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ATPL Theory - Module 1 > Instrumentation - Machmeters and ADCs > Flashcards

Instrumentation - Machmeters and ADCs Flashcards

1
Q

What does the Machmeter indicate to the pilot?

A

The Machmeter gives the pilot an indication of the speed relative to the speed of sound.

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2
Q

Why is it important to use the Machmeter at high altitudes?

A

To avoid dangerous areas of the flight envelope.

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3
Q

At what Mach number is it usual to cruise at high altitudes?

A

Often around M0.78 to 0.85.

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4
Q

What does M0.85 represent?

A

M0.85 means 85% of the speed of sound.

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5
Q

On what does the speed of sound depend?

A

The speed of sound depends on the temperature of the air.

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6
Q

How does temperature affect the speed of sound?

A

The warmer the air, the faster the speed of sound.

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7
Q

What is the formula to calculate the local speed of sound (LSS)?

A

LSS = 38.94 √T

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8
Q

What does T represent in the formula for LSS?

A

T is the static air temperature in Kelvin (K).

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9
Q

How is the Mach number related to the local speed of sound?

A

The Mach number is a decimal fraction of the local speed of sound and can be calculated if the TAS and LSS are known.

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10
Q

What is the formula to calculate the Mach number?

A

Mach number = TAS ÷ LSS

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11
Q

How do you calculate the Mach number with TAS = 390 KT and LSS = 584 KT?

A

Mach No = 390 ÷ 584 = M0.67

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12
Q

Is the local speed of sound dependent only on temperature?

A

No, the Mach number is a function of dynamic pressure divided by static pressure and is independent of air temperature.

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13
Q

Why is the Mach number independent of air temperature?

A

Because although the LSS is affected by temperature, so is the TAS, and the ratio between them (which is the Mach number) remains unchanged as temperature changes.

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14
Q

What is the consequence of the Mach number being independent of temperature changes?

A

At any given flight level (static pressure) and CAS (dynamic pressure), there is only one Mach number, and it is unaffected by temperature changes.

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15
Q

What components are involved in the Machmeter as shown in the diagram?

A

Main Shaft, Static Pressure, Pitot Pressure, Air Speed Capsule, Air Speed Link, Altitude Capsule, Ratio Arm, Pointer and Gearing, Ranging Arm, Hair Spring, Adjustable Index, Index Control, Pin and Spring.

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16
Q

What elements does the first generation direct reading Machmeter contain?

A

It contains the elements to find dynamic pressure (a capsule fed with pitot pressure inside a case fed with static), which is an airspeed indicator, and the element to find static pressure (an altitude capsule).

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17
Q

How does the airspeed capsule function in a first generation Machmeter?

A

It acts through a system of linkages to move a ratio arm in the direction marked AB.

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18
Q

What effect does the altitude capsule have on the ranging arm?

A

It moves the ratio arm in the direction CD, ultimately moving the pointer that indicates the Mach number.

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19
Q

What characterizes second generation Machmeter instruments?

A

They are either servo-driven indicators fed from the ADC or are EFIS displays. The ADC calculates the Mach number from dynamic pressure and static pressure inputs.

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20
Q

How can Mach number be displayed in the cockpit?

A

Mach number can be shown on an independent dial, as on the direct reading instrument, or can be incorporated with the IAS indication on a servo driven instrument.

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21
Q

What is a servo driven Mach indicator fed from? What does it display?

A

A servo driven indicator is fed from the ADC and includes not only IAS and Mach number but also a pointer indicating the aircraft maximum CAS as calculated by the ADC.

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22
Q

What can the aircraft maximum speed be?

A

The aircraft maximum speed can either be VMO (the maximum IAS) or calculated from MMO (the limiting Mach number), and it varies with altitude.

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23
Q

How do older instruments calculate the maximum airspeed?

A

Older instruments calculate the maximum airspeed, shown as the red and white striped “barber’s pole,” internally by adjusting the pointer position according to the output from an altitude capsule.

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24
Q

What are alternative forms of Mach number display?

A

Alternative forms of Mach number display are vertical straight scale, digital, or drum type arrangements.

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25
Q

Are mechanical vertical Mach indicators common on civilian aircraft?

A

No, they are not common on civilian aircraft.

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26
Q

On what type of aircraft were mechanical vertical Mach indicators found in the 1950s?

A

They were found on military aircraft such as the F-105 ‘Thunderchief’ supersonic fighter bomber.

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27
Q

Where can the vertical Mach indicator be found in the F-105 ‘Thunderchief’?

A

The vertical Mach indicator can just be made out to the left of the attitude indicator.

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28
Q

How is the Mach number shown on an EFIS aircraft?

A

The Mach number is shown digitally, normally located close to other speeds such as CAS and Groundspeed.

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29
Q

Where does the Mach number appear on the EFIS display in the image?

A

It appears in the bottom left corner, in this case as M0.745.

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30
Q

How is the VMO/MMO indicated on the airspeed tape?

A

It is indicated by the red and black alternating segments at the top of the airspeed tape.

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31
Q

What type of errors does the Machmeter suffer from?

A

The Machmeter suffers only from instrument and pressure error.

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32
Q

How significant are the errors in the Machmeter?

A

Because these errors are very small, the indicated Mach number can be taken to be the true Mach number.

33
Q

What happens if the pitot source becomes blocked in the Machmeter?

A

The Machmeter shows the same errors as an ASI. The Mach number will remain unchanged until static pressure changes in a climb or descent.

34
Q

What happens to the airspeed capsule in a climb with a blocked pitot source?

A

The airspeed capsule will have excess static pressure trapped, causing the instrument to over read.

35
Q

What happens to the airspeed capsule in a descent with a blocked pitot source?

A

The airspeed capsule will under read.

36
Q

What happens with blocked static sources in a climb?

A

Excess static pressure is trapped in the case, causing the Machmeter to under read.

37
Q

What happens with blocked static sources in a descent?

A

The Machmeter will over read.

38
Q

What happens if the static line fractures inside the pressure hull?

A

The static pressure will be too high, causing the instrument to under read.

39
Q

What happens if the pitot line leaks?

A

The instrument will under read.

40
Q

What are ECTM diagrams used for?

A

ECTM diagrams help to remember the relationship between EAS, CAS, TAS, and Mach number in climbs and descents.

41
Q

In a normal atmosphere, what is the sequence for drawing ECTM diagrams?

A

The sequence is ECTM from left to right.

42
Q

What should be drawn as a vertical line in an ECTM diagram?

A

The parameter that is to be held constant should be drawn as a vertical line.

43
Q

What do the lines in an ECTM diagram represent?

A

The lines represent the relationship between EAS, CAS, TAS, and Mach number in climbs and descents.

44
Q

What happens to the Mach number in a constant CAS climb below the tropopause?

A

The Mach number increases in a constant CAS climb below the tropopause.

45
Q

How do you interpret movement up and down the lines in an ECTM diagram?

A

Moving up the lines represents climbing, and moving down the lines represents a descent.

46
Q

What happens to Mach number, TAS, and EAS in a constant CAS descent?

A

In a constant CAS descent, Mach number decreases, TAS decreases, and EAS increases.

47
Q

How do airliners climb until they reach their crossover altitude?

A

Airliners climb on CAS (VMO as their limit) until reaching their crossover altitude, after which they climb using Mach number (MMO as their limit).

48
Q

What happens to the aircraft climb after reaching FL 100?

A

The aircraft is accelerated to 300 kt for further climb until it reaches the crossover altitude, in this case 29,314 PA. At that altitude, 300 kt = M0.78, which is the Mach number for the remainder of the climb to cruising altitude.

49
Q

What is the crossover altitude in the given example?

A

The crossover altitude is 29,314 PA.

50
Q

What happens in the descent regarding the crossover altitude?

A

Aircraft descend using MMO as their limit until crossover altitude is reached, and then VMO becomes the limiting factor.

51
Q

What could happen if an aircraft descends below crossover altitude using MMO as the limit?

A

The increasing CAS could easily mean that VMO is exceeded.

52
Q

What is the risk of maintaining a constant CAS during a climb?

A

Maintaining a constant CAS during a climb could mean that you stray into the transonic region inadvertently.

53
Q

What effect does increasing CAS to 325 kt have on the crossover altitude?

A

Increasing CAS to 325 kt means that the crossover altitude will be considerably lower for a modest speed increase.

54
Q

What should you do if asked about a climb or descent through an isothermal layer?

A

Draw the T and the M lines together, then use the ECTM diagram as before.

55
Q

What happens to CAS and Mach number when an aircraft descends through an isothermal layer with constant TAS?

A

Mach number stays unchanged, CAS increases.

56
Q

What should you do if asked about a climb or descent through an inversion layer where temperature increases with height?

A

Draw the ECTM diagram with T and M swapped round, inverted.

57
Q

What happens to CAS and Mach number when an aircraft climbs at constant TAS through an inversion?

A

Mach number and CAS will reduce in the climb.

58
Q

How were instruments in older aircraft designed?

A

In older aircraft, all the instruments were independent, with the working parts of the instrument just behind the dial itself.

59
Q

How do more modern aircraft handle pitot and static lines?

A

Modern aircraft feed the pitot and static lines to air data computers which calculate values of CAS, TAS, Mach number, SAT, and rate of climb and descent, and pass the information electronically to the instruments.

60
Q

What is the advantage of modern air data systems?

A

The data can be fed to the Autopilot and Flight Director System (APFDS), Flight Management System (FMS), ATC transponder, Ground Proximity Warning System (GPWS), area navigation aids, and an instrument comparison system.

61
Q

How are the feeds sent to the pilots’ instruments in modern aircraft?

A

The feeds go to the pilots’ instruments via EFIS symbol generators.

62
Q

What is the benefit of having two ADCs in modern aircraft?

A

The ability to compare outputs from two ADCs fed from multiple independent sensors reduces the possibilities of an air data error critically affecting the flight.

63
Q

What incident highlighted the potential limitations of air data systems?

A

In 2009, Air France flight 447 Airbus 330 was lost over the South Atlantic in an incident that started with unreliable airspeed readings, probably caused by icing of the pitot probes.

64
Q

What inputs does the ADC require from sensors?

A

Static pressure, Total pressure, TAT, Angle of attack, Flap and landing gear position.

65
Q

What outputs does the ADC compute and correct using stored aircraft data?

A

Pressure altitude/barometric altitude using static pressure, CAS (corrected for known pressure/position errors), TAS (computed from pitot and static pressure), SAT (by applying ram rise and recovery factors to the TAT), TAT (displayed as raw data if required), Angle of attack (displayed on a separate instrument or integrated into the EFIS and used in stall protection systems), Vertical speed (from rate of change of static pressure or an accelerometer in a strap down IRS, or a blend of both).

66
Q

How is CAS corrected in modern aircraft?

A

CAS is corrected for known pressure/position errors so that for the vast majority of modern aircraft, IAS is actually CAS.

67
Q

How is TAS computed?

A

TAS is computed from pitot and static pressure.

68
Q

How is SAT computed?

A

SAT is computed by applying ram rise and recovery factors to the TAT.

69
Q

What is the role of the ADC in stall protection systems?

A

The angle of attack is displayed either on a separate instrument or integrated into the EFIS and used in stall protection systems.

70
Q

How is vertical speed determined?

A

Vertical speed is determined either from the rate of change of static pressure or from an accelerometer in a strap down IRS, or a blend of both.

71
Q

What is the Air Data Inertial Reference Unit (ADIRU)?

A

The ADC is integrated with an inertial reference unit to create an ADIRU, with separate controls for the ADC part and Inertial Reference (IR) part.

72
Q

Why must care be taken when managing failure scenarios with the ADIRU?

A

Incorrect selections may lead to unintended and potentially irreversible consequences.

73
Q

What happens if the initial IR position is lost on an aircraft system?

A

All navigation capability on that system is lost.

74
Q

How many ADCs/ADIRUs do modern transport aircraft often have?

A

Modern transport aircraft often have two ADCs/ADIRUs, as well as multiple pitot/static and angle of attack sensors.

75
Q

What is the benefit of having dual ADCs in modern aircraft?

A

The redundancy provided by the use of dual ADCs makes the loss of an individual ADC less consequential than the loss of an individual analogue pressure instrument.

76
Q

What happens if a differential is reached between two ADCs?

A

A ‘miscompare’ caution is generated.

77
Q

What additional feature do most dual ADC systems have?

A

Most dual ADC systems have separate pitot/static feeds to standby instruments.

78
Q

What do older aircraft rely on for standby instruments?

A

Older aircraft rely on analogue standby instruments.

79
Q

What is more common nowadays for standby instrument systems?

A

Integrated standby instrument systems (ISIS), which are powered and supplied completely separately from the EFIS.

ATPL Theory - Module 1 (17 decks)

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