Altimeter Settings: When Do You Set 29.92?

Which region are you in?

Canada uses two altimeter regions, and the rule you fly by depends on which one you are in.

• Altimeter setting region: the low-level airspace, below 18,000 ft ASL, over southern Canada (TC AIM RAC 2.10). The Designated Airspace Handbook (TP 1820E) draws it as all low-level airspace within the Southern Domestic Airspace, which CARs 101.01 points to as its legal definition. Most PPL flying is here.
• Standard pressure region: everything else in Canadian Domestic Airspace (CARs 101.01). That is all airspace at or above 18,000 ft ASL, and also the low-level airspace outside the setting region's lateral limit: in the north, standard pressure starts at the surface (TC AIM RAC 2.11).

So there is no single "transition altitude" for every flight. 18,000 ft ASL is the boundary only while you are inside the setting region's lateral limit. TC AIM RAC Figure 2.9 maps both regions.

In the altimeter setting region: CARs 602.35

Three moments, three settings:

1. Before take-off: the aerodrome's altimeter setting. If you cannot get it, set the aerodrome elevation.
2. En route: the setting of the nearest station along the route. Where those stations are more than 150 NM apart, use a station near the route.
3. Before descending to land: the destination aerodrome's setting, if you can get it.

The en route update is the one candidates forget. A setting from a station 200 NM behind you can be badly wrong where you are now.

Crossing between the regions: CARs 602.37

You make the change while you are in the standard pressure region:

• Climbing out of the setting region: set 29.92 in Hg (1,013.2 mb) immediately after you enter the standard pressure region.
• Descending into the setting region: set the nearest station's setting immediately before you enter it. ATC normally gives you the setting with the descent clearance; if it does not, ask for it (TC AIM RAC 2.11(d)).

ATC can authorize something else, which is why the rule opens with "Except where otherwise authorized by an air traffic control unit". Drag the aircraft through a whole flight below.

Taking off inside the standard pressure region: CARs 602.36

When the aerodrome itself is in the standard pressure region, as in the north, the departure is the same as anywhere: the aerodrome setting, or its elevation, before take-off. You then set 29.92 before reaching the flight level you will fly. For a cruising level above FL 180, setting it at 18,000 ft ASL is acceptable (TC AIM RAC 2.11(b)).

Before descending to land, set the aerodrome's setting, if obtainable. If you hold first, keep 29.92 until immediately before you descend below the lowest flight level of the hold (CARs 602.36(2)).

Once 29.92 is set in the standard pressure region, you read flight levels, not altitudes: FL 250 is an indication of 25,000 ft (TC AIM RAC 2.11, Note).

Why the setting matters: 1,000 ft per inch

An altimeter measures pressure and turns it into height from the setting you give it. Each inch of mercury of error moves its "zero" by about 1,000 ft (TC AIM AIR 1.5.3).

• Setting too high for where you are: the altimeter reads too high, and you are lower than it shows. This is the dangerous one.
• Setting too low: the altimeter reads too low, and you are higher than it shows.

TC AIM AIR 1.5.3 works it through. A pilot sets 29.80 in Hg at departure and does not reset until 25 NM from the destination, where the setting is 29.20. The setting is 0.60 in Hg too high, so the aircraft is about 600 ft lower than indicated. Crossing a 4,800 ft ridge at an indicated 6,000 ft, the pilot expects 1,200 ft of clearance and has 600 ft.

Where candidates lose marks

• 29.92 on the ground in the setting region. It is the standard pressure setting, not a departure setting. Below 18,000 ft in the south you fly on local settings.
• When to set 29.92 on the climb. Coming up from the setting region, it is immediately after entering the standard pressure region (602.37(a)). "Before reaching the flight level" is the rule for a flight already operating in the standard pressure region (602.36(1)(b)).
• The destination setting before take-off. Before take-off it is the departure aerodrome's setting; the destination's comes before the descent to land.
• Skipping the en route update, or forgetting the 150 NM provision.
• The fallback. No setting available before take-off? Set the aerodrome elevation.
• Units. Standard pressure is 29.92 in Hg or 1,013.2 mb. Both appear in questions.
• Too high a setting reads too high. You are lower than the altimeter says.

Keep practising free

Studying for the PPL written after work, and you fly in southern Canada? Altimeter settings are on the PPL and CPL exam syllabus (TP 12880E and TP 12881E), and three steps cover them:

1. Learn the three moments of CARs 602.35, then the two transitions of 602.37. Work the figure above to the end of the flight.
2. Answer the four questions below, then one a day with the free Question of the Day.
3. On your next cross-country, say the setting out loud at each point: before take-off, at each station along the route, and before the descent to land.

Where exactly the setting region ends on a long northern route is drawn in TC AIM RAC Figure 2.9 and defined in the Designated Airspace Handbook. Look it up for your route; it is not something to guess.

• What the PPL written covers, and how it is marked: the PPL exam guide. For the commercial exam, the CPL exam guide.
• Which altitude do you cruise at, once the setting is right? VFR cruising altitudes.
• Which airspace are you in? Airspace classes A to G.

Practice

Original questions in the style of the Transport Canada written exams. Tap an answer to see why it is right or wrong.

1. Question 1 of 4

   You depart an aerodrome in the altimeter setting region and climb to cruise at FL 210. Under CARs 602.37, when do you set 29.92 in Hg?

   1Only when ATC includes it in the climb clearance2On levelling off at FL 210, the flight level to be flown3Before take-off, so that the climb is flown on standard pressure4Immediately after entering the standard pressure region

   Tap an answer to check it.

2. Question 2 of 4

   En route in the altimeter setting region, the nearest stations along your route are 200 NM apart. Which altimeter setting does CARs 602.35 require?

   1The departure aerodrome's setting, until the descent to land229.92 in Hg until a station is within 150 NM3The setting of a station near the route of flight4The destination aerodrome's setting, from the midpoint of the route

   Tap an answer to check it.

3. Question 3 of 4

   You leave with 29.80 in Hg set and do not reset it. Where you now are, the altimeter setting is 29.20 in Hg, and the altimeter reads 6,000 ft. About how high are you?

   15,940 ft ASL26,600 ft ASL35,400 ft ASL46,000 ft ASL

   Tap an answer to check it.

4. Question 4 of 4

   You are about to take off from an aerodrome in the north that lies in the standard pressure region. What do you set before take-off?

   1The setting of the nearest station inside the altimeter setting region2Nothing; in this region the altimeter is read as a flight level from the surface329.92 in Hg, because the aerodrome is in the standard pressure region4The aerodrome's altimeter setting, or its elevation if none is obtainable

   Tap an answer to check it.

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