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VFR Flight Into IMC

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Category: Loss of Control Loss of Control
Content source: SKYbrary About SKYbrary
Content control: EUROCONTROL EUROCONTROL


The purpose of this article is to provide advice on possible practical measures to maintain aeroplane control for a limited period of time in the event of a VFR flight encountering IMC conditions. The main goal is not to provide guidance on precision instrument flying; rather, it is to help the VFR pilot keep the aeroplane under adequate control until suitable visual references are regained.


Contents

Description

Visual flight rules (VFR) flight into instrument meteorological conditions (IMC), or unqualified flight into adverse weather, continues to be a significant safety hazard to general aviation (GA) flights. Although VFR flight into IMC accidents typically account only for a small proportion of the total number of GA accidents, such occurrences account for a considerable percentage of GA fatalities.

Unsafe Behaviour and Risks

Accident statistics show that the pilot who has not been trained in instrument flying, or one whose instrument skills have eroded, will lose control of the airplane in about 10 minutes once forced to rely solely on instrument reference.

Possible common explanations for pilots’ decisions to continue VFR flight into IMC are:

  • inclusion of expected prospective losses (e.g. loss of time, money, effort) into the decision making factors;
  • poor situation assessment as a result of pilots’ lack of experience in interpreting changing weather information (pilots with limited flight experience will not have had the same exposure to weather related hazards);
  • inadequate risk perception which means that pilots are overconfident in their abilities and do not fully appreciate the risks of flying into adverse weather.
  • external (social) pressure which may bias pilots’ decisions to continue the flight even though an assessment of the situation suggests they should do otherwise. For example, when passengers are onboard, a pilot may feel under pressure to reach their destination sooner rather than later or he/she may feel like impressing passengers with his/her flight skills, especially when faced with difficult flight conditions.

Defences

Possible measures for reducing the number of VFR flight into IMC accidents are the use of new cockpit technology and helping pilots avoid adverse weather or recover from hazardous situations should they encounter IMC.

The first steps to be taken by a VFR pilot to avoid encountering instrument meteorological conditions (IMC) are:

  • analyse the weather information
  • avoid flying into IMC if no certification and training has been received

Remedial actions when IMC is encountered:

  • recognition and acceptance of the seriousness of the situation and the need for immediate remedial action;
  • maintaining control of the airplane;
  • obtaining appropriate assistance in getting the airplane safely on the ground.

Analyse the Weather Information

Obtaining weather information is only the first step. The next is the practical weather analysis that should be made and which requires the understanding of the three basic elements of weather: Temperature, Wind and Moisture. Specifically the combination of these three elements can reduce visibility, create turbulence and reduce aircraft performance. The pilot should examine and review the weather conditions for departure, enroute and destination, and pay special attention to any information about windshear or convective activity.

It is best to have a carefully considered reserve plan which includes the alternate aerodromes and reserve fuel. Flight planning for only a legal fuel reserve could significantly limit the pilot's options if the weather deteriorates. More fuel gives access to more alternatives.

Avoid IMC if Not Certified

When the GA pilot does not hold an instrument rating he/she should avoid entering IMC. This could sometimes be challenging as the weather transitions are sometimes too subtle. The human eye can become so accustomed to progressive small changes in light, color, and motion that it no longer “sees” an accurate picture. In deteriorating weather, the reduction in visibility and contrast can occur gradually, and it may be quite some time before the pilot senses that the weather conditions have deteriorated significantly.

The pilot should avoid entering into IMC if not certified and should consider heading for the nearest airport if clouds are forming beneath the flight altitude, there are gray or black areas ahead, heavy rain or moderate turbulence, or clouds forming that require a descent.

Recognition

VFR pilots should assume they are in IMC conditions anytime they are unable to maintain airplane attitude control by reference to the natural horizon, regardless of the circumstances or the prevailing weather conditions. In addtion, a VFR pilot should accept that they are effectively in IMC anytime they, inadvertently or intentionally, are unable to navigate or establish geographical position by visual reference to landmarks on the surface for an indeterminate period of time. Such situations must be accepted by the pilot involved as a genuine emergency, requiring appropriate action.

Pilots should understand that unless they are trained, qualified, and current in the control of an airplane solely by reference to flight instruments, they will not be able to do so for any length of time.

Many hours of VFR flying using the attitude indicator as a reference for airplane control may give a pilot a false sense of security based on an overestimation of his personal ability to control the airplane solely by reference to instruments.

In visual meteorological conditions, even though the pilot may think he/she is controlling the airplane by instrument reference, the pilot receives an overview of the natural horizon and may subconsciously rely on it more than the cockpit attitude indicator. If the natural horizon were to suddenly disappear, the untrained instrument pilot would be subject to vertigo and spatial disorientation which in turn might lead to a complete loss of control.

Maintaining Control of the Aeroplane

Once the pilot recognizes and accepts the situation, he should understand that the only way to control the airplane safely is by using and trusting the flight instruments. Attempts to control the airplane partially by reference to flight instruments while searching outside the cockpit for visual confirmation of the information provided by those instruments would result in inadequate airplane control. The most important point to be made here is that the pilot must not panic. The task at hand may seem overwhelming, and the situation may be compounded by extreme apprehension. The pilot therefore should make a conscious effort to relax.

The pilot should understand that the most important concern is to keep the wings level. An uncontrolled turn or bank usually leads to difficulty in achieving the objectives of any desired flight condition. The pilot has to believe what the flight instruments show about the airplane’s attitude regardless of what the natural senses tell. The vestibular sense (motion sensing by the inner ear) could and would confuse the pilot. Because of inertia, the sensory areas of the inner ear cannot detect slight changes in airplane 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 attitude of the airplane has changed when, in fact, it has not. These false sensations result in the pilot experiencing spatial disorientation.

Attitude Control

An aeroplane is, by design, an inherently stable platform and, except in turbulent air, will maintain approximately straight-and-level flight if properly trimmed and left alone. It is designed to maintain a state of equilibrium in pitch, roll, and yaw. The pilot should be aware, however, that a change about one axis will affect the stability of the others.

Therefore, to achieve the needed emergency airplane attitude control, the pilot should:

  • trim the airplane with the elevator trim so that it will maintain hands-off level flight at cruise airspeed;
  • resist the tendency to over control the airplane, i.e. fly the attitude indicator 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 VFR pilot should remember that a small change as indicated on the horizon bar corresponds to a proportionately much larger change in actual airplane attitude.
  • make use of any available aid in attitude control such as autopilot or wing leveler.

The primary instrument for attitude control is the attitude indicator. Once the airplane is trimmed so that it will maintain hands-off level flight at cruise airspeed, that airspeed need not vary until the airplane must be slowed for landing. All turns, climbs and descents can and should be made at this airspeed. Straight flight is maintained by keeping the wings level using “fingertip pressure” on the control wheel. Any pitch attitude change should be made by using no more than one bar width up or down.

Turns

Turns are perhaps the most potentially dangerous manoeuvre for the untrained instrument pilot for two reasons:

  • the normal tendency of the pilot to over-control, leading to steep banks and the possibility of entering a “graveyard spiral.”
  • the inability of the pilot to cope with the instability resulting from the turn.

When a turn is 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.

A shallow bank will take very little vertical lift from the wings resulting in little if any deviation in altitude. It may be helpful to turn a few degrees and then return to level flight, if a large change in heading must be made. The process should be repeated until the desired heading is reached. This process may relieve the progressive overbanking that often results from prolonged turns.

Climbs

If a climb is necessary, the pilot should raise the miniature airplane on the attitude indicator no more than one bar width and apply power. The pilot should not attempt to attain a specific climb speed but accept whatever speed results. The objective is to deviate as little as possible from level flight attitude in order to disturb the airplane’s equilibrium as little as possible.

If the initial power application results in an inadequate rate of climb, power should be increased in increments of 100 RPM or 1 inch of manifold pressure until the desired rate of climb is attained. Maximum available power is seldom necessary.

Descents

Descents are very much the opposite of the climb procedure if the airplane is properly trimmed for hands-off straight-and-level flight. In this configuration, the airplane requires a certain amount of thrust to maintain altitude. The pitch attitude is controlling the airspeed. The engine power, therefore, (translated into thrust by the propeller) is maintaining the selected altitude.

Following a power reduction, however slight, there will be an almost imperceptible decrease in airspeed. However, even a slight change in speed results in less down load on the tail, whereupon the designed nose heaviness of the airplane causes it to pitch down just enough to maintain the airspeed for which it was trimmed. The airplane will then descend at a rate directly proportionate to the amount of thrust that has been removed.

Power reductions should be made in increments of 100 RPM or 1 inch of manifold pressure and the resulting rate of descent should never exceed 500 feet per minute. The wings should be held level on the attitude indicator, and the pitch attitude should not exceed one bar width below level.

Combined Manoeuvres

Combined manoeuvere, such as climbing or descending turns should be avoided if at all possible by an untrained instrument pilot already under the stress of an emergency situation.

Combining maneuvers will only compound the problems encountered in individual maneuvers and increase the risk of control loss. The Pilot shoul remember that the objective is to maintain airplane control by deviating as little as possible from straight-and-level flight attitude and thereby maintaining as much of the airplane’s natural equilibrium as possible.

Obtaining the Appropriate Assistance

The pilot should not hesitate to inform ATC that they are flying under VFR and enter IMC for a prolonged time. It is important to inform ATC in order to receive the best possible assistance. The controllers should be informed when the pilot is uncertain of his/her position and may experience difficulties maintaining separation from controlled airspace. It is also possible that a situation attributed to the VFR flight into IMC may arise and could result in airspace infringement and potential loss of separation.

When being assisted by air traffic controllers from the ground, the pilot may detect a sense of urgency as he or she is being directed to change heading and/or altitude. This sense of urgency reflects a normal concern for safety on the part of the controller but the pilot should not let this prompt his/her to attempt a manoeuvre that could result in loss of control.

Transition to Visual Flight

One of the most difficult tasks a trained and qualified instrument pilot must contend with is the transition from instrument to visual flight prior to landing. For the untrained instrument pilot, these difficulties are magnified.

The untrained instrument pilot may, after transition to visual flight, find the visibility still limited, the terrain completely unfamiliar, and altitude above terrain such that a “normal” airport traffic pattern and landing approach is not possible.

Additionally, the pilot will most likely be under considerable self-induced psychological pressure to get the airplane on the ground.

The pilot should take this into account and, if possible, allow time to become acclimatized and geographically oriented before attempting an approach and landing, even if it means flying straight and level for a time or circling the airport. This is especially true at night.

To summarize pilot’s actions when faced with IMC during VFR flight - use „I still want to fly” mnemomics:

I - Use flight instruments
Still - Small bank angles
Want - Wings level
To - Trim the plane
Fly - Fly easy, don’t overcontrol

Checklists

For more information regarding mitigation of the risks related to the conduct of VFR flights, see the following Checklists:

  1. Weather Go/No-Go VFR Flight Checklist
  2. Cold Weather Operations VFR Flight Checklist


Related Articles

Further Reading

EUROCONTROL Guidance Notes for GA pilots

FAA

NTSB Safety Alerts on General Aviation risks

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