Chap 4: Pitot-Static Instruments and Systems

Question 1

what is the mean sea level pressure and temperature by ISA?

Answer 1

pressure: 101.325 KPa / 14.7 PSI
temperature: 15 degC

Question 2

what does pitot-static system uses?

Answer 2

static air pressure & dynamic pressure (due to aircraft’s motion through the air)

Question 3

instruments that uses the pitot-static system

Answer 3

airspeed indicator
vertical speed indicator
mach meter

Question 4

standard pitot-static head or probe description

Answer 4

used on light aircraft
pressure and static probes are mounted parallel to another
liable to be blocked by sand and insects

Question 5

electrically-heated pitot-static head or probe description

Answer 5

used to prevent ice formation at high altitude or cold climate
head has a water trap and drain holes for excess moisture

Question 6

pitot head or probe with a separate static vent description

Answer 6

better reading from separating the static vent and pitot head

Question 7

3 types of pitot-static head or probes

Answer 7

standard probe
electrically-heated probe
separate static vent with pitot probe

Question 8

locations of pitot tubes and static vents

Answer 8

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

Question 9

types of primary flight instruments

Answer 9

airspeed indicator
attitude indicator
altimeter
turn & slip indicator
heading indicator
vertical speed indicator

Question 10

what does an altimeter do and how to read it?

Answer 10

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

Question 11

how does sensitive altimeters work?

Answer 11

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

Question 12

altimeter q codes

Answer 12

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)

Question 13

what is inside a servo assisted altimeters and its advantages

Answer 13

better accuracy compared to basic and sensitive altimeters, by using electronics and servo-mechanisms (motors)

Question 14

what does the Vertical Speed Indicator do, how does it work and what does it show?

Answer 14

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

Question 15

how does a VSI work?

Answer 15

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

Question 16

what does the airspeed indicator do?

Answer 16

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)

Question 17

how does an ASI work and what happens during acceleration/deceleration?

Answer 17

difference in pressure will move the capsule which operates the pointer

during acceleration, capsule expands

during deceleration, capsule contracts

Question 18

ASI reading when pitot tube/static port block or leak

Answer 18

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

Question 19

steps to determine true airspeed from indicated airspeed

Answer 19

indicated airspeed > calibrated airspeed > equivalent airspeed > true airspeed

Question 20

definition of IAS, CAS, EAS, TAS

Answer 20

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

Question 21

what does a machmeter do?

Answer 21

indicates the Mach Number, M which is a ratio between true airspeed and speed of sound

M = true airspeed / SOS

Question 22

how does machmeters work? and what happens when pitot & static ports are blocked

Answer 22

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

Question 23

what do pitot static system testers do?

Answer 23

check for leaks in the plumbing tubes by simulating pitot and static pressures at different airspeeds and altitudes to the flight instruments