The term avionics is a combination of aviation and electronics and refers to the electronic systems onboard a helicopter.  These systems may be used for communications, navigation, and display / control of other systems onboard the aircraft.  For military helicopters threat detection and weapon firing capabilities may also be included.  A familiar example of helicopter avionics is a GPS system that determines the position of the helicopter and displays it on a moving map.  Other avionic systems are described below.

Communications

Various communication systems may be onboard a helicopter.  First, due to the high levels of aircraft noise, an intercom is normally installed to facilitate communication between passengers.  Each person wears a headset with speakers and a microphone.

Very high frequency (VHF) radio communication is used for communication between two different aircraft, or for communication with air traffic control.  High frequency (HF) radio and/or satellite communication systems may also be installed on helicopters for longer distance communication, beyond line of sight.  In some cases (normally military aircraft), the communication systems may be encrypted.

Monitoring

Modern helicopters include “glass cockpits” – displays that show information from various systems in the aircraft.  For example, a multi-function display (MFD) may include pictures / animations of older mechanical gauges like attitude indicators.  More importantly, MFDs can change what's displayed during flight.  For example, if the engines are nearing a temperature limit, the MFD can change to show this temperature.  This improves safety by increasing a pilot’s situational awareness.  There are so many systems on modern aircraft that a pilot cannot monitor them all simultaneously.  A glass cockpit helps a pilot focus on what's important.

In most helicopters these displays are LCDs.    While designed to show the most relevant information to the pilot, they often have menus allowing the pilot to manually request alternate information to be shown on demand.  In this way the pilot can have a "customized" view that suits his mission or style.  An example system that is important to monitor is the engine.  Measured gas temperature, gas generator speed, power turbine speed and torque will normally be displayed to a pilot.  All these values have limits that should not be exceeded, and the displays will often change to highlight or otherwise call attention to the most critical parameters at a given moment of time.  Values are often displayed as percentages of limits or percentages of nominal values rather than physical units.  For example, rotor speed is almost always displayed as a percent of normal, operating rotor speed.  This offloads pilots from having to convert numbers in their head while flying.  

Navigation

A helicopter may include various devices to help a pilot navigate.  Examples include GPS, INS, and radio-based navigation.  Modern helicopters will compute their location from these instruments and display it on a map with other locations of interest.  Helicopters equipped with autopilot systems may use this information to automatically maneuver the aircraft towards a target location.

Flight control system

Most large, modern helicopters include a stability augmentation system (SAS).  Such systems include gyros that detect pitch, roll or yaw disturbances and add a correction to the pilot’s control positions to counter them.  Furthermore, SAS enhances intended attitude changes when the pilot moves the controls.  The SAS may be turned on or off, typically via a PBA, by the pilot.

Some helicopters have an autopilot system.  These systems typically have an attitude hold mode (ATT) and possibly hover hold, altitude hold, speed hold, etc...  As the name implies, attitude hold maintains pitch, roll and yaw allowing a pilot to completely release the controls.  This differs from the SAS which only arrests the rate of attitude change, but does not return the attitude to a reference value.  The higher-level modes typically have options:  speed hold may use ground speed or airspeed, altitude hold may be based on pressure altitude or radar altitude (height above ground). 

A flight director system may also be onboard, which computes steering commands for navigation.  An autopilot system may optionally be coupled to the flight director to automatically maneuver the aircraft to target locations.

Collision-avoidance systems

Many large helicopters have traffic alert and collision avoidance systems (TCAS).   These systems use transponders (devices onboard all aircraft to receive and respond to radio signals) to locate other aircraft and warn the pilot.  

Other systems may be included to warn the pilot of terrain collisions.  Ground-proximity warning systems use radar altitude to detect when the aircraft is approaching the ground.  More sophisticated terrain awareness systems use digital maps of terrain height in conjunction with navigation equipment to foresee potential terrain collisions.

Health monitoring systems

Data from sensors onboard helicopters is increasingly saved for “health monitoring.”  The goal of health monitoring systems is to increase safety and affordability by repairing / replacing parts at more appropriate times.  Historically parts were repaired / replaced according to simple schedules.   For example, replace the gearbox after five years of operation.  However, more sophisticated analysis may reveal that a gearbox was lightly used and can actually survive ten years safely.  Health usage and monitoring systems (HUMS for short) analyze such data and provide improved maintenance schedules.  The HUMS analysis may be done at a ground station, in which case the helicopter has a method (sometimes a removable memory card) to export data.  For more information about HUMS see this link.

Putting it together

To achieve the functionality described above, several sensors, computers and software applications must be installed on a helicopter.  Some of these things must communicate and many outputs must eventually be routed to displays in the cockpit.  The systems that achieve this are referred to as the aircraft network and sometimes mission computer.  For military helicopters, the MIL-STD-1553 standard is often used for this communication.