# Pressure

## References

• The following SKYbrary Articles:

## Objectives

Gain an understanding of:

• Static and Dynamic pressure
• Pressure settings
• Aircraft pitot-static system

## Static Pressure

Air has substance (mass). Not very much if compared to other matter, but nevertheless a significant amount. As a result of the weight of the mass of air, pressure acts on all bodies in the atmosphere - this pressure is called static pressure.

Static pressure:

• always acts equally in all directions;
• an aircraft always has static pressure acting on all surfaces;
• static pressure decreases with increase of altitude;
• static pressure at a given altitude will vary from day to day.

## Dynamic Pressure

Because air has mass, air in motion possesses kinetic energy, and will exert a force on any object in the path of the moving air particles. Conversely, an object (such as an aircraft) in motion through the air will have a force exerted on it by the air. This force is called dynamic pressure.

Dynamic pressure:

- acts only in the direction of movement; - it is proportional to the density of the air and the square of the speed of the air. (The denser the air, the more air particles will hit the object in a given period of time. The greater the speed, the more air particles will hit the object in a given period of time).

## Air Density

The mass of the air per unit volume is called air density (“number” of air particles in a given space).

Air density is:

• proportional to the static pressure;
• inversely proportional to the temperature;
• density decreases if humidity increases;
• density decreases with increasing altitude (Static pressure and temperature decrease with altitude, but the effect of decreasing static pressure is dominant).

The values of temperature, pressure and density are never constant in a given layer of the atmosphere. They are constantly changing. In order to compare aircraft performance, ICAO defined the International Standard Atmosphere (ISA). ISA assumes the following mean sea level values:

• Temperature: 15°C
• Pressure: 1013.25 hPa
• Density: 1.225 kg/m3

## Aircraft Pitot-Static System

An aircraft pitot-static system comprises a number of sensors which detect the ambient air pressure affected (pitot or dynamic pressure) and unaffected (static pressure) by the forward motion of the aircraft. These pressures are used on their own, or in combination with each other, to provide indications of various flight parameters. These include:

The Altimeter and VSI use the static pressure only whereas the ASI and Machmeter use pitot pressure and static pressure.

Pitot and static pressure are also used in other equipment, such as the Autopilot and the Cabin Altimeter.

## Indicated Air Speed

Indicated Airspeed (IAS) is a function of the difference between:

• the Total Pressure (Pt) (Dynamic/Pitot Pressure) measured by a forward facing pitot head or probe and;
• the Ambient or Static Pressure (Ps) measured at static plates or ports.

## Static Vents

Static pressure is measured through a number of vents, situated at aerodynamically neutral points on the aircraft fuselage. Vents are sited on either side of the fuselage and feed into a common tube; this has the effect of cancelling out to some extent errors arising from the position of the vents. A combination of careful vent siting and accurate calibration reduces errors to an acceptable degree. Commercial aircraft have at least two completely independent static systems to provide redundancy in the case of system failure.

Static vents are normally plugged when the aircraft is parked for more than a short period of time to reduce the chance of blockage or contamination. Vents may be electrically heated to prevent blockage by ice.

## Pitot Tubes

Pitot pressure is measured in a pitot tube or pressure head, which is an open tube facing forward along the axis of the aircraft. The pressure measured in the tube is a combination of static pressure and pressure due to the aircraft forward speed. Pitot tubes are carefully sited to reduce, to a minimum, error due to the flow of air over the aircraft.

Commercial aircraft have at least two completely independent pitot systems to provide redundancy in the case of system failure.

Pitot tubes are normally covered when the aircraft is parked for more than a short period of time to reduce the chance of blockage or contamination. They are invariably electrically heated to reduce contamination by moisture and to prevent blockage by ice.

## Pitot Tube and Static Ports Blockage

Complete or partial blockages of pitot heads and static ports caused by:

...can cause erroneous indications on flight instruments, especially air speed indicators, confusing the flight crew, reducing situational awareness and potentially leading to a loss of control situation.

In addition to airspeed indicators, systems which rely on information directly or indirectly (via Air Data Computers) from the pitot-static system are also unreliable if the pitot static system is blocked in some way.

Quiz Questions:

Q1: Static Air Pressure at a given altitude

1. varies from day to day
2. is usually greater during the summer period
3. is constant over time
4. is always greater than the static pressure at mean sea level

Q2: If the pitot tube is blocked then

1. the engine instruments will be unreliable
2. the Air Speed Indicator will be unreliable
3. the Altimeter will be unreliable

Q3: Complete or partial blockage of pitot heads and static ports may be caused by:

1. In-flight icing
2. heavy rain
3. insects
4. volcanic ash
5. any of the above