Exam (elaborations)
ATPL-A Instrumentation notes
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Intruments NotesMeasurements:
- The red pointer which is normally on the red line on EGT indicators moves when the corresponding value is
exceeded and remains positioned at the maximum value that has been reached.
- E = K x Th -> Electromotive force = Konstant x Hot junction temperature
- F = (C x 1,8) + 32
- To indicate a Temperature, a thermocouple doesn’t need power supply.
- Thermocouple -> no need of power supply -> only electrical power for the instruments lights in the cockpit
- In ight, If the Constant Speed Drive temperature indicator is in the red arc the pilot must disconnect it, and the
generator is no available for the rest of the ight.
- The probe used to mesure the air intake pressure of a gas turbine engined power plant is an aneroid capsule
- If manifold pressure consistently shows atmospheric pressure, the cause is probably a leak in pressure gauge line.
- In Transport category airplanes, the temperature is usually measured with resistance thermometers and
thermocouple thermometers.
- Low pressure fuel pump uses an aneroid capsule.
- Hydraulic pressure is sensed by a transducer that receives an electrical signal form a remote pressure sensor.
- The white arc of the temperature gauge correspond to a special operating range.
- 1BAR = 100000Pa = 15Psi.
Fuel measurements:
- The capacity fuel gauge provide information on mass whose indication is independent of the temperature of the
fuel
- Capacitance measurement is based on the variation of the dielectric of capacitor.
- Total fuel consumption is obtained by integrating the fuel ow with respect to time
- The mass fuel meter takes into account the fuel density.
- The dielectric constant of fuel is twice of the air and varies with density
- Capacitance is directly proporcional to: area of plates; relative permittivity
- Capacitance is inversely proporcional only to the distance between plates.
- C = Q /U C(ca p a cit a n ce) = Q(ch a rge)/U( p ote n ci a l, d i f er e n ce, b et wee n , pl a tes)
- A paddle-wheel placed in the fuel circuit of a gas turbine initially measures volumetric ow by a tally of impulses
- A basic mechanical fuel ow meter incorporates a spring loaded vane
- The principle of capacitance fuel gauges is based on the capacitance variation of a given capacitor with the type
of dielectric.
- If the tanks of your airplane only contains water, the capacitor gauges indicate a mass of water di erent from zero,
but inaccurate
Tachometer:
- The Rpm indicator or tachometer of a piston engine can include a small red arc, within the arc propeller generates
vibration, continuous rating is forbidden.
- The operating principle of the “electrical” type of tachometer is the rotation speed of a synchronous motor
energised by an alternator.
- Gas turbine engine speed is commonly indicated to the pilot in the form of percentage of a reference engine
speed.
Thrustmeter/FADEC:
- During take-o the front probe of an EPR system becomes blocked by ingested debris or icing but the rear probe
remains una ected . The displayed pEr will likely be higher than the actual EPR.
- N1 -> Rotational speed of the low pressure shaft.
- EPR-> the quotient of pressure at the exhaust nozzle and pressure at the compressor inlet.
- Sensors feeding EPR indicator are: pressure probes, one located upstream from the compressor inlet and other
downstream from the turbine outlet.
Torque-meter:
- Electrical: rotation speed
- Eletronic: twisting
- Oil torque meter : Helical
- Torque can be determined by measuring the computed oil pressure at the xed crown of an epicycloidal reducer of
the main engine gearbox.
- The operating principle of an electronic torque meter is increase in power causes twisting of the torque shaft.
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, Synchroscope and ENG Vibrating system:
- In a twin engined aircraft, if a synchroscope is installed, it works on the basis of a voltmeter which is fed by too
tachometer-gennies
- The output from an engine vibration transducer is always ltered to remove unwanted frequencies.
- A vibration indicator receives a signal from di erent sensors (accelerometers). It indicates the vibration amplitude at a
give frequency.
- Synchroscope give information about engine speed.
- Any vibration displayed on an engine vibration monitoring system for a turbojet indicates a rotor unbalance.
Pressure measurement:
- The alternate static source of a light non-pressurised airplane is located in the ight deck. As the alternate static
source is opened, the vertical speed indicator may indicate a momentary climb.
- Dynamic pressure measurement circuit is constituted of total pressure and static pressure probes.
- If alternate static port used: altimeter over-read; Airspeed indicator Over-read; Vertical speed indicator rst climb, then
zero, then normal.
- 1013,25hPa = 14,5Psi= 29,92 inHG
Temperature measurement:
- Total air temperature is higher or equal to static air temperature, depending on Mach number and Static air
Temperature.
- The airplane outside air temperature “probe” measures TAT minus Kinetic heating e ects in order to obtain the Static
Air Temperature.
- TAT = S AT * (1 + 0,2M 2 )
- The total air temperature is the temperature resulting from the aircraft motion in the air.
Altimeter:
- The altitude usually used by altimeter is pressure altitude
- Mach number make position error varie substantially
- Static pressures varies according TAS and angle of attack.
- Pressure altitude is the altitude corresponding in standard atmosphere, to the pressure Ps prevailing at
this point.
- If pressure setting is increased the indicated altitude increase
- Density altitude is the altitude of the standard atmosphere on which the density is equal to the actual
density of atmosphere.
- The purpose of vibration motor in sensitive altimeters is to increase the accuracy of the indication.
Vertical speed indicator:
- The response time of a vertical speed detector may be improved by adding a correction based on accelerometer
sensor.
Air speed indicator:
- The parameter that determines the relationship between EAS and TAS is density altitude.
- Steady ight—- Mconstant= CASconstant Tinc ->Mdec->LSSinc->TASinc
—- Tdec->Minc->LSSdec->TASdec
- To be as accurate as possible, an airspeed indicator must be calibrated according to the Saint-Venant Formula,
taking into account the air compressibility.
- In a twin-engine the blue radial line on airspeed indicator corresponds to the best single-engine rate of climb
- Vmo- Maximum Operating Speed. Calculated by EAS and shown as CAS.
- IAS-> - system error -> CAS -> - Compressibility error -> density error ->TAS
- A blocked pitot and open altimeter causes Airspeed indicator to react like an altimeter
- PUDSOD- Pitot underreads in descent | Static overheads in descent
- TAS=CAS+1%/600ft => ex.TAS=CAS+1%600ft->(6000ft)->10%.
- EAS is always lower than or equal to CAS considering a AIS, a second, striped needle, if installed indicates Vmo
- Vno is the maximum speed not to be exceed except still air and with caution.
MACHmeter:
- The compressibility factor is always lower or equal to 1
- During a descent at a constant MACH, TAS and CAS increase
- Mach number is de ned as the ratio of dynamic pressure to static pressure.
- Indication of MACHmeter is obtained from indicated speed and altitude using a speed indicator equipped with an
altimeter type aneroid.
- The reading of a Mach indicator is independent of the outside Temperature.
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