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 development 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 ensure that the aircraft is “Flight crew training is now used as “The old-type altimeters were not cor- operated within the cleared speed en- a means of solving this problem, but rected for static source error, which is a velope,” U.K. CAA said. consideration should be given to dis- function of airspeed,” Zachary said. “The In addition, during RVSM opera- continuing the use of some altimeter pilot would look at the altitude and look at tions, both the active autopilot and designs and standardizing the use of the airspeed and go to some chart and say, the operating transponder should be altitude and height reference systems ‘OK, I’ve got to do this correction, change selected to the same altimetry system, and altimeter-setting units of mea- my altitude, add 100 feet or 200 feet.’ “unless there is a systems limitation or surement.” Many of the Foundationʼs “That’s all done automatically now ... functionality which makes the require- recommendations have since been The new electronic altimeters have an inte- ment unnecessary and is detailed in the endorsed by ICAO, civil aviation au- grated ADM and are connected to pitot (for AFM [aircraft flight manual].” thorities and aircraft operators in many airspeed) and static pneumatics. All errors countries. Air Data Computers, are corrected internally. This is extremely ICAO has recommended procedures Glass Cockpit Displays important for the new, demanding require- for providing adequate vertical separa- Improve Accuracy ments for reduced separation of aircraft. ... tion between aircraft and adequate ter- It means that you have an altimeter that’s Despite the findings about ASE rain clearance, including what units absolutely correct.” drift, the precision of altitude informa- should be used to measure air pressure, — FSF Editorial Staff tion available on the flight deck has in- what settings should be used to display creased in recent years because of the the measurement and when during a development of the air data computer flight the settings should be changed; Notes (ADC), air data inertial reference unit nevertheless, many variations are used 1. Harris, David. Flight Instruments and Auto- (ADIRU) and digital displays. Mod- by civil aviation authorities in different matic Flight Control Systems. Oxford, England: ern systems may include an ADIRU countries (see “ICAO Prescribes Basic Blackwell Science, 2004. that receives information from air data Principles for Vertical Separation, Ter- 11 2. Carbaugh, Dave; Forsythe, Doug; McIntyre, modules (ADMs) connected to the rain Clearance,” page 74). Melville. “Erroneous Flight Instrument Informa- airplaneʼs pitot probes and static pres- Capt. David C. Carbaugh, chief tion.” Boeing Aero No. 8 (October 1999). sure sources; the unit incorporates the pilot, flight operations safety, Boeing best of that information (rejecting data Commercial Airplanes, said that, de- 3. Zachary, Jim. Telephone interview by Wer- that are incompatible with data pro- spite technological advances, “a hu- felman, Linda. Alexandria, Virginia, U.S. Nov. man still has to set the altimeter, and 12, 2004. Flight Safety Foundation, Alexandria, duced by the other sources) to provide Virginia, U.S. a single set of data to both pilots. Other itʼll display what itʼs asked to display; standby ADMs provide information if you ask it to display the wrong thing, for standby flight instruments.7,8 Continued on following page

AVIONICS NEWS • APRIL 2005 73 ICAO Prescribes Basic Principles RVSM for Vertical Separation, Continued from page 73 Terrain Clearance thatʼs what it will display. Itʼs well- documented that the human is the weak link in altimetry.”12 The International Civil Aviation Organization (ICAO) recommends a method of Altimeter mis-setting has been providing adequate vertical separation between aircraft and adequate terrain clear- identified as one of the top six causal 13 ance, according to the following principles:1 factors associated with level busts, • During flight, when at or below a fixed altitude called the transition altitude, an which are defined by the European aircraft is flown at altitudes determined from an altimeter set to sea level pressure Organisation for Safety of Air Navi- (QNH)2 and its vertical position is expressed in terms of altitude; gation (Eurocontrol) as unauthorized • During flight, above the transition altitude, an aircraft is flown along surfaces of vertical deviations from an ATC flight constant atmospheric pressure, based on an altimeter setting of 1013.2 hectopas- clearance of more than 300 feet out- cals [29.92 inches of mercury], and throughout this phase of a flight, the vertical side RVSM airspace and more than 14 position of an aircraft is expressed in terms of flight levels. Where no transition 200 feet within RVSM airspace. altitude has been established for the area, aircraft in the en route phase shall be “Level busts, or altitude devia- flown at a flight level; tions, are a potentially serious avia- • The change in reference from altitude to flight levels, and vice versa, is made, tion hazard and occur when an air- when climbing, at the transition altitude and, when descending, at the transition craft fails to fly at the level required level; for safe separation,” Eurocontrol said • “The adequacy of terrain clearance during any phase of a flight may be main- in the “Level Bust Briefing Notes,” tained in any of several ways, depending upon the facilities available in a particular a set of discussion papers included area, the recommended methods in the order of preference being: in the European Air Traffic Manage- – The use of current QNH reports from an adequate network of QNH reporting ment Level Bust Toolkit. (The tool stations; kit is designed to raise awareness of – The use of such QNH reports as are available, combined with other meteorologi- the level bust issue among aircraft cal information such as forecast lowest mean sea level pressure for the route or operators and air navigation service portions thereof; and, providers and to help them develop – Where relevant current information is not available, the use of values of the low- strategies to reduce level busts. Four- est altitudes of flight levels, derived from climatological data; and, teen briefing notes are a fundamental • During the approach to land, terrain clearance may be determined by using the part of the tool kit.) QNH altimeter setting (giving altitude) or, under specified circumstances ... a QFE3 “When RVSM applies, the poten- setting (giving height above the QFE datum). tial for a dangerous situation to arise ICAO says that these procedures provide “sufficient flexibility to permit variation in is increased. This operational hazard detail[ed] procedures which may be required to account for local conditions without may result in serious harm, either deviating from the basic procedures.” from a midair collision or from col- — FSF Editorial Staff lision with the ground (CFIT),” the briefing notes said. Notes Studies have shown that an aver- 1. International Civil Aviation Organization. Procedures for Air Navigation Services. Aircraft age of one level bust per commercial Operations, Volume 1: Flight Procedures. Part VI, Altimeter Setting Procedures. aircraft occurs each year, that one 2. QNH is the altimeter setting provided by air traffic control or reported by a specific station European country reports more than and takes into account height above sea level with corrections for local atmospheric pres- 500 level busts a year and that one sure. On the ground, the QNH altimeter setting results in an indication of actual elevation major European airline reported 498 above sea level; in the air, the QNH altimeter setting results in an indication of the true height level busts from July 2000 to June above sea level, without adjustment for nonstandard temperature. 2002.15 3. QFE is an altimeter setting corrected for actual height above sea level and local pressure Tzvetomir Blajev, coordinator of variations; a QFE altimeter setting applies to a specific ground-reference datum. On the safety improvement initiatives, Safe- ground, a correct QFE altimeter setting results in an indication of zero elevation; in the air, the QFE setting results in an indication of height above the ground reference datum. ty Enhancement Business Division, Directorate of Air Traffic Manage- ment Programmes, Eurocontrol, said

74 AVIONICS NEWS • APRIL 2005 that data is not sufficient to evaluate the QNH altimeter setting results in an Some operators require flight crews incorrect altimeter settings in Euro- indication of the true height above sea to set the altimeter to QFE in areas pean RVSM airspace.16 level, without adjustment for nonstan- where QNH is used by ATC and by Nevertheless, Blajev said, “An in- dard temperature. most other operators. correct altimeter setting is of concern (Another “Q code” is QNE, which The FSF Approach-and-Landing to us. ... Some of the 21 recommen- refers to the standard pressure altimeter Accident Reduction (ALAR) Task dations in the Level Bust Toolkit are setting of 1013.2 hectopascals [hPa], or Force said that using QNH has two designed to fight the risk of errors in 29.92 inches of mercury [in. Hg].) Continued on following page altimeter settings. One specifically is targeted at this: ʻEnsure clear procedures for altimeter cross-checking and approaching level calls.ʼ To support the implementation of this recommendation, we have developed a briefing note.”

Different Standards Lead to Confusion Some altimeter-setting errors that occur during international flights have been attributed to the fact that not all civil aviation authorities have the same altimeter-setting rules and requirements. C. Donald Bateman, chief engineer, flight safety systems, Honeywell, said, “We have so many different altimeter- setting standards. Obviously, thereʼs a good chance weʼre going to have errors, and weʼve had them.”17 For example, different altimeter-setting practices involving QFE and QNH can cause confusion. QFE is an altimeter setting corrected for actual height above sea level and local pressure variations; a QFE altimeter setting applies to a specific ground-reference datum. On the ground, a correct QFE setting results in an indication of zero elevation; in the air, the QFE setting results in an indication of height above the ground- reference datum. QNH is the altimeter setting provided by ATC or reported by a specific station and takes into account height above sea level with corrections for local atmospheric pressure. On the ground, the QNH altimeter setting results in an indication of actual elevation above sea level; in the air,

AVIONICS NEWS • APRIL 2005 75 Figure I Figure 2 Altimeter-setting Pressure/Altitude Conversion Table Conversion Table

RVSM �� Continued from page 75 �� advantages: “eliminating the need to change the altimeter setting during op- �� erations below the transition altitude/ �� flight level” and eliminating “the need �� to change the altimeter setting during a �� missed approach.” (Such a change usually is required when QFE is used.)18 �� Many civil aviation authorities use �� hectopascals (millibars), to measure �� barometric pressure; others use inches �� of mercury (Figure 1); if a pilot con- �� fuses the two and mis-sets the altimeter, the result can mean that the aircraft �� is hundreds of feet lower (or higher) �� than the indicated altitude (Figure 2; 19 �� Figure 3, page 77). �� The ICAO standard is for altimeter settings to be given in hectopascals, �� and in 1994, the Foundation recom- �� mended that all civil aviation authori- �� ties adopt hectopascals for altimeter �� settings to eliminate the “avoidable hazard of mis-setting the altimeter.”20 �� In 2000, the Foundation repeated �� the recommendation in its “ALAR �� Briefing Notes:” �� When inches Hg is used for the al- �� timeter setting, unusual barometric pressures, such as a 28.XX inches Hg �� (low pressure) or a 30.XX inches Hg �� (high pressure), may go undetected �� when listening to the ... ATIS [automatic terminal information service] or �� ATC, resulting in a more usual 29.XX �� altimeter setting being set. �� Figure 4, page 77 and Figure 5, page �� 78 show that a 1.00 in. Hg discrepan- cy in the altimeter setting results in a �� 1,000-foot error in the indicated alti- �� tude. �� In Figure 4, QNH is an unusually low 28.XX inches Hg, but the altim- �� eter was set mistakenly to a more usual �� 29.XX inches Hg, resulting in the true �� altitude (i.e., the aircraftʼs actual height �� above mean sea level) being 1,000 feet �� lower than indicated. �� In Figure 5, QNH is an unusually Source: U.S. Government Printing Office Source: U.S. Government Printing Office high 30.XX inches Hg, but the altim-

76 AVIONICS NEWS • APRIL 2005 eter was set mistakenly to a more usual the following: by the relief pilot using three digits 29.XX inches Hg, resulting in the true • The captain of an air carrier pas- with a decimal point. Since Frankfurt altitude being 1,000 feet higher than senger flight said that during descent normally issues both hectopascals and indicated.21 to Frankfurt, Germany, “the altimeters inches of mercury on the ATIS, I incor- Numerous reports about these prob- were incorrectly set at 29.99 inches rectly assumed that the decimal denot- lems have been submitted to the U.S. Hg instead of 999 hPa, resulting in ed the inches of mercury scale and an- National Aeronautics and Space Ad- Frankfurt approach control issuing an nounced ʻ2999ʼ and set my altimeter. ministration (NASA) Aviation Safety altitude alert. The reason I believe this The first officer did the same. ... In the Reporting System (ASRS),22 including happened is that the ATIS was copied Continued on following page

Figure 3 Effect of an Altimeter Mis-set to inches, Rather Than Hectopascals

��

�� AFL= Above Field level MSL=Mean seal level Hg=Mercury QNH= Altimeter setting that causes altimeter to indicate height avove mean seal level (thus, ﬁeld elevation at touchdown)

Source: Flight Safety Foundation Approach-and-landing Accident Reducation (ALAR) Task Force

Figure 4 Effect of a One-inch-high Altimeter Setting

��

AFL= Above Field level MSL=Mean seal level Hg=Mercury QNH= Altimeter setting that causes altimeter to indicate height avove mean seal level (thus, ﬁeld elevation at touchdown)

Source: Flight Safety Foundation Approach-and-landing Accident Reducation (ALAR) Task Force

AVIONICS NEWS • APRIL 2005 77 RVSM Continued from page 77 Figure 5 Effect of a One-inch-low Altimeter Setting

��

�� AFL= Above Field level MSL=Mean seal level Hg=Mercury QNH= Altimeter setting that causes altimeter to indicate height avove mean seal level (thus, ﬁeld elevation at touchdown)

Source: Flight Safety Foundation Approach-and-landing Accident Reducation (ALAR) Task Force

future, I will insist that all ATIS infor- proach, and I knew we were awfully tion altitudes. The reports included the mation is to be copied, and particularly close to some of the hills that dot the following: both altimeter settings. area ... but it was not until we landed • A flight crew on an air carrier “ ... Safety would also be greatly en- and our altimeters read 500 feet low cargo flight in Europe said that they hanced if ICAO standards were com- that I realized what had happened.”24 forgot to reset their altimeters at the plied with by the controllers (i.e., stat- unfamiliar transition altitude of 4,500 ing the units when giving the altimeter Transition Altitudes Vary feet. “Climbing to FL 60 ... we were setting) ... I believe this could happen Civil aviation authorities worldwide task-saturated flying the standard in- to almost any pilot, given similar cir- have established transition altitudes strument departure, reconfiguring cumstances. I feel that stating units by at which flight crews switch their al- flaps and slats, resetting navigation all concerned would eliminate most of timeter settings between the standard receivers and course settings, resetting the problem,”23 altimeter setting for flights at or above engine anti-ice, etc. The crew missed • Another pilot said that at the end the transition altitude and the altimeter resetting the Kollsman [barometric al- of a long overwater flight, “approach setting being reported by the near- timeter] window to 29.92 [inches Hg] control gave the altimeter as 998 hPa. est reporting station for flights below at 4,500 feet MSL [above mean sea I read back 29.98 [inches Hg]. [The] the transition altitude. The designated level] and leveled off at FL 60 indicat- approach controller repeated his origi- transition altitude varies from 3,000 ed altitude with a Kollsman setting of nal statement. Forgetting that our al- feet in Buenos Aires, Argentina, to 28.88 [inches Hg]. Departure [control] timeters have settings for millibars and 18,000 feet in North America.25 Tran- informed us of our error,”26 hectopascals (which I had only used sition altitudes can be specified for en- • A first officer on an air carrier pas- once in my career, and that was six tire countries or for smaller areas, such senger flight said, “Due to a distraction months ago), I asked where the con- as individual airports; in some juris- from a flight attendant, we neglected version chart was. ʻOld handʼ captain dictions, the transition altitude varies, to reset altimeters passing through FL told me that approach [control] meant depending on QNH. 180 from 29.92 [inches Hg] to 29.20 29.98 [inches Hg]. Assuming that he NASA said that numerous ASRS [inches Hg]. Extremely low pressure knew what he was doing, I believed reports have been submitted involving caused us to be at 12,200 feet when him. We were a bit low on a ragged ap- altimeter mis-setting events at transi- we thought we were at 13,000 feet.

78 AVIONICS NEWS • APRIL 2005 The controller queried us; we realized • “All digits, as well as the unit of 10. CFIT, as defined by the FSF CFIT our error and climbed to 13,000 feet measurement (e.g., inches or hectopas- Task Force, occurs when an airworthy air- after resetting the altimeter. We didnʼt cals) should be announced. craft under the control of the flight crew is flown unintentionally into terrain, obstacles accomplish the approach checklist on “A transmission such as ʻaltimeter or water, usually with no prior awareness descent, which would have prevented setting six sevenʼ can be interpreted by the crew. This type of accident can oc- 27 this,” as 28.67 inches Hg, 29.67 inches Hg, cur during most phases of flight, but CFIT • A first officer on an air carrier car- 30.67 inches Hg or 967 hPa. is more common during the approach- go flight said, “Received low-altitude “Stating the complete altimeter set- and-landing phase, which begins when warning, pulled up and discovered al- ting prevents confusion and allows an airworthy aircraft under the control of timeter ... was mis-set. Altimeter was the flight crew descends below 5,000 feet detection and correction of a previous above ground level (AGL) with the inten- set at 29.84 [inches Hg] and should error; [and,] tion to conduct an approach and ends when have been set at 28.84 [inches Hg]. • “When using inches Hg, ʻlowʼ the landing is complete or the flight crew Crew distracted with a [mechanical should precede an altimeter setting of flies the aircraft above 5,000 feet AGL en problem] about the time of altimeter 28.XX inches Hg, and ʻhighʼ should route to another airport. 28 11. ICAO. Procedures for Air Naviga- transition [through FL 180];” and, precede an altimeter setting of 30.XX tion Services (PANS). Aircraft Operations, • A first officer on an air carrier 32 inches Hg.” Volume 1: Flight Procedures. Part VI, Al- passenger flight said, “Just before we timeter Setting Procedures. began descent, the flight attendant Notes 12. Carbaugh, David C. Interview with brought up dinner for both of us at 1. International Civil Aviation Organi- Lacagnina, Mark, and Werfelman, Linda. the same time. Started descent as [we] zation (ICAO). Manual on Implementation Alexandria, Virginia, U.S., Oct. 28, 2004. of a 300 m (1,000 ft) Vertical Separation started eating. Because of distraction, Flight Safety Foundation, Alexandria, Vir- Minimum Between FL290 and FL410 ginia, U.S. we failed to reset altimeters at 18,000 Inclusive. Document 9574. Second edi- 13. U.K. CAA Level Bust Working feet. Descended to 17,000 feet with tion—2002. Group. On the Level Project Final Report, wrong altimeter setting. Resulted in 2. Ibid. Civil Aviation Publication 710. London, level-off 300 feet below assigned al- 3. Joint Aviation Authorities (JAA). England. December 2000. titude. Received [traffic advisory] of Guidance Material on the Approval of Air- 14. European Organisation for Safety of craft and Operators for Flight in Airspace Air Navigation (Eurocontrol). “Level Bust traffic at 16,000 feet. Controller sug- Above Flight Level 290 Where a 300-me- Briefing Notes: General.” June 2004. This 29 gested that we reset altimeters.” ter (1,000-foot) Vertical Separation Mini- is one of 14 briefing notes—related papers ASRS said, “The cure is strict ad- mum Is Applied. Leaflet no. 6, Revision 1. about level-bust issues—that are part of the herence to checklists and procedures Jan. 10, 1999. European Air Traffic Management Level (sterile cockpit,30 readback of ATC 4. U.S. Federal Aviation Administration Bust Toolkit, a package of informational (FAA). Guidance Material on the Approval clearances, etc.) and good CRM [crew materials produced by Eurocontrol and de- of Operations/Aircraft for RVSM Opera- signed to raise awareness of the level bust resource management] techniques tions. Document No. 91-RVSM. Feb. 20, issue among aircraft operators and air navi- for cross-checking with the other 2004. gation service providers and to help them crewmember(s).” 5. U.K. Civil Aviation Authority (CAA) develop strategies to reduce level busts. Another element that sometimes in- Safety Regulation Group. Flight Opera- 15. Law, John. “ACAS Provides an Ef- troduces confusion is the use of metric tions Department Communication 7/2004. fective Safety Net When Procedures Are April 28, 2004. Followed.” Flight Safety Digest Volume altitudes in some countries (for ex- 6. Sanders, David. E-mail communica- 23 (March 2004). ample, in Russia and China). The FSF tion with Werfelman, Linda. Alexandria, 16. Blajev, Tzvetomir. E-mail communi- “ALAR Briefing Notes” said that this Virginia, U.S., Nov. 1, 2004; Nov. 5, 2004. cation with Werfelman, Linda. Alexandria, requires standard operating procedures Flight Safety Foundation, Alexandria, Vir- Virginia, U.S., Nov. 5, 2004. Flight Safety (SOPs) for the use of metric altimeters ginia, U.S. Sanders, a U.K. CAA press of- Foundation, Alexandria, Virginia, U.S. ficer, said that the investigation is likely to or conversion tables.31 17. Bateman, C. Donald. Telephone in- continue for an “extended period.” terview and e-mail communication with The “ALAR Briefing Notes” said 7. Honeywell. Air Transport Avionics. Werfelman, Linda. Alexandria, Virginia, that, in general, to prevent many al- www.cas.Honeywell.com/ats/products/ U.S., Oct. 26, 2004; Oct. 29, 2004. Flight timeter-setting errors associated with nav.cfm. Nov. 5, 2004. Safety Foundation, Alexandria, Virginia, different units of measurement or ex- 8. Carbaugh, David C. “Erroneous Flight U.S. tremes in barometric pressure, the fol- Instrument Information.” Boeing Aero No. 18. FSF. “ALAR (Approach-and-land- 23 (July 2003). ing Accident Reduction) Briefing Notes.” lowing SOPs should be used “when 9. Flight Safety Foundation (FSF) Con- (“ALAR Briefing Note 3.1—Barometric broadcasting (ATIS or controllers) or trolled-flight-into-terrain (CFIT) Task Altimeter and Radio Altimeter.”) Flight reading back (pilots) an altimeter set- Force. CFIT Education and Training Aid. Safety Digest Volume 19 (August–Novem- ting: 1996. Continued on following page

AVIONICS NEWS • APRIL 2005 79 RVSM reported incidents to ASRS, such as al- Continued from page 79 titude deviations, have been remarkably ber 2000). stable over many years. Therefore, users The “ALAR Briefing Notes” are part of ASRS may presume that incident re- of the ALAR Tool Kit, which provides on ports drawn from a time interval of several compact disc (CD) a unique set of pilot or more years will reflect patterns that are briefing notes, videos, presentations, risk- broadly representative of the total universe awareness checklists and other tools de- of aviation safety incidents of that type.” signed to help prevent approach-and-land- 23. NASA ASRS. Report no. 295007. ing accidents (ALAs) and CFIT. The tool January 1995. kit is the culmination of the Foundation-led 24. Thomas, Perry. “International Al- efforts of more than 300 safety special- timetry.” ASRS Directline No. 2 (October ists worldwide to identify the causes of 1991). ALAs and CFIT, and to develop practical 25. Patten, Marcia; Arri, Ed. “The Low- recommendations for prevention of these Down on Altimeter Settings.” ASRS Di- accidents. The tool kit is a compilation of rectline No. 9 (March 1997). work that was begun in 1996 by an inter- 26. NASA ASRS. Report no. 206218. national group of aviation industry volun- March 1992. teers who comprised the FSF ALAR Task 27. NASA ASRS. Report no. 289818. Force, which launched the second phase of November 1994. work begun in 1992 by the FSF CFIT Task 28. NASA ASRS. Report no. 290122. Force. December 1994. 19. Hectopascal is the air-pressure mea- 29. NASA ASRS. Report no. 295619. surement recommended by ICAO. The February 1995. term is derived from the name of 17th-cen- 30. The “sterile cockpit rule” refers to tury French mathematician Blaise Pascal, U.S. Federal Aviation Regulations Part who developed a method of measuring 121.542, which states, “No flight crew- barometric pressure, and the Greek word member may engage in, nor may any pilot- for 100. One hectopascal is the equivalent in-command permit, any activity during a of 100 pascals, or one millibar. One inch critical phase of flight which could distract of mercury is equivalent to 33.86 hecto- any flight crewmember from the perfor- pascals. mance of his or her duties or which could 20. FSF. News Release: “Flight Safety interfere with the proper conduct of those Foundation Campaign to Reduce Con- duties. Activities such as eating meals, trolled-flight-into-terrain Accidents Pro- engaging in nonessential conversations duces Safety Checklist, Specific Preven- within the cockpit and nonessential com- tion Recommendations.” Nov. 2, 1994. munications between the cabin and cockpit 21. FSF. “ALAR Briefing Notes.” crews, and reading publications not relat- 22. The U.S. National Aeronautics and ed to the proper conduct of the flight are Space Administration (NASA) Aviation not required for the safe operation of the Safety Reporting System (ASRS) is a con- aircraft. For the purposes of this section, fidential incident-reporting system. The critical phases of flight include all ground ASRS Program Overview said, “Pilots, operations involving taxi, takeoff and land- air traffic controllers, flight attendants, ing, and all other flight operations below mechanics, ground personnel and others 10,000 feet, except cruise flight.” [The FSF involved in aviation operations submit re- ALAR Task Force says that “10,000 feet” ports to the ASRS when they are involved should be height above ground level during in, or observe, an incident or situation in flight operations over high terrain.] which aviation safety was compromised.” 31. FSF. “ALAR Briefing Notes.” ASRS acknowledges that its data have cer- 32. Ibid. tain limitations. ASRS Directline (Decem- ber 1998) said, “Reporters to ASRS may introduce biases that result from a greater tendency to report serious events than minor ones; from organizational and geo- graphic influences; and from many other  factors. All of these potential influences Reprinted with permission from the reduce the confidence that can be attached November 2004 Flight Safety Foundation to statistical findings based on ASRS data. Flight Safety Digest. However, the proportions of consistently

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