Plane & Pilot
Tuesday, October 8, 2013

Your Best Friend Or Worst Enemy

How well do you know your autopilot?

IAS. Indicated airspeed hold mode tells the autopilot to adjust pitch to hold the current indicated airspeed. In many systems, the pitch up/down control switch will allow target IAS to be changed. For a climb, select the proper speed and set climb power with the throttle. To descend, disengage altitude hold, trim for a descent, pull the power back and as the aircraft reaches the desired speed, engage IAS. Power can then be used to control the descent rate. IAS is particularly useful for making a climb.

FLC. Flight Level Change mode is a "smart" version of IAS mode. Its primary goal is to hold IAS; however, during a climb, the system will also try to hold altitude (up to a limit) while keeping airspeeds within limits. This is particularly useful when making a climb through thin air, where IAS may decrease considerably during the climb, or when turbulence may induce a sink. In the airline world, FLC mode may also be linked to auto-throttle systems to help manage airspeed.

CWS. Control wheel steering is activated by a push button on the yoke that temporarily disconnect the AP for as long as the button is pushed. In many systems, CWS also synchronizes the FD to the current flight attitude, although the behavior will depend on the current navigation mode, so check your manual.

GA. Go-Around mode disconnects the autopilot and displays a wing's level, preselected climb attitude on the flight director bars to help maintain situational awareness while hand flying during a missed approach. Once an acceptable altitude is reached, the AP can be re-engaged. Remember that, in a pinch, the GA button is another way to disconnect the autopilot.

YD. If you have it, the YD button toggles yaw damping on and off. Most AP systems come on line with the YD engaged. Some YD systems are very simple and just hold the rudder at its current position while actively damping yaw excursions. In these systems, the pilot must first set rudder trim for zero slip and then engage the yaw damper. More sophisticated systems will actively center the ball for zero slip and damp bumps in yaw.

Some Basic Do's and Don'ts
First, don't ever try to manually overpower any autopilot when it's connected. Fighting the yoke with the AP engaged can cause the system to react by running the pitch trim in the opposite direction, and even though the AP is connected to the control system through a clutch so that it can (in principle) be overpowered, the force needed to control the aircraft in a completely out-of-trim situation can be completely overwhelming. Try it in a simulator, and you'll quickly understand how difficult it is to safely operate the aircraft with 30, or more, pounds of force pulling on the yoke. NTSB records show that many folks who ignore this rule leave a smoking hole in the ground, so don't do it. If something goes wrong or you don't like what the AP is doing, disconnect it—never fight it.

Do know how to quickly disconnect your autopilot. Most airplanes have at least four or five ways to disconnect the AP. The main AP power switch, the big red button on the yoke, the electric trim switch, the CWS button, AFC or servo circuit breakers, the GA button, emergency bus switches, or even the master switch are all possible ways to disconnect AFCS systems. Study your system and put colored caps on your CBs, so that you can quickly find the important ones. Some of these options may disable electric trim (if you have it), so understand when you have to revert to manual trim. If you want to hand fly, bumping the electric pitch trim switch disconnects the AP on most systems while leaving the flight director active.

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