Calculating Alternate Weather Requirements for Aerodromes with GFAs

When filing an IFR flight plan in Canada, pilots must indicate an alternate aerodrome—a backup location other than the destination—that is suitable for landing if the destination becomes unavailable.

Photo by Artturi Jalli on Unsplash

When selecting an alternate, pilots must consider several factors:

  • Location and proximity
  • Approaches available
  • Airport services
  • Weather conditions

If an aerodrome would not be suitable as a destination, it is unlikely to be suitable as an alternate.

Regulatory Requirements

The Canadian Aviation Regulations (CARs) state:

CAR 602.122 Except as otherwise authorized by the Minister in an air operator certificate or in a special authorization issued under subsection 604.05(2), no pilot-in-command shall operate an aircraft in IFR flight unless the IFR flight plan or IFR flight itinerary that has been filed for the flight under section 602.73 includes an alternate aerodrome having a landing area suitable for use by that aircraft.

CAR 602.123 No pilot-in-command of an aircraft shall include an alternate aerodrome in an IFR flight plan or IFR flight itinerary unless available weather information indicates that the ceiling and visibility at the alternate aerodrome will, at the expected time of arrival, be at or above the alternate aerodrome weather minima specified in the Canada Air Pilot.

When the Alternate is Not Served by a TAF

Many Canadian aerodromes are not served by Terminal Area Forecasts (TAFs). However, this does not disqualify them from being used as alternates. Instead of using a TAF, pilots must refer to a Graphical Area Forecast (GFA).

According to the Canada Air Pilot:

Aerodromes served with a GRAPHIC AREA FORECAST (GFA) may qualify as an alternate provided the forecast weather contains:

  • no cloud lower than 1000 ft above the lowest useable HAT/HAA;
  • no cumulonimbus; and
  • a visibility is not less than 3 miles.

Example: Using Miramichi, NB as an Alternate

Let’s walk through an example using Miramichi, New Brunswick as an alternate aerodrome. This is only an example based on historic charts and approaches.

Available Approaches at Miramichi:
  • RNAV 09
  • RNAV 27
  • NDB/DME 09
  • NDB/DME 27
Based on a weather briefing and wind conditions from nearby Bathurst and Moncton airports, runway 27 is expected to be favoured. However, for added caution, we’ll also include runway 09 in our calculations.

Step-by-Step HAT Review:

Our first step is to calculate the rounded heights above touchdown zones for each approach. Remember we round down when the value is 20 or less, and round up for 21 or more.

Approach Lowest HAT (AGL) Rounded
RNAV 27 384′ 400′
NDB/DME 27 404′ 400′
RNAV 09 392′ 400′
NDB/DME 09 452′ 500′

The lowest usable HAT is 400'. This means we can't have any forecasted cloud lower than 1400' (lowest usable HAT + 1000'). 

The challenge is that cloud altitudes on GFAs are always in Above Sea Level (ASL). So we must convert 1,400' to ASL. I do this by adding the touchdown zone elevation (TDZE) to the AGL number. The TDZE for runway 27 is 96. So when rounded to 100' and added to the AGL number, we get 1,500' ASL.

This means no visibility less than 3 miles, no cumulonimbus, and no clouds can be forecasted on the GFA lower than 1,500' ASL.


You might notice that the approach plate already lists the LNAV minima in ASL—for example, 480′ ASL. So why don’t we just add 1,000 feet to that number? In this particular case, adding 1,000 to the ASL value will get you to the same result. However, to comply with the Canada Air Pilot (CAP) requirements, the proper method is to apply rounding to the HAT (Height Above Touchdown), which is measured in AGL.

Here's why it matters:

Consider an alternate airport where the LNAV minimum is 619′ ASL or 443′ AGL.

  • If we round the ASL value, 619′ rounds down to 600′.
  • But if we round the AGL value, 443′ rounds up to 500′.
This difference highlights why the AGL value must be used and rounded first, before converting to ASL for your final GFA ceiling calculation.



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