APPROACH-AND-LANDING ACCIDENT REDUCTION TOOL KIT fsf alar briefing Note 3.1 B arometric Altimeter and Radio Altimeter light crews on international routes encounter different units of measurement for setting barometric altimeters, When in. Hg is used for the altimeter setting, unusual baromet- • Inches of mercury (in. Hg). Fthus requiring altimeter cross-check procedures. ric pressures, such as 28.XX in. Hg (low pressure) or 30.XX in. Hg (high pressure), may go undetected when listening to the Statistical Data automatic terminal information service (ATIS) or ATC, resulting The Flight Safety Foundation Approach-and-landing Accident in a more usual 29.XX altimeter setting being set. Reduction (ALAR) Task Force found that lack of positional Figure 1 and Figure 2 show that a 1.00 in. Hg discrepancy in awareness was a causal factor1 in 51 percent of 76 approach- the altimeter setting results in a 1,000-foot error in the indicated and-landing accidents and serious incidents worldwide in altitude. 1984 through 1997.2 The task force said that these accidents In Figure 1, QNH is an unusually low 28.XX in. Hg, but the and incidents generally involved lack of vertical-position altimeter was set mistakenly to a more usual 29.XX in. Hg, re- awareness and resulted in controlled flight into terrain (CFIT). sulting in the true altitude (i.e., the aircraft’s actual height above mean sea level) being 1,000 feet lower than indicated. QNH or QFE? In Figure 2, QNH is an unusually high 30.XX in. Hg, but the altim- QNH (altimeter setting that causes the altimeter to indicate eter was set mistakenly to a more usual 29.XX in. Hg, resulting in height above mean sea level [i.e., field elevation at touchdown the true altitude being 1,000 feet higher than indicated. on the runway]) has the advantage of eliminating the need to Confusion about units of measurement (i.e., hPa vs. in. Hg) change the altimeter setting during operations below the transi- leads to similar errors. tion altitude/flight level (FL). In Figure 3, a QNH of 991 hPa was set mistakenly on the al- QNH also eliminates the need to change the altimeter set- timeter as 29.91 in. Hg (equivalent to 1012 hPa), resulting in the ting during a missed approach, whereas such a change usually true altitude being 640 feet lower than indicated. would be required when QFE (altimeter setting that causes the altimeter to indicate height above the QFE reference datum [i.e., Setting the Altimeter zero at touchdown on the runway]) is used. To help prevent errors associated with different units of mea- Some operators set the altimeter to QFE in areas where air surement or with unusual values (low or high), the following traffic control (ATC) uses QNH and the majority of operators SOPs should be used when broadcasting (ATIS or controllers) or use QNH. Standard operating procedures (SOPs) can prevent reading back (pilots) an altimeter setting: altimeter-setting errors. inches or hectopascals), should be announced. Units of Measurement • All digits, as well as the unit of measurement (e.g., The most common units of measurement for setting altimeters A transmission such as “altimeter setting six seven” can be in- are: terpreted as 28.67 in. Hg, 29.67 in. Hg, 30.67 in. Hg or 967 hPa. Stating the complete altimeter setting prevents confusion and and, allows detection and correction of a previous error. • Hectopascals (hPa) [previously referred to as millibars (mb)]; FLIGHT SAFETY FOUNDATION ALAR TOOL KIT | ALAR BRIEFING NOTE 3-1 | 1 Effect of a One-Inch-High Altimeter Setting Actual Intended approach path height 1,000 AFL IIndicated altitude Actual altitude 4,000 feet 3,000 feet MSL Field elevation 2,000 feet Sea level QNH: 28.XX inches Hg Altimeter error 1,000 feet Altimeter setting: 29.XX inches Hg AFL = above field level; MSL = mean sea level; Hg = mercury; QNH = Altimeter setting that causes altimeter to indicate height above mean sea level (thus, field elevation at touchdown) Source: FSF ALAR Task Force Figure 1 Effect of a One-Inch-Low Altimeter Setting Intended approach path Actual height Indicated altitude 3,000 AFL 4,000 feet Actual altitude 5,000 feet MSL Field elevation Altimeter setting: 29.XX inches Hg 2,000 feet Sea level QNH: 30.XX inches Hg Altimeter error 1,000 feet AFL = above field level; MSL = mean sea level; Hg = mercury; QNH = Altimeter setting that causes altimeter to indicate height above mean sea level (thus, field elevation at touchdown) Source: FSF ALAR Task Force Figure 2 of 28.XX in. Hg and “high” should precede an altimeter setting • When using in. Hg, “low” should precede an altimeter setting • High workload; of 30.XX in. Hg. • A deviation from defined task sharing; An incorrect altimeter setting often is the result of one or more of the following factors: • An interruption/distraction; • Inadequate cross-checking by flight crewmembers; or, 2 | FLIGHT SAFETY FOUNDATION ALAR TOOL KIT | ALAR BrIEFING NOTE 3-1 Effect of an Altimeter Mis-Set to Inches, Rather Than Hectopascals Intended approach path Actual height 1,360 AFL Indicated altitude Actual altitude 4,000 feet 3,360 feet MSL Field elevation 2,000 feet Sea level QNH: 991 hPa Altimeter error 640 feet Altimeter setting: 29.91 inches Hg (1012 hPa) AFL = above field level; MSL = mean sea level; Hg = mercury; hPa = hectopascals ; QNH = Altimeter setting that causes altimeter to indicate height above mean sea level (thus, field elevation at touchdown) Source: FSF ALAR Task Force Figure 3 Altimeter References The barometric-altimeter reference (“bug”) and the radio- • Adherence Confusion to about the defined units of task measurement. sharing (for normal conditions or abnormal conditions) and normal checklists are effective altimeter decision height (RA DH) bug must be set according to defenses to help prevent altimeter-setting errors. the aircraft manufacturer’s SOPs or the company’s SOPs. Table 1 shows some examples. For all approaches, except Category (CAT) I instrument land- Metric Altimeter ing system (ILS) approaches with RA DH, CAT II ILS approaches Metric altitudes in certain countries (e.g., Russia and China) also and CAT III ILS approaches, the standard call “minimum” will require SOPs for the use of metric altimeters (or conversion be based on the barometric-altimeter bug set at the minimum tables). descent altitude/height [MDA(H)] or decision altitude/height [DA(H)]. Crossing the Transition Altitude/Flight Level Radio-altimeter standard calls can be either: The transition altitude/flight level can be either: • Announced by the pilot not flying/pilot monitoring (PNF/PM) • Fixed for the whole country (e.g., FL 180 in the United States); or the flight engineer; or, or, Standard calls are tailored to the company SOPs and to the type • Fixed for a given airport (as indicated on the approach chart); • Generated automatically by a synthesized voice. of approach. To enhance the flight crew’s awareness of terrain, the stan- broadcast). • Variable, depending on QNH (as indicated in the ATIS dard call “radio altimeter alive” should be announced by the Depending on the airline’s/flight crew’s usual area of operation, first crewmember observing radio-altimeter activation at 2,500 changing from a fixed transition altitude/flight level to vari- able transition altitudes/flight levels may result in a premature The radio altimeter then should be included in the instrument feet above ground level (AGL). resetting or a late resetting of the altimeter. scan for the remainder of the approach. An altitude constraint (expressed in altitude or flight level) The radio altimeter indicates the aircraft’s height above also may delay or advance the setting of the standard altim- the ground, not the aircraft’s height above airport elevation. eter setting (1013.2 hPa or 29.92 in. Hg), possibly resulting The radar altimeter does not indicate height above trees or in crew confusion. towers. FLIGHT SAFETY FOUNDATION ALAR TOOL KIT | ALAR BRIEFING NOTE 3-1 | 3 altimeter indicates. Thus, the familiar axiom: “high to low, hot to Barometric-Altimeter and cold — look out below.” Radio-Altimeter Reference Settings The International Civil Aviation Organization (ICAO) publish- Approach Barometric Altimeter Radio Altimeter es altitude corrections (based on the airport surface tempera- Visual MDA(H)/DA(H) of 200 feet* ture and the height above the elevation of the altimeter-setting instrument approach or source) to be made to the published minimum safe altitudes.3 200 feet above For example, Figure 4 shows that when conducting an ILS ap- airport elevation proach with a published minimum glideslope intercept altitude Nonprecision MDA/(H) 200 feet* of 2,000 feet and an OAT of –40 degrees Celsius (–40 degrees ILS CAT I with no RA DA(H) 200 feet* Fahrenheit), the minimum glideslope intercept altitude should ILS CAT I with RA DA(H) RA DH be increased by 440 feet. ILS CAT II DA(H) RA DH The pilot is responsible for making this correction, except ILS CAT III with DH DA(H) RA DH when under radar control in a radar-vectoring area (because ILS CAT III with no DH TDZE Alert height the controller is responsible normally for terrain clearance, including accounting for the cold temperature correction). MDA(H) = minimum descent altitude/height; DA(H) = decision altitude/ height; ILS = instrument landing system; CAT = category; Nevertheless, the pilot should confirm this responsibility with RA DH = radio altimeter decision height; TDZE = touchdown zone elevation the air traffic services of the country of operation. * The RA DH should be set (e.g., at 200 feet) for terrain-awareness purposes. The Flight crews must apply the ICAO corrections for low tem- use of the radio altimeter should be discussed during the approach briefing.