Pressurization

Reference Turbine Pilot's Flight Manual Chapter 5

HOW DOES IT WORK?
Bleed air from the engines is continually distributed to the cabin.  Outflow valves modulate the exhaust of cabin air to obtain the desired level of pressurization.  

DEFINITIONS

• Maximum Differential (max diff): the maximum ratio of cabin pressure to outside (ambient) air pressure that the aircraft can sustain.
• Positive Pressure Relief Valves (Safety Valves): vent excess pressure overboard if max diff is exceeded (prevents overpressurization)
• Negative Pressure Relief Valves: ensure cabin never falls below ambient pressure
• Dump Valves: allow pilots to manually vent in an emergency (i.e. smoke)

• Squat Switch: ensures aircraft is depressurized on the ground

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

Wind

Reference ACWM Chapters 5 and 11

Wind is the result of changes in pressure gradient.

• flows from High to Low 
• steeper gradient = stronger wind

Coriolis Effect

• the deflection of wind caused by the Earth's rotation
• varies from zero at equator to maximum strength at poles

Curvature Effect

• when isobars are curved, air moves in an arc >> centrifugal force
• the pressure gradient around lows is generally stronger than around highs

Friction

• As you climb from the surface to 3000 feet, the wind veers (clockwise change in direction) and increases.
• As you descend from 3000 feet to the surface, the wind backs (counterclockwise change in direction) and decreases.
• We must also keep in mind that land and water create different amounts of friction.  There is less friction over water, so wind would blow at less of an angle across isobars and at faster speeds. 

Squalls and Gusts

• characteristics of turbulent flight conditions
• Gusts >> rapid peaks and lulls
• Squalls >> sudden increase lasting for a minute or more, then a decrease

Diurnal Effects

• surface winds are usually stronger and gustier during the day
• Sea Breeze during the day (higher pressure over water than land)
• Land Breeze at night

Topographical Effects

• hilly and mountainous terrain can contribute to mechanical turbulence
• Anabatic winds flow upslope during the day when mountain slops facing the sun are heated
• Katabatic winds flow downslope
• warm = Chinook wind, warms at DALR
• cold = glacier wind, cooling by underlying ice

Wind Shear

• increased performance (headwind) >> airspeed increases
• encountering increased performance on glideslope >> would have to reduce power to recapture glidepath, then increase power to maintain due to stronger headwind
• decreased performance (tailwind) >> airspeed decreases

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: