Instruments and Electonics - Coggle Diagram
Instruments and Electonics
Provides measurement of the total pressure created by the forward motion of the aircraft and the static pressure of the air surrounding the aircraft.
often called total pressure or ram pressure
Created by forward motion of the aircraft through the air
Required by the ASI and Machmeter
Air is brought to rest at the head of the tube - stagnation point
Total energy at the stagnation point is the sum of Potential Energy + Kinetic energy + Pressure Energy
Required by all four pressure instruments
Mostly affected by pressure of position error - which is reduced considerably by removing the static from the probe and installing static vents
Pressure Instrument Calibration
Pressure instruments are calibrated to a known set of atmospheric conditions (ISA)
International Standard Atmosphere (ISA)
Mean sea level Pressure - 1013,25hpa or 29,92 ic HG
Mean sea level temperature - 15C or 59F
lapse rate of 1.98C per 1000 ft up to 36090 ft where the temperature remains constant at -56,5C
From 20KM to 32KM temp increase at 1C per KM or 0,3C per 1000ft
Air Data Computer
Air data is the term used to describe the static and pitot pressure
The transmission of static & pitot pressures is achieved via very small pipelines, which in large aircrafts with ore instruments requires substantial plumbing
A system is used which transmits the pressures only as far as an Air Data Computer(ADC) which then converts the pressure into electrical signals which are transmitted to the various instruments
This eliminates mechanical linkage errors and corrections can be automatically applied.
Warning flags are provided in each indicator - activated by the loss of data signals and annunciator lights on the computer
The ASI measures the difference between Pitot Pressure (P) and Static Pressure (S)
A stationary aircraft is subject to atmospheric pressure which is Static Pressure (S), due to the weight of the column of air above the aircraft
In flight, the aircraft is subject to Dynamic Pressure (D) due to movement through the air
Dynamic Pressure is directly proportional to Airspeed
Pitot Pressure is measured by the Pitot head and is the sum of (D+S)
D + S = P
D = P - S (we must subtract static pressure from Pitot pressure to obtain dynamic pressure or airspeed.
ASI has a capsule which is sensitive to changes of Pitot Pressure which is supplied from the Pitot head.
Capsule is mounted inside an airtight case which is fed with static pressure from the static vent. and now that static pressure is inside and outside the capsule. they cancel out leaving only dynamic pressure (aka Airspeed)
Dynamic Pressure = 1/2 r v2 (1+v2/4c2)
V = Indicated Airspeed // c = speed of sound at sea level
ASI is calibrated according to ISA
an error arises when the actual air temperature differs from ISA due to speed of sound varying with temperature.
Instrument error is small due to minor imperfections from manufacturing or wear & tear
Correction card can be provided
Pressure or position error
Incorrect pressures being sensed by the pitot static probe or static vent
Instrument and Pressure errors are usually combined and a correction card placed near the ASi
IAS +/- Instrument and Pressure error = RAS or CAS
Manoeuvre induced error
Random error due to changes of aircraft attitude or when the pitot head is not aligned with airflow (climb and Descent)/ changes of aircraft configuration i.e flaps or landing gear being lowered
The ASI is calibrated for ISA, in such conditions, RAS = TAS. Under ANY other conditions, a density error will occur
Air density lower than standard will indicate an airspeed lower than TAS
Air is compressible, at high speeds (TAS 300 +) air is compressed and causes higher dynamic pressure than normal.
This causes an overread in the IAS
As air is compressed, its temperature rises and will have to be corrected before the correction is made on the nav computer
ASI Colour coding
Permissible speed range with flaps extended - Low speed end - stalling speed with flaps down
Low speed end - stalling speed with flaps up
Green yellow division
Vno Normal operating
Caution Range - Smooth air conditions only
Vne - Velocity never exceeded
If the pitot line from the pitot head to the ASI develops a leak, the ASI will underread (pitot pressure too low)
If the pitot head becomes blocked in level flight the ASI pressure line will hold the pressure and ASI reading will maintain the last recorded airspeed.
If there is a leak as well, airspeed will drop to zero
If the static source becomes blocked in level flight, the ASI will continue to read correctly.
Climbing and descending
refer to graph ASI page 3-4
An aneroid barometer calibrated to indicate altitude instead of pressure
Calibrated according to ISA
Static pressure is fed to the airtight altimeter case from the static vent
A capsule is supported in an open position by a leaf spring
As the static pressure decreases/increases, the capsule will expand or contract
The capsule is linked to a pointer
The altimeter becomes unreliable at extreme altitudes
Correction card can be prepared for position error
Time lag error
It may take time for a pressure change at the static vent to be shown on the altimeter - Rapid climb (underreads) - Rapid descent (overreads)
At sea level pressure changes, error is evident. Error is corrected by QNH set on the subscale
If the sea level temp or lapse rate is different then a temperature error exists. Error becomes significant at high altitudes - can be corrected on the nav computer
If the static vent becomes blocked, the old static pressure remains and no change of altitude will be shown.
Types of altimeters
Has two or three capsules
Three capsules give greater movement for a given pressure change
Jeweled bearings are fitted reducing friction and time lag in indications
Servo Assisted Altimeter
The accuracy of an altimeter decreases with altitude, which is why the servo was created
The capsules are retained, but their movement is measured electronically and a motor drives the altitude pointers.
Consists of a twin capsule unit which expands/contracts and affects the "I " and "E" bars
Pressure settings and definitions
The mean sea level pressure calculated from airfield pressure reduced to sea level using ISA.
With QNH set, altimeter will show ALTITUDE above mean sea level
Barometric pressure at an airfield
With QFE set, altimeter will show HEIGHT above airfield elevation
Standard setting 1013.25hPa
With QNE set, altimeter will show Pressure Altitude (aka HEIGHT above the 1013.25 pressure level
The height of an aircraft above the surface immediately below the aircraft
The height in ISA where the prevailing density will occur
Altitude alerting provides audio and visual warnings when the aircraft approaches or deviates from preset altitude by more than a certain amount.
Vertical Speed Indicator
Utilises the principle of differential pressure to indicate the rate of climb/descent
A capsule in an airtight case is supplied with static pressure
As an aircraft changes altitude, the resulting change in static pressure is fed directly to the capsule.
The pressure differential is proportional to the rate of climb/descent
Time Lag error
A short time interval must elapse before the new pressure reaches the instrument and a pressure differential is established
If static pressure is subject to position error, then the VSI can show momentary climb/descent if speed is rapidly changed
manoeuvre induce error
Fluctuations in pressure at the static vent during rapid changes of attitude can cause a false indication
Any blockages of the static pressure line will cause the VSI to read zero
Alternate static source
in unpressurised aircraft, cabin air can e fed to the static system from an alternate static source
Cabin pressure can be 10 hPa lower than true static pressure
If alternate source is used, Altimeter will overread by 300 feet
If alternate source is used, ASI will overread and VSI will show an initial climb due to lower cabin static pressure
Consists of the same basic elements but incorporates two small accelerometers which sense the initial acceleration when a climb or descent is initiated
Two small cylinders containing pistons are connected to the static pressure line
When a climb/descent is initiated the pistons are displaced by the inertia of the vertical acceleration force and create an immediate pressure change inside the capsule.
An aircraft in flights generates sound waves due to its motion
as the aircraft approaches the speed of sound, it catches up on its own sound wave and the pattern changes
Shock waves begin to form, drag increases and buffeting occurs which can stress the aircraft.
Speed of sound varies with altitude so conventional ASI's wont work
Mach Number = TAS/LSS (local Speed of Sound)
Speed of Sound