2. Pitot Static System

Pitot Head

Static Ports

Pitot-Static System

A system in which total pressure created by the forward motion of the aircraft and static pressure of the atmosphere surrounding it are sensed and measured in terms of:

SpeedAltitudeRate of change of altitude

Basic Pitot-Static consists of:

Pressure Head Known as pitot-static probe, consist of static slots,

pitot tube, drain hole and heating element. If only pitot pressure is collected through the tube, it

is called Pitot Tube or Pitot Head. It should be covered if the aircraft is park to avoid

foreign object entering the tube. Warning flag, red in colour, with word REMOVED

BEFORE FLIGHT to be attached to the pitot cover.

Pressure Head

Static Vent Also known as static port, it is located at the aircraft

fuselage and are position where there are least disturbance from the air flow.

Independent static vents, when fitted are always located one on each side of a fuselage and interconnected so as to balance out dynamic pressure effects resulting from any yawing or sideslip motion of an aircraft.

The static vent mounting plate should not painted and must be cleaned and smooth.

If aircraft is park the static vent should be blanked and the warning flag REMOVED BEFORE FLIGHT should be attached.

Static Port

Drain Trap

Pipeline Pitot and static pressures are transmitted through

seamless corrosion-resistant metal (light alloy/or tungum) pipeline.

Flexible pipes (maricon tubing) are used for connection of components mounted on anti-vibration mountings.

The minimum requirement diameter of the pipeline is 0.25 inch or 6mm.

The piping are marked by a letter P or S.Drain Trap/Drain Valve Allow draining of water accumulated in the piping

using spring loaded action. Drain traps are located at the lowest point of the

piping run/route.

Pitot-Static Instruments Airspeed Indicator (ASI)

Requires pitot and static pressure. ASI measure the differential pressure between pitot/static.

Altimeter Requires static pressure only Altimeter measures absolute pressure. If a leak occurs on a pressurised aircraft under read,

unpressurised aircraft - ???? Vertical Speed Indicator (VSI)

Requires static pressure only VSI measures rate of change of altitude pressure.

Machmeter Requires pitot and static pressure.

Pitot-Static System on Modern Aircraft

Altimeter

PURPOSETo indicate vertical distance from MSL (Mean Sea Level) or AGL (Above Ground Level) from minus 1,000 feet to 50,000 feet.To indicate pressure altitude when Baroscale is set to QNE (ICAO standard atmospheric pressure at Mean Sea Level)

Altimeter

PRINCIPLE OF OPERATIONAn altimeter measures air pressure, which decreases with increasing altitude, and from the pressure calculates and displays the corresponding altitude.It follows the aneroid barometric principle to measure absolute pressure (total pressure).

ConstructionA detecting element to collect static pressure.A measuring element which is the aneroid capsule.Mechanical coupling.An indicating element which is a pointer rotating over a scale.

Altimeter

AltimeterTypes of Altimeter

Non-sensitive Altimeter single pointer/one capsule.

Sensitive Altimeter multiple pointers/stack aneroid capsules.

Cabin Altimeter to measure cabin pressure.

Encoding Altimeter for ATC purposes.

Servo Altimeter to overcome lag error/more accurate.

Radio/Radar Altimeter measure vertical distance Above Ground Level (AGL) used in GPWS and autopilot.

Non-Sensitive Altimeter

Sensitive Altimeter

Cabin Altimeter

Encoding Altimeter

Air Traffic Control Radar Beacon System

Mode S Operation

Encoder Altimeter

Servo Altimeter

Radio/Radar Altimeter

Radar Altimeter Transmitter/Receiver

Uses antenna installed on aircraft belly.Transmitter sends out radio waves at 4.3GHz which strike the earth and bounce back to the receiver antenna.The system measures time taken for the wave to return back and calculation is made of AGL altitude.

Above 100,000ft Press 0 psi

37,000 ft Press 6.39 in Hg Temp 56.5C

30,000 ft Press 8.88 in Hg Temp 44.4C

10,000 ft Press 20.57 in Hg Tem p - 4.8C

5,000 ft Press 24.89 in Hg Temp 5C

Sea Level (0 ft) Press 29.92 in Hg Press 1013.2 Mb Press 14.7 psi Temp 15C

Minus 1,000 ft Press 31.01 in Hg Temp 16.98C

ALTITUDEThe vertical distance of an aircraft measured from mean sea level (MSL).

PRESSURE ALTITUDEWhen altimeter is set to Standard Mean Sea Level (ICAO) on the barometric window.When an aircraft altimeter is set at this pressure setting, it is flying at Flight Level.

INDICATED ALTITUDEDirect reading of altitude from altimeter when barometric window setting is not set to standard MSL (ICAO).

ABSOLUTE ALTITUDEIt is measured by radio, radar, or laser altimeter.It measured altitude above ground level (AGL).

DENSITY ALTITUDEIt is the pressure altitude corrected for non-standard temperature.

HEIGHTVertical distance of an aircraft from a reference plane (datum).

DATUMA reference line, from which calculations or measurements are taken.

Types of Altitude Measurement

Q-Code

The Q code is a telecommunication code. The codes in the range QAA QZZ (QAA-QNZ are for aeronautical use).

QFESetting aerodrome atmospheric pressure so that an altimeter reads zero on landing and take-off.

QNHSetting Mean Sea Level atmospheric pressure so that an altimeter reads the aerodrome altitude above Mean Sea Level.

QNESetting standard Mean Sea Level atmospheric pressure IAW the ICAO standard atmosphere, i.e. 1013.25 mb or 29.92 in Hg and altimeter will reads pressure altitude.

Error in Altimeter

Instrument error due to mechanical imperfection during manufacturing.

Position error due to incorrect location of pressure source.

Blockage error pitot static piping error.

Lag error instrument reading responds slower than the actual

altitude of the aircraft.Stiction error pointer sticking to the dial due to electrostatic effect. (Friction that tends to prevent relative motion between two movable parts at their null position.)

Error in AltimeterDuring calibration check in the shop these test are to be complied for IFR flying and any error to be recorded in the altimeter correction card at thousand-foot interval.

Scale error altimeter must follow reading from master indicator or manometer.

Hysteresis reading must agree when altitude is in increasing or decreasing.

After effect indication must return to original reading after performing test.

Friction reading taken before and after being vibrate.

Case leak test at 18,000 feet should not leak within tolerance.

Barometric scale error indication from barometric scale and altimeter reading must tally with

ICAO pressure table.

Airspeed Indicator (ASI)

Purpose:to indicate the speed of the aircraft relative to the airflow.to indicate the aerodynamic force acting on the aircraft surface.

Principle of operationuses a differential capsule to measure different in pitot and static pressure.

Constructiontwo detector units which senses pitot and static pressure.a differential capsule as a measuring unit.a mechanical coupling.an indicating element which is a pointer rotating over a scale graduated in knots. 1 knot = 1.1 miles.

A mechanical airspeed indicator has one differential capsule.A sensitive airspeed indicator has more than one differential capsule.

Air Speed Indicator

PITOT PRESSURE = DYNAMIC PRESSURE + STATIC PRESSUREDynamic pressure is representative of airspeed, so:

DYNAMIC PRESSURE = PITOT PRESSURE - STATIC PRESSURE

Square Law CompensatorA device that will compensates the non-linear deflection of the capsule when magnify to the pointer become linear. It will represent linear scale on the ASI.

Errors of ASIInstrument error due to imperfection of the instrument mechanism during

manufacturing.Position Error incorrect position of pressure sources. measured by trial and error and wind tunnel.

Compressibility Error results from air being compressed in the pitot tube inlet,

generally at altitudes above 10,000 feet and CAS in excess of 200 knots.

It generally produces IAS readings that are too high.Density Error due to decrease in the density of air as altitude increases.

Reading of ASI

ASIR (Airspeed Indicator Reading) reading not corrected for any error.

IAS (Indicated Airspeed) ASI corrected for Instrument error only.

RAS (Rectified Airspeed) IAS corrected for instrument and pressure errors.

CAS (Calibrated Airspeed) RAS compensated for non-linear (Square law)

EAS (Equivalent Airspeed) RAS corrected for compressibility error.

TAS (True Airspeed) EAS corrected for temperature and density error.

Vertical Speed Indicator (VSI)

It is also called Rate Of Climb (ROC) Indicator. Purpose:

To indicate to the pilot the vertical speed of the aircraft or rate of climb or dive of an aircraft in feet per minute.

Principle of operation It measures the rate at which the static pressure changes.

Construction A metering unit to established second pressure. A differential capsule sense pressure changes. An indicating element using pointer which rotates over a

scale graduated in feet per minute.

Vertical Speed Indicator

At level flightPrevailing static pressure is admitted to the interior of the capsule, and also to the instrument case through the metering unit. There is zero differential across the capsule and pointer indicates zero.When aircraft descendingMetering unit maintains case pressure lower than capsule pressure, changing it at the same rate and thereby creating a constant differential pressure across thecapsule.When aircraft climbMetering unit creates a constant differentialpressure across capsule by maintaining case pressure higher than capsule pressure.

Instantaneous Vertical Speed Indicator (IVSI)

It is also called Inertia Operated VSI.It is designed to overcome lag error in the normal VSI using accelerometer or dash pot.The inertia of the accelerometer piston will move upward (if descent) and downward (if ascent) so as to quicken the expansion and contraction of the differential capsule by pumping air into it.

Instantaneous Vertical Speed Indicator (IVSI)

Instantaneous Vertical Speed Indicator

Pitot-Static leak Test

To determine the rate of leak of a pitot-static system.Rate of leak is measured in feet per min or knot per minute.Different aircraft has different leak rate always refer to aircraft MM.Leak test on pitot-static system is carried out when; The pitot static piping and instrument are disturbed

(remove/install). Suspect of a leak in the pitot static system/instrument. It is called for in the task card when aircraft comes in for a

maintenance visit. (Every 24 month) When the drain holes of the pressure head is disturbed.

Procedures

Pitot Leak TestConnect up Pitotstatic Tester to the pitot head using adaptor.Pump in positive pressure slowly to make airspeed indicator pointer reads V ne red radial line. Caution: about PSI is required to make ASI read

150 knots.Retain pressure for 1 minute.If ASI reading drop more than 10 knots means there is a leak in the pitot pipelines.Trace leak from the indicator to source.

Pitot-Static Leak Tester

ProceduresStatic Leak Test

Connect up Pitotstatic Tester to the static port using adaptor.Pump in negative pressure slowly to make altimeter reads more than 1,000 feet above current reading. Caution: When applying suction to the system do not

exceed the vertical speed indicator maximum limit.Retain pressure for 1 minute.If altimeter reading drop more than 100 feet means there is a leak in the static pipelines.Trace leak from the indicator to source using soap water.

Caution when working on Pitot Static System

The amount of pressure required for 150 MPH indication is less than PSI . Avoid high pressure as instruments damage will result.

Do not apply suction to pitot pressure lines. When applying or releasing suction, take care not to

exceed rate range of vertical speed indicator. Do not apply pressure to static lines with instruments

connected. Never blow through the pitot or static lines toward the

instruments. Doing so may damage them.

Documents

2. Pitot Static System