|CHAPTER 13. Abnormal and Emergency Procedures
Attitude is defined as “The position of an aircraft as determined by the relationship of its axes and a reference, usually the earth’s horizon.” For WSC, the pitch and the roll are the relevant attitudes.
Most aircraft are generally, by design, inherently stable platforms and, except in turbulent air, maintain approximately straight-and-level fl ight if properly trimmed and left alone. They are designed to maintain a state of equilibrium in pitch, roll, and yaw. The pilot must be aware, however, that a change about one axis affects the other axes. The WSC aircraft is stable in the yaw and pitch axes, but less stable in the roll axis. The yaw and pitch axes of the WSC are easy to control, but the roll axis is the challenge for WSC aircraft control in IMC. The key to emergency aircraft attitude and directional control, therefore, is to:
The primary instrument for roll control is the attitude indicator if so equipped. [Figures 13-9 and 13-10]
For aircraft not equipped with an attitude indicator, a magnetic compass [Figure 13-11] or a GPS [Figure 13-12] are the instruments that can be used for roll control. The compass stays stationary and the WSC aircraft rotates around the compass dial. A pilot is fl ying wings level if the compass heading is not changing. If the compass is changing direction, the aircraft is banked into a turn. Similarly, the GPS provides ground track. If fl ying wings level, the GPS ground track is steady. If the GPS ground track is changing, the aircraft is in a bank and turning.
Turns are perhaps the most potentially dangerous maneuver for the untrained instrument pilot for two reasons:
As an example, a 180° turn would be the most likely turn to exit a cloud and return to where there is visibility with the surface. The direction the turn started should be noted in order to determine the direction needed to exit the IMC conditions. For example, if heading North when fl ying into the cloud, turn 180° and head South to exit the cloud.
When a turn must be made, the pilot should anticipate and cope with the relative instability of the roll axis. The smallest practical bank angle should be used—in any case no more than 10° bank angle. [Figure 13-13] A shallow bank takes very little vertical lift from the wings, resulting in little if any deviation in altitude, and the WSC aircraft can continue to be fl own at trim speed. It may be helpful to turn 90° and then reduce the bank and return to level fl ight. This process may relieve the progressive overbanking that often results from prolonged turns. Repeat the process twice until heading in the opposite direction of entry in order to exit. Once on the proper heading to exit the IMC conditions, maintain this heading until obtaining visual reference with the surface.
Turns with a magnetic compass or a GPS would be similar but the only indication of bank angle is the rate at which the compass or GPS is rotating. The rotation should be slow and steady and not increase in speed. Any increase in compass or GPS rotation should be slowed by decreasing the bank back to level fl ight to avoid increasing the bank. Practicing gentle turns and observing the rotational speed of the compass and GPS under VFR conditions will help a pilot recognize an acceptable rotational speed fl ying at trim speed should need ever arise.
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