Chap 4: Pitot-Static Instruments and Systems Flashcards
what is the mean sea level pressure and temperature by ISA?
pressure: 101.325 KPa / 14.7 PSI
temperature: 15 degC
what does pitot-static system uses?
static air pressure & dynamic pressure (due to aircraft’s motion through the air)
instruments that uses the pitot-static system
airspeed indicator
vertical speed indicator
mach meter
standard pitot-static head or probe description
used on light aircraft
pressure and static probes are mounted parallel to another
liable to be blocked by sand and insects
electrically-heated pitot-static head or probe description
used to prevent ice formation at high altitude or cold climate
head has a water trap and drain holes for excess moisture
pitot head or probe with a separate static vent description
better reading from separating the static vent and pitot head
3 types of pitot-static head or probes
standard probe
electrically-heated probe
separate static vent with pitot probe
locations of pitot tubes and static vents
pitot tube:
below speeds of sound:
ahead of wing tip / vertical stabiliser, side of fuselage nose section
above speeds of sound: at fuselage nose
static vents:
skin of fuselage, one on each side and interconnected to minimise dynamic pressure effects when yawing or turning
types of primary flight instruments
airspeed indicator attitude indicator altimeter turn & slip indicator heading indicator vertical speed indicator
what does an altimeter do and how to read it?
measures the altitude or height of the aircraft above mean sea level
like a clock, shorthand is thousands of feet, long is hundreds
crosshatch flag appears when altimeter is displaying an altitude below 10,000 MSL
how does sensitive altimeters work?
as altitude goes up, atmospheric pressure decreases
wafer capsule in altimeter expands when ascending (static pressure goes down)
capsule contracts when aircraft is descending (static pressure goes up)
when static ports are blocked, altimeter reading will:
underread if aircraft is ascending
overread if aircraft is descending
altimeter q codes
QFE: mean sea level pressure corrected for temperature (adjusted for specific site reference)
QNE: ISA sea-level pressure of 1013.25 mb (mean sea level)
QNH: pressure measured at a reference then reduced dow into mean sea level pressure
(different region, different pressures)
what is inside a servo assisted altimeters and its advantages
better accuracy compared to basic and sensitive altimeters, by using electronics and servo-mechanisms (motors)
what does the Vertical Speed Indicator do, how does it work and what does it show?
indicates if the a/c is ascending, descending or in level-flight
rate of climb/descent is in feet per minute (fpm)
VSi operates from static pressure, a differential pressure instrument
displays 2 types of information:
trend: immediate indication of increase or decrease in aircraft’s rate of climb or descent
rate: stabilised rate of change
how does a VSI work?
operated through a differential of static pressure
during level flight, casing Pa = capsule Pa, so no differential pressure (pointer is at 0)
during descend, Pa in capsule increases, but change of pressure is slowed by calculated leak, Pa surrounding the capsule < pressure in capsule, so capsule expands to show descend
during ascend, Pa in capsule decreases, but casing Pa > capsule Pa, capsule contracts and shows ascent
when static ports are blocked during ascending/descending, VSI will show 0
what does the airspeed indicator do?
measures the difference between pitot and static pressure to indicate the speed of aircraft
when on ground, the pitot and static pressure are equal (airspeed = 0)
when in flight, pressure on pitot line is greater than pressure in static lines
the difference in pressure is used by the pointer to show the speed in knots (nautical miles/hr)
how does an ASI work and what happens during acceleration/deceleration?
difference in pressure will move the capsule which operates the pointer
during acceleration, capsule expands
during deceleration, capsule contracts
ASI reading when pitot tube/static port block or leak
pitot tubes blocked:
ascending at constant speed, airspeed will overread
descending at constant speed, airspeed will underread
static ports blocked:
ascending at constant speed, airspeed will underread
descending at constant speed, airspeed will overread
pitot tube leak: airspeed will underread when ascending/descending
static port leak: airspeed will overread when ascending/descending
steps to determine true airspeed from indicated airspeed
indicated airspeed > calibrated airspeed > equivalent airspeed > true airspeed
definition of IAS, CAS, EAS, TAS
IAS: corrected for instrument error
CAS: compensated for square law response
EAS: calculated from measured pressure difference constant sea level density
TAS: EAS compensated for air temperature and density
what does a machmeter do?
indicates the Mach Number, M which is a ratio between true airspeed and speed of sound
M = true airspeed / SOS
how does machmeters work? and what happens when pitot & static ports are blocked
uses pitot-static system to generate M
when pitot tube are blocked:
ascending at constant speed, mach number will overread
descending at constant speed, mach number will underread
when static ports are blocked:
ascending at constant speed, mach number will underread
descending at constant speed, mach number will overread
what do pitot static system testers do?
check for leaks in the plumbing tubes by simulating pitot and static pressures at different airspeeds and altitudes to the flight instruments
