1. Instruments Pressure Altimeter
Chapter
Pressure Instruments
Pitot Static Probes and ADC.
Airspeed Indicator ( ASI ).
Mach Meter.
Pressure Altimeter.
Vertical Speed Indicator ( VSI ).
Gyroscopic Instruments
Direction Gyro Indicator ( DGI ).
Artificial Horizon ( AH ).
Turn And Slip Indicator.
Magnetism And Compasses
Magnetism General.
Direct Reading Magnetic Compass.
Sperry Compass System.
Miscellaneous Systems
Inertial Navigation System ( INS ).
Temperature Indicators.
Flight Data Recorders ( FDR ).
Angle Of Attack Indicators.
Flight Management System And Autopilots ( FMS ).

2. Pitot Static Instruments

3. Pressure Altimeter Introduction
The Pressure Altimeter is basically an Aneroid Barometer.
An Aneroid Barometer is used to measure the ambient Atmospheric pressure or Static pressure.
This pressure measurement is then used to display aircraft altitude.
The Altimeter is calibrated for ISA conditions.

4. Pressure Altimeter
The following is a summary of the operation of the Pressure Altimeter
The Pressure Altimeter measures the ambient Atmospheric pressure or Static pressure.
The Static pressure is measured by the Static port or vent and is fed into the airtight case of the instrument.
The Pressure Altimeter has a partially evacuated capsule supported by a leaf spring inside the case.
As the aircraft climbs the Static pressure decreases and the capsule will expand.
This expansion is transmitted through the linkages to indicate an increase in altitude.
As the aircraft descends the Static pressure will increase causing the capsule to contract.
This contraction is transmitted through the linkages to indicate a decrease in altitude.
A Bi- Metallic strip is incorporated into the suitable linkages to compensate for the expansion or contraction of the linkages with temperature changes.

5. Pressure Altimeter Construction
Sealed Case
Static Vent
Altimeter Scale
Capsule
Linkages
Static Port
Pointer
Baro Correction
Static pressure is measured at the Static port.
As the aircraft climbs the Static pressure decreases and the capsule expands.
This expansion shows an increase in altitude.
As the aircraft descends the Static pressure increases and the capsule will contract.
This contraction shows a decrease in altitude.
The Baro correction is used to compensate for any deviation in ISA conditions.

6. Air Data Computer ( ADC )
Central Air Data Computers ( CADC ) or Air Data Computers ( ADC ) are used in jet aircraft.
One ( CADC ) for the captain’s flight instruments and the second computer for the first officer’s flight instruments.
These computers convert mechanical energy from the Pitot and Static vents to an electrical equivalent by means of transducers and supply the information to the Airspeed Indicator, Mach Meter, Vertical Speed Indicator and the Altimeter.
The advantage of the ADC is that there are no mechanical linkages thus reducing errors to a minimum.
What is important in the next diagram is to note the inputs to and outputs from the ADC.

7. Air Data Computer ( ADC )
Static Defect Correction
Static
Pressure
Altitude
Static
Pressure
Vertical Speed
Transducer
Density
Density
28 Volts DC
Altitude
Memory
Altitude Hold
Mach
Number
Mach Number
Dynamic Pressure
Indicated Airspeed
Dynamic Pressure
Transducer
True Airspeed
True Airspeed
Total Air Temp TAT Probe
Static Air Temperature
Static Air Temperature
Angle Of Attack Probe
Angle Of Attack Correction
Angle Of Attack Corrected

8. Sensitive Altimeter
The Sensitive Altimeter has two or three capsules instead of one which gives the pointers a greater movement for a given pressure change.
The capsules drive three pointers showing different altitude calibrations.
A barometric setting knob is used to set the subscales.
Foot Increments
1000 Foot Increments
100 Foot Increments
Subscale Adjustment

9. Altimeter ( ALT ) Errors
Instrument Error :
These errors are caused by imperfections during manufacture and wear and tear of the instrument.
There is another consideration, and that is the rate of pressure change of the atmosphere with an increase in altitude and is assumed to be constant.
As this assumption is not true the altimeter will be unreliable at higher altitudes.
0 to 5000 Feet equates to approximately Mb.
35000 Feet to Feet equates to approximately 50 Mb.
60000 Feet to Feet equates to approximately 15 Mb.
Pressure and Position Error :
This error is caused by incorrect pressures being sensed at the Static vent due to the disturbed airflow over the airframe.
This error is also caused by the disturbed airflow due to the positioning of the Static vent on the airframe.
These errors are combined and a correction card can be produced.
Manoeuvre Induced Error :
This error is produced by the changes in aircraft attitude and configuration or when the Static vent is not aligned with the airflow i.e. a side slip or crabbed flight in crosswind.

10. Altimeter ( ALT ) Errors
Time Lag Error :
It may take time for pressure changes at the Static vent to register on the altimeter.
This is mainly due to the mechanical properties of the instrument.
During a rapid climb the Altimeter lags and therefore under reads.
During a rapid descent the Altimeter lags and therefore over reads.
Barometric Error :
The Altimeter is calibrated according to ISA conditions for atmospheric pressure.
If there is a deviation from the ISA pressure conditions an error will occur in the Altimeter indications.
The pressure error can be corrected for by means of the barometric correction scale.
Temperature Error :
The Altimeter is calibrated according to ISA conditions for atmospheric temperature.
If there is a deviation from the ISA temperature conditions an error will occur in the Altimeter.
The error is small at lower altitudes but significant at higher altitudes.
This error can be corrected by the altitude window on a navigation computer.

11. Servo Assisted Altimeter
The Servo Assisted Altimeter solves most of the problems found in the Pressure Altimeter.
The Pressure capsules are retained but their information is measured electronically by means of E and I bars.
A motor drives the pointers and altitude drums which improves accuracy and virtually eliminates time lag errors.
Foot Increments
1000 Foot Increments
100 Foot Increments
Subscale Adjustment

12. No Output to Secondary Coil AC Supply to Primary Coil
E and I Bar Theory
The transducer component of a Servo Assisted Altimeter consists of an E bar and an I Bar.
An Alternating Current is fed to the primary coil on the centre leg of the E bar.
Equal magnetic flux flows are produced due to the equal air gap between the E and I bars ( Level Flight ).
Equal but opposite voltages are produced in the secondary coils of the outer legs of the E bar.
This results in no output voltage.
No Output to Secondary Coil
AC Supply to Primary Coil

13. Directional Output to Secondary Coil AC Supply to Primary Coil
E and I Bar Theory
The transducer component of a Servo Assisted Altimeter consists of an E bar and an I Bar.
An Alternating Current is fed to the primary coil on the centre leg of the E bar.
When the aircraft climbs or descends the barometric capsules drive the I bar, thus causing the air gaps between the E and I bar to be different.
The different air gaps cause different amounts of flux to flow.
This results in an output voltage in the secondary coils.
The polarity of the output voltage depends on the direction the I bar is deflected i.e. climb or descend.
Directional Output to Secondary Coil
AC Supply to Primary Coil

14. Servo Assisted Altimeter Operation
Motor Drive
Voltage
Amp
Counters
Pointer
Motor
Output
Voltage
Cam Follower
Gear Train
Cam Mb
Scale
Worm Gear
Capsules
Anvil

15. Servo Assisted Altimeter Summary
Normal Operation ( Aircraft climbing and descending ):
As the aircraft climbs or descends the Capsules expand or contract as the Static atmospheric pressure changes.
The Capsules move the I bar thus changing the air gap between the E and I bar.
This change in air gap causes an output voltage from the outer legs of the E bar which is sent to the amplifier.
The amplifier determines the polarity of the output voltage and drives the motor in the correct direction depending on whether the aircraft is climbing or descending.
The motor, through the Gear Train, drives the altitude counters and the altitude pointer.
The motor, through the Gear Train, also turns the Worm Gear.
The Worm Gear drives the Cam.
The Cam drives the Cam Follower.
The Cam Follower moves the E bar to the null position.
In the null position the air gap between the E and I bar is the same.
No output voltage is produced and the whole operation stops at the aircraft altitude.

16. Servo Assisted Altimeter Summary
Barometric Adjustment.
To adjust the Altimeter for deviation in ISA pressure conditions the Baro correction is used.
If one increases the QNH value in the subscale the altimeter will indicate higher and the converse is true.
Turning the Baro correction drives the millibar scale through the Gear Train on the adjustment shaft.
Turning the Baro correction will also adjust the Anvil, either lengthening or shortening the shaft.
The Anvil will move the Adjustment Lever.
The Adjustment Lever will move the Worm Gear left or right from its null position.
The movement of the Worm Gear will move the Cam.
The Cam will move the Cam Follower.
Moving the Cam Follower will move the E bar with respect to the I bar.
This change in air gap causes an output voltage from the outer legs of the E bar which is sent to the amplifier.
The amplifier determines the polarity of the output voltage and drives the motor in the correct direction depending on which way the Baro correction was turned.
The motor, through the Gear Train, drives the altitude counters and the altitude pointer.
The motor, through the Gear Train, also turns the Worm Gear.
The Worm Gear drives the Cam.
The Cam drives the Cam Follower.
The Cam Follower moves the E bar to the null position.
In the null position the air gap between the E and I bar is the same.
No output voltage is produced and the whole operation stops at the aircraft altitude.

17. Altitude and Pressure Relationship
As the aircraft flies from a high pressure area to a low pressure area the atmosphere becomes less dense.
The Altimeter capsules sense this drop in pressure.
As the pressure drops the Altimeter capsules start to expand.
The expansion of the capsules is indicated as an increase in altitude.
The pilot tries to maintain his assigned altitude and thus starts to descend the aircraft.
The aircraft is thus moving closer to the ground for a constant assigned or maintained altitude.
The altimeter over reads.

18. Altitude and Pressure Relationship
The aircraft flies from a high temperature area to a low temperature area at a constant Pressure.
One must remember the QNH setting on the altimeter is a function of pressure at the aircraft reduced to sea level by means of a formula 1 Mb per 30 Feet assuming ISA pressure gradient.
In warm air, because of its lower density, the aircraft will have to be higher to obtain the same pressure difference in Mb from sea level. The altimeter under reads.
In cold air, because of its high density the displacement of the aircraft for the same Mb change will be less and the aircraft will be lower to the ground. The altimeter is over reading.

19. Altitude Definitions QNH:
This is the mean sea level pressure calculated from the airfield pressure reduced to sea level using the ISA formula.
The formula is 1Millibar change for every 30 Feet.
When QNH is set in the altimeter, the reading on the instrument is called an Altitude above mean sea level ( AMSL ).
Airfield Pressure
860 Mb
Airfield Elevation
5010 Feet
QNH is 1027 Mb
Mean Sea Level

20. Altitude Definitions QFE:
This is the barometric pressure at the airfield as measured by an Aneroid Barometer.
When the airfield pressure is set in the altimeter subscale the altimeter will read height above this reference.
When QFE is set in the altimeter subscale the altimeter measures height above the airfield elevation.
Airfield Pressure
860 Mb
Airfield Elevation
5010 Feet
QNH is 1027 Mb
Mean Sea Level

21. Altitude Definitions QNE:
This is the barometric pressure at the ISA reference point of Mb.
When the QNE is set in the altimeter subscale the altimeter will read the following:
Flight Level i.e. FL 090.
Pressure Altitude i.e Feet.
When QNE is set the altimeter reads altitude above the ISA reference level.
Airfield Pressure
860 Mb
Airfield Elevation
5010 Feet
ISA Reference Mb
QNH is 1027 Mb
Mean Sea Level

22. ISA Temperature Calculation
Lets consider this problem :
To calculate an ISA temperature given the following :
What is the ISA temperature at FL 350?
Flight Level 350
ISA Temperature at FL 350 = - 55°C
-55°c
55 degree temperature Drop
Total Drop 70°cCelcius
Temperature Drop = x 2 / = - 70°c
0°c
15 degree temperature Drop
15°c
Sea Level

23. Density Altitude Calculation
Density Altitude is a two part calculation; firstly to compensate for the pressure deviation from International Standard Atmosphere (ISA ) and secondly, the temperature deviation from International Standard Atmosphere ( ISA ).
Example No 1.
Airport elevation is 4517 feet, the QNH is hPa and the temperature is 22° Celsius. What is the density altitude of the airfield?
Step 1.
Correct the pressure deviation from ISA. ISA is Mb, for every Mb deviation the atmosphere deviation is 30 feet. If one turns the Mb subscale on the altimeter clockwise to increase the QNH the altitude will increase and conversely if the Mb subscale is turned anti-clockwise to reduce the QNH the altimeter will decrease.
Because the subscale is turned clockwise the numbers increase and the altitude increases thus the correction is added to the elevation to give the pressure altitude.

24. Density Altitude Calculation
Example No 1
Airport elevation is 4517 feet, the QNH is hPa and the temperature is 22° Celsius. What is the density altitude of the airfield?
Step 2
Correct the temperature deviation from ISA. For every 1° Celsius deviation from ISA there is a 120 feet altitude error. If the temperature is higher then the atmosphere is thinner and the converse is true.
Therefore density altitude is the pressure altitude compensated for temperature

25. Altitude Alerting System
The Altitude Alerting System is coupled electronically to the Altimeter System.
The system provides an audio and visual warning to the pilots when the aircraft is approaching a selected altitude.
The warning light remains illuminated until 200 feet before reaching the selected level.
The system also provides an audio and visual warning if the aircraft deviates above or below the selected altitude by more than 200 feet.
Selected Flight Level
FL330
Alert
800 Feet Before Selected Altitude

26. Altitude Alerting System
The Altitude Alerting System is coupled electronically to the Altimeter System.
The system provides an audio and visual warning to the pilots when the aircraft is approaching a selected altitude.
The warning light remains illuminated until 200 feet before reaching the selected level.
The system also provides an audio and visual warning if the aircraft deviates above or below the selected altitude by more than 200 feet.
800 Feet Before Selected Altitude
Alert
Selected Flight Level
FL330

27. Altitude Alerting System
The Altitude Alerting System is coupled electronically to the Altimeter System.
The system provides an audio and visual warning to the pilots when the aircraft is approaching a selected altitude.
The warning light remains illuminated until 200 feet before reaching the selected level.
The system also provides an audio and visual warning if the aircraft deviates above or below the selected altitude by more than 200 feet.
Alert
200 Feet Above Selected Altitude
Selected Flight Level
FL330
Alert
200 Feet Below Selected Altitude

28. Altimeter / Transponder Interface
In commercial aircraft the transponder is supplied with altitude information from an Altitude Encoding Altimeter.
In an Altitude Encoding Altimeter an encoding glass disk is driven by the Altimeter Transmission System.
The glass disk is encoded with a binary system that corresponds to the aircraft Pressure Altitude.
A lamp illuminates the glass disk and the information is read by a photo - electric cell.
After amplification the information is sent to the transponder.
The transponder transmits aircraft Pressure Altitude to ATC.

29. Aircraft Now Flying Flight Levels
Transition Altitude
The Transition Altitude is an altitude in the vicinity of the airfield at or below which the vertical position of the aircraft is expressed in altitude.
The Transition Altitude is usually 1000 Feet above the highest obstacle.
The Transition Altitude is found on the approach charts.
When climbing the QNE is set at the Transition Altitude.
Aircraft Now Flying Flight Levels
Set QNE Mb
Set QNE Mb
Transition Altitude
Transition Altitude
JNB
DBN

30. Aircraft Now Flying Flight Levels
Transition Level
The Transition Level is the lowest Flight Level available for the use above the Transition Altitude.
Vertical position of the aircraft is expressed in Flight Levels.
The Transition Level is usually 2000 Feet above the highest obstacle.
The Transition Level is given by the ATIS or ATC.
When descending the QNH is set at the Transition Level.
Aircraft Now Flying Flight Levels
Transition Level
Set QNH
Set QNH
Transition Level
Transition Layer
Set QNE Mb
Transition Layer
Transition Altitude
Transition Altitude
Set QNE Mb
JNB
DBN

31. Semi – Circular Flight Levels
In South Africa no VFR flights are allowed above Feet.
Flight Levels are assigned by means of the aircraft Magnetic Track.
In non RVSM airspace Flight Levels ensure 2000 feet separation between opposite direction traffic.
VFR
Flight Levels
IFR
Flight Levels
359°
359°
000°
000°
Even
Thousands
FL 280
then
Even
Thousands
+ 500 Feet to FL 285
then
Odd
Thousands
FL 290
then
Odd
Thousands
+ 500 Feet to FL 275
then
FL 280
FL 310
FL 350
FL 390
FL 285
FL 320
FL 360
FL 400
FL 290
FL 330
FL 370
FL 410
FL 275
FL 300
FL 340
FL 380
180°
179°
180°
179°

32. Altimeter Calculations
Identify the type of altimeter problem.
Always draw a sketch.
Using the formula 1Mb equal 30 Feet, convert altimeter settings so that all the units are the same.
Note that when the barometric value increases the altimeter will indicate higher.
Note that when the barometric value decreases the altimeter will indicate lower.

33. Altimeter Calculation No 1
An aircraft heading 003° magnetic and has 10° of left drift. The aircraft has to pass over high ground that is 2200 meters AMSL. Minimum clearance over high ground is 2000 Feet. Spot QNH is 1025 Mb. What is the lowest IFR flight level the aircraft must maintain?
FL X Mb
2000 Feet
Clearance
2200 M
QNH 1025
7218 Feet
Corrected
Altitude
6865 Feet
Sea Level

34. Altimeter Calculation No 2
En route at Flight Level 270, the altimeter is set correctly. On the descent the pilot fails to reset the altimeter to QNH Mb. If the airfield elevation is 1300 Feet, what will the altimeter indicate after landing?
If the altimeter was correctly set at Mb before landing it would read 1300 Feet after landing.
However, Mb is set, as the aircraft is at Flight Level 270.
FL 270 Mb
QNH
1300 Feet
Sea Level

35. Altimeter Calculation No 3
An aircraft leaves airfield Y, airfield pressure 960 Mb, and the altimeter reads airfield elevation of 1860 Feet. The aircraft lands at airfield Z, elevation 1000 Feet, where the altimeter reads 1270 Feet. What is the QNH at Z?
QNH
1022 Mb
1270 Feet
QNH
1022 Mb Y
1860 Feet
QFE 960 Mb
QNH 1022 Mb Z
1000 Feet
QNH 1013 Mb
62 Mb
Change
Sea Level

36. Altimeter Calculation No 4
An aircraft leaves an airfield X, elevation 510 Feet, with the QFE 999 Mb set on the altimeter, en route to airfield Y, which is 510 Nm from X. The QNH at Y is 1025 Mb. A spot height of 450 meters is 114 Nm from airfield X is cleared by 2000 Feet. What is the altimeter reading over the spot height?
QFE
999 Mb
2000 Foot
Clearance Z
450 Meters
1476 Feet
QNH 1018 Mb X
510 Feet
QFE 999 Mb
QNH 1016 Mb Y
QNH 1025 Mb
Change
17 Mb
Sea Level
114 Nm
510 Nm

37. Altimeter Calculation No 4
An aircraft leaves an airfield X, elevation 510 Feet, with the QFE 999 Mb set on the altimeter, en route to airfield Y, which is 510 Nm from X. The QNH at Y is 1025 Mb. A spot height of 450 meters is 114 Nm from airfield X is cleared by 2000 Feet. What is the altimeter reading over the spot height?
QFE
999 Mb
2000 Foot
Clearance Z
450 Meters
1476 Feet
QNH 1018 Mb
Corrected
Altitude
906 Feet X
510 Feet
QFE 999 Mb
QNH 1016 Mb Y
QNH 1025 Mb
Change
17 Mb
Sea Level
114 Nm
510 Nm

38. Altimeter Calculation No 5
During a preflight check the following details were noted. Airfield elevation was 5000 Feet, apron elevation was 4980 Feet, height of the Static Vent above the ground was 25 Feet, the altimeter was reading 45 Feet with the airfield QFE set in the subscale. What is the instrument error?
QFE Set
5 Feet
Airfield Elevation
5000 Feet
20 Feet
25 Feet
Apron Elevation
4980 Feet
QFE
Sea Level

39. Altimeter Calculation No 6
The following data is given
Pressure altitude is 8000 Feet.
QNH altitude is 7500 Feet.
OAT is 30° C.
Terrain elevation is 5700 Feet.
What is the Absolute Altitude?
Use altitude window on slide rule.
Set Pressure Altitude against temperature.
Read True Altitude on the outer ring of the slide rule against QNH altitude on the inner ring.
Subtract this True Altitude from the elevation to obtain Absolute Altitude or height above ground.
QNH Altitude = Feet -
Terrain Elevation = Feet =
Absolute Altitude = Feet