Altimeter Setting and Operating Procedures

Reference AIM RAC 2.11 and 2.12

ALTIMETER SETTING REGION

CAR 602.35 

• Departure: altimeter setting at aerodrome, or if not available the elevation of the aerodrome
• Enroute: altimeter setting of nearest station along route
• Arrival: altimeter setting at destination

STANDARD PRESSURE REGION

CAR 602.36

• Departure: altimeter setting at aerodrome, or if not available the elevation of the aerodrome
• Enroute: immediately prior to reaching the intended flight level, set 29.92
• Arrival: altimeter setting at destination
• Holding: once descending below lowest flight level of hold, altimeter setting at destination

TRANSITION BETWEEN REGIONS

CAR 602.37

• the simplest way to think about this is to always change the altimeter in the standard pressure region, whether it is a lateral or vertical transition

Atmospheric Pressure

Pressure Measurements

• Inches of mercury (Hg) >> for altimetry
• Hectopascals (hPa) and millibars (mb) >> for weather map analysis
• measured using a barometer (digital or aneroid type)

METARs give both an altimeter setting and a sea level pressure.  What's the difference?

• Station Pressure
• measured at the airport; the weight of the air above the station
• Altimeter Setting
• station level pressure reduced to MSL assuming ISA conditions
• MSL Pressure
• station pressure reduced to MSL using the average surface temperature for the last 12 hours
• useful in weather map analysis when patterns across different observation stations must be compared

And just for fun, here's 'Under Pressure' by Queen:

How High Are We, REALLY?

True Altitude was something that always seemed to trip us up in ground school, but it's not actually that difficult.  In fact, if you read the instructions on your E6B, you're already more than half-way there!

True Altitude is particularly important when there are abnormally cold temperatures.  Usually the scenarios presented involve leveling off at a MEA, flying over a ridge, and realizing that you don't have nearly the amount of altitude clearance as you might have expected because it's -40. 

4 simple steps:

1. set air temperature over pressure altitude (make sure you're using the correct side!)
2. read true altitude over calibrated altitude scale (calibrated = planned - height of altimeter source)
3. add result of (2.) to height of altimeter source
4. subtract height of ridge to get ridge clearance

I'm tagging this both SAMRA and SARON because it fits in both Meteorology and Flight Operations.  So it could come up anywhere!