Calculating Alternate Weather Requirements for Aerodromes With TAFs

When calculating alternate aerodrome weather requirements, it is important to understand the difference between precision approaches and non-precision approaches.

The CARs define a precision approach as: “an instrument approach by an aircraft using azimuth and glide path information.” In other words, this type of approach guides aircraft horizontally and vertically. Examples of precision approaches are Instrument Landing System (ILS), Microwave Landing System (MLS), and Precision Approach Radar (PAR). This means RNAV or GNSS approaches with vertical guidance are not considered precision approaches.

The CARs define a non-precision approach as: “an instrument approach by an aircraft using azimuth information.” In other words, this type of approach provides horizontal information to aircraft. Examples of non-precision approaches are the Non-directional Beacon Approach (NDB), VHF Omni-directional Range Approach (VOR), Localizer Approach (LOC) and Global Navigation Satellite System Approach (GNSS).

A suitable runway or approach means a runway or approach that is suitable to use considering wind direction, aircraft configuration, equipment, and runway surface.

Understanding that a runway and approach are suitable is important because if a runway is not suitable, we can not use it to calculate alternate weather requirements.

Always refer to the current Canada Air Pilot (CAP) for alternate weather minima. The following chart is an exert from the Aeronautical Information Manual (AIM) for spring 2020. This chart can be found in RAC 3.14. The following weather requirements only apply to aerodromes served by terminal area forecasts (TAFs). Note that in Canada we always use altitudes in increments of 100 feet for alternate weather requirements. Any number that is 20 or less will round down, and any number that is 21 or more will round up. So, 620 feet will round to 600 feet, and 621 feet will round to 700 feet.

ALTERNATE WEATHER MINIMA REQUIREMENTS
FACILITIES AVAILABLE AT SUITABLE ALTERNATE	WEATHER REQUIREMENTS
TWO OR MORE USABLE PRECISION APPROACHES, each providing straight-in minima to separate suitable runways	400-1 or 200-1/2 above lowest usable HAT and visibility, whichever is greater.
ONE USABLE PRECISION APPROACH	600-2* or 300-1 above the lowest usable HAT and visibility, whichever is greater.
NON-PRECISION ONLY AVAILABLE	800-2* or 300-1 above the lowest usable HAT/HAA and visibility, whichever is greater.
NO IFR APPROACH AVAILABLE	Forecast weather must be no lower than 500 ft above a minimum IFR altitude that will permit a VFR approach and landing.
FOR HELICOPTERS, where instrument approach procedures are available	Ceiling 200 ft above the minima for the approach to be flown, and visibility at least 1 SM, but never less than the minimum visibility for the approach to be flown.

*600-2 and 800-2, as appropriate, are considered to be STANDARD ALTERNATE MINIMA.

Let’s look at some examples to help understand this information.

• ILS 29, 200 feet, 1/2 mile
• LOC 29, 380 feet, 1 mile
• LNAV 29, 230 feet, 1 mile
• VOR 24, 360 feet, 1 1/2 miles
• NDB 24, 460 feet, 1 1/2 miles

Step 3: I note that one precision approach is available at my ETA. Referring to the Alternate Weather Minima Requirements chart above, with one usable precision approach, the lowest allowable ceiling in the TAF is 600 feet, and the lowest visibility is 2 miles. The chart also says to add 300 feet to the lowest usable HAT, (300 + 200′ HAT = 500). Since 500 feet is lower than the standard 600 feet, I must use 600 feet. I also add 1 mile to the lowest visibility (1 + 1/2 miles = 1 1/2 miles). Since 1 1/2 miles is less than the standard 2 miles, I must use 2 miles.

In summary, if I want to use ABC Airport as my alternate, I must have a ceiling of not less than 600 feet and a visibility of not less than 2 miles in my TAF at my ETA.

Example 2:

• NDB 06, 580 feet, 2 mile
• LNAV 06, 330 feet, 1 1/2 mile
• VOR 11, 360 feet, 1 1/2 miles
• NDB 11, 460 feet, 1 1/2 miles

Step 3: I note that only non-precision approaches are available at my ETA. Referring to the Alternate Weather Minima Requirements chart above, with non-precision approaches, the lowest allowable ceiling in the TAF is 800 feet, and the lowest visibility is 2 miles. The chart also says to add 300 feet to the lowest usable HAT, (300 + 400′ HAT = 700). (Note that the lowest HAT, 330′, was rounded to 400′). Since 700 feet is lower than the standard 800 feet, I must use 800 feet. I also add 1 mile to the lowest visibility (1 + 1 1/2 miles = 2 1/2 miles). Since 2 1/2 miles is greater than the standard 2 miles, I must use 2 1/2 miles.

In summary, if I want to use DEF Airport as my alternate, I must have a ceiling of not less than 800 feet and a visibility of not less than 2 1/2 miles in my TAF at my ETA.

Example 3:

Step 1: I have chosen ABC Airport as my alternate by considering its location in relation to my planned destination, the services available, the large number of approaches available, NOTAMS, and the fact that the aerodrome has a TAF.

Step 2: After looking at the TAF, I determined that based on forecasted winds at my ETA, there will be nine available and suitable approaches. The winds are very light today. Looking at the approach plates for these approaches, I note the approach types, the height above the touchdown zone (HAT), and the recommended viability.

• ILS 29, 200 feet, 1/2 mile
• LOC 29, 380 feet, 1 mile
• LNAV 29, 230 feet, 1 mile
• VOR 24, 360 feet, 1 1/2 miles
• NDB 24, 460 feet, 1 1/2 miles
• NDB 06, 580 feet, 2 mile
• LNAV 06, 330 feet, 1 1/2 mile
• ILS 11, 200 feet, 1/2 miles
• NDB 11, 460 feet, 1 1/2 miles

Step 3: I note that two precision approaches are available at my ETA. Referring to the Alternate Weather Minima Requirements chart above, I see that with two precision approaches, the lowest allowable ceiling in the TAF is 400 feet and the lowest visibility is 1 mile. But I also note that the chart says, “TWO OR MORE USABLE PRECISION APPROACHES, each providing straight-in minima to separate suitable runways.” Since ILS 11 and ILS 29 both approach the same runway surface, we can only take credit for one precision approach. For one precision approach, the lowest allowable ceiling in the TAF is 600 feet, and the lowest visibility is 2 miles. The chart also says to add 300 feet to the lowest usable HAT (300 + 200′ HAT = 500′). Since 500 feet is lower than the standard 600 feet, I must use 600 feet. I also add 1 mile to the lowest visibility (1 + 1/2 miles = 1 1/2 miles). Since 1 1/2 miles is less than the standard 2 miles, I must use 2 miles.

In summary, if I want to use ABC Airport as my alternate, I must have a ceiling of not less than 600 feet and a visibility of not less than 2 miles in my TAF at my ETA.

Finally, if your chosen alternate aerodrome requires 600-2 or 800-2, which are considered standard, you can apply a sliding rule. As seen below, reduced visibility is acceptable for a higher forecasted ceiling, but only what is seen in the chart.

• 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