Instruments Flashcards
What is the difference between a Potentiometer and a rheostat?
In a potentiometer current flows both ends of the coil and in a rheostat only one
Explain a control transformer?
A primary coil that produces a continuously charging magnetic flux in the iron core and a secondary coil where the flux induces an AC voltage
How is a synchro built up?
A rotor and 3 stator coils. The coils are at 120 degrees to each other and act like 3 transformers contained in one unit.
What voltages do synchros use for excitation of the rotor and what does this do in the rotor coil?
26Vac or 115 Vac and it will create a magnetic field in the rotor.
When would you use a synchro with a servo loop?
When the torque or a synchro receiver is not high enough.
What is a differential synchro?
A synchro that has 3 coils in the rotor and 3 coils in the stator and is possible to add or subtract angles.
What is the build-up of a resolver?
It has 2 stator coils and a rotor coil at 90 degrees with respect to each other. From the signals in the stator coils it makes sine and cosine signals.
Name two possible applications for the resolver?
Angular transmitter (Thrust leavers) and Phase Angle Shifter (0 - 360 degree selector)
What is the build-up of a Linear Variable Differential Transformer?
A Primary Coil with a linear moveable iron core then 2 secondary coils connected in series.
How does a LVDT work?
The iron core moves in one direction, one of the secondary coils receives more of a magnetic flux and induces a higher coil voltage, the secondary coil receives less magnetic flux, therefore a lower voltage. The difference between the voltage in the secondary stator coil is proportional to mechanical position.
What type of demodulation is necessary for position reading with LVDT/RVDT?
Phase-dependant-demodulation
Explain the build-up of a DC servo loop?
A DC source connected to a variable control potentiometer and a follow up potentiometer. An amplifier amplifies the ratio signal and drives a motor with it.
How do you drive an AC motor in a DC servo Loop?
You must add a chopper circuit to make AC from the DC signal and to drive the AC motor from this signal we need an extra amplifier.
Why would you want to use an AC motor in a DC servo loop?
When we need even more torque.
Absolute pressure is measured from what reference?
zero pressure or vacuum
Gauge pressure is measured from what reference?
Existing atmospheric pressure
What pressure sensing devices work electrically?
Strain Gauges and Piezo-resistive sensors (Piezo-resistive is more accurate)
Name some quantity sensing systems?
Variable resistance, Mechanical Float Type, Float and Reed switches, Capacitance and Direct Read.
What is the dielectric constant of Air and Fuel?
Air = 1 and Fuel = 2 (but varies due to temp)
What does the capacitance of a capacitor depend on?
- Area of the plates
- Separation between the plates
- Dielectric Constant
Why is compensator in a FQIS?
The dielectric constant of the fuel will change with rep so it is placed in parallel with the probes and cancels changes in the dielectric constant.
How does the capacitance bridge read fuel quantity?
It compares the impedance of the probe to a reference probe, when the 2 are equal there is no flow through the indicator. When the capacity of the tank changes the bridge unbalances and the indicator shows the amount of charge.
In which units is fuel quantity indication represented?
Number of pounds or kilograms on board.
Explain a Negative Temperature Coefficient Resistor?
Its resistance decreases at increasing temperatures. So it is called a High Temperature Conductor.
Explain a Positive Temperature Coefficient Resistor?
Its resistance increases at increasing temperatures. So it is called a Low Temperature Conductor.
What materials does a thermocouple consist of?
Chromel and Alumel
How long is the recording capacity of a FDR?
25 - 50 hours
What is the purpose of the FDAU?
Provides the FDR with converted information from the aircraft systems
When is the FDR recording?
When one engine is started and will stop 5 minutes after last engine shutdown
What is the purpose of the EVENT pushbutton in relation to the FDR?
It will record an event mark in the FDR memory
What does a FDR ULB do?
Sends a 37Khz pulse 2 miles in any direction for about 30 days
Where would you find the Three Axis Accelerometer of a FDR?
At the centre of gravity of the aircraft
What does ADAS stand for?
Air Data Acquisition System
What is the purpose of the ADAS system?
To accept large amounts of engine and aircraft system data for the analysis of troubleshooting.
In the ADAS system where is the data stored?
On magnetic tapes or optical disks, and a printer prints the aircraft data.
How can you read the data stored in ADAS?
It can be printed, sent via ACARS or read on the MCDU
What is the standard air pressure and Temp at sea-level?
1013.2 MB 29.92 Hg and 15 Degrees
Does a leak check have to be carried out when a pitot static system is disturbed?
Yes
What is the reason for a left and right static port?
To eliminate instrument errors when aircraft yaws
What energy does a pitot tube measure?
Kinetic energy of the airflow due to the motion of the aircraft.
What temperature measuring device does a TAT probe use?
A temperature dependant resistor (platinum)
Explain a pressure altimeter?
Static pressure is directed into an aneroid chamber that is connected to a gear??
QNH is set what will your pressure altimeter read when landing at an airfield?
Airfield elevation
QFE is set, when landing the altimeter will show?
0 feet
During cruise the altimeter setting is …………. and shows the ………. ?
QNE, Flight Level
The pneumatic vertical speed indicator uses the ………. Pressure?
Static
What is incorporated in a Instantaneous VSI?
An accelerometer-operated air pump across the capsule.
What does an airspeed indicator measure?
The difference between pitot and static pressure
Ram air pressure is the difference between?
Total pressure and static pressure. If the aircraft is on the ground total pressure an static pressure would be equal.
What is IAS?
Indicated airspeed is the uncorrected reading of an air speed indicator. Little use to navigation purposes
What is CAS?
Calibrated airspeed (CAS) is the speed shown by a conventional airspeed indicator after correction for instrument error and position error
What is TAS?
True Airspeed is CAS but with a correction applied for temperature and altitude. (Atmospheric conditions)
What is GS?
Groundspeed represents the actual speed of an aeroplane over the ground. It is TAS adjusted for wind.
What is TAT and Static Air Temperature used for?
TAT - Power settings of the engines
SAT - Navigational purposes and to inform passengers of OAT
Mach number is?
Velocity / Local Speed Of Sound
What do Vmo / Mmo mean?
The maximum operating limit speeds, and warning will alert the pilots if the limit is exceeded.
An aircraft climbs with a constant TAS, its mach number?
Increases
What are the inputs for an air data computer?
Discretes, TAT, Total pitot and static pressures, Baro settings, AOA.
How does a ADC convert pneumatic pressure to electrical data?
It goes through a pressure transducer/oscillator the to an A to D converter.
