# Pitot & Static Pressure System

## Pitot & Static Pressure System

The pilot static pressure system consists of the altimeter, ASI and VSI (vertical speed indicator). Because pressure, temperature, density and humidity vary from place to place and experience a constant rate of change, measuring aircraft performance through its related indicators is very difficult. As is established- temperature, density, humidity and pressure all equate to one thing, air. The ICAO having establishes the guideline ISA provides this as a strict standard by which pilots can calibrate their instruments and calculate actual conditions; as ISA conditions are unlikely to exist in reality. Standards set by the ICAO for ISA include the following:

• atmospheric sea level (MSL) pressure is 1013.25hPa;
• temperature at sea level is +15°
• air temperature decreases at 1.98° per 1000ft; and
• temperature decreases to -56.5° at 36,089ft (isothermal layer) and then remains constant

When studying the atmosphere we must understand that atmospheric pressure is caused by the weight of air existing above us. As air is compressible, the weight of air increases nearer to the Earth’s surface and then decreases with height. The reduction of pressure with height is non-linear due to atmospheric variations in density. Atmospheric pressure decreases at approximately 1hPa per 30ft after 5000ft and at about 1hPa every 225ft between 40, 000 and 45, 000ft. Minor fluctuations in atmospheric pressure will occur at higher altitudes and this makes reading the atmosphere very difficult. Any major fluctuation of atmosphere from high to low pressure will result in either ASI’s reading slightly over speed in high density atmosphere and under speed in low density atmosphere. The pilot static pressure system misconstrues the increase or decrease of atmospheric pressure as a change in airspeed.

Static pressure is always exerted, but in order for dynamic pressure to be exerted there must be motion of body relative to air. ‘Total pressure’ is the sum of static and dynamic pressure; this is called ‘pitot pressure” (pronounced pi-tow). The instrument used to measure dynamic pressure is called a ‘pitot tube’, usually built onto some forward section under the wing or around the nose. The pitot tube utilizes a compressible metal capsule called an ‘aneroid’ which reacts to changes to dynamic pressure. Static vents built into the fuselage of an aircraft measure static pressure. The VSI senses the rate of change of static pressure in the pitot tube and translates it in to rate of descent in ft/min. The altimeter senses static pressure in the pitot tube and translates the information into KIAS. The VSI measures the difference between pitot pressure and static pressure.

There are two pitot tubes and two static vents on every aircraft to ensure against false readings. This is known as the ‘balanced static system’. In the case of a damaged or obstructed static vent, an aircraft has ‘alternate static sources’ in the cabin or/and cargo hold which measure static pressure. This alternate system is not balanced and will provide false readings. Pitot tube covers and static vent covers should be placed on after every flight to prevent insects from forming blockages; icing is also a common risk. Pitot tubes and static vents should also be checked before flight. Pitot tubes are fitted with electric heaters activated by a pitot heat switch. This should be switched to the on position for moments of precipitation and when ice could be formed (high density low temperature atmosphere).

Pressure instruments are prone to two types of error; ‘position error’ and ‘manoeuvre induced error’. Position error occurs due to the placement of pitot tubes and static vents being at various angles to the relative air flow, due to where they are installed on an aircraft. The size of the error will rely on angle of attack, airspeed and Mach speed (% of speed of sound). The errors will read even greater when using an alternate static pressure source. The flight manual for a specific aircraft will contain ‘pressure error correction’ chart (PEC).

Manoeuvre induced error are the result of a change in direction of air flow over the pitot tubes and static vents, which occur shortly after and during a manoeuvre.

ALTIMETER

Because aviation has the added element of altitude, pilots learn specific forms of navigation across the vertical plane which ground vehicles do not. Due to the nature of weather, its fluctuation requires a pilot to adapt to these changes in pressure and therefore remain on the correct flight path. Pilots use information regarding height, pressure altitude and flight level. One must emphasise how they are all different. It is literally a matter of life and flying into the side of a mountain, this is somewhat irreversible.

Height is the aircrafts vertical position to a specific datum, whether it is MSL, buildings an aerodrome or a mountain.

Altitude is an aircrafts altitude above MSL.

Flight level is the aircrafts vertical distance from ISA datum 1013.25 hPa.

Density altitude is pressure altitude corrected for non standard temperature.
Pressure altitude is indicated altitude corrected for non standard pressure.

QNH referrers to the MSL at a given location; this is measured by the local weather station and passed on to the control towers. Pilots tune in to the ATIS (airport terminal information service) before takeoff in order to calibrate their altimeters to the area QNH. The ISA (international standard atmosphere) as set by the ICAO (International Civil Aviation Organization) for MSL is 1013.25hPa (hectopascal) which is the equivalent to 1atm (atmosphere). However due to the nature of weather, this is rarely ever the case. Aircraft below 10,000ft must refer to the immediate area QNH in order to obtain accurate altitude readings for clearing objects close to ground i.e. mountains. Area QNH values are usually accurate within 5hPa of the actual QNH. A plane taking off at a non-controlled airfield means that there is no ATIS broadcasting the area QNH. A pilot can figure out the area QNH by setting the altimeter to the aerodromes elevation above MSL and seeing what the altimeter sub-scale comes up with. The aerodromes elevation will be stated on maps or charts. Without maps or charts for the area a pilot should not fly.

All aircraft flying at and over 10,000ft have their altimeters set to ISA and this gives us our ‘flight level’. High flying jets will work according to flight level and not direct altitude readings. Altitude up to 10,000ft is referred to as ‘transition altitude’. All flight levels are expressed as FL, e.g. FL180 is pronounced “flight level one eighty”, meaning 18,000 ft.

Height refers to your vertical distance measured from a specific datum i.e. high mountains and terrain. QFE refers to the use of the subscale setting on an altimeter as a means of indicating clearance height above a specific datum; the base datum being set to QNH allows for exact readings. The pilot is able to adjust the altimeter subscale by using the rotational knob just under the indicator. Without the correct QNH a disaster is imminent. Note the clearance height at label (b), given the correct QNH there should be 1500ft of clearance between the aircraft and the mountain.

Pressure altitude refers to your actual altitude above MSL. Label (a) displays the potential difference between pressure height and flight level, in this case 19hPa variation. Note that if MSL was 1025 hPa or more, than FL110 would not be available. As we graduate from one area to the next, pressure changes according to weather. Just as if a pilot does not adjust his altimeter under 10, 000ft to the area QNH, the consequences could be fatal. It would be almost undetectable to find that you were in fact on a gradual ascent or descending towards the ground. If the altimeter is set to a certain pressure altitude and the pressure changes, the plane will follow the indicated sea level pressure settings and either descend gradually or climb. Remember that density altitude is pressure altitude corrected for temperature error.

Note:’ elevation’ refers to an aerodromes height above MSL (c) 700ft).

Pressure height is important because the AIS (airspeed indicator) is calibrated by the altimeter. Taking into account that at altitudes above 10, 000ft we refer to flight level, it has henceforth come to a pilots understanding that our IAS will differ slightly from our TAS. This is due to our altimeter being set to flight level which is calibrated by ISA and not by pressure altitude; the actual pressure at sea level MSL. To calculate pressure height, we convert the arithmetic difference in hPa between the QNH and ISA MSL. ISA states that after 5000ft, the approximate rate of fall of hPa is 1hPa per 30ft. We use this knowledge in our sum:

(ISA – QNH) x 30 = Pressure Height

Example 1-

Difference between ISA (1013hPa) and 994hPa (QNH) = (1013hpa – 994hPa) x 30ft/hPa =

+19hPa x 30ft/hPa = +570hPa

*Using laws of algebra: one negative and one positive make a negative.

Example 2-

Difference between ISA (1013hPa) and 1031hPa (QNH) = (1013hPa – 1031hPa) x 30ft/hPa =
-18 x 30ft/hPa = -540hPa

Therefore, when QNH is less than ISA, pressure height is greater than elevation,
when QNH is less than ISA; pressure height is less than elevation.

Taking in to account that ISA states up to about 5000ft 1hPa is equal to 30ft, if an altimeters subscale setting is in error by one hPa, an aircraft will be 30ft above or below the assigned or chosen altitude. E.g. if flying at 4000ft with an altimeter subscale setting of 1024hPa instead of the actual atmosphere of 1026hPa (2hPa difference) than the altimeter will be out by 60ft, reading 3940ft. This is called ‘barometric error’. Barometric error is induced when MSL pressure is different from ISA.

• If the subscale setting is too low, than the altimeter will read too low;
• if the subscale setting is too high, than the altimeter will read to high.

‘Temperature error’ occurs when atmospheric temperature is different to that of ISA. An air column with a higher temperature than ISA will cause an altimeter to under read, as warm air is less dense. A column of air colder than ISA will cause an altimeter to over read as colder temperatures denote a higher atmospheric density.

‘Instrument error’ occurs due to small manufacturing imperfections and the large mechanical amplification necessary for small sensed movements. This error increases with altitude as the aneroid capsule movement is smaller due to the diminished atmosphere. Atmosphere at altitude shows smaller variations for a given pressure change which would otherwise show greater at MSL.

‘Lag error’ occurs during a rapid descent or climb; the aneroid capsule senses pressure change but the relay of this information to the indicators is not instantaneous.

Regarding the effects that the environment has on pressure instruments there are ‘orographic’ considerations which a pilot must always be wary of. Because of the mechanical effects of strong winds near mountainous areas atmospheric pressure in the lee of the mountain is reduced, indicating a higher than true altitude. While this is already a very dangerous situation, the over reading altimeter is compounded on light aircraft. Also the down draught of wind which occurs between mountains may exceed the climb capabilities of a light aircraft.

ASI (AIRSPEED INDICATOR)

The ASI has no knowledge of air density; it only measures dynamic pressure (pitot pressure – static pressure). The calibration of the ASI is under the assumption that air density corresponds to ISA, so immediately we know that IAS (indicated airspeed) is only ever accurate under ISA conditions. Normally aircraft are use to flying in low density atmosphere; so aircraft fly faster to achieve a set dynamic pressure. This set dynamic pressure is translated as a specified airspeed, however due to the low density atmosphere TAS will be higher than the IAS. This is ‘density error’. Density error becomes greater with increased altitude, at 20, 000ft the TAS may be as much as IAS + 35%. Density error does not occur at MSL or in ISA conditions. Note that air density is directly affected by temperature. By programming the outside temperature in to a flight computer, an aircraft can be calibrated to maintain IAS by adjusting the TAS accordingly.

At high speeds relative airflow becomes compressed in the pitot tubes and this causes a higher pressure reading than ambient atmosphere. This becomes a ‘compressibility error’ and the ASI has a tendency to over read. For this reason more sophisticated aircraft have also a digital ASI as well as the pressure ASI indicator. The digital ASI is fixed with a rotatable knob which switches the airspeed from displaying in KIAS and in to MACH number (% speed of sound); MACH has no compressibility errors and will therefore display TAS. To understand MACH numbers use this formula:

• MACH = airspeed ÷ 60
• KIAS = MACH x 60

e.g. 320 KIAS ÷ 600 = 0.53 MACH,
0.53 MACH x 600 = 320 KIAS

• 60 KIAS = 0.1 MACH

• MACH in to nm/min (nautical miles) = MACH x 10

• nm/min into MACH = ÷ 10

e.g. 0.53 MACH x 10 = 7.5 NM per minute
7.5 nm per minute ÷ 10 = 0.53 MACH

(Note: 1 nm is equal to 1852 m; nautical miles per minute expressed in meters per minute is just the answer x 1852).

The ASI like all pressure instruments suffers from position error, usually no more than 2 KIAS. To find position error a pilot will refer to the aircrafts manual. Knowing position error is vital in the process of obtaining TAS; as IAS becomes CAS calibrated for position error and CAS becomes EAS calibrated for compressibility error, we can finally know our TAS with the added corrections of density error.

A way to remember compressibility error and density error is to realize how they are opposite’s i.e. compressibility error produces over read at low altitudes and/or high density atmosphere and density error increases with altitude as atmospheric density decreases causing an under read.

• CAS (calibrated airspeed) refers to IAS corrected for position error, also known as RAS (rectified airspeed). CAS can usually be found in the aircrafts manual.

• EAS (equivalent airspeed) refers to CAS corrected for density error. This is the purest form of expression of true airspeed and will not be found in an aircraft manual.

ASI MARKINGS

ASI’s are colour coded in reference to the aircrafts ‘V speeds’ which state how it is to be handled under specific configurations. These coloured arcs also display the aircrafts vital limitations.

VS0 – maximum stall speed with flaps extended, (low end white arc)

VS1 – maximum stall speed with flaps retracted, (low end green arc)

VFE – maximum airspeed with flaps extended, (high end white arc)

VNO- maximum speed for normal operations, (high end green arc, beginning of yellow arc)

VNE – do not exceed speed ‘max speed all opps’ (indicated by red line at the top the yellow arc).

The yellow arc between VNO and VNE is the caution zone.

Vmc – minimum indicated airspeed at which the aircraft can be controlled with the critical engine inoperative (indicated by red line near bottom of green arc for multi engine aircraft only).

VYSE – best climb speed for aircraft with critical engine inoperative (indicated with a blue line, for multiengine aircraft only)

VSI (VERTICAL SPEED INDICATOR)

The VSI measures an aircrafts rate of climb and descent which is displayed on the indicator as ft/min. Unlike the ASI which measures dynamic pressure, the VSI measures the rate of change in static pressure.

The outside atmosphere is let in to an aneroid capsule which acts as a metering unit. When the atmosphere surrounding the capsule changes, the pressure differential will cause the air inside the capsule to either flow in or out accordingly. However, air flowing in or out of the capsule is metered by a calibrated release valve, this valve is set to slow down the process of diffusion long enough in order to display the change of pressure as a climb or descent on the VSI. When the atmosphere both inside and around the capsule have once again reached equilibrium, the VSI will read zero. Mechanical stops are set in place to give the VSI its maximum reading for normal operation - feet per minute and let it fall back to 0 when equilibrium of pressure differential is reached. If the VSI is maxed out than a pilot will take caution in maintaining an acceptable climb/descent rate. In the diagram below the needle is showing a climb rate of about 200 ft/min. The number 10 would show a climb rate of 1,000 ft/min, if it was the number 10 on the bottom end it would mean a descent rate of 1, 000 ft/min.

IVSI (INETRIAL LEAD VERTICAL SPEED INDICATOR)

The IVSI is fitted with an accelerometer pump enabling an artificial instantaneous response between the rate of pressure differential and the needle indicator. The artificially generated pressure change from the accelerometer will fade after a few seconds allowing normal operation to take place. Although this eliminates lag error the IVSI is still prone to instrument and manoeuvre induced errors. The accelerometer pump is sensitive to changes in bank angle and is in that sense un-reliable. The accelerator pump is shown in the figure below; some aircraft IVSI are fitted with more than one.

Author: David G. P. Martin

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