Molly Ayala
Flight instruments are vital to the safe operation of an aircraft, providing the pilot with the information they need to maintain control and identify potential issues. Most aircraft are equipped with a standard set of flight instruments that fall into four basic categories: pitot-static systems, compass systems, gyroscopic instruments, and engine instruments. Some simple single-engine aircraft do not have an electrical system, but most aircraft built since the 1940s have adopted a standardized pattern of flight instruments, known as the
| Characteristics | Values |
|---|---|
| Types of Instruments | Altimeter, Airspeed Indicator, Heading Indicator, Attitude Indicator, Turn Coordinator, Vertical Speed Indicator, Tachometers, Temperature Gauges, Fuel and Oil Quantity Displays, Engine Pressure Gauges |
| Power Source | Direct Current (DC) or Alternating Current (AC) |
| Backup Power | Batteries or Ground Power Unit (GPU) |
| Function | Provide information on aircraft's in-flight performance, help pilots maintain control and identify potential issues |
| Gyroscopic Instruments | AI, HI, and Turn Indicator |
| Example of Electrical Malfunction | Loss of display on EFIS, electrical fumes, loss of significant electrical system functionality |
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What You'll Learn
The 'six pack'
The "six pack" is a term used to refer to the six main flight instruments in a cockpit. These instruments are crucial for pilots, providing essential data for navigating the skies and helping pilots maintain proper orientation. While the specific instruments in the six pack can vary, they typically include a combination of pitot-static and gyroscopic instruments.
The pitot-static instruments include the Airspeed Indicator (ASI), Altimeter, and Vertical Speed Indicator (VSI). These instruments draw information from the aircraft's static and dynamic air pressures. The ASI, for example, measures the aircraft's speed by comparing ram air pressure from the pitot tube and static air pressure from the static port. The Altimeter provides altitude readings, and the VSI helps manage ascent or descent rates.
The gyroscopic instruments, on the other hand, include the Attitude Indicator (AI), Heading Indicator (HI), and Turn Coordinator. These instruments rely on spinning gyroscopes to determine the aircraft's orientation, direction, and turning. The AI, for instance, uses a gyro to display the aircraft's attitude relative to the horizon, helping pilots maintain spatial orientation. The HI uses a rotating gyro to inform the pilot of the aircraft's direction, similar to a compass.
While not all six pack instruments are electrically powered, some, like the gyroscopic instruments, can be powered by electricity. These instruments may also be powered by vacuum or pressure. Modern digital avionics displays, which can replace traditional six pack instruments, typically require electrical power.
Understanding and effectively utilising the six pack instruments is essential for pilots, both during training and in real-world scenarios. By interpreting the data provided by these instruments, pilots can ensure safer and more efficient flights.
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Gyroscopic instruments
Gyroscopes within these instruments are usually electrically or vacuum-driven. In an electrical system, a gyro is powered by an electric motor, which is itself powered by the battery and alternator. In the case of a failure of the alternator, the gyroscopic instruments will work off the battery for a limited time. In vacuum-driven systems, a vacuum pump, driven by the engine, reduces the pressure within the instrument case. However, vacuum failure can be fatal, and vacuum pumps are susceptible to under-reading due to rotor deceleration if the vacuum pressure drops.
Gyroscopes are used in attitude indicators, heading indicators, and turn coordinators. The attitude indicator, also known as the artificial horizon, uses a gyro to display the aircraft's attitude relative to the horizon. The heading indicator uses a rotating gyro to show the direction the aircraft is flying in, similar to a magnetic compass. The turn coordinator uses gyroscope precession to provide a visual depiction of the direction and rate of heading change, as well as any slipping or skidding during a turn.
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Electric starters
The electric starter motor is powered by an onboard battery, ground electrical supply, or both, and is operated by a key or switch in the cockpit. There are two types of electric starter motors: direct-cranking and starter-generator systems. Direct-cranking systems disengage as internal combustion engines, while starter-generator systems are permanently engaged.
The introduction of electric starters came with some challenges, including increased weight and complexity. Oil dilution systems and engine pre-heaters were developed to address the challenges of cold ambient conditions, which caused high load on the starting system due to viscous engine oil and the reluctance of fuel to vaporize and combust at low temperatures.
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Electrical system failures
The first step is to identify the problem and its severity. This can include resetting essential circuit breakers, cycling the alternator, and turning off all non-essential equipment to conserve energy. A dead alternator or generator means the battery becomes the only source of electrical power, so managing the electrical load is crucial to prolonging its lifespan.
Pilots should inform Air Traffic Control (ATC) about the electrical failure, declare an emergency, and request vectors to the nearest suitable airport. They may also need to make a final broadcast to ATC before powering down. It is important to leave the transponder on to receive heading and altitude information from controllers.
In Visual Flight Rules (VFR) weather conditions, an electrical failure is less problematic as pilots can rely on visual cues and basic instruments like a compass and airspeed indicator. However, in Instrument Meteorological Conditions (IMC), an electrical failure can be more challenging, especially at night, as pilots may lose access to critical systems such as runway lights, landing lights, and position lights.
In any case, backup navigation tools and handheld radios are essential to ensure communication and safe landing during electrical failures.
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Back-up power supplies
Aircraft electrical systems are self-contained networks of components that generate, transmit, distribute, utilise, and store electrical energy. They are present in almost all aircraft, although their complexity varies. Some very simple single-engine aircraft do not have an electrical system and are started using a flywheel and crank arrangement or by "hand-propping" the engine.
In the event of a complete electrical system failure, certain components such as standby flight instruments and aircraft emergency floor path illumination have their own backup power supplies. These backup power supplies ensure that essential instruments continue to function. The definition of "essential" depends on the type of flight (day VFR, night VFR, or IFR).
For example, in the event of a complete loss of electrical power in a Robin DR401, backup instruments such as the altitude, airspeed, and vertical speed indicators will still work as they don't require any power to operate. Additionally, the attitude indicator gyro is powered by a vacuum pump linked to the engine, so it would also remain operational.
In the case of a bus failure, which is typically caused by a failure of the power source supplying the bus rather than the bus itself, provisions for an alternate power source may allow the bus to be restored. Multiple layers of redundancy are built into aircraft electrical systems to greatly reduce the potential for a complete loss of all electrical generation capability.
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Frequently asked questions
The six pack refers to a set of six critical instruments that are installed in nearly all aircraft. This includes instruments that give information on the aircraft's in-flight performance, such as the Altimeter, the Airspeed Indicator, the Heading Indicator, the Attitude Indicator, the Turn Coordinator, and the Vertical Speed Indicator.
Most modern aircraft have electronic flight instrument systems. The Heading Indicator, for example, uses a rotating gyro to depict the direction the aircraft is flying in and is driven by suction from a vacuum pump or an electrical system. The Attitude Indicator, also known as the Artificial Horizon, is another instrument that may be powered by electricity.
In the event of an electrical fire or complete electrical system failure, there are backup power supply procedures in place. Standby Flight Instruments and Aircraft Emergency Floor Path Illumination, for instance, have their own backup power supplies.
