Feb 14, 2020
All pilots must learn how to understand and interpret aircraft instruments in order to fly safely. These instruments are vital to the operation of the aircraft, helping the pilot maintain control and identify any potential issues at a glance.
Understanding the way these instruments work means the pilot is able to recognise when the equipment is malfunctioning, avoiding unnecessary mistakes during flight and on take-off and landing. Read on for more information on aircraft flight instruments and how they work.
What Are Aircraft Instruments?
Aircraft instruments are the sometimes confronting array of dials, gauges and gadgets located in the cockpit of an aircraft. Pilots rely on these instruments to understand where the plane is, how fast it is travelling and what it is doing as well as a large amount of other information.
There are four basic kinds of aircraft instruments grouped according to the job they perform. These are flight instruments, engine instruments, navigation instruments and miscellaneous position/condition instruments.
These are the instruments that give information on the aircraft’s flight attitude (orientation relative to the horizontal plane). Examples are the Altimeter, the Airspeed Indicator, and the Heading Indicator, the Attitude Indicator (artificial horizon), Turn Coordinator, and Vertical Speed Indicator.
These are instruments designed to constantly measure operating parameters relating to the aircraft’s engine(s). Examples are tachometers, temperature gauges, fuel and oil quantity displays, and engine pressure gauges.
These instruments provide guidance information to enable the aircraft to follow its intended path. Examples include various kinds of navigational devices ranging from the simple compass and radiolocation to GPS location devices.
Miscellaneous Position/Condition Instruments
This category covers a range of miscellaneous gauges and indicators not included in the first three groups that provide data on positions of moveable components on the aircraft, and the condition of various aircraft components or systems. Examples include cabin environment (pressure, temperatures etc.) flight control position, and auxiliary power units etc.
The Original Aviation 6 Pack
Sometimes referred to as the ‘aviation six pack’, these are the basic 6 ‘Flight Instruments’ that are found in almost every aircraft in some way, shape or form – whether as individual instruments or merged together as part of the newer ‘glass cockpit’ technology. For more in-depth information on the altimeter and the airspeed indicator see below.
- Airspeed Indicator
- Vertical Speed Indicator
- Attitude Indicator
- Heading Indicator
- Turn Coordinator
These basic Flight Instruments can be further classified as:
- Pitot-Static Systems. Using air pressure differences, namely ambient air pressure affected (pitot pressure) and unaffected (static pressure), to determine flight parameters such as the speed and altitude of the aircraft.
- Gyroscopic Instruments. Using gyroscopic principles to provide information on the attitude of the aircraft during flight (the orientation of the aircraft in relation to its surroundings).
An Altimeter displays the aircraft’s current height above sea level (not ground level). A traditional Altimeter has three hands measuring hundreds, thousands, and ten thousands of feet. These three hands move at different speeds and when the readings are added together they give an indication of the aircrafts current altitude.
5 Types Of Altitudes
- Indicated Altitude. The altitude indicated on the altimeter when the correct barometric pressure is set.
- True Altitude. Height above sea level (MSL).
- Absolute Altitude. Height above ground level (AGL).
- Pressure Altitude. The altitude indicated on the altimeter based on a ‘standard atmospheric level’, this is sometimes used in flight planning calculations.
- Density Altitude. This is the Pressure Altitude adjusted for temperature variations (density altitude affects aircraft performance).
How Does An Altimeter Work?
The Altimeter’s readings are based on barometric pressure, however due to the constantly changing nature of barometric pressure the altimeter needs to be pre-set prior to, and also during every flight as the barometric pressure changes.
As a very basic description, the altimeter works by utilising a static port on the outside of the aircraft, increases and decreases in altitude cause the device to expand and contract altering the reading on the gauge. This information is used in conjunction with the pre-set barometric pressure to provide a more accurate altitude reading.
5 Common Errors Associated With Altimeters
- Inconsistent Airflow. Interrupted airflow to the external static port during flight can cause the altimeter to give inaccurate readings. This is commonly associated with gusty wind conditions, or during certain manoeuvres.
- Elasticity. The continual expansion and contraction of the altimeter’s operating parts during normal use can result in the parts losing some of their rigidity, becoming naturally more flexible resulting in inaccurate readings.
- Pilot Error. The correct barometric pressure must be entered into the altimeter in order for it to give accurate results. Pilot error is one of the most common reasons altimeters fail to give accurate readings; a difference of 1″ Hg can cause an altitude deviation of 1,000 feet.
- Air Density. The density of air alters from one area to the next, just as it does on the ground. Errors in altimeter readings over long flights are commonly associated with changes in air density.
- Static Port Blockages. Something blocking the external static port would obviously prevent the altimeter from detecting and changes to altitude.
Airspeed Indicator (ASI)
The Airspeed Indicator (ASI) is classified as a Pitot Static System, it measures the speed of the aircraft as it moves through the air using air pressure differences from both a static port and a pitot tube. A traditional ASI has graduated numbers over a round dial with a single clock-like hand indicating the aircrafts current speed. This measurement is usually given in knots (Nautical Miles per Hour) but sometimes in other forms such as kilometres per hour.
4 Types Of Airspeeds
- Indicated Airspeed (IAS). The Airspeed Indicator reading without any consideration for atmospheric conditions or potential installation and instrument errors. The Indicated Airspeed is used to give the manufacturers recommendations for aircraft performance indications relating to take off, landing, and stall speeds.
- Calibrated Airspeed (CAS). The Indicated Airspeed corrected for installation error and instrument error. Under certain operating conditions installation and instrument errors may total several knots.
- True Airspeed (TAS). The Calibrated Airspeed corrected for altitude related atmospheric conditions such as temperature variations and air density. The True Airspeed is used for flight planning calculations.
- Groundspeed (GS). The aircraft’s actual speed over the ground, or the True Airspeed adjusted for wind resistance factors (headwind, tailwind etc.).
How Does An Airspeed Indicator Work?
Utilising both the static and pitot systems on an aircraft, the ASI takes into account the airflow and equalising pressure differences from an external pitot tube and static port to provide speed indications during flight.
While on the ground the Airspeed Indicator will show a reading of zero as the pressures are equal, when airborne, air entering the external pitot tube places pressure on an internal diaphragm causing the Airspeed Indicator to move upwards.
2 Common Errors Associated With Airspeed Indicators
- Static Port Blockages. Debris, insects, water or ice blocking the external static port prevents the Airspeed Indicator from giving a correct reading as air is unable to enter the port. If the static system is blocked but the pitot tube remains clear, it is important to note that the Airspeed Indicator will continues to operate but it will be giving you inaccurate readings.
- Pitot Tube Blockages. As with above, any debris or blockages to the external pitot tube will result in incorrect readings.