Cruising Altitudes and Altimeter Settings

Why it matters

Two aircraft converging head-on at the same altitude is the worst geometry in aviation: the closing speed is highest and the other aircraft is hardest to see. The cruising-altitude rule attacks this problem with arithmetic. Eastbound and westbound traffic are assigned different thousands of feet, VFR and IFR traffic are offset from each other by 500 feet, and everyone in a region measures altitude against the same pressure reference. Follow the rule and an opposite-direction aircraft on a collision course is already a thousand feet away from you before either of you has spotted the other.

The two halves of this unit — where to cruise (CAR 602.34) and how to set the altimeter (CAR 602.35 to CAR 602.37) — only work together, which is why the exam tests them together.

Odd or even, plus 500

In level cruising flight above 3,000 feet AGL, a VFR aircraft must fly at a cruising altitude appropriate to its track unless ATC assigns something else. IFR aircraft follow the rule at all altitudes. The track that matters is the magnetic track in the Southern Domestic Airspace and the true track in the Northern Domestic Airspace, where the compass becomes unreliable.

Below 18,000 feet the table in CAR 602.34 works like this:

A mnemonic many pilots use: "East is odd" — heading east (000° to 179°) you fly odd thousands, and as a VFR pilot you add 500. Westbound (180° to 359°) you fly even thousands plus 500. So a VFR flight on a magnetic track of 095° cruising above 3,000 feet AGL belongs at 3,500, 5,500 or 7,500 feet; the same aircraft coming home on a track of 275° belongs at 4,500 or 6,500.

Three refinements round out the rule:

• 3,000 feet AGL, not ASL. Over terrain at 2,000 feet elevation, the rule only bites above 5,000 feet indicated. Below 3,000 feet AGL you choose any altitude that satisfies the minimum-altitude rules of the previous unit.
• Track, not heading. In a strong crosswind your heading may be 170° while your track is 185°: the track decides, and you would fly an even-plus-500 altitude.
• Above 18,000 feet the same odd/even logic continues as flight levels (FL 190, FL 210 eastbound; FL 200, FL 220 westbound, with further spacing rules at very high levels), but PPL holders without an IFR rating stay well below that.

An ATC clearance to another altitude overrides the table, and aerial survey and mapping flights may be exempted under strict conditions.

Two regions, two ways to set the altimeter

An altimeter is a barometer with a dial in feet. To read the correct altitude it must be told the current sea-level pressure, and Canada uses two different conventions depending on where you are.

The altimeter-setting region is the Southern Domestic Airspace below 18,000 feet ASL — where nearly all PPL flying happens. Here you keep the altimeter set to the local altimeter setting, a pressure value supplied by a weather station, so that the altimeter reads true altitude above sea level near that station. CAR 602.35 gives the procedure:

1. Immediately before take-off, set the aerodrome's altimeter setting. If none is obtainable, set the altimeter to the aerodrome elevation.
2. In flight, set the altimeter setting of the nearest station along your route. If the stations along the route are more than 150 nautical miles apart, use a station near the route.
3. Immediately before starting the descent to land, set the destination aerodrome's altimeter setting, if it is obtainable.

The standard pressure region is the Northern Domestic Airspace, plus all airspace at or above 18,000 feet ASL. Here everyone cruises with the altimeter set to the standard pressure of 29.92 inches of mercury (1013.2 millibars), and altitudes are called flight levels. CAR 602.36 keeps the take-off and landing steps the same as in the south — set the aerodrome altimeter setting (or elevation) before take-off, and the aerodrome setting before descending to land — but adds: before reaching your cruising flight level, set 29.92. During a holding procedure before landing, you keep 29.92 until you descend below the lowest flight level of the hold.

Crossing from one region to the other

CAR 602.37 fixes the moment you change the setting, unless ATC authorises otherwise:

• Flying into the standard pressure region, set 29.92 immediately after entering it.
• Flying into the altimeter-setting region, set the nearest station's altimeter setting immediately before entering it.

Either way, the aircraft is on the local setting while in the altimeter-setting region and on standard while in the standard pressure region; the switch happens on the standard-pressure side of the boundary.