1,000 ft between levels and very accurate navigation, aircraft on the same or opposite tracks can pass through each other's wake. The AIM notes that most such encounters have been moderate or less, and tells pilots to expect wake about 10 to 30 miles after passing 1,000 ft below opposite-direction traffic, or 10 to 30 miles behind and 1,000 ft below same-direction traffic. Some are severe: in January 2017 a Bombardier Challenger 604 passing about 1,000 ft below an A380 on an opposite-direction track over the Arabian Sea was rolled several times and lost roughly 10,000 ft before the crew recovered. Over the North Atlantic, the Strategic Lateral Offset Procedure (SLOP) lets aircraft with automatic offset capability fly the centerline or an offset to the right of it, up to 2 NM and in 0.1 NM steps where the system allows, which also helps avoid wake; crews can agree mutual offsets on the air-to-air frequency, 123.450 MHz. In US domestic airspace a crew meeting or expecting wake may ask for a vector, a level change or, if capable, a lateral offset, which there needs a clearance (see wake turbulence and jet blast).

Equipment failures and contingency procedures

If a required system fails, or the remaining equipment no longer meets the requirements, the crew tells ATC UNABLE RVSM DUE EQUIPMENT. The phrase gives the first notice and is then repeated on initial contact on every frequency within RVSM airspace until the problem is cleared or the aircraft has left the airspace. ATC applies 2,000 ft vertical separation or a horizontal minimum, or clears the aircraft out of the band. If both air data computers fail, the standby altimeter and airspeed indicator remain, but the crew must expect to leave RVSM airspace. Severe turbulence or mountain wave that causes altitude deviations of about 200 ft is reported with UNABLE RVSM DUE TURBULENCE (or MOUNTAIN WAVE); in the United States the controller then vectors the aircraft clear of traffic at adjacent levels, or changes its level or route, traffic permitting.

When an aircraft in oceanic airspace cannot keep to its clearance and cannot obtain a revised one, for example in an emergency descent, it follows the in-flight contingency procedures of PANS-ATM and NAT Doc 007:

1. turn at least 30° right or left of track;
2. establish and keep a parallel, same-direction track offset by 5 NM;
3. keep the assigned level, if possible, until established on the offset, then take up a vertical offset from the levels normally used, either after descending below FL290 or within the band: 500 ft at or below FL410, 1,000 ft above it. From FL410 that means FL415 or FL405.

The 45° turn and 15 NM offset of older texts no longer apply, and a deviation around weather follows a separate weather deviation procedure. NAT aircraft monitor 121.500 MHz and the air-to-air frequency 123.450 MHz, the channels used to alert nearby aircraft when something goes wrong.

Non-RVSM aircraft

An aircraft without RVSM approval may be accommodated in RVSM airspace only in limited cases, with 2,000 ft vertical separation from all other traffic. In the United States the AIM lists them: Department of Defense, foreign State and MEDEVAC flights, certification and development flights, and aircraft climbing through the band to or descending from levels above FL410 without leveling off. They are handled with FAA approval and workload permitting, never routinely. Such an aircraft does not file W and uses NEGATIVE RVSM in its calls.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.