6 - Aircraft Instruments and Systems Flashcards
Pressure Flight Instruments
What are the pressure flight instruments, and how do they work?
The pressure flight instruments are
- Airspeed indicator (ASI)/Mach meter (MM)
- Altimeter
- Vertical speed indicator (VSl)
Pressure instruments measure atmospheric pressure by using the
pitot-static system, which is a combined sensor system that detects the
following:
- The total pressure - (static and dynamic pressure), also called pitot
pressure, which is measured by a pi tot probe - Static pressure - alone, which is measured by either the static port
on a pitot probe or by a separate static vent
The difference between the two will give a measurement of the dynamic
pressure. That is,
Dynamic pressure = total pressure - static pressure
Dynamic and/or static pressure measurements are the basis of the
flight instrument readings.
Pressure Flight Instruments
How does the airspeed indicator (ASI) work?
The ASI measures dynamic pressure as the difference between the total pitot pressure measured in the instrument’s capsule/diaphragm
and the static pressure measured in the case.
The dynamic pressure represents the indicated airspeed (lAS) as knots per hour.
(See Q: Describe indicated airspeed (lAS), page 120.)
The ASI instrument is calibrated to international standard atmosphere (ISA) mean sea level (MSL) density of 1225 g/mJ.
Pressure Flight Instruments
What are the airspeed indicator (ASI) instrument errors?
The ASI instrument suffers from the following errors:
- Instrument error
- Pressure error
- Density error
- Compressibility error
- Maneuver error
- Blocked pitot static system
Pressure Flight Instruments
How is VMO displayed on the airspeed indicator (ASI)?
On the ASI display, a red/black striped pointer indicates the VMO speed.
Pressure Flight Instruments
Describe how a Mach meter works.
The Mach meter measures the airspeed relative to the local speed of sound.
In essence, the Mach meter is a combined airspeed indicator (ASI) and
altimeter that comprises the following:
- A capsule feed with pitot pressure inside an ambient static pressure
feed case that acts as anASI and measures dynamic pressure as the
airspeed. - A sealed capsule containing international standard atmosphere (ISA)
mean sea level (MSL) conditions inside the ambient static pressure
feed case, which acts as an altimeter by measuring the static
pressure, which it relates to an altitude.
Mach meter ratio = (pitot-static)/static
These two functions inside the Mach meter are linked via a ratio arm
that itself acts on a ranging arm to ultimately move the pointer/digital
Pressure Flight Instruments
What errors does a Mach meter suffer from?
The Mach meter only suffers from the following errors:
- Instrument error, which is caused by the inaccuracies in the Mach
meter’s construction - Pressure error, also known as position or configuration error
However, these errors are extremely small, and therefore, the
indicated Mach number speed can be read as the true Mach number
speed. - Blocked pitot static system (see Q: What are the airspeed indicator
(ASI) / Mach number (Mn) indications and actions for a blocked pitot
static probe? page 129).
Note: The Mach meter does not suffer from density or temperature
errors because its built-in altitude capsule design and its ratio to the
dynamic pressure measured compensate for density and temperature
variations.
Pressure Flight Instruments
What are the airspeed indicator (ASI)/Mach meter indications and actions
for a blocked pitot and/or static probe?
A static line blockage means that the static pressure in the ASI/Mach
meter instrument case remains a constant value. Therefore,
- At a constant altitude, the ASI/MM will read correctly.
- During descent, the ASI/MM will over-read due to an increase in the
pitot pressure in the capsule against the trapped low static pressure
of the higher altitude. - During climb, the ASI/MM will under-read due to a decrease in the
pitot pressure in the capsule against the trapped high static pressure
of the low altitude.
A pitot line blockage means that the total pressure in the ASIIMM
instrument capsule remains a constant value. Therefore,
- At a constant altitude, the ASIIMM reading will not change even if
the airspeed does due to the trapped pitot pressure in the capsule
against a constant altitude static pressure. - During descent, the ASI/MM will under-read due to an increase in
the static pressure in the case against a constant pitot pressure. - During climb, the ASI/MM will over-read due to a decrease in the
static pressure in the case against a constant pitot pressure.
The actions for a blocked pitot/static system causing an unreliable
ASl/MM reading would be to
1. Ensure that the pitot static probe anti-ice heating (pitot heat) is on,
if applicable.
2. Use an alternative, such as a static source or an air data computer,
if applicable.
3. Use a limited flight panel, i.e., standby ASI.
4. Fly at a correct attitude and power setting.
Pressure Flight Instruments
How does a pressure altimeter work?
A simple pressure altimeter is designed to measure static air pressure,
which it relates to an indicated altitude.
As the aircraft ascends,
the static pressure in the instrument case decreases, which allows the
enclosed capsule to expand, and this in turn moves the needle on the
instrument face to indicate a corresponding altitude.
For a descent, the opposite function applies.
A sub-scale setting device is included so that the instrument can be
zeroed to various datum elevations.
(See Q: Give the definitions of the
following altimeter sub-scale settings, page 130.)
The altimeter capsule is calibrated to full international standard
atmosphere (lSA) mean sea level (MSL) conditions, i.e., + 15C,
29.92 in/1013 millibars, and 1225 g/m3.
Pressure Flight Instruments
Give the definitions of the following altimeter sub-scale settings?
QNH
QFE
QFF
QNE
QNH - is a local altimeter setting that makes the altimeter indicate the aircraft’s altitude above mean sea level (AMSL) and
therefore airfield elevation.
There are two types of QNH:
1. Airfield QNH
2. Regional QNH, which is the lowest forecast QNH in an altimeter
setting region.
QNH is QFE reduced to sea level using international standard atmosphere (ISA) values for the calculation.
QFE. This zeros the altimeter on the airfield elevation datum. There
are two types of QFE:
1. Airfield QFE is measured at the highest point on the airfield.
2. Touchdown QFE is measured at the touchdown point of the runway
in use for precision approaches.
QFF. This is similar to QNH except that it uses the actual conditions
(not ISA) to find the sea level pressure. It is used more commonly by
meteorologists than by pilots.
QNE. This is not an altimeter setting but is the height shown at
touchdown on the altimeter with 29.92 in or 1013 millibars (hPa) set
on the sub-scale. It is used at very high aerodromes where QFE pressure
is so low that it cannot be set on the altimeter sub-scale.
Standard setting. 29.92 in or 1013 hPa millibars standard setting
will give altimeter readings as a pressure altitude or flight level and
is used for traffic controlled airspace above the transition layer.
Pressure Flight Instruments
What are the aviation definitions of height, altitude, and flight level?
Altitude - is the measured distance above the local pressure setting (i.e.,
QNH) or altitude above mean sea level (MSL).
Flight level - is the measured pressure level above the 29.92-in/1013-
millibar datum.
Pressure Flight Instruments
What are the altimeter instruments errors?
The altimeter instrument errors are as follows:
- Instrument error
- Pressure error (also known as position or configuration error)
- Time-lag error
- Barometric error
- Temperature/density error
- Blocked static port
Pressure Flight Instruments
What are the altimeter indications and actions for a blocked static port?
A static line blockage means that the static pressure in the altimeter
instrument case remains a constant value.
Therefore, the altimeter will
display the altitude where the blockage occurred regardless of any actual change in the aircraft’s altitude.
The actions for a blocked static line causing an unreliable altimeter
reading would be
- To ensure that the pitot static probe anti-ice heat is on (pitot heat),
if applicable. - To use an alternative, such as a static source or an air data computer,
if applicable. - ‘Ib use a limited flight panel, i.e., standby altimeter or vertical speed
indicator (VSI), if available. - To fly correct attitude and power settings, especially for level flight.
Pressure Flight Instruments
Given a temperature deviation from ISA of -36°C, the pressure altimeter will
(a) over-read,
(b) under-read, or
(c) read correctly, and why?
The altimeter will over-read because the temperature deviation is
colder than the international standard atmosphere (ISA); i.e., the
altimeter reads an altitude higher than the actual altitude of the
aircraft.
(See Q: What density errors are commonly experienced?
page 119.)
Pressure Flight Instruments
What do you know about servo-assisted altimeters?
A servo-assisted altimeter increases the accuracy of a simple pressure
altimeter because its design no longer relies on a direct mechanical
link between its capsule and the altitude pointer on the instruments
display dial.
Instead, the servo-assisted altimeters use an electrically
conducted E&I bar arrangement.
Pressure Flight Instruments
How does a vertical speed indicator (VSI) instrument work?
The VSI instrument measures the rate of change of static pressure and
displays this as a rate of climb or descent (expressed as feet per minute, or fpm) on the VSI instrument face.
The capsule is fed with static pressure and reacts immediately to any change in static pressure,
whereas the static pressure feed into the case is restricted or slowed
by a metering unit, thus creating a differential static pressure between
the capsule and the case.
As long as the aircraft continues to climb
or descend, the VSl will translate this as a rate of climb or descent
measurement on the instrument dial face.
Pressure Flight Instruments
What errors do the vertical speed indicator (VSI) instrument suffer from?
The errors that the VSI instrument suffers from are
- Instrument time-lag error
- Pressure error (also known as position error)
- Maneuver error
Pressure Flight Instruments
What do you know about an instantaneous vertical speed indicator (IVSI)?
The IVSI was designed to counter the time-lag error experienced by
simple VSI.
The IVSI uses two spring-loaded dash pots in the static
line before the capsule that cause an immediate differential pressure
to be sensed due to their inertia at the start of a climb or descent.
Once the aircraft is established in a climb or descent, the dash-pots are
centered by their springs, and when the aircraft starts to level out,
the opposite inertia of the dashpots produces an immediate change in the
reading on the IVSI display.
Pressure Flight Instruments
What are the advantages of an instantaneous vertical speed
indicator (IVSI)?
The advantage of an IVSI is the immediate display of any change in
the aircraft’s rate of climb or descent (ROC/ROD).
Pressure Flight Instruments
What are the disadvantages of an instantaneous vertical speed
indicator (IVSI)?
The disadvantage of an IVSI is that the dashpots, which sense the vertical
acceleration of the aircraft, are also affected by the acceleration
in a turn.
Therefore, the IVSI has an error that it initially shows as a
rate of climb (ROC) when applying large angles of bank, i.e., over 40
degrees of bank.
However, if the turn is maintained, the IVSI will stabilize
to zero but then indicates a rate of descent (ROD) as the aircraft
rolls out of the turn.
Pressure Flight Instruments
What are the vertical speed indicator (VSI) indications and actions for a
blocked static port?
A static line blockage means that the static pressure in the VSI instrument
capsule and case via the metering unit both remain a constant value.
Therefore, the VSI display will read zero at all times regardless
of any actual change in the aircraft’s rate of climb of descent
(ROC/ROD).
The actions for a blocked static line causing an unreliable VSI reading
would be
1. To ensure that the pitot static probe anti-icing heating (pitot heat)
is on, if applicable.
- To use an alternative, such as a static source or an air data computer,
if applicable. - To use a limited flight panel, i.e., altimeter, if available.
- To fly correct attitude and power settings.
Pressure Flight Instruments
How is air temperature measured?
Either a total head thermometer or a rose-mount probe that is
extended into the free airstream commonly measures air temperature.
Usually this temperature is displayed to the pilot on a total air
temperature (TAT) gauge.
(See Q: Describe total air temperature,
page 224.)
In flight, TAT is only a function of the ram effect ofthe air entering
the probe, and pitot heat is not considered when calculating outside air
temperature (OAT).
Pressure Flight Instruments
What do you know about air data computers?
Modern aircraft feed their static and pitot lines into an air data
computer (ADC) that calculates the RAS, TAS, MN, TAT, ROC, and
ROD and then passes the relevant information electronically to the
servo-driven flight instruments (not the standby instruments, which
retain their own direct static / pitot feeds).
The advantage of the ADC system is that the data calculated can be feed to the following: 1. Autopilot (AP) 2. Flight director system (FDS) 3. Flight management system (FMS) 4. Ground proximity warning system (GPWS) 5. Navigation aids 6. Instrument comparison systems
Gyroscopic Flight Instruments
What are the gyro flight instruments?
The gyroscopic flight instruments are
- Directional indicator (DI)
- Artificial horizon (AH)
- Turn and slip indicator or tum coordinator
Gyroscopic Flight Instruments
What is a gyroscope?
A gyroscope is a body (usually a rotor/wheel) rotating freely in one or
more directions that possesses the gyroscopic properties of rigidity and
precession