Airspeed Indicators

The airspeed at which an helicopter is travelling through the air is essential to the pilot, both for the safe and efficient handling of the helicopter and as a basic input to the navigation calculations.

Principle of the Airspeed Indicator (ASI)

When an helicopter is stationary on the ground it is subject to normal atmospheric or static pressure, which acts equally on all parts of the helicopter structure. In flight the helicopter experiences an additional pressure due to the aircraft's motion through the air, which is known as dynamic pressure, and is dependent upon the forward motion of the helicopter and the density of the air, according to the following formula:

PT = 1/2ρV2 + PS

Where:

PT = total or pitot pressure (also known as total head pressure or stagnation pressure)

PS = static pressure

ρ = air density

V = velocity of the helicopter (TAS)

Re-arranging the formula, the difference between the pitot and static pressures is equal to ½ ρV2 (dynamic pressure). The airspeed indicator thus measures the pressure differential between the two sources, and provides a display indication graduated in units of speed.

Operation of a Simple ASI

In the simple ASI, a capsule acting as a pressure sensitive element is mounted in an airtight case, as shown by the diagram. Pitot pressure is fed into the capsule and static pressure is fed to the interior of the case which, when the helicopter is in motion, will contain the lower pressure.

A pressure difference will cause the capsule to open out with any movement being proportional to the pressure differential across the capsule skin (pitot - static). A mechanical linkage is used to transfer the capsule movement to a pointer that moves over a dial, and which is normally calibrated in knots.

A bi-metallic strip is also incorporated in the mechanical linkage to compensate for any expansion/contraction of the linkage caused by temperature variations.

The pitot and static system on an aircraft is used to measure the total pressures created by the forward motion of the aircraft, and the static pressure surrounding the airframe. These pressures are then fed to instruments, which convert the pressure differentials into speed, altitude and rate of change of altitude. The system is alternatively referred to as a ‘Manometric' or ‘Air Data System'.

Pitot Tube

The Pitot tube (pitot probe) is used to sense the total or pitot pressure, which is the combined static and dynamic pressure of the airflow. The tube is fitted to the airframe with its opening facing directly into the airflow, and the airflow comes to rest, i.e. stagnates inside the tube entrance.

A baffle plate is fitted inside the tube entrance, and is designed to intercept much of the moisture in the airflow. A pinhole drain also allows any moisture to leak away to atmosphere without significantly affecting the sensed pressure. A further drain is additionally provided at the back of the probe where the pitot pressure feeds into the pitot line. This drain may either be a pinhole, or a larger capacity moisture trap, which may only be drained on the ground, by activating a manual release button.

The probe is mounted on a part of the aircraft where there is minimal disturbance to the airflow, and is designed to extend well forward into the airflow. These probes are typically mounted close to the nose, at the wing tips, on a pylon extending well below the wing, or at the top of the fin.

The probe is also fitted with a heater, which is powered from the aircraft electrical supply (usually 28 Volt DC or 115 Volt AC), and is switched on as required by the flight deck crew to prevent the formation of ice.

An indicator light gives the operative state of the system. Some types show an amber light when switched ‘OFF', or alternatively with the system switched ‘ON' and the heating element failed. Most transport category aircrafts have at least two pitot tubes.

Static Source

The ambient pressure of the air mass surrounding the aircraft, or "static pressure", is obtained via a static source. The static source or ‘static vent' senses the static pressure of the atmosphere, which is unaffected by the airflow. To achieve this, the source (vent) is located on a part of the aircraft where the airflow will be undisturbed by its passage, and is also positioned with its entrance perpendicular to the airflow. The vent is manufactured and attached to the surface of the aircraft so that it does not create local disturbances in the airflow.

Vent pipe connections are installed with a slight downward angle to ensure adequate drainage, and it is also important that the vent plates are not painted, as this would impair their thermal efficiency. This may be indicated on the aircraft structure next to the plate by a placard.

The direction of the airflow around the static vent may vary as the airspeed and configuration of the aircraft changes, and may also induce errors known as position (or pressure) errors. These errors can be minimised by carefully positioning the static vent, or by using multiple vents to average out the errors. This is known as ‘Static Balancing', and is achieved by fitting vents on either side of the aircraft fuselage.

The purpose of this is to even out any differences of pressure that may be caused by the sideways motion of the static vents, such as will occur during a yaw or side-slip condition. Any residual position (pressure) errors are recorded during initial flight tests and a correction table is produced, for various airspeeds and configurations. These readings are incorporated into the Aircraft Operating Manual (AOM).

If failure of the primary pitot/static pressure source should occur, for example icing up of a pitot or pressure head due to a failed heater circuit, errors may be introduced in the instrument readings and other areas dependent on such pressure.

As a safeguard against partial failure, a standby system may be installed in some aircrafts, whereby static pressure and/or pitot pressure from alternate sources can be selected and connected into the primary system. A blockage of the pitot source is not serious, as it will only affect the ASI. A blockage of the static source will however affect all of the instruments, and it is thus common practice to provide an alternate static supply.

Alternate Static Source

The changeover to an alternate static source is normally achieved by selector valves located in the static lines, which are located on the flight deck, within easy reach of the flight crew. A typical internal alternate static source installation. Such a system will only operate satisfactorily if the cabin is unpressurised and the air within the cabin is relatively undisturbed.

When calibrating the pressure/position errors of the alternate system, the manufacturer will lay down the conditions required in respect of the position of such items as windows, heating/ventilation and doors, all of which must be observed if the system is to work correctly.

Moderate or large aircrafts will normally have a minimum of two separate static systems, and each will be fed by a pair of balanced static vents. The second pair of static vents is normally referred to as the auxiliary static ports.

Combined Pitot-Static (Pressure) Head

In some light aircrafts the complication and expense of separate pitot head(s) and static sources is avoided by often incorporating both functions into a combined pressure head. The combined head is usually mounted on a pylon below the wing towards the tip, thus reducing the position/pressure error to an acceptable level although accuracy is well below the standards achieved by more sophisticated systems.

Static pressure is admitted to the pressure head through slots or holes cut into the static casing at 90° to the airflow, whilst pitot pressure enters the head via the pitot input port that faces directly into the airflow. The different pressures are then fed to the flight instruments via separate, seamless and corrosion-resistant metal pipelines. An electric heating element is also connected to the aircraft electrical supply to prevent ice forming inside the pressure head, which may obstruct the airflow.

This sensor is particularly prone to errors during manoeuvres, when changes in pitch can result in the pitot/static sources being presented to the relative airflow in such a way that the full dynamic pressure is not fully sensed. More significantly, dynamic pressure effects may also intrude into the static supply.

Systems using a combined pressure head thus tend to suffer from increased pressure errors due to the positioning of the sensor, and in particular during aircraft manoeuvres. The combined pressure head is thus only suitable for use on relatively small, low performance aircraft.

Calibration of the ASI

Standard datum values are used in the calibration of air speed indicators since dynamic pressure varies with air speed and air density. Density also varies with temperature and pressure. The values used are the sea level values of the ICAO International Standard Atmosphere (ISA).

Colour Coding of the ASI

The scale is calibrated in terms of speed, usually knots or miles per hour (MPH), but in some cases may be kilometres per hour (KPH). It is thus essential that you know which terms are being displayed on the ASI. On light helicopters the dial is normally colour coded with the coloured segments indicating the following:

• Green Arc: This arc is the normal operating speed range.
• Red Radial Line: This line marks VNE.

ASI Errors

The dial of the ASI is calibrated to a formula, which assumes constant air density (standard mean sea level) and no instrument defects. Any departure from these conditions, or disturbance in the pitot or static pressures being applied to the instrument, will result in a difference between the indicated and true air speeds. The following sources of error exist.

Instrument Error

This error is caused by the manufacturers' permitted tolerances in the construction of the instrument. This error is determined by calibration and if it is found to be significant is recorded on a calibration card. This correction is normally combined with that for pressure error.

Pressure Error

This error arises from the movement of the air around the helicopter and causes disturbances in the static and pitot pressure. The causes of this error are:

Position of the Pitot-Static Sensors

This can alter the pressures being fed to the instrument, and is particularly so in the case of a combined pitot-static head where the dynamic pressure component may significantly affect the static supply. To minimise this source of error separate static vents are positioned well away from the pitot head, which can result in a 95% reduction in the overall pressure error.

The position/pressure error is normally determined by calibration, and a pressure error card is tabulated in the Helicopter Flight Manual. This card may also incorporate any instrument error calibrations.

Turbulence Error

This results in random accelerations, which will vary in magnitude, and will make indications on the pressure-fed instruments extremely unreliable.

Manoeuvre Induced Error

This is caused by changes in the helicopters attitude and/or configuration and is normally only short term. The main sources of error are normally in the static supply, but since the transient effects of manoeuvre induced error are not predictable or avoidable, the flight crew must be aware of this problem.

The pressure error will change if any of the following vary:

• Airspeed
• Angle of attack
• Configuration (flap setting, undercarriage etc)
• Position of the pitot/static sources and sideslip

Compressibility Error

The calibration formula for most airspeed indicators does not contain any compensation for the fact that the air is compressible. At low airspeeds this is insignificant but at airspeeds over 300KTAS this factor becomes significant. This is especially so at high altitudes where the less dense air is easily compressed.

Compressibility causes an increase in the measured value of dynamic pressure, which will cause the ASI to over-read. Thus, compressibility varies with airspeed and altitude. The error and correction can be compensated on some mechanical navigation computers but is tabulated against altitude, temperature and CAS in the handbooks of others.

Density Error

Dynamic pressure varies with airspeed and density of the air. In calibration, standard mean sea level pressure is used; thus, for any other condition of air density, the ASI will be in error. As altitude increases, the density decreases and the indicated airspeed (IAS), and thus equivalent air speed (EAS) at speeds in excess of 300 KTAS, will become progressively lower than the true air speed (TAS). For example at 40,000 ft the density is only ¼ of its msl value.

The dynamic pressure, which is proportional to TAS2, will thus be ½ the msl value for the same TAS, ie. an helicopter flying at 400 KTAS will have an IAS of 200 knots. The following formula will help to establish the relationship:

For accuracy, the correction of CAS to TAS is done on a navigational computer using the ambient temperature (outside air temperature), at the required pressure altitude.

The relationship between the various air speeds is as follows:

• Air Speed Indicator Reading (ASIR) + Instrument Error Correction=Indicated Air Speed(IAS)
• IAS + Pressure Error Correction = Calibrated Air Speed (CAS)
• CAS + Compressibility Error Correction = Equivalent Air Speed (EAS)
• EAS + Density Error Correction = True Air Speed (TAS)

In practice, the corrections are combined to give:

• ASIR + Instrument Error Correction + Pressure Error Correction = CAS
• CAS + Compressibility Error Correction + Density Error Correction = TAS

ASI Faults

The following faults may occur in the ASI:

Blockages

A blockage of the pitot tube, as shown on the next page, possibly due to ice, will cause the ASI not to respond to changes of speed in level flight. The capsule will however behave as a barometer or altimeter capsule, and will react to changes in the static pressure. If the helicopter climbs, the ASI will indicate an increase in airspeed (over-read) and if it descends, it will indicate a decrease in airspeed (under-read).

If the static line is blocked, the ASI over-reads at lower altitudes, and under-read at higher altitudes than that at which the line became blocked.

Leaks

A leak in the pitot system will cause the ASI to under-read, whilst a leak in the static line will cause the ASI to over-read in an unpressurised fuselage (cabin pressure is usually lower than the atmospheric static pressure), and under-read in a pressurised helicopter (cabin pressure higher than static).

Alternate Static Vents

Use of the alternate static vents will almost always result in a different pressure error profile.