6. INSTRUMENTS

What happens to the reading when it is extremely cold?

(below -16C) The true altitude will be a lot lower than shown, hence corrections need to be applied.

White lighting is usually combined with grey cockpit interiors because?

- you have unrestricted use of colour - warning indicators become more prominent - black instrument cases against a grey background will emphasise their size and shape

What is the RAT if the SAT is -20C and the stagnation rise is 10C and the recovery factor is 80%?

-20 + 8 = -12C

To find the airspeed you need to...?

... to compare the general pressure outside the aircraft (static pressure) with the pressure created from its movement through air, so this instrument is connected to both the static and the pitot pressure systems.

Parallax error is due to...?

... to the angle from which you read the instrument.

If your system cannot measure TAT correctly you must..?

...you must use a Recovery Factor.

Because the atmosphere gets less dense as you climb, the IAS must be corrected. What is the rate?

1,75% per 1000ft.

On average the TAS increases by 2% over the IAS for every how many feet?

1000 ft.

How many ADC:s are there in most modern transport aircrafts?

2 to provide redundancy

If a temperature sensor with a recovery factor of 0.75 indicated 30C and the SAT is 25C, what is the RAM rise?

6.7C. The trick is to remember that 5C is 0.75.

Altimeter suffer from what kind of errors?

(page 6-10) MECHANICAL ERRORS - scale error - friction error - position error TEMPERATURE ERROR ELASTIC ERROR - Hysteresis (=an irregular response to pressure changes) - drift - secular error TIME LAG REVERSAL ERROR

There are several types of pressure, including?

- Absolute pressure or the difference between the pressure of a fluid and absolute zero (a vacuum). It is usually measured in inches of mercury, as on a Manifold Absolute pressure gauge. It would be the sum of gauge pressure and atmospheric pressure, and is what forces the fuel and air charge into the cylinders of a piston engine. - Gauge Pressure This on the other hand, is measured against ambient air pressure, so it is absolute pressure minus atmospheric pressure. For example, fuel and oil pressure instruments indicate the amount that the pump has raised the pressure of the fluid above that of the atmosphere, positive or negative. If the absolute pressure stays constant, gauge pressure varies with atmospheric pressure. - Differential pressure it is just the difference in pressure between two points, as represented by the airspeed indicator. Two inlet ports may be used, with each connected to one of the sealed volumes whose pressure is to be monitored.

Instruments cover four areas of aircraft operation, which are?

- Control (such as the artificial horizon and engine instruments) - Performance (ASI, VSI, altimeter, compass) - Navigation ( VOR, DME, ADF) - Miscellaneous (Warning flags, gear position indicators, pressure and temperature etc.)

Instruments can be classified in four groups, after variations of properties of certain materials against variations of temperature:

- Expansion - Vapour- pressure - Electrical; based on: - resistance; or - increase in electromotive force - Radiation (Most will be electrical)

Instruments have up to 4 sub-systems, not all of which will be in the same case, these are?

- detection (e.g. temperature probe) - measurement (aneroid capsule) - coupling (suitable linkage between measurement and indication) - indication (Pointer, or digital display)

Individual instruments may be lit by: ?

- integral lighting, which is built into the instrument - ring, eyebrow, or post lighting, all of which are fitted to the outside of the instrument case - floodlighting

What are the most significant advantages of the ADC?

- position error correction - remote data transmission capability

What happens if the static system fails?

- the ASI will over-read in the decent and under-read in the climb - the altimeter will read the same in the climb or decent - the VSI will read zero

What happens if the pitot system fails?

- the ASI will under-read in the decent and over-read in the climb

What happens at the point of measurement?

A measuring body absorbs some energy and converts it to a quantity that has a functional relationship with the quantity measured. As some energy is absorbed, that quantity will never be the same as the true value. Corrections are usually included with amplification signals because the sample is small.

Primary instrument vs secondary instruments?

A primary instrument is one which gives instant and constant readouts (also called direct). A secondary instrument is one that you have to deduce things from, such as the altimeter increasing, telling you that the pitch must have changed. Note also that a primary instrument will tell you at what rate things are changing but the secondary will only indicate that change is taking place.

What will the pitot tube failure affect?

ASI. A static system failure affects the ASI, VSI and the altimeter.

What is true altitude?

Actual height above mean sea level (MSL)

What is static pressure?

An aircraft is acted on from all directions by static pressure, which is fed into the system through static lines that are connected to static ports or static vents on both sides of the machine to ensure that they balance out when it yaws , or performs strange manoeuvres. They may or may not be heated (generally not on smaller machines) Warning lights associated with pitot/static heating systems usually come on when the heating element or the power relay has failed, so one light can have two meanings. Information from the static ports may also be fed to non-flight systems, such as autopilots or flight directors.

What is dynamic pressure?

An equal volume above the level of flow, which is dynamic pressure, and a measure of airspeed. In simple terms, dynamic pressure of the air against the front surfaces of an aircraft (as detected by the pitot tube) is greater than the pressure of the undistributed air sensed through static ports. The difference is proportional to the square of speed, so the instruments can be calibrated in terms of speed, such as knots. The formula of dynamic pressure is : Dynamic pressure = 1/2pV^2 Here the p (rho) is air density and V the true velocity. As you can see the strength depends on the speed of the relative airflow as its density

The ADC data can also be fed to the...?

Autopilot, flight director, flight management system, GPWS, area navigation aids, instrument comparison systems and the EFIS symbol generators to be converted for electronic display. Standby instruments use pitot-static plumbing.

What are bellows?

Bellows are an extension of the capsule (think of. them as several unsealed capsule joined together), but operate like a helical compression springs - indeed, there may even be a spring inside to increase the spring rate and to help the bellows return to its normal length once the source of pressure is removed. They are used for higher pressures and commonly used for example, to measure the output of a low pressure booster pump. Two would be used in a manifold pressure gauge, one open to the induction manifold and the other evacuated and sealed.

What is a bimetallic strip thermometer?

Below about 150kts, a thermometer like that shown below is good enough for getting the OAT (ADD PICTURE 6 -4) The probe sticks out to the airstream, and the dial is inside the cockpit. The works consist of a helical (coil-shaped) bimetallic strip that twists as the temperature changes and moves the pointer. The probe cannot be shrouded from the sun and it is necessarily next to the fuselage skin, so its readings can be affected by kinetic heating, even at low speeds - at 150kts, the rise can be around 3degrees. Being crude instruments, they are also subject to other errors, so a professional rule of thumb is to assume an error of about 2-3 degrees

Give an example of good rules of thumb when it comes to altimeter readings.

From high to low pressure, your altimeter will over read (from HIGH to LOW, your instrument is HIGH) Going from HIGH temperature to LOW temperature, your instruments will be HIGH

What happens if the fluid (air) is an ideal one?

In that case total pressure is equal to the sum of the potential energy, kinetic energy and pressure energy but the first is ignored in a pitot tube and the kinetic energy is converted to pressure energy anyway.

Height vs altitude

Height refers to the vertical distance from a particular datum, usually the surface on an airfield. Altitude means the distance above sea level

When is the only time that your altimeter will indicate true altitude?

In ISA conditions

Flight instruments are grouped in a --- arrangement...

In a T- arrangement.

How is information extracted in a thermistor?

In the same way as the coil.

What are aneroid capsules?

In gauges used for small measurements, or for absolute pressure, the gear train an needle may be driven by an enclosed and sealed chamber, called an aneroid, used in aneroid barometers, altimeters, altitude recording barographs and the altitude telemetry instrument sin weather balloon radiosondes. The sealed chamber is used as a reference pressure and the needles are driven by the external pressure. A capsule consists of two diaphragms placed face to face and fined at their edges to form a chamber that may be completely sealed or left open to a source of (absolute) pressure. They are also used for low(ish) pressures, but are more sensitive than diaphragms.

How frequently are pitot -static systems checked?

During regular maintenance, usually something like every 2 years for IFR machines.

How can corrections be made?

Normally, down to -15C you would apply 4% increase for every 10C below standard, when the temperature is well below ISA correct altitudes by ADD PICTURE 6-11

What sort of errors are there when it comes to instruments?

Instrument error comes from the usual imperfections in manufacturing and can be sorted out by fine calibration. Environmental error is caused by solar heating or icing, for which the Rosemount probe has a heater. Probes are usually mounted to keep them in shadow but the residual effects of environmental error can only be minimised and not corrected for.

What is the pitot-static system in short?

It consists of a series of pipes around the cockpit through which air flows to feed three common instruments: the altimeter, airspeed indicator and the VSI ADD PICTURE 6-7

What is a simple altimeter?

It is a little more basic, with only one capsule - they are commonly used as cabin altimeters on pressurised aeroplanes since, at high altitudes, the capsule's movements are difficult to detect.

What is altimetry?

It is a science of measuring vertical distances in the atmosphere. The decrease of pressure with altitude depends on gravity and air density pressure = g x density x heigh diff

What is the difference between RAT and SAT?

It is called the Ram Rise.

What is a transition level?

It is determined by the ATS unit concerned, as it varies with pressure from day to day and is always higher than the transition altitude.

What is an transition altitude?

It is normally specified for an aerodrome by the State in which it is located. It is as low as possible and it is rounded up to the nearest 1000ft. Above transition altitude, separation from other aircraft is achieved if all altimeters read the same, in this case 1013.25.

What is an ADC and what is its task?

It is the Air Data Computer. It was developed in an attempt to reduce the plumbing and improve reliability and accuracy, by allowing the instruments to be operated electrically from remote places. It is a "black box" in a central location that receives inputs from the usual sources and translates them into electrical equivalents for transmission to the relevant indicators, which have no pressure sensing elements so they can be simpler and or cheaper to make

What is the stagnation pressure?

It is the combination of static and dynamic pressure, because airflow is being brought to rest inside the pitot tube or stagnating. ADD PIC 6-16.

What is the transition layer?

It is the difference between the transition altitude and the transition level, it will be more than zero and less than 500ft.

What is pressure altitude?

It is the height in the standard atmosphere that you may find a given pressure, usually 29,92" or 1013 hPa , but actually whatever you set on the altimeter - if you set 1013 on the sub scale and the needles read 6000ft , the PA for that setting is 6,000 ft. So what is indicated is the height of the pressure selected.

What is calibrated altitude

It is the indicated altitude corrected for instrument and position error.

What is TAT?

It is the temperature that would be recorded if you would stop dead during flight (i.e. with nothing frictionally induced - on the ground, TAT/RAT = SAT) TAT is higher than or equal to SAT, depending on the Mach number and the SAT.

What is elevation?

It is the vertical distance of a point on the Earth's surface from the mean sea level.

What is an encoding altimeter?

It is used with transponder mode C (S?????) so that your altitude can be shown on a radar display. The encoding assembly is mechanically activated by the aneroid capsule. Older versions consist of a light source , various lenses and an encoder disc with a special pattern on it (in eleven concentric circles) that works like a bar code when the light is reflected from it to produce binary inputs that correspond to 100-ft increments in altitude.

What is indicated altitude

It is what is shown on the dial at the current altimeter setting.

What is indicated air temperature?

It is what the aircraft feels, which is the same thing as OAT plus adiabatic heating.

With the recovery factor of 1, this means?

It means that a thermometer is measuring TAT, which is SAT + 100 % of stagnation rise. If a more basic thermometer has a recovery factor of 0.8 it is only measuring SAT + 80% of the RAM rise or the temperature of air has been brought only partially to rest, so the measured temperature is called the Ram Air Temperature (RAT) If the recovery factor is 0, only SAT is measured.

What is a aneroid?

It means without fluid or not wet (depends on which book you read) to distinguish between aneroid and hydrostatic gauge, which do use fluid, although aneroid gauges can be used to measure liquid pressure. The pressure sensing element may be a Bordoun tube, a diaphragm, a capsule or bellows, all of which will change their shape in response to pressure

What is an alternate static source?

It takes its feed from inside the aircraft in case the main one starts leaking or gets blocked, either through ice, a bird strike, or whatever. When it is used, some error will be introduced into the instrument readings because the cabin air pressure is affected by the airflow over the cabin (there are also different pressure errors) so indicated airspeeds and altitudes will read slightly higher than normal. The VSI will show momentary reverse as the alternate source is selected, then it will stabilise and produce normal readings.

Dynamic pressure varies....airspeed?

It varies with the square of the airspeed.

There are several variations of the airspeed theme, which are?

More on 6-16 - Indicated airspeed IAS The uncorrected direct reading. - Calibrated airspeed CAS The IAS corrected for pressure (system) errors, which are the highest at low speeds and high angles of attack. Also known as RAS (rectified airspeed) it is a measure of dynamic pressure at low speeds. An aircraft always takes off at the same CAS. - True airspeed TAS The CAS corrected for altitude and temperature or density. It is only speed and the only figure used for navigation.

Can you measure dynamic pressure in isolation?

No, as static pressure is always present, so you should really write: q = (q + ps) - ps

When would RAT = TAT?

Only when the RAM rise is equal to the full stagnation rise (using a thermometer with a K factor 1) This is assumed with the Rosemount probe, hence its other name; Total Air Temperature Probe.

What is a Rosemount probe?

Otherwise known as the Total Air Temperature Probe, this has a smakl (i.e quick reacting) platinim basef resitance coil inside concentric cylinders, mounted on a streamlined strut around 50mm or so from the fuselage skin, which therefore has little influence on it (skin temperature can be increased by kinetic energy) Rosemount probes are used on larger helicopterd eg pumas.

What is the task of the pitot tube?

Otherwise known as the Total Pressure Probe or the pitot tube is used to detect total pressure . Total pressure (sometimes called stagnation pressure) is the pressure obtained when a moving gas is brought to a stop through an adiabatic process - in this case, it includes the static pressure that affects the aircraft from all sides and an extra element that comes from forward movement, since the pitot tube is pointed towards the direction of flight (within 5 degrees)

Instruments are further grouped under three headings?

PITCH - artificial horizon (primary pitch instrument) - Altimeter (primary pitch instrument) - Airspeed indicator (secondary pitch instrument) - Vertical speed indicator (VSI) (secondary pitch instrument) BANK - artificial horizon (primary bank instrument) - heading indicator (secondary bank instrument) - turn indicator (secondary bank instrument) POWER - Airspeed indicator (secondary power instrument) - engine instruments - temperature instruments

Cruising levels are expressed in the terms of?

Page 6-12 - flight levels - altitudes - heights

The total pressure error comes in two categories, which are?

Position- or configuration error and manoeuvre error

What is the formula for pressure and what is the unit?

Pressure is the force per unit per area, or the force exerted on an area divided by the size of that area: P = F/A where F is Force (N) and A is the area in m2. The result: N/m2 is equal to 1 Pascal (Pa) which is the standard unit of pressure under the SI system.

What kind of altimeter checks can one do?

Rotating the knob through +/- 10 hPa must produce a corresponding heigh difference or about +/- 300ft in the relevant directions.At known elevation on the aerodrome vibrate the instrument by tapping, unless mechanical vibration is available: - Set the scale to current QNH, the altimeter should indicate the elevation. - Set the scale to QFE, it should indicate the height of the altimeter in relation to the QFE reference point - Both should set to the aerodrome QFE and should indicate within +/- 80 ft of zero - With No 1 on QFE and no 2 on aerodrome QNH , the difference should equal the aerodrome altitude (+/-80ft9 -With both on QNH, indications should be within 80 feet of aerodrome elevation and 80 feet of each other.

What is the basic principle of the altimeter?

Static pressure is inversely proportional to altitude, so if you know the static pressure, you can figure out how high you are ( in the standard atmosphere) As you go up, pressure is less, so the altimeter translates air pressure into an estimate of altitude, although it will be better sealed than a barometer, so that the air pressure in the cockpit doesn't affect it - the only pressure that should be there is the static pressure from the pitot-static system.

In systems that require that pressure is measured in one way or the other, such systems would need what and why?

Such systems would have a transmitter at the pressure source and an indicator on a panel. This means you won't have yucky fluids in the cockpit, and you don't have to have a lot of plumbing. Indicators can be based on synchronous receivers, DC or AC radiometers or servos

How does the ASI work?

The ASI is similar to the altimeter inside, except that the capsule is fed directly with pitot pressure, and its size will vary I'm direct proportion to any increase or decrease. As the capsule expands under the pressure of RAM air, static air in the case can escape through the static port. It captures total pressure, then subtracts static pressure to get dynamic pressure, which is proportional to forward speed so it measures differential pressure. The needle is connected to the capsule through the usual suitable linkage.

What is the difference between True and Indicated altitude called?

The D value.

What is the definition of position error?

The amount by which the local static pressure differs from that in the free stream airflow. Hence it will vary substantially with the Mach number. 95% of it is caused by turbulent flow around the pressure head.

How does the capsule itself work?

The capsule's movement as you go up and down are magnified through a spring by a "suitable linkage" that connects directly to the pointer, using jewelled bearings.

What is a recovery factor?

The difference between TAT and SAT is the stagnation rise, and the proportion of the stagnation temperature that can actually be sensed by the aircraft instruments is the recovery factor or the K value, which is governed by the thermometer, so the recovery factor expresses the sensitivity of a temperature sensor as a percentage. It is determined by flight testing and will be found in the manual.

What sort of different displays are there?

The displays can be circular or straight (like a tape) or digital or even a combination.

What is absolute altitude?

The geometric height above terrain - what would be measured by radar altimeter.

What is a remote bulb thermometer?

This consists of a bulb and a Bourdon tube filled with liquid or vapour, so the Bourdon tube could also loosely be regarded as measuring temperature, but it is still really measuring pressure. Expansion of the liquid causes the tube to lengthen, which moves the indicator, using the usual suitable linkage, as described above.

Hoe does a bourdon tube work?

The pressure differential from the inside to the outside causes the tube to change from an elliptical to a more circular shape and to straighten out, rather like an uncoiling hose. Which way it moves is determined by the curvature of the tubing, as the inside radius is slightly shorter than on the outside, and the ratio between the major and minor axis depends on what sensitivity you need - the larger the ratio, the greater it is. The pressure range is governed by the tubing each thickness and the radius if the curvature. ( ADD PICTURE 6-3) The end result is that a specific pressure causes movement for a specific distance. When the pressure is removed, the tube returns to its original shape. To do this, the material used requires a form of heat treating (spring tempering) to make it closely retrain its original shape while allowing some elasticity under a load. Beryllium copper, phosphor bronze and various alloys of steel and stainless steel are good for this purpose, but steel has a limited service life due to corrosion. Most gauges use phosphor bronze. In summary, a Bourdon -based gauge uses a coiled tube which causes the rotation of an indicator arm connected to it, as it expands from pressure changes.

What is a servo altimeter?

The servo-assisted altimeter typically uses a digital readout and is connected to the ADC. It is more accurate than the simple pressure altimeter due to its use of an induction pick-off device. In this instrument, the aneroid capsules are connected to one end of a pivoting magnet (an I-bar) which influences an E-bar that has windings on each of its arms. At sea level, on a typical servo altimeter, the tolerance in ft from the indicated altitude must not exceed +60ft. In fact the quoted accuracy is 1hPa at mean sea level (+/- 27ft and +/- 50 ft at 10 000 ft)

Accuracy of the instruments depend on what?

The shape of the probe and where it is placed.

What is a resistive coil thermometer?

The small, but stable, resistance of the nickel or platinum coil changes with absolute temperature. The coil is in a circuit with a fixed voltage, changes in which (from resistance) are measured with a meter calibrated in degrees Celsius.

What is a manometer?

The term manometer is often used to refer specifically to liquid column hydrostatic instruments. These consist of a vertical column of liquid in a tube whose ends are exposed to different pressures, with the difference in fluid height being proportional to the difference in pressure. However, the simplest design is a closed-end U shape, with one side connected to the region of interest. A force equal to the applied pressure multiplied by the area of the bore will force the liquid downwards until, eventually, the two levels will stand the same distance above and below the original level. Any fluid can be used, but mercury is preferred for its high density and low vapour pressure, so the tube can be shorter. Manometers are used for calibration purposes.

What is there inside a sensitive altimeter?

There are two aneroid capsules (vacuums) which are corrugated for strength and kept open with a large leaf spring.

What are diaphragms?

They are simply circular metal disks that are corrugated to give them strength, to provide larger deflections. They are used to detect low pressures. One side of the disk is exposed to the pressure to be measured and the other side is linked to the indicating mechanism.

What is a Wheatstone Bridge?

This device measures an unknown electricak resistance by balancing two legs of a bridge circuit, one of which includes the unknown component. They are commonly used in Air Data Computers that use solid state capsules. ADD PICTURE 6-5 As the bridge becomed unbalanced, the varying voltage across the middle can be measured with a voltmeter. For temperature measurement purposes, you can replace the voltmeter with a wiper arm that is postitioned by a servo loop and how far the arm moves in a measure of the temperature change. It will centre at 15degrees C.

What is density altitude?

This is the altitude in the Standard Atmosphere at which the prevailing density occurs, meaning your real altitude from the effects of height, temperature and humidity and is used to establish performance, as it is a figure that expresses where your machine thinks it is as opposed to where it actually is. For now it is pressure altitude corrected for non-standard temperature.

what should one do to find the DA?

To find the DA on the flight computer, set the aerodrome elevation or pressure altitude against the temperature in the airspeed window. Or: formula: PA +/- (118.8 x ISA dev) (in exams multiply with 120)

A static blockage causes the altimeter to...?

To stay at the height at which the blockage occurred. A partial blockage would cause a significant time delay.

How much does the true altitude change in percentages relative to temperature?

True altitude changes by 4% for every 10degreeC deviation from ISA conditions or 2% for every 5,5degreeC. 4% is correct for the stratosphere, but it is closer to 3,5% for lower altitudes. 4% for every 11C is more accurate.

What is meant by temperature compensation?

Various methods can be used to make an instrument over- or under-read according to which way the temperature is going. For example, a thermal junction can get hot by itself, which will vary the emf it produces and give you false readings. In mechanican terms, a bimetal strip made out of invar and brass or steel can be attached to a capsule to make it expand or contract lightly or you could arrange to vary the resistance of an electrical current.

What happens to the static system while accelerating?

When accelerating, the static system is less sensitive, so the ASI and altimeter will over-read and the VSI will show a climb.

The greatest pitot-static errors are found when?

When manoeuvring. If the left static port for example, the altimeter over-reads when sideslipping to the left but is otherwise OK in symmetric flight.

What is SAT?

Where the air has only partially been brought to rest (as it would be if you used a more basic thermometer than the Rosemount) you don't get so much of a temperature rise. The difference is called RAM rise and the indicated temperature is RAM Air Temperature, which is equal to SAT + a percentage of RAM rise. The formula is: SAT = (TAT) / (1 + 0.2KM2) where K = recovery factor and M = Mach no

What are flat plate sensors?

With their sensing element flush with the aircraft skin, are susceptible to environmental errors because of their relative lack of shielding. They are affected by frictional heating in the boundary layer(not compressibility)