GPS Approaches

Recall the basic operation of GPS: it triangulates your position by measuring distances from satellites by precise timing of radio signals.  4 satellites are required to obtain a 3D position fix. 

Reference AIM COM 3.14

GPS approaches are generally more efficient because they allow pilots to bypass procedure turns and proceed directly to the FAF.   GPS approaches must be retrieved from a current avionics database.  Pilot-generated waypoints are not approved for approach procedures.  There are two types of GPS approaches: stand-alone and overlay.

Stand-Alone

• approach design is usually based on a 'T' pattern 
• charted as "RNAV (GPS) RWY XX"

Overlay

• the underlying navaids do not have to be monitored
• you can use the GPS when the traditional navaid is out of service

There is always the requirement for a RAIM check (for +/- 15 minutes of the ETA) Without it, you have no assurance of the accuracy of the GPS position!

If you want to take credit for a GPS approach at an alternate aerodrome (AIM COM 3.14.12):

• there must be a usable approach at the planned destination which is served by a functioning traditional aid
• the published LNAV minima are the lowest landing limits for which credit may be taken when determining alternate weather minima requirements (not LNAV/VNAV or LPV)
• approach-level RAIM must be available at the ETA for the alternate
• periodically during the flight, and at least once before the mid-point of the flight to the destination, verify that approach-level RAIM is expected to be available at the planned alternate at the ETA

Note: There are GPS and WAAS NOTAM files which can advise of outages / failures

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:

Pressure Levels

In my January 2 post about the Earth's atmosphere, I said "some things you just have to memorize" with regards to which pressures correspond to which levels of the ICAO standard atmosphere.  As it turns out, memorization is not required if you have a CX-2 flight computer (and possibly others, but this is the one I have)!   Here's how:

• press 'FLIGHT'
• select (1) Altitude
• select (3) Std Atmos
• enter the altitude
• read the OAT, as well as pressure in "Hg and mb!

Small wins :)

Lapse Rates and Stability

Adiabatic Processes

• no heat is added or removed from air
• rising air >> lower pressure >> expands >> temperature decreases
• sinking air >> higher pressure >> compresses >> temperature increases

Lapse Rates

• Dry Adiabatic Lapse Rate (DALR): 3 C / 1000 ft
• Saturated Adiabatic Lapse Rate (SALR): 1.5 C / 1000 ft
• Average: 2 C / 1000 ft

• Environmental Lapse Rate (ELR): indicates the temperature of the surrounding air; comparison to lapse rate of air parcel determines if it is stable or unstable
• shallow lapse rate compared to SALR: absolute stability
• lapse rate between DALR and SALR: conditional instability
• steeper lapse rate than DALR: absolute instability

Characteristics of Stable Air

• sustained low visibility (i.e. haze layers, drizzle, fog)
• continuous precipitation
• strato-form clouds (layers)
• steady winds

Characteristics of Unstable Air

• good visibility
• showery precipitation
• cumulo-form clouds (vertical development)
• gusty winds

DO YOU KNOW... why rising saturated air cool less rapidly than rising unsaturated air?

• heat is released during the condensation of water vapour

V Speeds

I knew the definitions of V1 and V2, but what about V3? Turns out we just call it something different on the Dash 8 - it's Vfri (flap retraction speed).  There are a ton of V Speeds, but I'll pick out some key ones. 

V1: engine failure recognition speed

• abort the take-off and bring the aircraft to a stop on the runway plus stopway OR
• continue and be at V2 at 35 feet above the departure end of the runway
• increases with weight

V2: take-off safety speed

• a referenced airspeed obtained after the aircraft lifts off and at which the required OEI climb performance can be achieved

Vmca: aim minimum control speed

• lowest calibrated airspeed at which control of an aircraft can be maintained following the failure of the critical engine with the remaining engine(s) operating at take-off power
• determined at gross weight with the C of G at the aft limit, flaps in take-off position, landing gear retracted, and the propeller windmilling if no autofeathering system is installed

Va: maneuvering speed

• maximum speed at which full deflection of the primary flight controls will not cause overstressing of the aircraft

Fuel Requirements

Fuel is probably one of the highest costs associated with the operation of an aircraft, so it makes sense that operators are always looking for ways to be more fuel efficient.  Extra fuel also means extra weight, which could otherwise be designated to paying passengers or cargo.  That being said, we know there are legal requirements for the minimum fuel on-board to cover various 'what if' scenarios.  

CAR 602.88 Fuel Requirements (General)

• Day VFR: destination + 30 min at cruise
• Night VFR: destination + 45 min at cruise

• IFR (prop)
• destination, approach, missed approach, alternate + 45 min
• if no alternate: destination, approach, missed approach + 45 min
• IFR (jet)
• destination, approach, missed approach, alternate + 30 min
• if no alternate: destination, approach, missed approach + 30 min

• All aircraft also need enough fuel to provide for:
• taxi, foreseeable delays prior to take-off
• weather
• foreseeable traffic delays
• loss of cabin pressure
• loss of engine at critical point
• any other foreseeable delays

CAR 704.20 Fuel Requirements (Commuter)
In addition to above,

• IFR
• descent to lower of single-engine service ceiling or 10000
• cruise at lower altitude to suitable aerodrome
• approach, missed approach
• hold for 30 min at 1500 AAE

CAR 705.25 Fuel Requirements (Airline)

• VFR: destination + 45 min at cruise

• IFR
• over designated routes / areas: destination + 5%
• descent to lower of single-engine service ceiling or 10000
• cruise at lower altitude to suitable aerodrome
• approach, missed approach
• hold for 30 min at 1500 AAE