Pdfcoffee.com chandan-instrumentation-pdf-free PDF

Preview

Summary

FOR DGCA CPL,ATPL EXAMINATIONUshaPilotinAcademyCapt(+9 1 )9 939063185Capt Kumar+91 9939063185Page 2AIR TEMPERATURE MEASUREMENTThe measurement of air temperature is important to a pilot Aircraft performance and flight planning and necessary for safety, economy and maximum performance.If the temperat...

Description

FOR DGCA CPL,ATPL EXAMINATION

Usha Pilotin Piloti n Academy Acade my

Capt.CHANDAN (+91)9939063185

USHA PILOTING ACADEMY

+91 9939063185

Capt.Chandan Kumar

AIR TEMPERATURE MEASUREMENT The measurement of air temperature is important to a pilot  Aircraft performance and flight planning and necessary for safety, economy and maximum performance. If the temperature sensor fitted aircraft fuselage When Aircraft speed increases; the air close up to the aircraft becomes compressed. Due to this compression the air becomes heated. This means that the temperature sensed will too warm and not representative of the actual air temperature outside of the aircraft.

 Static Air Temperature (SAT) is the temperature of the undisturbed air through which the aircraft is about to fly.  Total Air Temperature (TAT) is the maximum temperature attainable by the air when brought to rest, adiabatically.  The increase of air temperature at higher speeds as a result of the adiabatic compression of the air is known as the “Ram Rise”.  The percentage of the “Ram Rise” sensed, and recovered, by a TAT probe is termed the Recovery Factor (Kr).

Thus a TAT probe having a factor of 0.90 would measure SAT plus 90% of the ram rise. A recovery factor of 1.0 would produce a reading of SAT plus 100% ram rise = TAT. Modern air temperature probes have recovery factors approaching 1.0. For example, if a sensor has a recovery factor of 0.80, it measures SAT + 80% of the RAM Rise.

AIRCRAFT INSTRUMENTATION

Page 2

USHA PILOTING ACADEMY

+91 9939063185

Capt.Chandan Kumar

Air Temperature Thermometers may be divided into two basic types: Direct Reading Thermometer used in low speed aircraft uses a bimetallic strip consisting of two metals, such as Invar and Brass, bonded together.

When this strip is heated, the brass, having a higher coefficient of expansion than the Invar, will expand much more than the Invar How much the strip bends depends on the temperature rise to which the strip is subjected, and is therefore a measure of the temperature. Bi-Metallic Helix Thermometer

AIRCRAFT INSTRUMENTATION

Page 3

USHA PILOTING ACADEMY

+91 9939063185

Capt.Chandan Kumar

The principle of the bi-metallic strip is used to provide a direct indication of temperature. If the bimetallic strip is wound into a helix (coil) then any temperature change will cause the helix to rotate. TOTAL AIR TEMPERATURE PROBE

 In flight, the air flows through the probe in the manner indicated; separation of any water particles from the air is effected by the airflow being caused to turn through a right angle before passing round the sensing element.  The bleed holes in the intake casing permit boundary layer air to be drawn off under the influence of the higher pressure that is created within the intake and casing of the probe.  A pure platinum wire resistance-type sensing element is used. The probe has an almost negligible time-lag, and a high recovery factor of approximately 1.00.  A heating element is mounted integral with the probe to prevent the formation of ice, and is of the self-compensating type in that as the temperature rises so does the element=s resistance rise, thereby reducing the heater current. ERRORS Aircraft thermometers used for the measurement of air temperature are subject to the following errors:  Instrument errors - imperfections in manufacture.  Environmental error - solar heating of the sensor. Ice accretion on the probe.  Heating error - adiabatic and kinetic (friction) heating

The relationship between heating error, SAT and TAT is Where: V is the true airspeed in knots SAT is the Static Air Temperature TAT is the Total Air Temperature RAT = SAT + RAM Rise or SAT = RAT - RAM Rise RAT(TAT)- Ram Air Temperature

TAT = SAT + Ram Rise

AIRCRAFT INSTRUMENTATION

Page 4

USHA PILOTING ACADEMY

+91 9939063185

Capt.Chandan Kumar

 Total Air Temp (TAT) = Static Air Temp (SAT) + Ram Rise  Total Air Temp (TAT) = Outside Air Temp (OAT) + Ram Rise  Indicatted Outside Air Temp (IOAT) = Corrected Outside Air Temp (COAT) + Ram Rise

Temperature conversion formula

AIRCRAFT INSTRUMENTATION

Page 5

USHA PILOTING ACADEMY

+91 9939063185

Capt.Chandan Kumar

AIRCRAFT INSTRUMENTATION

Page 6

USHA PILOTING ACADEMY

+91 9939063185

Capt.Chandan Kumar

THE AIRSPEED INDICATOR  An aircraft on the ground in still air is subject only to atmospheric (static) pressure (S)  An aircraft in forward motion are subject to an additional (dynamic) pressure.  This results in a total (pitot) pressure (P) on the leading edges of dynamic pressure plus static pressure.

Pitot(P) = Dynamic(Dy) + Static(S)  The dynamic pressure is often called ‘pitot excess’ pressure (PE) so we have:P = PE + S  The pitot head senses pitot pressure and the static/vent senses static pressure. These two pressures are fed to the airspeed indicator, a differential pressure gauge, which measures their difference PE (the dynamic pressure). Now dynamic pressure is a measure of airspeed  Dynamic Pressure = ½ ρV2 Where V is true airspeed and ρ is density of the surrounding air.

ASI CONSTRUCTION

AIRCRAFT INSTRUMENTATION

Page 7

USHA PILOTING ACADEMY

+91 9939063185

Capt.Chandan Kumar

An airtight box divided by a flexible diaphragm, with pitot pressure fed to one side and static pressure to the other side. The pressure difference across the diaphragm is (Dy + S) - S, which is Dy, the dynamic pressure. Accordingly, the diaphragm deflects by an amount proportional solely to this dynamic pressure, its movement being transmitted by a system of levers to the indicating needle on the face of the ASI.  Dynamic pressure depends not only on the speed of the aircraft but also on the air density. This density varies with temperature and pressure and so with altitude  The ASI is calibrated to read true airspeed for the air density of 1225 grammes per cubic metre which would be produced by the ISA MSL pressure of 1013.25 mb and temperature + 15°C (dry air conditions) ASI ERRORS Density Error as climb density will decrease so ASI will under read The ASI under-reads the true speed at altitude, the discrepancy being called ‘density error’. If below MSL, the ASI will over read the true speed. As climb ASI

=

½

ρV2 then ASI will under read

CAS + correction for density error (nearly always +) = TAS TAS = CAS + (1.75% of CAS per 1000 ft of altitude) Example, for a CAS of 100 knots at 10 000 ft: TAS = 100 + (1.75/100 x100 x 10) = 117.5 knots Instrument Error Manufacturing imperfections and usage result in small errors which Position Error Alternatively known as ‘pressure’ error, this arises mainly from the sensing of incorrect static pressure Position errors throughout the speed range are determined by the aircraft manufacturer during the test flying programme for a particular aircraft type. IAS (indicated air speed) ± P and I correction = CAS

AIRCRAFT INSTRUMENTATION

Page 8

USHA PILOTING ACADEMY

+91 9939063185

Capt.Chandan Kumar

Manoeuvre Induced Error The pressure error changes if any of the following vary:    

Airspeed Angle of attack Configuration (flap setting, undercarriage etc) Position of the pitot/static sources and sideslip

Compressibility Error At low airspeeds this error is insignificant, but at airspeeds over 300 KTAS, this error becomes significant.

More ASI Definitions  VS0 = The stall speed or the minimum steady flight speed in the landing configuration.  VS1 = The stall speed or the minimum steady flight speed in a specified configuration.  VFE = The maximum Flap Extension speed  VNO = The maximum normal operating limit speed.  VNE = The Never Exceed speed  VLO = The maximum Landing Gear Operation speed (up or down).  VLE = The maximum speed Landing Gear Extended speed  VYSE = Best rate of climb when Single Engine (2 eng a/c)

AIRCRAFT INSTRUMENTATION

Page 9

USHA PILOTING ACADEMY

+91 9939063185

Capt.Chandan Kumar

 The White Arc denotes the flap operating range, VS0 to VFE  The Green Arc denotes the normal operating speed range VS1 to VNO  The Yellow Arc denotes the caution range, which extends from VNO (normal operating limit speed) up to VNE (the never exceed speed).  A Red Radial Line denotes VNE, the never exceed speed.  A Blue Radial Line denotes the best rate of climb speed for one engine out, maximum weight, at mean sea level (VYSE).  A Red Radial Line denotes the minimum control speed at maximum weight (V MCA).

TOLERANCE - ± 3% or 5 Knots whichever is the greater

AIRCRAFT INSTRUMENTATION

Page 10

USHA PILOTING ACADEMY

+91 9939063185

Capt.Chandan Kumar

BLOCKAGES AND LEAKS Pitot head blocked  In Straight and Level -----ASI Reading Correct  In Climb-------------------ASI will Over Read  In Descent---------------ASI will Under Read

Static Head Blocked  In Straight and Level-------ASI Reading Correct  In Climb---------------------ASI will Under Read  In Descent------------------ ASI will Over Read

due to position error. Any dynamic, or turbulence, effects would usually result in a higher static pressure and thus produce an under-reading. Leaks  A leak in the pitot system causes the ASI to under-read,  A leak in the static line causes the ASI to over-read in an unpressurised fuselage (cabin pressure is usually lower than the atmospheric static pressure)  Under-read in a pressurised aeroplane (cabin pressure higher than static).

AIRCRAFT INSTRUMENTATION

Page 11

USHA PILOTING ACADEMY

+91 9939063185

Capt.Chandan Kumar

THE PRESSURE ALTIMETER  The Earth is surrounded by a gaseous envelope, which is divided into several concentric layers that extend outward from the Earth's surface, up to a height of approximately 900 km (500 miles).  Pressure which decreases steadily with increasing distance from the Earth's surface  Air density is directly proportional to atmospheric pressure and similarly reduces with increasing altitude.  Increasing Temperature Pressure Decrease and Vice-Versa also Decrease when increase altitude

 The pressure altimeter is a simple, reliable, pressure gauge calibrated to indicate height

 The pressure at a point depends on the weight of the column of air which extends vertically upwards from the point to the outer limit of the atmosphere.

AIRCRAFT INSTRUMENTATION

Page 12

USHA PILOTING ACADEMY

+91 9939063185

Capt.Chandan Kumar

 Greater the height, the lower the pressure, and by measuring the pressure the altimeter measures height.  The relationship between pressure and height is not a linear one so we need calibration.

CALIBRATION International Standard Atmosphere (ISA)  At mean Sea Level Pressure 1013.25 millibars Temperature +15°C Density 1225gm m3  From MSL up to 11 km (36,090 feet) Temperature falling at 6.5°C per km (1.98°C/1000 feet)  From 11 km to 20 km (65,617 feet) A constant temperature of - 56.5°C  From 20 km to 32 km (104, 987 feet) Temperature rising at 1°C per km (0.3°/1000 feet).  1013.25 mb/hPa = 29.92”Hg = 14.7 psi SIMPLE ALTIMETER

 Static pressure is fed into the case of the instrument from the static source. As height increases, static pressure decreases and the capsule expands under the control of a leaf spring. A mechanical linkage magnifies the capsule expansion and converts it to a rotational movement of a single pointer over the height scale  The simple altimeter has a setting knob which is geared to the pointer.

AIRCRAFT INSTRUMENTATION

Page 13

USHA PILOTING ACADEMY

+91 9939063185

Capt.Chandan Kumar

SENSITIVE ALTIMETER

The principle of operation is similar to that of the simple altimeter but there are the following refinements: Two or three capsules gives the increased movement necessary to drive three pointers. These are geared 100:10:1, the smallest indicating 100,000 feet per revolution, the next 10,000 feet per revolution and the largest 1,000 feet per revolution.

 Jewelled bearings are fitted, reducing friction and the associated lag in indications. The pilot turns the knob until the desired pressure level (say, 1005 mb. appears on a pressure sub-scale on the face of the instrument.) As he turns the knob, the height pointers rotate until, the sub-scale showing the desired 1005, the altimeter indicates the aircraft's height above this pressure level.

AIRCRAFT INSTRUMENTATION

Page 14

USHA PILOTING ACADEMY

+91 9939063185

Capt.Chandan Kumar

EXAMPLES OF ALTIMETERS

AIRCRAFT INSTRUMENTATION

Page 15

USHA PILOTING ACADEMY

+91 9939063185

Capt.Chandan Kumar

SERVO-ASSISTED ALTIMETERS  servo-assistance overcome the frictional resistance with consequently enhanced instrument accuracy The principle of the servo-altimeter is that the small movements of the capsules are detected bya very sensitive electro-magnetic pick-off. This produces an electric current which is amplified and used to drive a motor which rotates the counters and pointer.

AIRCRAFT INSTRUMENTATION

Page 16

USHA PILOTING ACADEMY

+91 9939063185

Capt.Chandan Kumar

 AC is fed to the middle leg of the E bar, setting up alternating magnetic fields in the outer legs  The coils on these two legs are wound 180° out of phase. The exciter therefore induces a current in each leg, but since these are 180° out of phase and of equal strength, they cancel each other out when the I bar is equidistant from the legs of the E bar (that is when no pressure change acts on the capsules).  With a change of pressure the capsules expand or contract, moving the I bar on its pivot, closing the gap between the I Bar and E Bar at one end and opening it at the other.  This causes an imbalance of magnetic fields and therefore of the currents induced in the Coils (E). The imbalance causes an error signal which is passed to the amplifier, where it is amplified and rectified, and thence to the servo motor  The servo motor drives the counter-pointer system of the altimeter and at the same time, via a cam drive, re-aligns the E Bar with the I Bar.  Once re-aligned, the error signal ceases and the altimeter indicates the correct height. Note 1 mb(milibars) change of pressure about 30 ft

ALTIMETER ERRORS Time Lag The response of altimeter to change of height is not instantaneous. This causes the altimeter to under-read in a climb and over-read in a descent. The lag is most noticeable when the change in altitude is rapid and prolonged. Servo-altimeter does not suffer from the linkage friction Instrument Error Manufacturing imperfections, including friction in the linkage, Residual errors may be listed on a correction card

With the sensitive altimeter the error increases with altitude Position (or Pressure) Error Pressure errors arise because the true external static pressure is not accurately transmitted to the instrument. A false static pressure arises because of disturbed airflow near the pressure head or static vent. Pressure error is negligible at low altitudes and speeds, but becomes more significant with increasing airspeed. Air Data Computers are designed to compensate for this type of error.

AIRCRAFT INSTRUMENTATION

Page 17

USHA PILOTING ACADEMY

+91 9939063185

Capt.Chandan Kumar

Manoeuvre-Induced Error This is caused by transient fluctuations of pressure at the static vent during change of, mainly, pitch attitude Barometric Error Barometric error occurs when the actual datum level pressure differs from that to which the Subscale is set. If the aeroplane flies from an area of high pressure into an area of low pressure it descends even though the altimeter reading remains constant.

 If the subscale is set to 1030 hPa.  A subscale error of 1 hPa is equivalent to an indicated altitude error of 28 to 30 ft.  The QNH has reduced to 1010 hPa, which represents an altitude change of approximately 600 ft.  The subscale datum is now at appoint that is effectively 600 ft below sea level, and the altimeter measures from this level. If you are flying from Low Pressure area to high pressure area the altimeter Read Under read. If you are flying from High pressure area to Low pressure area altimeter will Over Read

AIRCRAFT INSTRUMENTATION

Page 18

USHA PILOTING ACADEMY

+91 9939063185

Capt.Chandan Kumar

Temperature Error

When flying in colder air (with an air density greater than ISA at that altitude), the altimeter will over-read. Note- If temperature increase Air Density will Decrease but If Temperature Decrease the air Density will Increase if the air below the aeroplane is warmer than standard, the air is less dense (low pressure) and the aeroplane is higher than indicated. Conversely if the air is colder than standard, it is more dense (high pressure), and the aeroplane is lower than standard.

 HIGH -----LOW----HIGH (Over Read)  LOW------HIGH----LOW (Under Read)

Hysterisis Error The capsules suffer from hysterisis, which causes a lag in the instrument reading during a climb or descen

AIRCRAFT INSTRUMENTATION

Page 19

USHA PILOTING ACADEMY

+91 9939063185

Capt.Chandan Kumar

Orographic Error Differences from standard may occur when air is forced to rise/descend over hills or mountains. Low pressure tends to occur in the lee of mountains with high pressure on the windward side

BLOCKAGES AND LEAKAGES  If the static source becomes blocked, the altimeter will not register any change in height the height at which the blockage occurred will still be indicated regardless of any climb or descent.  If the aircraft is CLIMBING then the altimeter will UNDER READ  If the aircraft is DECSCENDING then the altimeter will OVER READ  If a leakage In static line on a pressurised aircraft, the altimeter will show the (lower) cabin altitude rather than aircraft altitude  If a Leakage in the static line within an unpressurised aircraft will normally result in the altimeter over reading, due to the pressure in the cabin being lower than ambient due to aerodynamic suction

SOME DEFINITIONS

AIRCRAFT INSTRUMENTATION

Page 20

USHA PILOTING ACADEMY

+91 9939063185

Capt.Chandan Kumar

Height The vertical distance of a level, point or object measured from a specified datum. (Normally associated with QFE and height above aerodrome level). or The vertical dimension (size) of an object. Altitude The vertical distance of a level, point or object measured from MSL. (Normally associated with QNH). Cruising Level This is a vertical position for a significant portion of the flight and can be a height, altitude, or flight level depending on the altimeter setting procedure in force. Flight Levels When in sub scale we set 1013.25 Hpa then its gives constant pressure level or Flight level associated with QNE A flight level is expressed For example, with 1013.25 set and 25 000 feet indicated, the flight l...