Autopilot.  An aircraft flight control system that automatically manipulates the roll, pitch, and, in some cases, the yaw control surfaces of the airplane to capture and track the route programmed into the FMS/RNAV, or altitudes, vertical speeds, headings, and courses selected by the pilot. 

Autopilot flight mode annunciator.  A display that presents the names of autopilot functions that are either armed or engaged. It is the only reliable source of information about what autopilot functions are in use. 

Autothrottle system. A system that automatically manipulates the thrust setting of the airplane to help follow the vertical trajectory portion or selected airspeed of the planned flight route. 

Autotrim system. The system that automatically adjusts the pitch trim of the airplane in response to trim commands generated by the autopilot. 

Bottom-of-descent point. The end point of the descent, as calculated by the FMS/RNAV. 

Broadcast weather service.  A weather service that prepares weather products and transmits them to participating aircraft, also known as a data link weather service. 

Chapter. Associated group of electronic “pages” of information from databases found in FMS and GPS RNAV units similar in contents, such as airports, VORs, software/ unit settings, and feature selections. 

Command bars.  A flight director display that presents roll and pitch instructions (generally, V‑shaped visual cues) to help the pilot maintain the flightpath/flight track to the selected point. The pilot keeps the airplane symbol aligned with the command bars on the flight director, or centered on the FD crossbars (e.g., older Cessna units). 

Crossing restriction. A directive issued or published by air traffic control that instructs the pilot to cross a given waypoint at a specified altitude, and sometimes at a specified airspeed. 

Cursor mode. The function offered by the FMS/RNAV that allows data entry into an avionics unit such as the FMS and RNAV. 

Data link weather service.  See broadcast weather service. 

Deceleration segment. A planned portion of a descent designed to permit the aircraft to slow to meet a terminal area speed restriction, crossing restriction, or other speed restriction. 

Desired track. The great circle course computed by the FMS/RNAV, it goes from the past waypoint to the next (active) waypoint. 

Distance measuring equipment (DME). Line‑of‑sight limited airborne equipment (transceiver) using paired pulse replies from ground‑based transponder to determine slant range distance by time between airborne transmission of pulses and return of pulses from the ground transponder. 

DME.  See distance measuring equipment. 

EHSI.  See electronic horizontal situation indicator. 

Electronic flight instruments.  Flight instruments that use electronic devices to prepare and/or present information such as airspeed, attitude, altitude, and position. 

Electronic horizontal situation indicator (EHSI). Electronically generated HSI display, either CRT or LCD type, indicating all standard HSI functions on a video screen instead of using mechanical components. 

eLORAN.  See long range navigation. 

Engaged. A system mode or function that is actively performing its function. 

Error-evident display.  Any display that presents information in a way that makes errors more obvious and detectable. 

FD.  See flight director. 

Faildown. The substitute display or backup instrument mode available if the primary component fails. In some systems, for example, the MFD can substitute for the PFD if the PFD fails. The PFD information “fails down” to the MFD. In other systems, the substitute for the PFD might be the conventional standby instruments and the standby or secondary navigation CDI. 

Flight director. Electronic flight calculator that analyzes the navigation selections, signals, and aircraft parameters. It presents steering instructions on the flight display as command bars or crossbars for the pilot to position the nose of the aircraft over or follow. 

Flight management system (FMS). A computer system containing a database to allow programming of routes, approaches, and departures that can supply navigation data to the flight director/autopilot from various sources, and can calculate flight data such as fuel consumption, time remaining, possible range, and other values. 

