RVSM Heightens Need for Precision in Altitude Measurement Part 1 of a 2-Part Series

TECHNOLOGY RVSM Heightens Need for Precision in Altitude Measurement Part 1 of a 2-part series Technological advances have honed the accuracy of aircraft altimeters, but as weʼll explore in part 1 of this 2-part series, false indications still can occur at any altitude or flight level. Next monthʼs issue will examine limitations of the altimeters themselves, most associated with the ʻweak linkʼ in altimetry—the human. BY FLIGHT SAFETY FOUNDATION STAFF ith the expanding use of studies in the 1980s, that RVSM was plementation says that before flight in reduced vertical separa- technically feasible and developed a RVSM airspace, a flight crew should Wtion minimum (RVSM) manual for RVSM implementation.2 conduct a ground check to ensure that airspace, precise aircraft altitude infor- Further guidance for aircraft operators the required two main altimeter sys- mation has become increasingly impor- is contained in two ICAO-approved tems are within the prescribed toler- tant. The reduction of standard vertical documents: European Joint Aviation ances. separation of aircraft to 1,000 feet/300 Authorities Leaflet No. 63 and U.S. During flight, “generally flight crew meters between Flight Level (FL) 290 Federal Aviation Administration Doc- operating procedures in RVSM air- (approximately 29,000 feet) and FL ument 91-RVSM.4 space are no different than those in 410 means that deviation from an as- Included in these documents are any other airspace,” the ICAO manual signed flight level presents greater risks minimum equipment requirements for says. than existed with vertical separation of RVSM operations: Nevertheless, the manual says, “It 2,000 feet/600 meters. • Two independent altitude- is essential that the aircraft be flown RVSM standards and advanced flight measurement systems; at the cleared flight level (CFL). This deck technology on transport category • One secondary surveillance radar requires that particular care be taken aircraft are designed to help minimize transponder with an altitude-reporting to ensure that air traffic control (ATC) those risks. Nevertheless, hazards—in- system that can be connected to the clearances are fully understood and volving malfunctioning instrument sys- altitude-measurement system in use complied with ... During cleared transi- tems as well as human error—remain. for altitude-keeping; tion between [flight] levels, the aircraft RVSM implementation has become • An altitude-alerting system; and, should not be allowed to overshoot possible in part because of improve- • An automatic altitude-control or undershoot the new flight level by ments in the accuracy of modern altim- system. more than [150 feet/45 meters].” eter systems, compared with the baro- In addition, an ICAO minimum air- In addition, flight crews should con- metric (pressure) altimeters that were craft system performance specification duct regular hourly cross-checks be- used in jet transports in the late 1950s (MASPS) requires that the altimetry tween the altimeters, and “a minimum (see “The Evolution of Altimetry Sys- systems in RVSM-approved aircraft of two RVSM MASPS-compliant systems,” page 72).1 Because the accuracy have a maximum altimeter system er- tems must agree within 60 meters (200 of conventional pressure altimeters is ror (ASE) of 80 feet/25 meters and that feet). Failure to meet this condition reduced at higher altitudes, the interna- the automatic altitude-control systems will require that the system be reported tional standard established in 1960 was must be able to hold altitude within 65 as defective and notified to ATC,” the for vertical separation of 2,000 feet be- feet/20 meters. (ICAO defines ASE as ICAO manual says. tween aircraft operated above FL 290. “the difference between the altitude Height-monitoring is another RVSM As technological advances in altim- indicated by the altimeter display, as- requirement, and the U.K. Civil Avia- eters, autopilots and altitude-alerting suming a correct altimeter barometric tion Authority (CAA) said in mid-2004 systems led to more precision in mea- setting, and the pressure altitude cor- that height-monitoring had revealed suring and maintaining altitude, the In- responding to the undisturbed ambient the problem of “ASE drift,” a phenom- ternational Civil Aviation Organization pressure.”) enon in which, over time, most aircraft (ICAO) determined, after a series of The ICAO manual for RVSM im- Continued on page 73 AVIONICS NEWS • APRIL 2005 71 The Evolution of Altimetry Systems Figure I ltimeters have provided pilots with es- Typical Flight Instrumentation on Early Jet Transports Asential flight information since the de- velopment in 1928 of an accurate baromet- �� ric (pressure) altimeter. Altimeters indirectly measure the height �� of an aircraft above mean sea level or �� above a ground reference datum by sens- ing the changes in ambient air pressure that accompany changes in altitude and �� provide a corresponding altitude reading in �� feet or meters. Static air pressure typically is derived from static sources mounted on the sides of the fuselage. �� Figure 1 shows how the system typically works in early jet transports. A static AC=Alternating current AI=Attitude indicator ALT=Altimeter ASI=Airspeed indicator line connects the static ports to the altimeter, mounted in an airtight case in which a Source: Adapted from Carbaugh, David C. “erroneous Flight Instrument Information.” In sealed aneroid barometer reacts to changes Enhancing Safety in the 21st Century: Proceedings of the 52nd Annual International Air Safety Seminar. Alexandria, Virginia, U.S.: Flight Safety Foundation, 1999. in static air pressure. When static air pressure increases, the barometer contracts; Figure 2 when static air pressure decreases, the Typical Flight Instrumentation on Modern, barometer expands. The movement of the Fly-by-wire Airplanes barometer causes movement of height-in- �� dicating pointers, which present an altitude �� indication on the face of the altimeter.1 �� Also on the face of a conventional baro- �� metric altimeter is a barometric scale, cali- �� brated in hectopascals (hPa; millibars) or �� inches of mercury (inches Hg). The scale can be adjusted by a pilot to the local �� barometric pressure (e.g., within 100 nauti- � � �� cal miles [185 kilometers]) or to standard �� barometric pressure—1013.2 hPa or 29.92 inches Hg—as required by applicable regu- �� lations. �� The system changed as new airplane �� models were introduced with air data com- �� puters and other advanced electronics and digital displays. ADIRU=Air data inertial refercen unit ADM=Air data module Figure 2 shows how the system typically AIMS= Airplane information managemetn system ALT=Altimeter works in modern transport category air- ASI=Airspeed indicator LCD=Liquid crystal display PFD= Primary flight display Ps= Static pressure Pt= Total pressure SAARU= Secondary attitude craft, in which an air data inertial reference air data reference unit unit (ADIRU) is the primary source for altitude (as well as airspeed and attitude), and Source: Adapted from Carbaugh, David C. “erroneous Flight Instrument Information.” In the information is displayed on the pilots’ Enhancing Safety in the 21st Century: Proceedings of the 52nd Annual International Air Safety Seminar. Alexandria, Virginia, U.S.: Flight Safety Foundation, 1999. primary flight displays. Pitot and static pres- 72 AVIONICS NEWS • APRIL 2005 RVSM Improvements in the accuracy of Continued from page 71 modern altimeter systems, however, sures are measured by air data modules begin to fly lower than their displayed have not eliminated the possibility of 5 (ADMs) connected to three independent altitude.” critical altimeter-setting problems, air pressure sources; ADM information U.K. CAAʼs continuing investiga- which often result from human error. 6 is transmitted through data buses to the tion of ASE drift has found that likely Several factors related to barometric ADIRU. The ADIRU calculates altitude and causes include changes over time in altimeters often have been associated airspeed by comparing information from the performance of air data computers with a flight crewʼs loss of vertical the three sources, and provides a single and erosion of pitot-static probes. situational awareness, which in turn set of data for both the captain and the The investigation also has found that has been associated with many con- first officer. If an ADIRU fails, an electronic ASE can be exacerbated by inadequate trolled-flight-into-terrain (CFIT) acci- 9,10 standby altimeter and an electronic stand- operational practices by flight crews, dents. These factors include confu- by airspeed indicator receive pitot-static especially noncompliance with aircraft sion resulting from the use of different data from standby ADMs.2 operating restrictions contained in the altitude and height reference systems The newest systems are “far more accu- RVSM airworthiness approval. and different altimeter-setting units of rate” than the altimeters that were installed “In particular, if the approval was measurement. in early jet transports, said Jim Zachary, based on adherence to speed limits, In 1994, the Flight Safety Foun- president of ZTI, an avionics consulting the flight crew must be aware of those dation (FSF) CFIT Task Force said, firm.3 limits and

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