December 1, 2012
This Garmin G1000-equipped aircraft is in heading- and altitude-hold modes, armed to track localizer and glideslope. Autopilot and yaw damper are engaged, and the autopilot is looking at the left PFD (green arrow) for information.
The autopilots in modern light jets are technological wonders. Capable of coupling to both lateral and vertical guidance, they offer an array of modes and sub-modes for each axis. This very capability, however, makes it critically important for pilots to be hyper-aware of what exactly the auto-pilot is trying to do with the airplane, as well as what the autopilot is planning on doing next.
Pilots have a long track record of thinking they’ve asked the autopilot to do one thing, while in fact it’s doing something totally different. “What’s it doing now?” is a question heard so frequently in modern automated cockpits that it’s become a punch line at training centers.
Consider the pilot who pushes the button next to the intended button on the autopilot control panel. Or the one who pushes the correct button, but inadvertently pushes it twice—causing a very different mode to become active. Even with correct button pushing, autopilots automatically pass through several mode changes during certain phases of flight, and changes made to the state of the aircraft during these transitions often may not occur as planned.
To help pilots keep track of the automation, the primary flight displays (PFD) of many light jets give the prime real estate directly above the attitude indicator to a box called the status bar, or informally, the “scoreboard.” The status bar displays the basic activation state of the autopilot, as well as what lateral and vertical modes are currently being used by the autopilot, and what modes are armed. An armed mode is one awaiting a specific trigger to become active—reaching a given altitude, altitude hold mode may be triggered, or intercepting the final approach course, localizer tracking will engage.
Text color and positioning on the status bar tells the pilot which modes are active versus armed. Garmin-based avionics place the active modes closest to the screen center, with the armed modes further to the edges of the screen. Collins systems place the active modes on the top line of the status bar, with armed modes beneath. Green is commonly used to indicate an active mode, while white indicates modes that are armed.
In the status bar shown below, for example, the autopilot is currently in an active lateral mode of heading hold (HDG)—any change of the heading bug by the pilot will result in the aircraft turning. Localizer tracking (LOC) is armed; once the localizer needle is nearly centered, the mode will automatically switch from heading hold. Vertically the autopilot is holding altitude (ALT), while awaiting glideslope needle centering to switch to glideslope tracking mode (GS).
The importance of including the status bar in the primary scan cannot be overstated. A pilot’s first action after pressing any button on the autopilot control panel should be to confirm the status bar now shows the mode requested, in armed or active status, as appropriate. Designated examiners often cite a lack of confirming mode changes in the status bar as contributing to a loss of mode control—and subsequent checkride failures.
The autopilot is a fantastic servant and a terrible master, so ensuring it is following orders is a top job for the PIC.
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