Inadvertent Flight into Instrument Meteorological Conditions (IMC)
Proper flight planning using available weather resources should allow a pilot to avoid flying when the probability of low visibility is high. It is expected that WSC pilots exercise good judgment and not attempt to fly when the visibility is questionable. However, this section is included as background for this emergency procedure for inadvertent flight into instrument meteorological conditions (IMC), flight without visual reference to the horizon.
Although it is possible to get an attitude indicator installed in a WSC aircraft, there are no training requirements for flying by instruments for sport or private pilot WSC ratings. Samples of these instruments are shown in Figures 13-9 and 13-10.
Sport pilots are not allowed to fly unless there is visual reference to the surface and three miles visibility. This is different for private pilots for whom there is not a requirement for visual reference to the ground and the minimum flight visibility is only one statute mile (SM).
Accident statistics show that the average airplane pilot who has not been trained in attitude instrument flying, or one whose instrument skills have eroded, will lose control of the aircraft in about 10 minutes once forced to rely solely on instrument reference. WSC pilots without any instrument training attempting to use instruments in IMC conditions would lose control much sooner. No WSC pilot should attempt flight into IMC conditions.
The purpose of this section is to provide guidance on practical emergency measures to maintain aircraft control in the event a VFR pilot encounters IMC conditions. The main goal is not instrument flying; it is to help the VFR pilot keep the aircraft under adequate control until suitable visual references are regained.
The first steps necessary for surviving an encounter with IMC by a VFR pilot are:
- Recognition and acceptance of the gravity of the situation and the need for immediate remedial action.
- Maintaining control of the aircraft.
- Obtaining the appropriate assistance in getting the aircraft out of IMC conditions.
A VFR pilot is in IMC conditions anytime he or she is unable to maintain aircraft attitude control by visual reference to the natural horizon, regardless of the circumstances or the prevailing weather conditions. Additionally, the VFR pilot is in IMC any time he or she is inadvertently or intentionally and for an indeterminate period of time unable to navigate or establish geographical position by visual reference to landmarks on the surface. These situations must be accepted by the pilot involved as a genuine emergency requiring immediate action.
As discussed earlier, when entering conditions in which visibility is decreasing or IMC, the pilot should turn around, climb, or descend immediately and return to where ground visibility is known. Do not continue assuming that conditions will clear and visibility will be regained.
Maintaining Aircraft Control
Once the pilot recognizes and accepts the situation, he or she must understand that the only way to control the aircraft safely is by using and trusting the flight instruments. Attempts to control the aircraft partially by reference to flight instruments while searching outside the flight deck for visual confirmation of the information provided by those instruments results in inadequate aircraft control. This may be followed by spatial disorientation and complete loss of control.
The most important point to be stressed is that the pilot must not panic. Recognize the situation and take immediate action. The task at hand may seem overwhelming, and the situation may be compounded by extreme apprehension. The pilot must make a conscious effort to relax and understand that the only concern at this point is to fly toward known visibility. If climbing into a cloud, reduce throttle and descend. If descending into a cloud, increase throttle and climb out of the cloud. If visibility is suddenly lost (e.g. flying into a cloud), turn 180° and fly toward known visibility.
The pilot should remember that a person cannot feel control pressures with a tight grip on the controls. Relaxing and learning to control with the eyes and the brain instead of muscles usually takes considerable conscious effort.
The pilot must believe that the flight instruments show the aircraft’s pitch attitude and direction regardless of what the natural senses tell. The vestibular sense (motion sensing by the inner ear) can confuse the pilot. Because of inertia, the sensory areas of the inner ear cannot detect slight changes in aircraft attitude nor can they accurately sense attitude changes which occur at a uniform rate over a period of time. On the other hand, false sensations are often generated, leading the pilot to believe the pitch attitude or direction attitude of the aircraft has changed when, in fact, it has not. These false sensations result in the pilot experiencing spatial disorientation.
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 flight 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:
- Fly at the normal trim speed. To climb, increase throttle; to descend, decrease throttle. To fly level, fly at the throttle setting that provides level flight. The vertical speed indicator or altimeter provides information regarding pitch attitude.
- Resist the tendency to overcontrol the aircraft. Fly with fingertip control. No attitude changes should be made unless the flight instruments indicate a definite need for a change.
- Make all attitude changes smooth and small, yet with positive pressure.
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 flying 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 flying 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:
- The normal tendency of the pilot to overcontrol, leading to steep banks.
- The inability of the pilot to cope with the instability resulting from the turn.
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 flying 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 flight. 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 flight 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 flying at trim speed should need ever arise.