Inadvertent VFR Flight Into IMC

A VFR pilot is in IMC anytime he or she cannot maintain airplane control by reference to the natural horizon.

Inadvertent IMC encounters can be caused by:

Improper preflight planning.
Unforecast weather conditions.
Poor judgment of deteriorating weather conditions.
Operating in marginal VFR conditions (scud running).

The steps necessary for surviving an encounter with IMC by a VFR pilot are:

Recognize and accept the seriousness of the situation and the need for immediate remedial action.
Maintain control of the airplane.
Obtain the appropriate assistance.
Exit IMC.

Recognizing an Entry into IMC

The first clue of deteriorating weather can often be the need to gradually reduce the cruising altitude to maintain VMC. At night, the first indication is usually the gradual disappearance of ground lights.

Establishing and Maintaining Control

The key to surviving an inadvertent IMC encounter is maintaining control of the aircraft.

Initial actions:

Do not panic; make a conscious effort to relax.
Trust what the flight instruments show, not what the senses tell.
Level the wings and center the ball/brick: good bank control has the effect of making pitch control much easier.
Stop the rate of climb or descent.

Actions to maintain attitude control:

Use the attitude indicator as the primary instrument for attitude control.
Trim the airplane with the elevator trim so that it maintains hands-off level flight at cruise airspeed.
Resist the tendency to over-control the airplane. 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.
Make use of any available aid in attitude control, such as the autopilot or wing leveler.
Deviate as little as possible from the level flight attitude to prevent disturbing the airplane’s equilibrium.
Avoid combined maneuvers (e.g., climbing while also turning).

Actions to avoid icing:

Turn the pitot heat and carburetor heat ON, if equipped and the conditions are conducive to icing.
Apply additional power to compensate for the reduced engine performance.

Obtaining Assistance

An inadvertent IMC encounter is an emergency. ATC should be contacted and an emergency declared as soon as possible. However, pilots must not let the tuning of radios or other tasks distract them from maintaining control of the airplane.

ATC can help by:

Moving other aircraft out of the way.
Identifying nearby terrain and obstructions.
Guiding airplanes back to VFR conditions.
Talking pilots through a descent or an instrument approach.

Exiting IMC

There are two basic ways to exit IMC:

180° Turn: An inadvertent entry into IMC is typically best resolved by making a 180° turn to fly back into VFR conditions.
Straight Descent: If conditions are unfavorable for making a 180° turn, a descent to VFR conditions may be appropriate.

180° Turn Procedure

Note the target heading to rollout on.
Decide on the direction to turn. Consider the proximity to terrain, obstructions, and airspace.
Establish a coordinated turn in the selected direction.
Keep the bank under 20° and maintain a level pitch attitude.
Rollout on the target heading and maintain a straight-and-level attitude until breaking out of clouds.

Straight Descent Procedure

Establish a straight-ahead, stabilized descent by gradually reducing power to set up a 500′ to 800′ FPM rate of descent.
Monitor the turn coordinator. Correct for deviations using rudder inputs alone.
Apply trim so that it is possible to briefly remove the hands from the flight controls without altering the flightpath.

Upon breaking out of clouds, resume normal cruise flight.

Sensory Systems for Maintaining Orientation

The body uses three integrated sensory systems to determine posture, orientation, and maintain balance: the visual, vestibular, and somatosensory.

The brain interprets signals from the sensory systems to provide the sense of kinesthesis (the ability to sense movement through the body) and proprioception (unconscious perception of movement and spatial orientation).

Visual System

During flight in visual conditions, the eyes are the major orientation source and usually prevail over false sensations from other sensory systems. When these visual cues are removed, false sensations can cause a pilot to become disoriented quickly.

Vestibular System

The vestibular system in the inner ear helps identify rotational movements (pitch, roll, and yaw) and linear acceleration (horizontal and vertical).

Semicircular Canals: The inner ear consists of three semicircular canals. They are positioned to detect rotational movement in three axes, corresponding to pitch, roll, and yaw in an airplane.

Each canal is filled with fluid and has a section full of fine hairs. Rotation of the inner ear in any direction causes the tiny hairs to deflect, which stimulates nerve impulses, sending messages to the brain.

Otolith Organs: Each ear also has two other organs, called otolith organs, mounted at right angles. The utricle is mounted horizontally, and the saccule is mounted vertically.

Each otolith organ consists of a gel-like fluid full of fine hairs. On top of the gel are otoliths, which act as weights to pull on the gel during acceleration. The gel’s movement deflects the hairs, sending signals to the brain through nerves.

Illusions: Usually, sensations from the vestibular system are interpreted accurately in flight, but without visual references outside the aircraft, many situations create convincing illusions that are difficult to overcome.

Somatosensory System

The somatosensory system sends signals from the skin, joints, and muscles to the brain that are interpreted in relation to the Earth’s gravitational pull. These signals determine posture. “Seat of the pants” flying is largely dependent upon these signals.

These sensations can be reliable when used with visual and vestibular clues. However, the body cannot distinguish between acceleration forces due to gravity and those resulting from maneuvering the aircraft, leading to sensory illusions and false impressions of an aircraft’s orientation and movement.

Vertigo

Vertigo is a sensation of feeling dizzy or off-balance that can create or increase illusions in flight. It is often triggered by a change in the position of the head (e.g., Coriolis illusion). In many cases, it goes away without any medical treatment. Inner ear problems, such as an ear infection or disease, can also cause or increase a person’s sensitivity to vertigo.

Spatial Disorientation

Spatial disorientation is a state characterized by a false sense of one’s position and motion relative to the plane of the Earth’s surface. When it occurs, pilots cannot see, believe, interpret, or prove the information derived from their flight instruments. Instead, they rely on the false information that their senses provide.

Weather Planning

Aviation weather planning is a cycle:

Start with an overview (“get the big picture”).
Receive a formal briefing to make a final go/no-go decision.
Acquire updates in flight and consider actual versus forecast weather encountered.

Go/No-Go Decision-Making Factors

Pilot: Personal minimums, proficiency, currency, and physical and psychological readiness
Aircraft: Operating limitations and installed equipment including datalink weather
Environment:
Hazardous weather conditions, including icing and turbulence
“VFR not recommended” by an aviation weather specialist [VFR]

Lack of alternate airports where the weather is good [IFR]

The easiest point to cancel a flight is before you leave the flight planning room.

Updating Weather Briefings in Flight

Pilots can update weather briefings in flight by contacting an FSS or via datalink. FSS frequencies are listed in the Chart Supplements.

Personal Minimums

Personal minimums act as a buffer between the demands of a situation and the extent of the pilot’s skills. Pilots should establish their own “hard” minimums unique to their current level of experience and proficiency.

Personal minimums should be thought of as the human factors equivalent to reserve fuel. Never attempt a flight that requires the use of skills within your reserve level of piloting ability.

Clouds

The stability of the atmosphere determines which of two types of clouds are formed: cumuliform or stratiform.

Cumuliform clouds are billowy-type clouds having considerable vertical development, which enhances the growth rate of precipitation. They are formed in unstable conditions, and they produce showery precipitation made up of large water droplets.

Stratiform clouds are flat, more evenly based clouds formed in stable conditions. They produce steady, continuous light rain and drizzle made up of much smaller raindrops.

Estimating Cloud Bases

The dew point and temperature can be used to estimate the above-ground altitude of the cloud bases, provided that lifted unsaturated air creates the clouds.

Dew point decreases about 1°F per 1,000′ while unsaturated air decreases at 5.4°F (dry adiabatic lapse rate). The two lapse rates converge at approximately 4.4°F per 1,000′. When the temperature and dew point converge, cloud bases form.

Cloud Base Height = ((Temperature in °F – Dew Point in °F) ÷ 4.4°F) × 1,000

Fog

Fog is a surface-based cloud composed of either water droplets or ice crystals. A small temperature-dew point spread is essential to the formation of fog. Fog may form by cooling the air to its dew point or adding moisture to the air near the ground.

Types of Fog

Radiation fog (ground fog) is formed when terrestrial radiation cools the ground (land areas only), which in turn cools the air in contact with it. When the air is cooled to its dew point, or within a few degrees, fog forms. Radiation fog forms most often in warm, moist air over low, flatland areas on clear, calm nights.

Advection fog (sea fog) is formed when warm, moist air moves (wind is required) over colder ground or water. It is usually more extensive and much more persistent than radiation fog. It deepens as wind speed increases up to about 15 knots. Winds much stronger than 15 knots lift the fog into a layer of low stratus clouds.

Upslope fog is formed when moist, stable air is cooled to its dew point as it moves upward against sloping terrain. Cooling is at the dry adiabatic lapse rate of approximately 3°C per 1,000′.

Precipitation-Induced fog (frontal fog) is formed when relatively warm rain or drizzle falls through cool air. Evaporation from the precipitation saturates the cool air and forms fog. It is most commonly associated with warm fronts but can occur with slow-moving cold fronts and stationary fronts.

Steam fog is formed in winter when cold, dry air passes from land areas over comparatively warm ocean waters. Condensation takes place just above the surface of the water and appears as steam rising from the ocean.

Ice fog occurs in cold weather when the temperature is much below freezing, and water vapor forms directly into ice crystals. Conditions favorable for its formation are the same as for radiation fog except the temperature is colder, usually -25°F colder.

Fronts

A front is a boundary between two air masses. Fronts develop when air masses of differing temperatures, pressures, or relative humidity meet. The formation of a front is called frontogenesis. A frontolysis occurs when a front dissipates.

Weather that is hazardous to flight is commonly associated with fronts, which in turn are associated with low-pressure systems. Pilots should examine the types and locations of fronts and low-pressure systems in relation to their flight path to anticipate the possible weather conditions that could be encountered.

Warm Fronts

A warm front is the leading edge of an advancing warm air mass and moves about half as fast as a cold front. Warm fronts are associated with stratus and nimbostratus clouds, and steady rain and a shallow boundary develops between the two air masses.

The physical characteristics of a warm or cold front can be different with each front. They vary according to the air mass’s speed and the degree of stability of the air mass being overtaken. A stable air mass forced aloft continues to exhibit stable characteristics. An unstable air mass forced to ascend continues to be characterized by cumulus clouds, turbulence, showery precipitation, and good visibility.

Occluded Fronts

An occluded front occurs when a cold front catches up to a warm front. When the warm front is overtaken, the cold air lifts the warm front aloft. Occluded fronts are associated with cumulonimbus and nimbostratus clouds, showers, and thunderstorms.

Stationary Fronts

A stationary front is a boundary between two air masses that are not moving. A vertical boundary between the two air masses develops. Frontal waves and low-pressure areas usually form on slow-moving cold fronts or stationary fronts, resulting in extended periods of rain.

Managing Priorities During Emergencies

“Aviate, navigate, communicate” is a phrase used by pilots to remember the priorities of tasks during emergencies.

The priorities are:

Aviate: Maintaining positive aircraft control has priority over all other considerations, including airplane configuration and checklists.
Navigate: Know where you are and where you intend to go.

Communicate: Let someone know your position and intentions on the emergency radio frequency (121.5 MHz) or by contacting a nearby ATC facility. If already in radio contact with a facility, do not change frequencies unless instructed to change.

Emergency Deviation from FAA Regulations

In an emergency requiring immediate action, pilots may deviate from a regulation to the extent necessary to meet that emergency. Upon request, the PIC must send a written report of the deviation to the FAA.

Emergency Deviation from ATC Clearances and Instructions

Pilots may deviate from an ATC clearance or instruction during an emergency. ATC must be notified as soon as possible.

If requested by ATC, the PIC must submit a detailed report (written) of the emergency within 48 hours to the manager of the ATC facility.