Weather Theory 6 April 2005

Heating of the Earth All weather on earth is caused by differential (unequal) heating of the earth’s surface! Large-scale atmospheric circulation (warm air rises; cooler air settles) Add rotational effects of the Coriolis Force and winds are deflected to the right (in the Northern Hemisphere) Coriolis force causes wind to flow parallel to isobars.

Circulation and Wind Air circulates clockwise around a high and counterclockwise around a low. When isobars are closer together, wind speed increases. Surface friction causes winds to not exactly parallel the isobars but rather move outward from high pressure areas. (e.g., more favorable winds will be found south of high-pressure areas when flying west.)

Temperature Standard Temperature is 15 Celsius Temperature decreases an average of 2 degrees per 1000 feet. (Standard lapse rate) When temperature increases with altitude, there is a temperature inversion. The most frequent ground-based TI is produced by radiation on a still night.

Altimeter As static pressure decreases, indicated altitude increases Altimeter setting is adjustable in “Kohlsman Window”, aka Altimeter Setting Window Local altimeter setting will cause the instrument to read the approximate field elevation when located on the ground at the airport Reset altimeter to 29.92 when climbing through 18,000 feet.

Altitude Terminology Indicated Altitude Absolute altitude Altitude read on the altimeter when it is set to the current local altimeter setting Absolute altitude Height above the surface True altitude True height above Mean Sea Level (MSL) Pressure altitude Altitude indicated whenever the altimeter setting dial is set to 29.92 (Standard Datum Plane) Density altitude Pressure altitude corrected for non-standard temperature and/or pressure.

Altimetry Standard day 29.92” Hg and +15 deg. C On a standard day at sea level, pressure altitude, true altitude, indicated altitude, and density altitude are all equal.

“High to low…look out below” When flying from an area of low pressure/low temperature to an area of higher pressure/higher temperature without adjusting the altimeter setting, the altimeter will indicate lower than the true altitude setting…and vice versa.

Density vs. True Altitude

Vertical Speed Indicator (VSI) Operates only on static pressure, but is a differential pressure instrument Operates on the principle of a calibrated leak… Face of VSI outputs change in pressure over time displayed in feet per minute.

Airspeeds and Airspeed Indicator Displays difference between pitot (impact) pressure and static pressure Pressures are equal when airplane is parked on ground in calm air.

Airspeeds Indicated airspeed (IAS) Calibrated airspeed (CAS) Uncorrected reading from the airspeed indicator Calibrated airspeed (CAS) Indicated airspeed corrected for installation and instrument error. True airspeed (TAS) Calibrated airspeed corrected for temperature and pressure variations. Groundspeed (GS) Actual speed of the airplane over the ground – this is the TAS adjusted for wind.

Airspeeds – color coded VSO – stall speed / minimum steady flight in landing configuration (lower limit of white arc) VFE – max. flap-extended speed (upper limit of white arc) VS1 – stall speed in specified configuration (lower limit of green arc) VNO – max. structural cruising speed (top of green arc, bottom of yellow arc) VNE – never exceed speed (upper limit of yellow arc, marked in red)

Airspeeds, others VLE – max. landing gear-extended speed. VA – design maneuvering speed (flown in rough air or turbulence to prevent overstressing airframe) VY – Best rate-of-climb airspeed (creates most altitude in a given period of time) VX – Best angle-of-climb speed (airspeed resulting in most altitude in a given distance.)

Gyroscopic Principles Rigidity in space Axis of rotation points in a constant direction regardless of the position of its base. Precession Tilting or turning of a gyro in response to a deflective force.

The Attitude Indicator Relies on rigidity in space Direction of bank determined by relationship of miniature airplane to the horizon bar. Miniature airplane remains stationary – horizon moves

Turn Coordinator Relies on precession As an airplane enters a turn, the TC indicates rate of roll. When bank is held constant, TC indicates rate of turn. Most TCs display an index on the “Standard-rate turn”, wherein the airplane takes 2 minutes to turn 360 degreers. The “ball” or inclinometer indicates quality of turn (skid/slip status).

Heading indicator “Gyroscopic compass” Magnetic compasses are difficult to read and suffer from errors; the heading indicator (also known as a directional gyro or DG) DGs suffer from precession due to bearing friction – the indicator must be realigned with the magnetic compass during straight-and-level, unaccelerated flight.

Magnetic Compass Compass points to magnetic north Susceptible to several errors

Compass Errors Variation Deviation Magnetic Dip

Dip errors Magnetic dip: When turning north from an easterly or westerly heading, the compass lags behind the actual aircraft heading. When a turn is initiated while on a northerly heading, the compass first indicates a turn in the opposite direction. When turning south from an easterly or westerly heading, the compass leads the actual heading. When a turn is initiated on a southerly heading, the compass immediately leads ahead. Mnemonic: UNOS – undershoot north, overshoot south

Dip errors continued Accelerating or decelerating while heading either east or west will also cause compass errors. When accelerating on an east or west heading, the compass indicates a turn to the north. When decelerating on an east or west heading, the compass indicates a turn to the south. Mnemonic: ANDS – accelerate north, decelerate south. Compass accurate only in S&L, unaccelerated flight.

Variation Errors Magnetic poles do not coincide with geographic poles. Most places on Earth, the compass needle does not point to True North. Angular differences between magnetic north and true north are called variations and are displayed on aeronautical charts.

Deviation Errors The metal, electrical systems, and operating engine all create magnetic fields from the aircraft. Aircraft manufacturers install compensatory magnets to prevent most errors. Remaining errors are called deviation. A card in the aircraft will list the deviation at various different compass points.

Next Week… Regulations (FAR/AIM & Test Prep)