Plane & Pilot
Tuesday, October 8, 2013

Your Best Friend Or Worst Enemy

How well do you know your autopilot?

BC/REV. Back-course or reverse sensing is used to fly an approach using the back-course signal of an ILS. In most systems, this switch simply sets up reverse sensing so that you can set your HSI to the inbound course, and the autopilot can track the reversed navigation signal. Back-course setup and operation often varies considerably between systems, so be sure to check your manuals.

Vertical Navigation Modes
Many AP systems allow flight control in the vertical direction, so let's look at how those modes work.

Pitch Mode. Holding a constant pitch angle is the "wake-up" mode when engaging many autopilots. Most AP systems will engage to hold the current pitch angle shown on the AI, so if you're level, that's what will be held. The pitch angle is normally adjustable through a preset range—typically about +/- 10-15 degrees. Most systems revert to pitch mode when altitude hold or vertical descent modes are canceled. Some simple systems are set to simply hold a level pitch angle. Pitch mode is generally a safe option for climbing because it's unlikely to cause a stall.

ALT. Altitude hold mode does what it says—holding the current altitude shown on the altimeter. Altitude hold goes active on systems with altitude capture capability when the altitude is the same as set on the altitude preselect controller. Most altitude hold systems have a safety G-switch to release the autopilot in rough air. Autopilot disconnect is usually signaled by a blinking light and an audible warning. A few systems also include a switch that determines how hard the autopilot will try to hold altitude in turbulence. This kind of switch might be labeled "soft ride" (e.g., King KFC 275 and 325 systems).

VS. Vertical Speed mode tells the AP to hold a preselected vertical speed. In systems with altitude capture, the vertical speed will be held until reaching the altitude set and armed on the altitude pre-selector panel. There are normally upper and lower limits on what VS values you can select. Great care is needed when using VS mode to make a long climb since it's possible to run out of climb capability at higher altitudes where a stall can occur (IAS, FLC, or Pitch modes are preferred).

VNV. Vertical Navigation mode commands the autopilot to follow a descent profile generated by an FMS (flight management system) or GPS navigator. Although most systems work only for a descent, some more capable systems allow both climb and decent profiles to be followed.

GS. Glideslope mode allows the aircraft to track a glideslope beacon or glide path (GP) signal—most commonly from a WAAS navigator. All systems have rules about capturing the GS/GP signal and most require that the signal be captured from below, while in ALT mode. If you are vectored onto an approach above the GS (or GP), you must descend to a point below the GS/GP and then level off to capture the signal. Make sure that the GS light goes "active" as you capture the glideslope. There's something seriously wrong if you're descending below the GS without an active GS indication. Know the rules for your system and practice operations in good weather before you head into the clag.

Other Common Modes
Not all airplanes have these features, but here are a few additional capabilities found on more advanced systems.


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