When calculating an IFR alternate for an IFR flight plan in Canada a pilot must ensure that the terminal area forecast (TAF) does not fall below the calculated alternate minima.
There is sometimes confusion with BECMG, TEMPO and PROB in regards to IFR alternates. The Transport Canada Aeronautical Information Manual defines them as follows:
BECMG-Permanent Change Group (Gradual): If a permanent change in a few weather elements is forecast to occur gradually, with conditions evolving over a period of time (normally one to two hours, but not more than four hours), the new conditions that differ from those immediately prior are indicated following “BECMG.” Normally only those elements for which a change is forecast to occur will follow “BECMG.” Any forecast weather element not indicated as part of the “BECMG” group remains the same as in the period prior to the onset of the change.
TEMPO-Transitory Change Group: If a temporary fluctuation in some or all of the weather elements is forecast to occur during a specified period, the new conditions that differ from those immediately prior are indicated following “TEMPO.” In other words, when an element is not indicated after “TEMPO,” it shall be considered to be the same as that for the prior period. The time period, as with”BECMG,” is indicated by two four-digits date/time groups following “TEMPO.” The first two digits of each group indicate the date, while the last two digits of each group indicate the time in whole UTC hours.
PROB-Probability Group: In order to indicate the probability of occurrence of alternative values of forecast groups, PROB30 (a 30% probability) or PROB40 (a 40% probability) is placed directly before the change group’s validity period and alternative value(s) to indicate that different conditions will occur within the specified time period. The time period is given in whole UTC hour values. For example, “PROB30 2817/2821” would indicate that between 1700Z and 2100Z on the 28th day of the month there is a 30% probability that the indicated weather will occur. The weather elements used in the PROB group are restricted to hazards to aviation, which include, but are not limited to, the following:
- freezing precipitation;
- low-level wind shear below 1 500 ft AGL; or
- ceiling and visibility values important to aircraft operations (e.g. threshold such as alternate limits, lowest approach limits).
There are two ways to look at a BECMG period. The weather is either improving, or deteriorating. We never know exactly when the weather will improve or deteriorate during the BECMG period, so we must interpret the two differently. Remember also that “improving” or “deteriorating” weather is subjective and depends on the situation.
When the BECMG is forecasting improving weather we must presume that the weather will not improve until the end of the BECMG period. When the BECMG is forecasting deteriorating weather we presume that the weather will deteriorate at the beginning of the period.
TAF CYDP 171938Z 1720/1723 33018G28KT P6SM -SHRA FEW025 OVC080
BECMG 1720/1722 30012G22KT
RMK NXT FCST WILL BE ISSUED AT 180945Z=
In this example TAF we can see the BECMG period forecasted for the 17th of this month, between 20-22z. The winds will change from 330º at 18 gusting 28 kts. to 300º at 12 gusting 22 kts.
If the wind change in this BECMG period is an improvement, the wind is within the aircraft’s crosswind limit, the wind change is considered to take effect at 22z.
If the wind change in this BECMG period is a deterioration, wind is outside of the aircraft’s crosswind limit, the wind change if considered to take effect at 20z.
TAF CYLL 171738Z 1718/1806 14018G28KT P6SM SCT100 BKN220
FM172200 16015KT BKN080
BECMG 1801/1803 BKN007
FM180300 16010KT P6SM BKN007
RMK NXT FCST BY 180000Z=
In this example TAF we can see the BECMG period forecasted for the 18th of this month, between 01-03z. The ceiling will go from broken at 8000′ to broken at 700′.
As the ceiling in this BECMG period is a deterioration, the new ceiling is considered to take effect at 01z.
The weather during a TEMPO can not fall below your alternate minima for an aerodrome. Sometimes a TEMPO forecast will show an improvement. Credit for improving weather in a TEMPO can not be taken.
There is a level of uncertainty when it comes to legally defining what to do when TEMPOs show improving weather conditions. Expand the information below to see how I derive this interpretation.
TEMPO Interpretation[expand] The explanation of a TEMPO in the AIM under MET 3.9.3 (page 145) explains that a TEMPO “is only used when the modified forecast condition is expected to last less than one hour in each instance, and if expected to recur, the total period of the modified condition will not cover more than half of the total forecast period.” When the TEMPO conditions are not happening, the forecasted conditions prior to the TEMPO are what is expected. Since we don’t know when these TEMPO conditions will occur we have to use worst case scenario. That being said, in the opposite situations when an element of a TEMPO improves we also have to use worst case scenario since we don’t know when in the TEMPO period this will happen.[/expand]
TAF CYQM 171738Z 1718/1818 34006KT P6SM SCT006 OVC012
FM172300 01008KT 6SM BR OVC006 TEMPO 1723/1806 3SM -DZ BR BKN003 OVC006
FM180600 04005KT 3SM BR OVC003 TEMPO 1806/1813 OVC006
FM181300 07008KT 5SM BR OVC006
FM181600 08006KT P6SM OVC010
RMK NXT FCST BY 180000Z=
In the first TEMPO of this example TAF temporarily between 23z and 06z the visibility will drop from 6 miles to 3 miles, light drizzle and the ceiling will drop from 600′ to 300′. Since the ceiling is below required alternate minima, this aerodrome could not be used as an alternate during this period.
In the second TEMPO of this example TAF temporarily between 06z and 13z the ceiling will climb to 600′. Normally, an aerodrome with a forecasted ceiling of 600′ can be used as an alternate, if the aerodrome has a usable ILS approach, but since the 600′ is a TEMPO during a forecast ceiling of 300′, credit can not be taken.
PROB is like a TEMPO but with one distinct difference. When a PROB is found in a TAF it can not fall lower then the landing minima not alternate minima.
Like a TEMPO, credit for improving weather in a PROB can not be taken.
TAF CYDQ 171839Z 1719/1805 03008KT P6SM SCT040 BKN080 TEMPO 1719/1803
5SM -SHRA BR SCT020 BKN040 PROB30 1721/1803 2SM RA BR OVC007
FM180300 VRB03KT P6SM SCT040 BKN080
RMK FCST BASED ON AUTO OBS. NXT FCST BY 180100Z=
In this TAF the PROB30 between 21z and 03z is 2 miles visibility, rain, mist and a ceiling of 700′. This aerodrome, Dawson Creek, has only two non-precision approaches. Without the PROB30 this aerodrome would be a legal alternate. Normally, 2 miles visibility and a ceiling of 700′ would disqualify Dawson Creek from being a legal alternate. Since these values fall in a PROB and they don’t fall lower then the landing minima, Dawson Creek is still a legal alternate.
Below is an exert from an NDB approach in Dawson Creek. The landing minima are highlighted in the red boxes. Remember that a value of 552 would be rounded to 600.
- Canada Air Pilot – Published by Nav Canada and is available for purchase from many Canadian pilot supply stores and Nav Canada’s online store: http://products.navcanada.ca/
- Canadian Aviation Regulations – http://www.tc.gc.ca/eng/civilaviation/regserv/cars/menu.htm
- Aeronautical Information Manual – http://www.tc.gc.ca/eng/civilaviation/publications/tp14371-menu-3092.htm