I-IMC (Inadvertent-Instrument Meteorological Conditions) is a situation where pilots lose visual contact with the ground due to deteriorating meteorological conditions. In simple terms, we have lost our visual cues that allow us to control the aircraft attitude and flightpath by looking outside at the relationship between the ground and sky (the natural horizon).

Why does the loss of visual cues we suffer when we inadvertently enter into IMC have such rapid and disastrous consequences? It’s all to do with spatial disorientation.

Humans were designed to be firmly fixed to the earth, with clear vision, gravity giving as a sense of “up” and our inner ears providing balance.

You mainly achieve orientation through your eyes with the remaining 20% split between your inner ear and your somatosensory system, the seat of your pants or what you “feel” as weight.

Keeping yourself oriented in VFR flight is easy - you have visual reference to a horizon outside the aircraft; therefore, your eyes can easily work out which way is up. Even in a steep turn with a bit of G applied, you still know which way is up. But when you lose that horizon, 80% of your orientation goes with it. But that’s okay, your inner ear and seat of your pants will help…won’t they?

Sadly, they can actually make it worse. A change in flight attitude could add some more G making you think gravity is below you or you’re flying a loop when in fact you could actually be in a steep turn or spiral dive. Without that visual backup, the inner ear can be easily tricked into thinking you’re not turning when actually the aircraft is gently rolling but at a rate that your body simply can’t pick up!

So therefore, if you end up in IMC, you can’t trust your internal sense of direction and balance as it could be lying to you. Seasoned IR pilots can tell you about the internal argument between eyes and ears when flying IMC, commonly known as “the leans”, when the instruments report you as straight and level but your body is telling your otherwise. Ultimately, if you enter IMC you need to ignore your senses, trust your instruments, and concentrate on flying the aircraft as your life depends on it!

How can these events happen to us?

The risk comes from two main areas (that may combine):

The first area of risk is: Errors in pre-flight planning meaning unsuitable weather conditions during our flight are not anticipated

This may be that we fail to obtain sufficient information about the weather to make a good decision on a safe routing or decision to fly, or we misinterpret the information we have obtained when making our pre flight planning decisions. Both of these can be due to deficiencies in our knowledge or training, or can be the result of human factors errors such as perceived pressure, fatigue, distraction or complacency.

The second area is: Errors during execution of our flight that cause us to continue into unsuitable weather

This could be due to a loss of situational awareness that means we do not realise the threat that the aircraft is now entering or about to enter unplanned IMC conditions. This could be us flying into bad weather or allowing bad weather to develop around us without taking action.

Personal overconfidence or complacency about our ability to continue flight in poor weather conditions can also lead to this. People may believe they can cope, or ‘have got away with it before’. There may also be peer pressure from other pilots flying in the same weather or with the pilot in command.

Finally, external pressures or ‘Press-on-itis’ as it is sometimes called can be a contributory factor. This could be perceived or real pressure to get back home, or get a rental aircraft back to base or to complete a promised flight with passengers who often may not appreciate the dangers of the weather and the risks involved.

How can we avoid these events happening to us?

During pre-flight planning, we should aim to use accurate and up to date weather sources to make sure we at least obtain and understand:

·       An area forecast covering the intended route. What is the general frontal situation, cloud and freezing level that will affect our flight? Are there any local threats such as CBs in the forecast?

·       Forecast of the wind speed and direction for the departure, route and arrival. Is it going to be within our comfortable limits? Is it associated with changeable weather situations that could catch us out?

·       Relevant TAFs (Terminal Aerodrome Forecasts) and METARs (Meteorological Aviation Reports) for departure, enroute and arrival. Even if our airfields don’t have their own, nearby airfields can be great indicators of likely conditions. Are we comfortable with what conditions we may experience? Is there a risk of a significant change? What PROBs are there that could happen? 

GA ‘EFBs’ such as Skydemon and Airbox can provide excellent route briefings and right up to date information, make sure you are connected to the internet and pulling the latest info though. There are also colour coded graphical overlay tools that can also help us, but make sure you understand what the app is using as filters and what it is telling you.

In flight, as part of our LOOKOUT scan we should be looking ahead to try and spot poor weather on our intended route. In particular, a continuing need to descend closer to terrain to maintain clear of cloud is a key indicator that we are potentially likely to Inadvertently enter IMC to avoid CFIT (controlled flight into terrain). In any case, when becoming aware of worsening conditions, an early and decisive move to either alter our route, turn back or divert is critical.

At or Below 3000ft AMSL and 140kts IAS, clear of cloud with the surface in sight during the day - 1500m visibility is entirely legal for UK Class G VFR flight. However, in reality most VFR pilots will find anything below 5km visibility very challenging. In low visibility the lack of a natural horizon and visible navigational features can make workload extremely high and increase the risk of disorientation. This can be especially so over water or featureless terrain.

What should I do if I inadvertently enter IMC?

The absolute number one priority is to ‘STAY IN CONTROL’, then have a plan to regain visual conditions / seek assistance.

Once you recognise you are losing visual references, you must make a firm decision to trust and control the aircraft by using the available flight instruments. 

·       Concentrate on the attitude indicator as your primary instrument

·       Try and maintain the correct attitude for straight and level flight

·       The priority is to keep the wings level (a turn can quickly develop into a spiral dive and departure from controlled flight)

·       Limit looking away from the attitude indicator to glances at other instruments and displays

·       Only make small, progressive control inputs.

·       Consider the escape plan; is a 180 turn to return the way you have come appropriate? Are you at a safe height? (note: altitude vs terrain height)

·       Seek assistance from ATC or D&D. Do not downplay the situation, an early MAYDAY call will often be most appropriate. Be clear you are in I-IMC and not qualified or trained to fly in these conditions.

To perform a 180 degree turn, gently select the turning attitude on the AI. A bank angle of about 15 degrees should be sufficient and will guard against the likelihood of overbanking and a spiral dive developing. Some back pressure may be required to prevent the nose pitching down in the turn. Concentration should be maintained on the AI with occasional glances at the DI. When reaching your desired exit heading, gently roll the wings level to select the straight and level attitude again. Remember straight and level may have to maintained for a few minutes to exit IMC again as you will have travelled some distance during the entry and exit manoeuvres.