In RVSM airspace, the operator must guarantee every aircraft carries two independent altitude measurement systems and an SSR transponder wired to the altimeter in use. State airworthiness authorities certify that these installations can sustain the required stringent performance.
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Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is the role of an altimeter in RVSM?
In RVSM airspace, the altimeter is the sole source of flight-level information. EU OPS 1.872 obliges the aircraft to carry two independent altitude measurement systems. Each system measures static pressure from a cross-coupled static source, applies static-source-error correction, converts the data to pressure altitude and shows the value to the flight crew. The same altimeter simultaneously supplies a digitally encoded pressure-altitude signal to the transponder for secondary-surveillance-radar reporting and to the automatic altitude control system for height-keeping. Because altitude alerting is required for RVSM compliance, the altimeter also provides signals referenced to the flight-crew-selected altitude. These signals drive the alerting device and permit automatic control and alerting. A standby altimeter, although not explicitly mandated for RVSM, is implicitly protected by the rule that the transponder can be switched to either of the two primary altimetry systems, assuring continued altitude reporting if one fails. An operator must guarantee that aeroplanes operating in RVSM airspace are equipped with two independent altitude measurement systems, an altitude alerting system, and an automatic altitude control system.
What is the maximum altimeter difference allowed in RVSM?
The maximum altimeter difference allowed in RVSM varies with altitude. On the ground during the preflight check, altimeters must be within 75 feet (22.86 meters) of each other. The difference between the known elevation and the elevation displayed on the altimeters must not exceed 75 ft (22.86 meters). In flight, a minimum of two altimeters need to be within 60 m (200 ft), and the system must be capable of controlling altitude within 65 ft (20 m). At the point in the basic RVSM envelope where ASE mean plus three standard deviations reaches its largest absolute value, the absolute value must not exceed 200 ft (60 m). At the point in the full RVSM envelope where mean ASE reaches its largest absolute value, the absolute value must not exceed 120 ft (37 m). The operator must report occurrences equal to or greater than an assigned altitude deviation of 90 m (300 ft).
What is the RVSM altimeter error tolerance?
The ICAO minimum aircraft system performance specification (MASPS) requires that the altimetry systems have a maximum error of 80 ft (25 m). This means the altimetry system error (ASE) must not exceed 80 feet (25 meters) at the point in the full RVSM flight envelope where mean ASE reaches its largest absolute value. For reportable height-keeping events, the operator must report altimetry system error (ASE) equal to 75 m (245 ft), while total vertical error (TVE) and assigned altitude deviation (AAD) limits for reporting are set at 90 m (300 ft).
What are the altimeter requirements for RVSM airspace?
The altimeter requirements for RVSM airspace are detailed below.
- Two primary altimeters must be operational at entry into RVSM airspace
- One automatic altitude control system must be operational at entry into RVSM airspace
- At cruise FL the two primary altimeters should agree within 200 ft of the RVSM airspace
- Altimetry systems must have a maximum error of 80 feet within the RVSM airspace
- Automatic altitude control systems must be able to hold altitude within 65 feet/20 meters of the RVSM airspace
A secondary surveillance radar (SSR) transponder with an altitude reporting system that is connected to the altitude measurement system in use for altitude keeping is also mandatory. If the transponder becomes inoperative, 14 CFR section 91.215 regulates the procedure for continued operation. 14 CFR part 91 governs operations in RVSM airspace, and operators must comply with these standards and be authorized by the Administrator.
What is the RVSM altimeter check procedure?
Before flight, flight crew conduct ground check per ICAAO manual for RVSM implementation to guarantee two main altimeter systems within prescribed tolerances remain operational and that altitude alert is operational. Pilot ensures RVSM equipment operative and compares altimeters to known ground elevation. Manual details altimetry system checks and contingency plans for failures. During external inspection, check static ports and condition of fuselage around pitot static sources. Operators must guarantee MEL required systems are operational: two primary altitude measurement systems, automatic altitude control system, autopilot with altitude hold mode, and secondary surveillance radar transponder with altitude reporting system, as regulated by 14 CFR section 91.215 if the transponder is inoperative. Initial altimeter cross check of primary and standby altimeters must be recorded.
During flight, hourly cross checks between primary altimeters are made, and if one primary altimeter remains operational, pilot shall cross check standby altimeter and record primary and standby altimeter readings at approximately one hour intervals. Pilots must inform ATC if severe turbulence impacts ability to maintain cleared flight level. Aircraft are flown at cleared flight level in cruise and do not intentionally depart from cleared flight level without clearance. Automatic altitude control is utilized except to disengage for turbulence or trim. Pilots must request new clearance if equipment fails prior to cruise. If altitude equipment fails, the pilot shall contact ATC and state ‘Unable RVSM Due Equipment’ and apply contingency procedures. When the transponder fails, pilots shall contact ATC and request authority to continue to operate at cleared flight level.
