Aircraft General Knowledge Ref 6

Aircraft General Knowledge Ref 6.5.1 to 6.5.5 as per the CATS Appendix 1.0.

6.5 Instruments
6.5.1 Pitot/static system (Suggest a diagram like this one:

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– pitot tube, function
The function of the pitot tube is to measure dynamic pressure which is used by the Airspeed indicator to display IAS (Indicated Airspeed)

– pitot tube, principles, and construction
The pitot tube is a very simple device, it is a tube, made of metal, one end is connected to a pipe which is routed to the ASI the other end is open and faces in the direction of forward flight. It also includes a static port that measures static pressure at the pitot tube. When the aircraft starts to move forward the dynamic pressure inside the tube starts to increase, the ASI uses the resultant difference between dynamic and static pressure to display the airspeed on the instrument. Most pitot tubes are heated electrically to prevent icing of the pitot tube.

– static source
the static source of an aircraft is an opening that is 90 degrees to the relative airflow and measures the static pressure of the air, the static pressure is fed to the Altimeter, ASI (Airspeed Indicator) and VSI (Vertical Speed Indicator)
– alternate static source
Due to gross errors that can occur if the static source were to be blocked, unpressurised aircraft are fitted with an alternate static pressure source in the cockpit, which can be switched on by the pilot in case the primary static source is blocked. Alternate static source pressure is slightly lower than outside static pressure, this can cause the Altimeter to indicate an altitude hight than actual, there will be a sudden climb indicated on the VSI and then it returns to normal and the ASI indicates a speed higher than the actual speed.

– position error
Static system: As an aircraft moves through the air, the streamlines of the air are affected by the movement of the aircraft. Therefore, the static pressure on various points of the fuselage varies from point to point. The ideal position of the static port would be where the air flowing past the aircraft does not affect the static pressure measured for all angles of attack, this position unfortunately does not exist. Aircraft designers place the static port on the aircraft where it will be affected least by the changes in angle of attack, the errors that are caused by the changes in angle of attack and the movement of the aircraft through the air are called position error. Position error affects Indicted Airspeed and Indicated altitude. Aircraft manufacturers use the aircraft operating manual to publish the difference for indicated airspeeds and Indicated altitudes for a range of speeds. In low speed aircraft, the position error is the difference between the indicated and equivalent airspeeds.

– system drains
The pitot and static system are built with water drains usually at the lowest part in the system to drain any water that has ingressed into the system.

– heating element
Most pitot tubes and static ports are heated electrically to prevent icing of the pitot tube and static ports, blocking of these ports can cause serious errors

– errors caused by blockage or leakage
The range of errors that can occur with a blocked static and Pitot are detailed in the table below:

Situation Airspeed Altimeter VSI
Blocked Pitot Reads Airspeed when blockage occurred
Not Affected
Not Affected
Blocked Pitot and drain hole. Open Static High in Climb

Low in Decent
Not Affected
Not Affected
Blocked pitot, open static Low in Climb

High in Descent Reads Altitude when blockage occurred Slowly reduces to Zero
Using Alternate cockpit static source
Reads high
Reads High Momentarily shows a climb
Broken VSI Glass Reads High Reads High Reverses

6.5.2 Airspeed indicator – principles of operation and construction
The ASI (Airspeed indicator) works on the principle of measuring difference between static and dynamic pressure and then displaying this difference on an instrument that as been calibrated in airspeed, normally mph or knots:
The ASI consists of a sealed case which is vented to the atmosphere via the static port, inside the case is a metal diaphragm that is connected to the pitot tube, this diaphragm can expand, this expansion moves a suitable linkage, which in turn moves a needle that is displayed on a suitable scale.

– the relationship between pitot and static pressure
The ASI indicator is connected to the Pitot tube and static vent, the difference between the pitot pressure and static pressure is called dynamic pressure, dynamic pressure is pressure due to movement and ASI displays this as speed.

– definitions of indicated, calibrated and true airspeed
Indicated airspeed (IAS) is the speed that is displayed on the ASI, this speed includes minor errors, such as position and instrument error.
Calibrated airspeed (CAS) is Indicated airspeed corrected for instrument and position error. Although manufactures attempt to keep these errors as small as possible, small errors are present, they can be corrected by using the airspeed calibration chart in the aircraft flight manual.
True Airspeed (TAS) is calibrated airspeed that has been corrected for altitude and non-standard temperature, it is the speed relative to the airmass it is flying in. TAS is equal to CAS at sea level in International Standard Atmosphere (ISA) conditions, if air density decreases for a constant IAS, TAS will increase.

– instrument errors

– airspeed indications, colour coding
An ASI has certain colour coded bands that are standard on most aircraft:
– Green band – Normal operating range, lower end, stall speed at maximum all up weight and flaps retracted known as Vs1. Upper end, max structural cruising speed, do not exceed this speed unless in smooth air.
– White band – Operating range with flaps extended. Lower end, Stall Speed in the landing configuration, known as Vs0.
– Yellow band – caution range, these speeds may only be flown in smooth, non-turbulent air.
– Red Mark – Never exceed speed or VNE, this speed is to never be exceed under any circumstance, or it may result in structural damage.

– pilot’s serviceability checks
The ASI can only be checked if it is serviceable once the aircraft achieves a certain speed, this is typically done during the take-off ground roll, where one of the checks is to ensure the airspeed is “alive”.

6.5.3 Altimeter
– principles of operation and construction
The altimeter measures altitude above a particular pressure datum, this pressure datum can be set in the altimeter setting window. The altimeter is connected to the static port and uses the current static pressure to calculate its altitude above the set pressure datum. The altimeter has three needles, one for 10000’s of feet, one for 100’s of feet and one for 10’s of feet
The altimeter is constructed of a pressure sealed cylinder that is connected to the static port, inside the cylinder is a diaphragm capsule that is sealed with a particular pressure inside it. Due to the changing pressure on the outside of the diaphragm the diaphragm capsule expands or contracts and the moves a suitable linkage which in turn rotates the needless on the face of the altimeter
(Example of a good diagram to put here)

(Taken from the FAA pilots handbook of aeronautical knowledge FAA-H-8083-25B)
– the function of the subscale
Due to the pressure at a particular point in the atmosphere changing all the time, we need the ability to calibrate the altimeter for the current pressure in the atmosphere. We do this by setting a particular pressure in the altimeter setting window, we typically set QNH or QNE in the altimeter setting window. QNH is typically given to the pilot by Air Traffic Control or received from an Automatic Terminal Information Service

– effects of atmospheric density
The decrease in pressure in the atmosphere as we increase our altitude is not linear i.e. the pressure change is not constant, at high altitudes the pressure decrease per 1000′ is less than compared to lower altitudes. Most altimeters used in light aircraft are calibrated to 20000 feet. As can be seen in the diagram below the change in pressure at higher altitudes is less.
(Example of a good diagram to put here)

By Geek.not.nerd – Own work, CC0,

– pressure altitude
Pressure altitude is indicated when the altimeter is set a standard datum, this datum is 1013.25mb or 29.92 inHg. In south Africa and most of the world we use 1013 as the standard subscale setting.

– true altitude
True altitude is the height above Mean Sea Level or MSL, for the altimeter to indicate true altitude we need to set QNH in to the subscale setting window. QNH can be obtained from aviation weather reports and air traffic control towers.
(Example of a good diagram to put here)

A diagram as above would be great to explain the relationships between the different types of altitudes.

– international standard atmosphere
This is a standard atmosphere that we use to compare changes in the atmosphere, the ISA consists of the following at sea level:
– Pressure of 1013.2 millibar – Decreasing at about 1 millibar per 30 feet in the lower atmosphere (up to about 5,000 feet).
– Temperature of +15 °C – Temperature falls at a rate of 2 °C per 1,000 feet until the tropopause is reached at 36,000 feet above which the temperature is assumed to be constant at -57 °C.
(The precise numbers are 1.98 °C, -56.5°C and 36,090 feet)
– Density of 1,225 gm/m3.

– flight level
When we are in the cruise and above the transition altitude and level, we put the standard setting of 1013 in the subscale of the altimeter, we are now flying on Flight Levels, the reason for this is that all aircraft in the cruise are on the same setting and therefore ensures vertical aircraft separation. 15000 feet would be Flight Level 150 and 7500 feet would be Flight Level 075

– presentation (three needles)
The standard altimeter has three needles of different shapes and lengths, the longest need moves in increments of 10000 feet, the medium length needle moves increments of 1000 feet and the shortest needle moves in increments of 100 feet.
(An illustration like this would explain the three needles very well:)

– instrument errors
If the static port were to become blocked the altimeter would indicate the altitude for the pressure that was trapped inside the static system when the static port was blocked, therefore even if the aircraft were to climb or descend it would not indicate this on the altimeter.
Non-standard Pressure:
If we were unable to adjust the altimeter it would indicate the incorrect altitude unless the current atmospheric conditions were the same as ISA. If for example we were not able to change the pressure setting in the subscale window on the altimeter and we flew from and area of atmospheric high pressure to an area of atmospheric low pressure our altimeter would stay the same but our true altitude would decrease, therefore we say the altimeter starts to overread. There is a saying that says “High to Low, Beware Below” Therefore altimeters are built with a subscale setting windows so that the pilot can regularly update the to the current atmospheric pressure.
Non-Standard Temperature:
If we were to fly from and area of high temperature to an area of lower temperature, our aircraft would descend while the altimeter indicates a constant altitude, therefore our altimeter starts to overread, the reason this happens is because cold air is more dense than warm air an the pressure that is set on the subscale setting window is lower in the atmosphere because of the colder temperatures. We use the saying “Hot to Cold, Beware Below”

– pilot’s serviceability checks
To check the accuracy of the altimeter we compare the displayed altitude, once QNH is set, to the airfield elevation.

6.5.4 Vertical speed indicator
– principles of operation and construction
The VSI operates on the principle of pressure deferential, inside the instrument case is a sealed capsule that is connected to the static port, outside of the capsule but inside the instrument case is connected to the static port port but through a valve that delays the change in pressure inside the instrument case, as the aircraft climbs or descends the pressure inside the capsule changes immediately, but the pressure inside the case, but outside of the capsule, changes very slowly and therefore the needle in the front of the instrument indicates a climb or descend.

(Example of a good diagram to put here)

(Taken from the FAA pilots handbook of aeronautical knowledge FAA-H-8083-25B)

– function
The function of the VSI is to indicate to the pilot whether the aircraft is climbing or descending and at what rate the aircraft is climbing or descending. The rate of clib or descend is typically indicated in feet per minute.

– inherent lag
Due to the design of the VSI in particular the valve that slows down the pressure change in the instrument case there is an inherent lag in the system, therefore the aircraft needs to be in a stable climb or descend for a few seconds before an accurate rate of climb or descend is displayed on the VSI.

– instantaneous VSI
To overcome the inherent lag in a VSI an Instantaneous VSI or IVSI was developed, this instrument can sense acceleration in the vertical plane and moves the needle immediately as a climb or descend is initiated, as the climbs becomes more stable the pressure differential system starts takes over and displays a stable rate.
(Example of a good diagram to put here)

(Taken from the FAA pilots handbook of aeronautical knowledge FAA-H-8083-25B)

– presentation
The VSI has a scale that shows climb or decent in feet per minute, if the needle is above the zero mark, it is indicating a climb and if it is below the zero mark it is indicating a climb.
(A diagram if the VSI instrument face will be very useful here.)

– pilot’s serviceability checks
When stationary on the ground the VSI should show zero rate, it should also be checked just before take-off to ensure it is indicating zero and a climb should also be indicated soon after lift-off.

6.5.5 Gyroscopes – principles
Gyroscopes are used in a few instruments in aircraft they are the heading indicator, the attitude indicator and the rate of turn instrument, referred to as a turn co-ordinator if it has a balance ball included.
A gyro in essence is a spinning wheel, any wheel that spins possess gyroscopic properties, i.e. a bicycle wheel. A gyro has two properties that are used in our instruments, they are rigidity and precession.

– rigidity
Rigidity is the property of the gyro where by it maintains its orientation in space, the faster and more mass a gyro has the more rigidity it has.
– precession

If a force where to be applied to a spinning gyro the gyro would not react in the direction that the force is applied, the force would process 90? in the direction of rotation and the gyro would react as if the force was applied there, this is called precession.
(Example of a good diagram to put here)

(Taken from the FAA pilots handbook of aeronautical knowledge FAA-H-8083-25B)


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