Inadvertent VFR Flight Into IMC
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Note that for the purposes of this article the term “IMC” implies meteorological conditions exist that require flight by sole reference to instruments.
This article discusses weather-related flight risk management and practical measures if inadvertently encountering instrument meteorological conditions (IMC) conditions during a VFR flight. This article does not provide guidance on precision instrument flying; it is intended only to help the VFR pilot to keep their aeroplane under adequate control until suitable visual references are regained. Thorough pre-flight planning and full awareness of personal skill levels and experience should make this article, in the most part, unnecessary.
Every human – including weather forecasters and pilots - is liable to make errors. Therefore, the pilot should identify the risks that deteriorating en-route weather could present, particularly when the route is associated with any high ground, surface obstructions or proximity to controlled airspace, and consider how to manage those risks before the flight. In pre-flight planning, the pilot should consider not only what alternate courses of action will be available if the weather were to deteriorate, but also when those courses of action should be taken. For example, what would be the minimum visibility or cloud base needed to continue on track? Once airborne, these decisions are likely to be made under stress so pilots must also be aware of their own abilities and limitations, which may well be more limiting than the local regulations may require or the law demand.
This article assumes that:
- The pilot holds a PPL(A) licence with SEP Class rating and no additional qualifications. Therefore, the pilot has received only limited basic training in “flight by sole reference to instruments” as part of their PPL initial training.
- The aeroplane is equipped with a basic instrument fit (Attitude Indicator, Airspeed Indicator, Sensitive Altimeter, Directional Gyro/HSI, Turn Indicator/Coordinator and Vertical Speed Indicator) For other aircraft types/classes and instrumentation fits, additional advice and/or alternative techniques may be more appropriate.
All aircraft instruments are subject to pilot (mis)interpretation and have errors that generally increase with wear and age. Therefore, the pilot must understand correctly the flight instrument indications and limitations. Pre-flight checks to ensure the flight instruments are fully serviceable are critical. Encountering adverse flight conditions is not the time to discover an instrument(s) is unserviceable!
Visual flight rules (VFR) flights that inadvertently or intentionally enter into instrument meteorological conditions (IMC) continue to be a significant safety hazard to general aviation (GA) flights. Although loss of control (LOC) and controlled flight into terrain (CFIT) accidents by VFR pilots in IMC typically account only for a proportion of the total number of GA accidents, such occurrences account for 75% of weather related GA fatalities.
Errors and Risks
Accident statistics show that a pilot who has not been trained and qualified in instrument flying, or one whose instrument skills have eroded, will soon lose control of the aeroplane if forced to fly solely by reference to flight instruments.
VFR Pilots may attempt to continue a VFR flight in IMC conditions for many reasons, such as:
- Placing priority on the wrong things (e.g. loss of time or additional expense of a diversion or delay to the flight) when making decisions (sometimes referred to as “get-home-itis”)
- Poor situational awareness perhaps as a result of pilots’ lack of experience in interpreting changing weather conditions once airborne.
- Over-confidence leading to poor risk awareness because pilots overestimate their own abilities and are complacent about flying into adverse weather. They may believe that their limited PPL course instrument training will enable them to cope in instrument conditions for a sustained period.
- Internal (personal) and external (social) pressure may be allowed to bias pilots’ decisions to continue the flight even when objective assessment of the situation suggests they should do otherwise. For example, when passengers are on board, a pilot may feel a strong responsibility to reach their destination sooner rather than later. Passengers may apply pressure to fly and this must be resisted.
To reduce the risk of an accident from inadvertent VFR flight into IMC, pilots need strategies to avoid adverse weather and a framework of actions to assist recovery if they inadvertently fly into IMC.
The first step to be taken by a VFR pilot to avoid encountering instrument meteorological conditions (IMC) is effective pre-flight planning. Pilots must be able to access comprehensive weather information to use at the planning stage. For a typical VFR flight, pilots should ensure that they obtain an area forecast covering the route; a forecast of the wind speed and direction valid for the duration of the flight; at the lower flight levels local conditions can have a dramatic effect on the weather. Where available, Terminal Aerodrome Forecasts (TAFs) and Meteorological Terminal Air Report (METAR) for the destination, destination alternate and all other airfields en-route should be obtained. If the specific airfield or strip does not have these available, the data for surrounding airfields will still be a useful source of information to aid decision making. All this information can then be comprehensively analysed and from this the pilot can make a holistic decision on whether the flight can be made safely and which route is most suitable. Once airborne, the pilot should adopt pre-planned alternative courses of action as necessary to avoid flying into IMC. If, despite these precautions, the pilot enters IMC the priorities are: (1) Maintain control of the aeroplane; (2) Obtain appropriate ATC assistance in turning back or otherwise heading the aeroplane safely back to VMC conditions.
Each of these is considered in detail below:
- Ascertain the likely cloud base that could be encountered, as this will determine the maximum altitude that the pilot can expect for the cruise. Generally, for VFR flight a pilot should plan to be at least 1000’ above the typical terrain along the route. If this is not possible on the preferred routing, then an alternative routing will be needed.
- Having chosen the route then a “Minimum VFR Altitude to Continue” can be calculated. This will typically be at least 500’ above the expected terrain/obstructions and should be noted for each leg of the route on the Plog.
- Once airborne, this acts as a trigger to alert the pilot to the fact that if he/she has to descend to this level due to a lowering cloud base, then it is no longer safe to continue on the planned route. The risk of an inadvertent entry into IMC whilst trying to maintain terrain clearance in this scenario is substantial. A diversion will be required, to either the left or right of the original planned track or possibly a 180 degree turn to return to the last turning point or point of departure.
- PPL(A) holders are permitted to operate VFR in certain classes of airspace at 1500m visibility. However, if the in-flight visibility is forecast at less than 5km, the VFR pilot needs to give serious thought as to the viability of a safe flight. In low visibility it is very likely that there will not be much of a natural horizon to work with to enable normal flight by reference to visual attitude; additionally, the navigational task will be further complicated by difficulty in identifying features for off track errors and turning points/waypoints along the route.
- The general weather conditions en-route should be considered. Are there any weather fronts along the route or forecast to affect the route? Are there any CBs forecast? What will the temperature be at cruising altitude – will any of the precipitation freeze onto the windscreen/airframe? All of these factors should be considered as all could contribute to the failure of the pilot to see and avoid an unintended excursion into IMC.
- Allowing a sensible amount of fuel for the flight increases the options available to the pilot should a diversion be required. Careful fuel planning will allow sufficient fuel for start/taxi/take-off, climb, en-route fuel, descent and landing. In addition to the fuel calculated as required to fly the route, contingency fuel of 10% in recommended. Fuel then needs to be added to cover any diversion to an alternate destination. Additionally it is recommended to plan to have at least ¼ tanks or 45 minutes of fuel (whichever is the greater) left in the tanks on reaching the destination as a final reserve fuel.
- Airborne monitoring of fuel consumption against planned fuel usage will give the pilot an appreciation of fuel capability (endurance and/or range) at any point during the flight, so assisting with the diversion planning process should any diversion prove necessary.
Avoiding IMC Once Airborne
Typically, VFR rules require the aircraft to remain 1000’ vertically from cloud. In certain classes of airspace subject to additional conditions, an aircraft can operate VFR “clear of cloud with the surface in sight”. In this situation the cruising level should be adjusted to be no closer than 300’ below the cloud base. This allows for any up draught that may cause an inadvertent increase in altitude as well any issues such as not having the aircraft in an accurate trimmed condition (and the fact that close to the cloud base, relative humidity is likely to be close to 100%, and it is possible that cloud may form around the aircraft).
Whilst cruising, the standard working cycle of Lookout/Attitude/Instruments should be maintained with regular checks on the weather ahead as part of the lookout scan. This could be challenging, as the weather transitions are often rather subtle. The human eye can become so accustomed to progressive small changes in light, colour, 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 first clue of deteriorating weather can often be the need to gradually reduce cruising level to maintain VMC. Reference to the pre-planned "Minimum VFR Altitude to Continue" figure will alert the pilot as to when further descent is not safe and a diversion is required before an inadvertent entry into IMC occurs.
Diversion options should have been considered and established at the pre-flight planning stage. When planning a flight across higher ground it is useful to have identified ‘escape route’ options towards lower ground.
Recognition of Inadvertent Entry into IMC
VFR pilots should assume they are in IMC conditions anytime they are unable to maintain aircraft attitude control by reference to the natural horizon, regardless of the circumstances or the prevailing weather conditions. In addition, a VFR pilot should accept that they are effectively in IMC anytime they are unable to navigate or establish geographical position by visual reference to landmarks on the surface unless they have planned and are legally able to operate “VFR on top”. Such situations must be accepted by the pilot involved as a genuine emergency, requiring immediate action.
Pilots must 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 in conditions of low visibility 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 impression of the natural horizon and may subconsciously rely on it more than the cockpit attitude indicator. If all vestiges of the natural horizon subsequently disappear, the untrained instrument pilot is likely to become spatially disorientated, which is likely to lead to loss of control.
Maintaining Control of the Aeroplane
Once the pilot recognises that they have entered IMC conditions, they should understand that the only way to control the airplane safely is by using and trusting the flight instruments. The pilot should make a firm decision to change from visual reference to instrument flight if possible before visual references are completely lost. The pilot should initially concentrate on the attitude indicator and select and maintain the correct attitude for straight and level flight. Once this is achieved, the pilot must concentrate on the AI limiting glances away from the AI for essential checks only – e.g. the RPM to confirm appropriate power is set. It is important not to try to mix searching for external clues as to the attitude of the aircraft with instrument flying.
The pilot should understand that the most important concern is to keep the wings level. An uncontrolled bank may lead to spiral dive and loss of control. The pilot must believe what the AI shows about the aeroplane’s attitude regardless of sensory input. vestibular organs (in the inner ear) is unreliable in flight when unaccompanied by relevant visual information. The sensory areas of the inner ear cannot detect slight changes in angular acceleration, nor can they accurately sense changes that occur at a uniform rate over a period. On the other hand, false sensations are often generated due simply to movements of the head and may lead the pilot to believe the attitude of the airplane has changed when, in fact, it has not. If the pilot has managed to achieve straight, stable flight, do not make any control movement unless the AI shows that it is necessary, and even then make only small control inputs.
Other receptors found throughout the body, known as somatosensory receptors (commonly referred to giving feeling and G forces) located all over the skin, bones, joints, skeletal muscles, internal organs and the parts of the cardiovascular system, will also be providing information to the brain. This information can be in conflict with the visual and vestibular senses. These false or conflicting sensations can result in the pilot experiencing spatial disorientation even when in VMC. Particularly during flight in poor visibility or in cloud all this can be catastrophic. The outcome may develop into what is often called a “graveyard spiral”. This name is self explanatory.
Having established stable straight and level flight, the next step is to consider ice protection. In most light single-engine aircraft this may be limited to selecting the pitot heat on and selecting the carburettor heat to “Hot”, then restoring the power back to the cruise value.
In most cases, an inadvertent entry into IMC will be best resolved by a 180 degree turn to fly back into the VFR conditions behind. This manoeuvre needs to be planned before attempting to execute the turn,
Firstly, the direction of turn should be considered taking into account factors such as terrain on each side of the flight path. For example, if before entry into IMC the pilot was aware of terrain to the left of track then a turn to the right might be the best plan.
Before entering the turn, the pilot must decide on a target heading to roll out on. This can be achieved by rotating the heading bug to the bottom of the DI/HSI. Where there is not a heading bug, the pilot should note the heading indicated at the bottom of the DI/HSI – this can be noted down, or a suitable Navigation Aid (such as a VOR OBI or rotating card ADF indicator) can be set with the desired heading at the top of the indicator. The pilot will know the turn is completed when the numbers on the DI/HSI match those set on the Navigation Aid indicator.
Alternatively, if the stopwatch is started as the roll into the turn is initiated, a heading change of approximately 180 degrees will be achieved in 1 minute.
Whichever technique is used, it must not distract the pilot from keeping their attention on the AI throughout.
A bank angle of not more than 15 degrees will be sufficient. Controlling the bank angle is important. Keeping the bank angle low will mean that no additional back pressure will be required to maintain an effectively level turn. In addition, this will guard against the possibility of entering a spiral dive.
When entering and whilst maintaining the turn, the pilot should concentrate primarily on the attitude indicator. The DI should be checked occasionally to monitor the progress onto the desired heading but otherwise the concentration needs to be on the AI. The pilot should anticipate reaching the desired heading then concentrate solely on rolling the aircraft by reference to the attitude indicator to re-establish straight and level flight. This should be maintained until the aircraft returns to VFR conditions.
A period of time will have elapsed before the pilot recognised that entry in IMC had occurred, further time will have been utilised establishing straight and level flight and yet more time taken to plan the escape turn. Therefore, once straight and level on the planned heading to exit, it may take a few minutes to get back to VFR conditions.
If the 180 degree turn fails to achieve a return to VMC then the pilot will need to accept that he will have to continue to manoeuvre the aircraft by sole reference to instruments.
Obtaining the Appropriate Assistance
If the escape turn does not result in return to VMC, the pilot should not hesitate to declare “Mayday”. In doing so he/she must inform ATC that they are not qualified to fly on instruments but have entered cloud. It is important to alert ATC to allow them to offer the best possible assistance.
All controllers are trained for such occurrences and will be best placed to enable the pilot to maintain terrain clearance and return to VFR conditions. Often they will request the pilot to set the transponder to a discrete code, unless the pilot has already set 7700 as part of the mayday. The pilot must not allow setting the transponder code to distract from maintaining close attention on the AI. Setting the transponder will identify the aircraft on radar so allowing the aircraft to be vectored.
In this situation the pilot is best to remember the three “C’s” – Contact, Confess, and Comply. It would be unusual for a controller to ask the pilot in these circumstances to do combined manoeuvres such as descending turns. Do not attempt to make anything other than a simple heading change or a climb/descent to keep the flying task as simple as possible.
For more information regarding mitigation of the risks related to the conduct of VFR flights, see the following Checklist:
- Flight Instrument Presentation of Aircraft Attitude
- Visual Navigation
- Navigation by Radio Aids
- Entering Controlled Airspace
- Spatial Disorientation
- Somatogravic and Somatogyral Illusions
- Safety Sense Leaflet 23: Pilots - It's Your Decision, published January 2013.
- FAA "Got Weather" campaign
- Airplane Flying Handbook, FAA-H-8083-3B, 2016.
- General Aviation Pilot’s Guide to Preflight Weather Planning, Weather Self-Briefings, and Weather Decision Making, June 2008.
- FAA Risk Management Handbook
NTSB Safety Alerts on General Aviation risks
- Safety Alert 20 - Reduced Visual References Require Vigilance
- Safety Alert 23 - Pilots: Manage Risks to Ensure Safety
- EGAST Safety Promotion Leaflets on GA operations:
- Helicopter Airmanship; training leaflet published in 2011.