CFI Instrument Practical Test Standards, FAA-S-8081-9B, June 2001

II. Technical Subject Areas

A. Aircraft Flight Instruments and Navigation Equipment

1. Flight instrument systems and their operating characteristics
   a. Pitot-static system
   1) Includes three basic pressure-operated instruments:
   a) Sensitive altimeter
   b) Airspeed indicator (ASI)
   c) Vertical speed indicator (VSI)
   
   2) All three pressure-operated instruments depend on accurate sampling of static (ambient) pressure at two or more locations outside the aircraft
   
   a) Static pressure is pressure of the air that is still, or not moving, measured perpendicular to the surface of the aircraft
   b) Static ports may be on the side of an electrically heated pitot-static head
   
   
   c) Static pressure may be measured through flush ports on the side of the fuselage
   i) Ports are normally paired, one on each side of the aircraft
   ii) Area around ports may be heated to help prevent ice blockage
   d) If static ports block, pilot may open the static system alternate source to provide static pressure from a source inside the aircraft
   3) Pitot pressure (impact or ram air pressure) is used to measure airspeed
   a) Measured through an open-eyed pitot tube pointed directly into the relative wind
   b) Pitot tube connects only to ASI
   4) Position error
   a) Under some flight conditions, expecially at higher angles of attack in landing configuration, disturbances in the air around the static port(s) may cause errors in pressure-operated instrument indications
   b) The POH contains any position error corrections to apply to airspeed indications under specified conditions
   5) Sensitive altimeter
   a) The sensitive altimeter is an aneroid barometer that measures absolute pressure of ambient air and displays it in terms of height above a selected pressure level
   b) Principle of operation
   i) Sensitive element is a stack of evacuated, corrugated bronze aneroid capsules the thickness of which changes with air pressure
   ii) An adjustable barometric scale, visible in the Kollsman window, allows the pilot to set reference pressure from which altitude is measured (from 28.00 to 31.00 inches Hg)
   • A change in scale of 1 inch Hg changes altimeter reading by 1,000 feet
   • To read pressure altitude, set scale to 29.92 inches Hg
   • Set scale to local altimeter setting to indicate existing height above sea level
   c) Altimeter errors
   i) Preflight check for mechanical error: with barometric scale set to local altimeter setting, altimeter should indicate within 75 feet (maximum error) of airport elevation
   ii) Nonstandard temperature effects
   • Warmer than standard air: pressure levels are farther apart; altimeter indicates lower; aircraft will be higher (than in standard air)
   • Colder than standard air: pressure levels closer together; altimeter reads higher; aircraft is lower ("flying from high to low, look out below!")
   • Very cold air: altimeter error may be significant; increase minimum terrain clearance altitudes; notify ATC if flying higher than assigned altitude
   • ICAO Cold Temperature Error Table (AIM)
   iii) Nonstandard pressure effects
   • When flying into lower pressure, altimeter reads higher than true altitude and aircraft is lower than indicated ("flying from high to low, look out below!")
   6) Encoding altimeter
   a) A sensitive altimeter with an internal computer that measures pressure altitude (referenced from 29.92 inches Hg) and delivers data to Mode C transponder which transmits data to ATC ground controller
   b) 14 CFR part 91.217 requires that the transponder transmitted altitude must be within 125 feet of the pressure altitude (29.92 in Hg) indicated by the altimeter that the pilot is using to maintain flight altitude
   7) Absolute altimeter (radar or radio altimeter)
   a) Measures time taken by transmitted signal to travel from aircraft to ground below and back (not a pressure-operated instrument)
   b) Most have provisions for setting a minimum altitude at which a warning light or sound will be produced
   8) Airspeed indicator (ASI)
   a) ASI is a differential pressure guage that measures dynamic pressure, the difference between static and total, or ram, pressure caused by movement of the aircraft through the air
   b) Corrugated phospor-bronze aneroid, or diaphragm, receives ram pressure from the pitot tube, while the sealed instrument case receives static pressure
   c) Indicated airspeed (IAS) is the uncorrected airspeed shown on the ASI dial
   d) Calibrated airspeed (CAS) is IAS corrected for instrument and position errors; POH provides CAS for IAS with various flap and landing gear configurations
   e) Equivalent air speed (EAS) is CAS corrected for compression of air in the pitot tube (same as CAS in standard atmosphere at sea level; less than CAS at higher altitudes and faster airspeeds)
   f) True airspeed (TAS) is CAS corrected for nonstandard temperature and pressure
   i) TAS = CAS in standard atmosphere at sea level
   ii) Some true airspeed indicators have temperature-compensated aneroid bellows
   iii) Others have a knob requiring pilot input to align outside air temperature (OAT) with pressure altitude so that TAS is then indicated
   g) Mach number is the ratio of TAS to the speed of sound in the same atmospheric conditions; it may be indicated by a Machmeter
   h) Maximum allowable airspeed indicator has a additional pointer indicating never-exceed apeed which changes with altitude
   i) Airspeed color codes:
   • White arc: flap-operating range (from flaps-down stall speed to maximum airspeed with flaps down)
   • Green arc: Normal operating range (from flaps-up stall speed to maximum airspeed in rough air)
   • Blue radial line: Best single-engine rate of climb speed
   • Yellow arc: Structural warning area (from maximum rough air speed to never-exceed speed)
   • Red radial line: Never-exceed speed
   
   9) Vertical speed indicator (VSI) (vertical velocity indicator, VVI, formerly rate-of-climb indicator)
   a) Rate-of-pressure-change instrument indicates deviation from a constant pressure level; indicates rate of climb or descent
   b) Aneroid and instrument case both receive static pressure, but case is vented through a calibrated orifice (a hole of specific diameter used to delay pressure change)
   c) VSI pointer lags a few seconds behind actual pressure change, but is more sensitive than an alitimeter
   d) An instantaneous VSI (IVSI) has two aircraft pitch-sensitive accelerometers that eliminate the lag time of the VSI

AIM
Instrument Flying Handbook, FAA-H-8083-15, 1999

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