Aircraft Instruments and Systems Flashcards
What are the pressure flight instruments? (3)
ASI
Altimeter
VSI
How do pressure instruments work in general?
Pressure instruments work by measuring Static and Pitot pressure which provides us with the dynamic pressure.
Total / Pitot pressure = Static + dynamic pressure
How does an ASI work?
An ASI works by measuring the difference between the pitot pressure within the capsule and the static pressure within the case. The Pitot pressure - static pressure provides us with the dynamic pressure which is represented as IAS. An ASI is calibrated in ISA conditions at MSL and therefore needs to be corrected to provide us with TAS.
What are the ASI errors?
Pressure / Position error
Instrument error
Manoeuvre
Compressability error (IAS >300KTAS)
Density error
How is VMO displayed on an ASI?
A red / black striped pointer (barbers pole).
Describe how a Machmeter works?
A machmeter compares the aircrafts TAS to the LSS.
It has 2 capsules, one altitude capsule and one airspeed capsule.
The first capsule is the airspeed capsule which measures pitot pressure (total pressure), the second is the Altitude capsule whixh expands and contracts depending on the static pressure within the case.
From the formula MN = TAS / LSS we can see that:
MN is proportional to: Dynamic Pressure / Static Pressure which = Pitot - Static (Dynamic) / Static pressure.
What errors does a Machmeter have?
Position / Pressure error
Instrument
Manoeuvre
What are the ASI / Mach Meter indications and actions for a blocked pitot or static probe?
Constant alt?
Pitot climb / desc?
Static climb / desc?
At a constant altitude they will read the same.
Pitot blockage: Underread in descent / Overread in ascent.
Static blockage: Overread in descent / Underread in ascent.
What are the three different types of altimeter?
Simple:
Static pressure is fed in to the instrument case and as the aircraft climbs, an anneroid capsule expands and pushes up a “leaf spring” which controls the pointer via a linkage. It can be adjusted to zero or airfield elevation but does not have a subscale.
Sensitive
This works on a similar principle to the simple altimeter but uses more than 1 anneroid capsule as well as jeweled bearings which reduces the lag. A pressure subscale can also be set on this altimeter.
Servo Assisted
The principle of this is that small movements of the capsules are detected by a very sensitive electro-magnetic pick off (EI bar) which produces a current and is amplified. This reduces instrument error and reducing lag, allowing it to feed into other systems such as the autopilot etc.
What are the altimeter instruments errors?
Position
Instrument
Manoeuvre
Barometric (Calibrated for ISA)
Lag
Temperature (Calibrated for ISA)
What are the altimeter indications / actions for blocked static port?
The altimeter will display the the altitude where the blockage occured and will remain there regardless of any change in altitude.
Ensure pitot heat is on or use the alternate static source.
How does a VSI work?
A VSI measures the present static pressure with a recent static pressure. Each side of a metering unit through a restricted choke which transmits the change at a lower rate.
This shows the difference in static pressure inside the capsule as well as inside the case.
As there is more feet per hPa at altitude the rate of pressure change would be different at different altitudes. This is corrected by an orifice which provides a constant rate of change regardless ofaltitude.
What are the errors of the VSI?
Position
Instrument
Manoeuvre
Lag (6 seconds)
Blockage (crack glass if need to, if alt static source is selected there will be a momentary climb)
How does an IVSI work?
An IVSI works by incorporating a “dashpot” or a “dynamic vane” which responds very quickly to change in altitude due to the inertia. This causes and instant rise or reduction in the pressure of the capsule.
What are the disadvantages of an IVSI?
When applying a larger angle of bank such as 40 degrees or more the IVSI will indicate a climb.
The IVSI is also very sensitive when in turbulent conditions.
What is the indication of a blocked static port?
A blocked static port would mean that the value of the static pressure inside the capsule as well as the case would be the same and therefore would indicate zero on a VSI.
What are the two types of thermometers? (2)
Direct Reading:
(bimetallic strip) made from brass and invar
Total Air Temperature probe:
This is fitted away from the aircrafts boundary. As the air passes through the tube, separation of water partucles is achieved by hving the airflow turn at right angles before passing the sending element.
What are the errors of air temperature gauges? (3)
Instrument error (imperfections)
Environmental errors (solar heating / ice accreation)
Heating error (adiabatic / compression)
What is the TAT formula?
TAT = SAT + RR
What is the recover factor? (Kr)
The percent of the ram rise recovered by the TAT probe.
How does an Air Data Computer (ADC) work?
An ADC uses analogue or digital computing techniques to convert pressure and temperature data into electrical signals which are transmitted to the display instruments and other systems.
When an ADC has a failure? What does it use to give a prompt indication?
BITE (Built In Test Equipment)
Power up BITE (start up)
Continuous BITE (every second)
Maintenance BITE (current or post failure for maintenance crew)
What are the advantages of an ADC?
Improved displays
Reduced lag
Error correction
Integration
Clean design
What is a Gyro?
A gyroscope consists of a rapidly spinning disc (4,000-25,000 rpm) which has 2 basic properties, rigidity and precession.
How does a gyroscope work?
A gyroscope maintains its rigidity in space by the inertia of its spinning mass, unless subject to an external force. The rotor is usually susended in a series of frames called gimbals, 1 gimbal = 1 degree of freedom. Any force applied to the gyro will create precession 90 degrees in the direction of rotation. The rigidity of a gyroscope depends on the mass of the rotor, the effective radius of the rotor as well as the rpm of the rotor.
What are the two types of gyroscopic wander?
Horizontal = drift
Vertical = topple
Describe Real / Apparent wander?
Real (random) wander: This is where the axis of the gyro moves with respect to inertial space. For example, manufacturing imperfections and bearing friction etc which are reduced by higher quality engineering.
Apparent wander (Earth Rate and Transport Wander):
Earth Rate - this is caused by the rotation of the earth. At the equator the gyro axis remains fixed in space and aligned to the local meridian. At the pole the gyro still remains aligned in space but the meridian direction changes as the earth rotates at the following rate with a negative value in the northern hemisphere:
Earth Rate = 15 x sine mean latitude
Transport Wander:
This is caused by flight east or west at latitudes other than the equator and is actually meridian convergence.
Transport Wander = (GS / 60) x tan mean latitude
What are the two main type of gyros? (2 main, 1 has 3 subtypes)
Displacement Gyros:
These have 2 gimbals and 2 degrees of freedom (DGI / AI)
Rate Gyros:
These have 1 gimbal and 1 degree of freedom (Rate of turn Indicator / Yaw dampers)
Describe the directional gyro indicator (DGI)?
Type of gyro?
Tied to what plane?
How many gimbals?
Advantages?
Errors? (5)
The DGI uses a tied gyroscope in the yawing plane and has 2 gimbals. It needs to be regularly synchronised with the magnetic compass and is not to replace it but compliment it.
It’s advantages are that it does not suffer from acceleration errors or other magnetic errors caused by the earths magnetic field.
However it does suffer from:
- Transport error
- Earth Rate
- Random wander
- Gimballing error
- Errors from varying RPM
How is apparent wander corrected for on a DGI? (2)
This depends on how sophisticated the instrument is but principally it is correted by 2 methods:
Every 10 to 15 minutes manually synchronise with the magnetic compass using the slaving knob on the DGI.
Use of a latitude nut which provides an opposite error to the earths rotation (ER = 15 x sine mean latitude per hour).
What is transport wander on an uncorrected gyroscope on a DGI?
Transport Wander = Groundspeed / 60 x tan mean latitude
This is effected by the aircraft east / west position on the globe.
Transport Wander + Apparent Wander = Total Apparent Wander
Describe the air driven artificial horizon?
How many degrees of freedom and what are they?
Axis of gyroscope and what gyroscope?
How is gyroscopic precession overcome?
The artificial horizon is the primary attitude instrument which measures and displays the pitch and roll of the aircraft about the horizon.
There are 2 degrees of freedom (pitch and roll)
Uses an earth tied gyroscope which is alligned to the earths vertical axis and rotates about a vertical exis.
4 pendulous vanes are used in order overcome gyroscopic precession.
What errors do the artificial horizon have?
Turning errors (main)
Acceleration errors
Real wander of gyro
What are the turning errors of the Artificial Horizon?
Through 90: Under read bank - Pitch climb
Through 180: Correct bank - Pitch climb
Through 270: Over read bank - Pitch climb
Through 360: Correct bank - Pitch correct
What are the indications of a failed air driven artificial horizon? (2)
How do you re-erect the gyro?
- Low suction shown on the suction gauge.
- Possible warning flag
To re-erect the gyro can be accomplished by caging and uncaging the gyroscope, but if there is low suction then it will probably topple again and the VSI and turn coordinator should be monitored.
Describe the electrically driven artificial horizon?
Type of motor?
Speed in RPM?
Advantages?
What is its freedom in pitch and roll in degrees?
Electrically driven artificial horizons have the same basic principles as the air driven one, with the earth tied gyro and 2 gimbals (pitch and roll).
However, with use of a squirrel-cage motor, the gryoscope is driven to a higher RPM of around 22,000 RPM which increases its rigidity and means that it does not require pendulous suspension as well as minimising turning and acceleration errors.
It has complete freedom in roll and around 85 degrees in pitch.
What is a servo assisted artificial directional indicator (ADI) or also known as a remote artificial horizon?
Advantages? (3)
This uses data from the INS / IRS systems to display pitch and roll information in modern aircraft.
The advantages of this system is that it is free of turn and acceleration errors. Also, it uses a dual system whereby INS 1 feed the captain’s instruments and INS 2 feeds the co-pilots instruments which can be compared to separate standby insturments in order to detect errors and deselect incorrect systems.