Icing and Autopilot Use
One of the sample questions for the Instrument Rating is:
A generally recommended practice for autopilot usage during cruise flight in icing conditions is
A) keeping the autopilot engaged while monitoring the system.
B) periodically disengaging the autopilot and hand flying the airplane.
C) periodically disengaging and immediately reengaging the altitude hold function.
This is an example of Knowledge Test questions where none of the answers is satisfactory. What has been left out is whether the aircraft is certified for known ice, the degree of icing, and whether it is day or night. I searched through FAA publications to see what they have to say about icing and autopilot use and none of the documents clearly support any of the answers. I’ve quoted the relevant passages below but my best guess is that the answer to the question is B.
The autopilot can reduce the workload of a pilot, but in icing conditions, it can mask the effects of icing until it can no longer cope with them and then it disengages. When this happens, the pilot can be caught off guard with an airplane in an extreme nose-up trim condition. A stall-spin condition can easily develop. Using the autopilot to level the wings and give headings will reduce the pilot’s workload without causing inadvertent nose high attitude and potential stall conditions. By controlling the pitch manually, the pilot will be able to detect decreased performance of the aircraft and react accordingly.
AC 60-22 Aeronautical Decision Making
3. CONVENTIONAL DECISION MAKING.
a. In conventional decision making, the need for a decision is triggered by recognition that something has changed or an expected change did not occur. Recognition of the change, or non-change, in the situation is a vital step in any decision making process. Not noticing the change in the situation can lead directly to a mishap. The change indicates that an appropriate response or action is necessary in order to modify the situation (or, at least, one of the elements that comprise it) and bring about a desired new situation. Therefore, situational awareness is the key to successful and safe decision making.
FAA-H-8083-9A Aviation Instructor’s Handbook
Highly reliable automation has been shown to induce overconfidence and complacency. This can result in a pilot following the instructions of the automation even when common sense suggests otherwise. If the pilot assumes the autopilot is doing its job, he or she does not crosscheck the instruments or the aircraft’s position frequently. If the autopilot fails, the pilot may not be mentally prepared to fly the aircraft manually
AC 91-74B Flight In Icing Conditions
5-6. TAKEOFF AND CLIMBOUT
b. Ice Accumulation. Airplanes are vulnerable to ice accumulation during the initial climbout in icing conditions because lower speeds often translate into a higher Angle of Attack (AOA). This exposes the underside of the airplane and its wings to the icing conditions and allows ice to accumulate further aft than it would in cruise flight. At rotation and climbout, some aircraft occasionally are susceptible to stall warning horn activation in icing. Pilot awareness of this hazard in his or her particular aircraft is important to maintain situational awareness.
c. Vigilance. Consequently, any ice that forms may be out of the pilot’s view and go undetected. Extreme vigilance should be exercised while climbing with the autopilot engaged. Climbing in Vertical Speed (VS) mode in icing conditions is highly discouraged.
d. Monitor Airspeed. When climbing with the autopilot engaged in the vertical speed mode, ice accretion will result in a loss of climb performance. If the vertical speed is not reduced, the autopilot will maintain the rate until stall. It is critical that the pilot monitor airspeed to assure that the aircraft maintains at least the minimum flight speed for the configuration and environmental conditions.
FAA H 8083-15 Airplane Flying Handbook
Unless otherwise recommended in the AFM/POH, the autopilot should not be used in icing conditions. Continuous use of the autopilot masks trim and handling changes that occur with ice accumulation. Without this control feedback, the pilot may not be aware of ice accumulation building to hazardous levels. The autopilot suddenly disconnects when it reaches design limits, and the pilot may find the airplane has assumed unsatisfactory handling characteristics.
AC 61-67C Stall and Spin Awareness
102 a In some icing conditions there are adverse changes to the stall speed, stall characteristics, performance, and handling characteristics of the airplane. These adverse changes are potentially hazardous for several reasons. First, aerodynamic stall may occur with little or none of the usual cues in advance of the stall or at the occurrence of stall. These cues include airframe or control surface buffet, reduced control effectiveness, and activation of the stall warning horn, stick shaker, and stick pusher. Next, because of high drag on unprotected surfaces and residual ice on protected surfaces, there may be insufficient power or thrust to increase speed while holding constant altitude to reduce the AOA. Finally, post stall recovery of a contaminated airplane may be complicated by gross changes in control effectiveness, airplane response characteristics, and abnormal control forces. As a result of these factors, large losses in altitude can occur during recovery.
102 c. Further complications involve use of the autopilot. The autopilot may apply control inputs that will mask detection of some of these tactile cues by the pilot or attempt to control the airplane in the stall. Sudden autopilot self-disconnect with control surfaces trimmed into extreme positions or with controls trimmed into uncoordinated flight will complicate post stall recovery and may lead to a spin or spiral.
No Mention Of Autopilot Use In Icing
AIM
FAA_H_8083-15B Instrument Flying Handbook 2012
FAA-H-8083-16 Instrument Procedures Handbook
FAA-H-8083-25B Pilots Handbook of Aeronautical Knowledge