Instrumentation - Principles and Sensors Flashcards
How has the quality and quantity of aircraft instrumentation changed over the last hundred years?
It has steadily improved.
What was the original aircraft instrumentation like at the beginning of the 20th century?
It was fairly basic, restricted to a few engine instruments, some form of airspeed indicator, and a compass.
What is a direct indicator in aircraft instrumentation?
It is a system where input data in the form of pressure or rotational speed is fed directly to the instrument.
Where are direct indicators still used?
In light aircraft, such as the Cessna cockpit.
What are servo-driven indicators?
Remote systems used since the 1960s where data is fed to a central computer, converted to electrical signals, and then used to drive needles or indicators in the cockpit.
Why did servo-driven indicators become necessary?
Due to the complex use of data and the increasing size of aircraft, making direct reading instruments impractical.
What are the four basic instruments required for safe flight in the ‘basic T’ arrangement?
Airspeed indicator, attitude indicator, altimeter, and compass or gyro-driven direction indicator.
What are primary flight instruments?
Essential instruments for safe flight; other instruments are considered useful extras or standby instruments.
What is the ‘six pack’ in modern aircraft instrumentation?
A classification that includes the turn coordinator and vertical speed indicator in addition to the primary instruments.
What are Electronic Flight Instrument Systems (EFIS)?
Fully electronic systems with LCD screens that allow selective display of flight parameters, replacing the basic T arrangement.
Why do modern aircraft still retain standby instruments?
In case the primary electronic systems fail, with standby instruments fed from discrete sensors and separately powered.
How is temperature described?
As a comparative measure of hot or cold, with absolute zero as the point where all molecular activity ceases.
What are the common temperature scales used?
Degrees Celsius (°C), degrees Fahrenheit (°F), and Kelvin (K).
What is the freezing point of water on the Celsius scale?
0°C
What is the boiling point of water on the Celsius scale?
100°C
What is absolute zero on the Celsius scale?
-273°C
What is the Kelvin scale used for?
The Kelvin scale is used to measure temperature and is based on absolute zero, which is -273°C.
How is a Kelvin different from a degree Celsius?
A Kelvin is the same size as a degree Celsius, but the Kelvin scale starts at absolute zero (-273°C).
How do you convert Celsius to Kelvin?
Add 273 to the Celsius temperature.
How do you convert Kelvin to Celsius?
Subtract 273 from the Kelvin temperature.
What is -55°C in Kelvin?
218 K
What was an old name for Kelvin?
Degrees absolute or °A.
What is the Fahrenheit scale?
The Fahrenheit scale is a temperature scale widely used in the US, with 0°F as the freezing point of salt water and 100°F as the average temperature of the human body.
At what temperature does fresh water freeze and boil on the Fahrenheit scale?
Fresh water freezes at 32°F and boils at 212°F.
How do you convert Celsius to Fahrenheit using a formula?
°F = (°C × 9/5) + 32
What is 9°C in Fahrenheit?
48°F
What are the four types of temperature sensors?
Bimetallic sensors, Resistance thermometers, Thermocouples, and Radiation pyrometers.
How does a bimetallic sensor work?
It relies on two different metals bound together. When heated, one metal expands more than the other, causing a strip to bend or a coil to uncoil.
How does a resistive sensor or RTD work?
It relies on the change of electrical resistance of a pure metal like platinum, nickel, or copper as temperature changes.
What is a Wheatstone bridge?
It is an arrangement of resistors used to measure small changes in resistance.
What is a thermocouple?
A sensor made of two dissimilar metals that creates an electrical potential at their junction proportional to temperature.
Where are thermocouples commonly used?
In environments with very high temperatures, such as the exhaust of a jet engine.
How does a thermocouple measure temperature?
It uses the potential difference created at the junction of two dissimilar metals and measures it with a galvanometer or millivoltmeter.
What is the formula to express the operation of a thermocouple?
E = K x Th, where E is electromotive force (emf), Th is hot junction temperature, and K is a constant.
How are thermocouples wired?
They are wired in parallel so that the failure of one does not cause a total failure of the temperature monitoring system.
What are the advantages of thermocouples?
They do not need to be powered by any external source of electricity and can sense temperatures between -200°C and 1250°C.
What is the disadvantage of thermocouples?
They are not that accurate.
What is a radiation pyrometer used for?
It measures the frequency of emitted radiation from the area to be examined and is used to measure turbine blade temperatures at the turbine inlet area of a jet engine.
What are the two types of air temperature probes used in aircraft?
The expansion type (bimetallic strip) and the electrical wire/resistance type.
What type of temperature sensor is normally used in light aircraft?
The bimetallic type, with a spring-like coil of bimetallic strip fixed internally to the probe.
How is the probe mounted in light aircraft?
Through the windshield with the indicator inside and the sensor outside the aircraft.
What are the limitations of the probe used in light aircraft?
It has a sunshield but no anti-ice protection and is only sufficiently accurate for low-speed aircraft.
What is the problem of speed in temperature sensing for faster aircraft?
If air temperature is sensed as it flows past the aircraft, frictional heating will distort the result; if previously static air is brought up to speed, the extra energy will increase the sensed temperature.
What terms are used to describe air temperature in faster aircraft?
Static Air Temperature (SAT) and Total Air Temperature (TAT).
How is the increase in temperature caused by ‘ram rise’ calculated?
Using the formula: SAT (1 + 0.2M²) = TAT, where M is the Mach number.
What is the constant in the SAT/TAT formula called?
The recovery factor, shown as K or Kr, with typical values around 0.9.
How do you derive SAT from TAT in practical use?
SAT = TAT ÷ (1 + 0.2KM²).
What is an alternative method for calculating temperature rise?
Using the aircraft’s True Air Speed (TAS) on the navigation computer.
Give an example of temperature rise using TAS.
At 200 KT TAS, the temperature rise is 4.25°C; at 300 KT TAS, it is 9.5°C.
What are some names for temperature sensors on faster aircraft?
They can be called ‘total head thermometer’, ‘Rosemount probe’, or just ‘temperature probe’.
What feature does a Rosemount probe have to protect the sensor?
It has a heater to protect the sensor during flight in icing conditions.
How does a temperature probe work?
Air is accelerated through a venturi, some of it is drawn off into a sensing chamber, where water and dust particles are separated. The temperature is measured by a resistive element.
What is the role of the heating elements in a temperature probe?
They could affect the air temperature reading, so the sensing element is isolated from the effects of the de-icing heater.
When will a temperature probe indicate correctly?
Only when the aircraft is moving because it relies on air being drawn into the sensing chamber.
What happens to a temperature probe when stationary on the ground with the heater on?
The gauge may go to full-scale deflection, and the heated probe itself may reach temperatures in excess of 300°C.
What are flat plate temperature sensors and how do they work?
They have the sensing element mounted flush with the aircraft skin, do not suffer ram rise, but are affected by frictional heating.
What is displayed by temperature gauges?
They may display TAT (Total Air Temperature) and/or apply calculated corrections to display OAT (Outside Air Temperature).
What are the three sources of error in air temperature measurements?
Instrument error, environmental error, and heating error.
How can instrument error be compensated for?
By fine calibration of the instrument.
How is environmental error caused and how is it managed?
Caused by solar heating or ice accretion; probes incorporate heaters and are mounted on the underside of the aircraft to stay in shadow.
What causes heating error and how is it managed?
Caused by ram rise or frictional heating; can be compensated for by calculation, usually within aircraft computers.
What is the time standard across aviation?
Coordinated Universal Time (UTC), virtually identical to Greenwich Mean Time (GMT).
Why is time important in engine and systems maintenance?
Because some maintenance is carried out by date or by hours flown or engine running time.
What is a system clock in modern digital aircraft?
An electronic clock providing ‘system time’ as the onboard reference for anything requiring a time reference.
How can system clocks be updated?
Manually reset or updated from external time references, such as GPS data.
How is time measurement accomplished on basic aircraft?
As part of a specific instrument or as a separate timer not feeding into any systems.
What is a Hobbs meter?
A separate meter recording hours and tenths of an hour, either elapsed engine hours or flight time.
How can a Hobbs meter be activated?
Electrically from the moment the aircraft is powered up, by oil pressure signifying engine running, by a weight-on-wheels switch, or by an airspeed sensing vane.
What is a simple accelerometer used for in a cockpit?
It is used to display load factor and in some simple data recorders.
How does a simple accelerometer work?
It uses a weight mounted on a spring; acceleration extends or compresses the spring, and a scale records the value of the acceleration.
What do more sophisticated accelerometers consist of?
A weight suspended on a thin metal blade.
How does a sophisticated accelerometer detect acceleration?
Acceleration in one axis moves the weight off-center, affecting the I-bar and E-bar system, generating a magnetic flux.
What happens when the weight in an accelerometer is displaced?
The flux in the ends of the E-bar will no longer be equal, generating an output signal.
How is the error signal in an accelerometer processed?
Through a feedback loop applied to electromagnets to push the weight back to the neutral position.
What is the feedback signal in an accelerometer equivalent to?
It is a direct equivalent of the force felt by the weight and measures the acceleration along the input axis.
What technology do modern Inertial Reference Systems (IRS) use for accelerometers?
They use Micro Electro Mechanical System (MEMS) accelerometers on chips.
How do MEMS accelerometers work?
They sense the displacement of a small mass as either a change of resistance or a change of capacitance.
How many accelerometers does an IRS have and why?
Three accelerometers mounted at right angles to each other to sense acceleration in all planes.
What systems can the IRS provide acceleration outputs to?
Flight data recorder, autopilot, flight director system, yaw dampers, autobrake system, and any other system that requires acceleration information.
