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52 Table 35. Comparison of HOT generated from different pilot aids. HOTDS Visibility METAR Visibility METAR Snow Test Daylight/ OAT Fluid Site 1 Site 2 (Statute Report and Snow Intensity # Darkness (C) Analyzed HOT HOT Miles) Visibility Chart Report (min) (min) 95 Darkness 3/4 0 Octagon no HOT 40 - 90 86 104 MaxFlo 107 Daylight 1/2 -4 100/0 25 - 60 25 - 60 36 47 97 Darkness 1/2 0 no HOT 30 - 55 93 120 ABC-S 75/25 123 Darkness 3 -8 25 - 50 25 - 50 79 112 The differences in snow intensity derived from the various sites is longer than the applicable HOT derived from METAR sources have a large impact on holdover guidelines: sources. The range in HOT determined by METAR snow intensity (e.g., for Test 95, 90-40= 50 min) compared to the variance The maximum snow intensity covered in the HOT table is in HOT (e.g., for Test 95, 104-86= 18 min) from the two moderate, thus for any snow intensity indications of heavy, test sites is shown in Table 36. there is no HOT value available to the pilot. Similarly, in cases where the METAR report leads to an in- dication of light snow, the HOT table for Types II and IV HOTDS Implementation Strategy fluid will provide a HOT time based on moderate snow. and Timeline This HOT time will usually be notably shorter than is re- The examination of HOT generated from METAR indica- ally necessary for light snow. tions showed that there is a genuine possibility that very dif- In Test 95, where the METAR reported the intensity of ferent values can result from the two alternative ways of ap- snow as light, the corresponding HOT table provides a plying METAR forecasts. range of holdover time from 40 to 90 minutes, based on The use of METAR indications to generate HOT has some moderate snow. The interpretation of this range can lead inherent shortcomings: to further shortening of holdover times. The Transport Canada Holdover Time Guidelines caution as follows: An important one is its frequency of issue, generally on an "The only acceptable decision-making criterion, for take- hourly basis; off without a pre-takeoff contamination inspection, is the The HOT values generated from METAR indications have shorter time within the applicable holdover time table airport-wide application, regardless of airport size; cell." Thus, in Test 95, the applicable holdover time based The precipitation rate reported in METARs (as light, moder- on METAR would be 40 minutes. ate, or heavy) is not correlated with the liquid water equiva- For the two test sites, the HOT values shown are based on lent (LWE) used during fluid testing to establish HOT guide- the actual test data. In the case of Test 95, the HOT values lines; and are 104 and 86 minutes, much longer than that based on Pilots must use subjective judgment when using METAR the METAR report. indications or when using personally estimated visibility The same observations apply to the other tests selected for distance in conjunction with HOT Guidelines to establish comparison. In all cases, the shortest HOT of the two test a HOT value. Table 36. Range in HOTS. Fluid Analyzed Test # Range in HOTS (minutes) From METAR From Test Data Octagon MaxFlo 95 50 18 100/0 Octagon MaxFlo 107 35 11 100/0 ABC-S 75/25 97 25 27 ABC-S 75/25 123 25 33