You’re flying back to base after a long day. The weather is slowly deteriorating, you’re running out of daylight, and your legal maximum duty period limit is approaching quickly. You can’t afford to turn back, right?

You decide to push on, trying to get back to base. You certainly don’t want to end up stuck in the middle of nowhere! The cloud base reduces even further, the Radalt you set at the start of your flight is going off. Visibility is reducing to the point where your horizon and the clouds in front of you are starting to merge together.

A few minutes later, you find yourself disoriented, stressed, and wishing that you had never taken off. Your references are almost gone, you’re tight on the controls, and your instruments are making little sense. You’re now in Instrument Meteorological Conditions (IMC).

Dozens of pilots every year have to deal with inadvertent IMC entries, both in the fixed wing and rotary industry. What can we do about this? Let’s dive in.

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What is Inadvertent IMC?

Inadvertent IMC is an unplanned transition from Visual Meteorological Conditions (VMC) to Instrument Meteorological Conditions (IMC). This can very easily lead to spatial disorientation, confusion, and a distrust in flight instruments.

Humans rely on visual cues in order to make sense of the world around them. If we take that ability away, it becomes harder to maintain an accurate sense of where we are, and what direction we’re facing. This leads to multiple threats that we’ll discuss below.

The United States Helicopter Safety Team (USHST) have published a video that shows the danger perfectly:

What are the Causes of Inadvertent IMC?

Inadvertent entry into IMC has multiple causes. While poor weather is usually one of the main factors, we’re not really interested in that as it’s not the root cause. If we’re really trying to zoom into why pilots fly into IMC inadvertently, we need to take a step back and look at the big picture. Let’s go over the main causes:

Inadvertent IMC

Plan Continuation Bias

Plan continuation bias is defined as:

NASA in cooperation with the Ames Research Centre reviewed 37 accidents that the National Transportation Safety Board (NTSB) investigated. They determined that 75% of decision errors in these accidents were caused by the decision to continue with the original plan, despite many variables telling the crew to choose a different one.

Inadvertent IMC is a huge threat for instances were plan continuation bias is a factor. In many cases, it can be prevented by timely decisions to divert or abort the flight. Whether you call it get-home-itus, push-on-itus or get-there-itus, deciding to continue when all odds are stacked against you is often just not a safe move.

Self Induced Pressure

The FAA worded self induced pressure very wisely:

Self induced pressure can result from many things. From low self-esteem, your level of agreeableness, how assertive you are, the safety culture you’re in, and your overall personality.

Some pilots are at a higher risk of being exposed to self induced pressure than others. Reflect on how likely you could experience this, and whether you have had instances in the past where you’ve put yourself under pressure to ‘get the job done’.

This is a recipe for disaster if you combine it with deteriorating weather and plan continuation bias.

Commercial Pressure

Commercial pressure is defined by ICAO as:

Commercial pressure by itself doesn’t have to result in a bad outcome, but it sure is a negative influence on flight safety. Decisions in the cockpit should strike a balance between safety and efficiency, but safety should always be the main priority.

To tackle commercial pressure, there are 2 things that can be done. Either

1) Stand your ground and don’t give in to organisational pressures
2) Foster a safety culture in your organisation, so that it becomes less of a threat

Both options are obviously quite hard, and easier said than done. This is why it’s still a very common occurrence that still leads to many accidents worldwide. We’ve covered this extensively in our article about commercial pressure in aviation.

Lack of In-Flight Situational Awareness

Being a few steps ahead of the aircraft is a really efficient antidote for Inadvertent IMC situations. If you’re aware of the upcoming aircraft flight path, as well as the changes in weather, you could mitigate for it before it happens.

This could be in the form of having extra backup plans available, slowing down, or having a discussion with the flight crew about their perspective on the situation.

No Plan B

A lack of a backup plan is another major red flag for potential inadvertent IMC situations. If you have a pre-briefed backup plan that you can fall back on when shit hits the fan, it’s easier to make the decision to turn back.

When flying around high ground, or into weather that you know could easily present issues along the way, try to consciously have multiple escape plans. If any of them become no longer an option, it’s a good indicator that you should probably think of the next plan B or C.

Complacency or Over-Confidence

Our mindset while flying plays a big part in inadvertent IMC situations. Before we’ve even started the aircraft, there will have been multiple choices that you’ve already made that will present more or less risk.

Have you briefed properly, did you assess the weather in depth, does your aircraft have any MEL items that will having a plan B or C harder to execute?

The amount of vigilance vs complacency you have on any given flight has a strong correlation to accidents caused by Controlled Flight Into Terrain (CFIT), IIMC, and Loss of Control (LOC). We’ve discussed this here.

What are the Dangers of Inadvertent IMC?

About 75% of general aviation weather related fatalities are caused by Controlled Flight into Terrain (CFIT) and Loss of Control in Flight (LOC-I):

Inadvertent IMC

Controlled Flight Into Terrain (CFIT)

For the purposes of this article, we’ll use the ICAO defition of CFIT:

CFIT is still a massive threat to General Aviation, helicopters, and also airline traffic (depending on the airport and part of the world). The risk of CFIT becomes much greater after flying into IMC inadvertently.

When you enter IMC inadvertently, it’s harder to keep a high level of situational awareness, which can easily lead to CFIT.


Well, if most of your workload goes to keeping control of the aircraft, you’re struggling to keep calm, and are not sure which way is up, the mental model you have about the environment around you is not going to be as accurate as you’d like it to be.

Loss of Control In Flight (LOC-I)

While Loss of Control in Flight (LOC-I) can have a similar effect as CFIT, it doesn’t necessarily have to end because you’ve hit the ground. With LOC-I, it’s usually the attitude of the aircraft itself (whether fixed wing or rotary) that is incompatible with safe flight.

Of course, if things do go wrong, the end result is similar to CFIT, but whether the flight itself was still ‘controlled’ is the difference here.

Everything could be perfectly fine, with the autopilot engaged for instance, but you’re not aware of the mountain ahead of you (CFIT). Or you could get completely disoriented, not know which way is up, causing extreme aircraft attitudes that aren’t sustainable with the power available to you (LOC-I).

A mountain, ridge line, obstacle or the ground can be inches away after prolonged periods of not knowing which way is up! IATA published an interesting document that analyses LOC-I accidents.

How to Prevent Inadvertent IMC?

Flight in IMC can be perfectly safe, but only if it’s properly planned. This means an IFR flight plan + briefing, using an aircraft that is Instrument Flight Rules (IFR) approved, with aircrew that all have instrument ratings.

But, for those who are not engaged in planned IFR flight operations, things are slightly different. Even for those with IFR aircraft and IR rated aircrew. There are lots of ways we can mitigate the risks of entering IMC inadvertently. Let’s go over the most effective methods:

Inadvertent IMC

Maintain IFR Competency and Recency

While most companies encourage pilots to keep their skills ‘legally’ current (i.e 3 instrument approaches every 90 days), this doesn’t mean you as a pilot are therefore completely competent at flying IFR!

Ask yourself if you feel confident and competent at converting to IFR, and flying an ILS at a diversion airfield? If not, get in the books and engage with your employer to see if you can get the training you need to feel confident. It may save your life one day.

Learn to say NO

This is probably the most effective (but also the hardest for some) mitigation. Being able to say no is very important. Especially when all the signs point at the fact you are likely going to struggle with low cloud bases and visibility (depending on your legal and personal limits).

If you find saying no particularly difficult, consider training yourself to become more assertive and confident in your decisions. At the end of the day, you’re the one in charge of flight safety. If you deem it unsafe, don’t go!

Recognise incoming IMC signs

Sometimes the reason IIMC happens is because we’re too slow at recognising that things are about to take a turn for the worse. By recognising incoming IMC in time, we can act and maintain safe flight parameters. If you’re behind the aircraft or your situational awareness is low, recognising this in time becomes a lot harder.

If you’re flying on night vision goggles, flip them up regularly (or look underneath them) to check what the conditions outside are actually like. Night vision goggles often make things look a lot more forgiving than they actually are!

Scenario-based training

Simulator training is changing a lot worldwide. The airlines and other operators are implementing evidence based training (EBT), as well as line oriented flight training (LOFT) and scenario based training (SBT).

If you’re training IIMC situations in the simulator as part of what is an otherwise normal operation, you will develop the tools and the ability to remain calm to deal with IIMC effectively if it happens for real.

Consider the Risk of Inadvertent IMC

Are you aware of the dangers? Yes we know, this might be a silly question. But you’d be surprised how many pilots are very ‘relaxed’ about the risks, even when they’re not IFR qualified.

From R22 pilots proudly showing off flying in IMC to other small GA traffic taking off when bigger IFR equipped air ambulance helicopters are offline..

Ask yourself if you’re aware of the risks, how it can result in accidents, and how comfortable you are with it, especially if you’re IFR rated!

The crew, the aircraft, and the airport / procedures all need to legally be able to get you down safely for it to be considered ‘normal’ or ‘safe’.

Have a Plan B

A lack of a plan B is a big contributor to these accidents, so actually having a plan B (and preferably a plan C as well!) can be a good mitigator.

How often do you take off with an actionable plan B in mind? How bad does the weather or situation need to get for you to start thinking about this? Try to reflect on this and see where you can improve.

At any time, you’ll always have 3 options:

1) Divert
2) Return back to base
3) Land & live

Don’t fall for Plan Continuation Bias

Push-on-itis, get-there-itis, get-home-itis. There are dozens of names for what essentially comes down to plan continuation bias. Plan continuation bias is slightly bigger than these though.

If the flight crew have agreed on what plan A is, plenty of research in human factors has discovered that we’re more inclined to continue with plan A, even if plan A makes no logical sense anymore.

Adapting to changing circumstances is the key here. If you’re stuck in your ways and are not willing to change when the situation demands it, you could be in for a rough ride (literally).

Know Your Personal Limits

If you’re a military pilot used to flying at 50’ and below, your personal limits are likely very different than a civilian trained pilot who is used to flying around at 2000’ in the summer.

What are your personal limits, have you ever defined them? If it’s not easy to define, just imagine what needs to be thrown on your lap for you to say ‘no, that’s it’.

Everyone has their own limits, and knowing them can be extremely valuable as it helps you recognise the situations where your limits are reached.

Maintain Terrain Awareness

Navigation databases, Flight Management Systems and Terrain Awareness Warning System (TAWS) are all great tools to help with terrain awareness. However, nothing beats a proper briefing and evaluation of what your upcoming flight will bring in regards to terrain.

Will you have to cross ridge lines, mountains, rising terrain? What is the elevation of the site you’re going to land in?

If your required separation from the ground is at least 500’ for instance, you will have to know your terrain height throughout your planned route. Mainly to make sure you’re aware of your required altitudes vs heights.

How to Deal with Inadvertent IMC?

Right, so you’ve entered cloud inadvertently. What should you do next? You don’t want to be another statistic! Let’s cover our recovery options step by step:

Inadvertent IMC

Stay Calm, Don’t Panic

Step 1, whichever way you’re going to spin this is to stay calm. Nothing good will come if you’re not calm when things start going wrong. Your ability to stay calm (or not) will heavily influence the outcome of what’s about to happen. Take a deep breath, remember your training, and carry out plan B (or create one if you’ve taken off without one pre-briefed!)

Trust Your Instruments

Step 2: Trust your instruments! We all have a natural tendency to trust our instincts and incoming sensory information. But guess what, we’re really not designed for any of this. None of us are.

While we might be slightly flustered and confused right now, your instruments are working just as well as 1 minute ago!

Admit that you’re in IMC

Don’t be in denial. You’ve inadvertently entered IMC, it’s time to deal with it now. You’ll often read in investigation reports that the flight crew was still ‘pretending’ that everything was fine, and that no adjustments were needed.

‘I can see the moon’, ‘I can still see the ground’, and ‘I think I still have references in the 11 o’clock’ are often simply not good enough.

You’re in IMC, start acting like it, time to perform a safe IMC recovery!

Maintain Control of the Aircraft

So you’ve stayed calm and kept an active scan going. Your primary concern now is to maintain a safe aircraft attitude at a sustainable airspeed. In addition to this, you’d preferably fly at a speed that allows for a potential climb in the near future, if you happen to be near rising terrain.

Use the Autopilot if available

If you’re flying an aircraft with an autopilot, make sure you make the most of the resources available to you. A simple altitude or heading hold can free up mental workload that you could be using right now in formulating your action plan.

Carry out your Inadvertent IMC Recovery Plan

Now it’s time to think about what you’re going to do about this. Your operations manual will often mention the operator’s preferred way for you to recover from this. The truth is however that every situation is different and might need a different plan of action.

In general, there are 2 main options:

1) Climb to a minimum safe altitude and divert IFR to an instrument approach-equipped airport (if you’ve been trained and have the equipment required).

2) Perform a level rate 1 turn by using the autopilot or trim system (avoid using force trim release in helicopters while in cloud) and return back to where you came from (if the situation allows for it and you’re confident you can break cloud this way).

Depending on the terrain and situation you’re in, you might be able to come up with different solutions, but these 2 are the most conventional ways to recover.

Tell ATC and ask for Assistance

Once you’ve established a plan and you’re executing it, establish 2 way comms with ATC and ask for assistance if required. This will be required anyway if you’ve picked option 1 above.

What Accidents have been Caused by Inadvertent IMC?

We’ll focus on the helicopter related crashes, but fixed wing is definitely not exempt from this. The most notable helicopter crashes in recent times are:

HEMS Helicopter Crash, Alabama

Right before a Eurocopter AS350 B2 took off in fog and darkness from a motor vehicle accident site in Alabama (U.S), the experienced pilot texted a friend that “the other guys would have turned around” rather than fly the HEMS mission, according to the final NTSB investigation report.

The helicopter crashed shortly after takeoff. It killed the pilot, the patient, and two medical crew members. The pilot was trained but not current for Instrument Procedures.

The report said the probable cause of the accident, which occurred just after midnight on March 26, 2016, was:

The NTSB identified two contributing factors: “the pilot’s self-induced pressure to complete the mission despite the weather conditions and the operator’s inadequate oversight of the flight by its operational control center.”

S76 VIP Helicopter Crash (Kobe Bryant), California

A Sikorsky S-76B helicopter, N72EX, entered a rapidly descending left turn and crashed into terrain in Calabasas, California.

The NTSB commented:


You can find the report here.

AW139 VIP Helicopter Crash (Lord Ballyedmond), United Kingdom

An AW139 took off in what was essentially IMC, lost references, and crashed. Everyone on board was killed. There were lots of different variables at play here, and the report is well worth a read.

The AAIB concluded:


General Aviation Pilot Behaviours in the Face of Adverse Weather

Surviving IIMC

56 Seconds to Live

Safety Study on VFR Flight into Adverse Weather

Inadvertent IMC Actions (Paolo Dal Pozzo)


Inadvertent Entry into IMC remains a big issue, particularly for helicopters and General Aviation. We’ve gone through what IIMC is, how to avoid it, and what to do to get back on the ground safely. Have we missed anything, or do you have other tips that pilots can benefit from? Let us know!

Categories: Safety

Jop Dingemans

AW169 HEMS Commander | Founder of Pilots Who Ask Why | Aerospace Engineer | Flight Instructor


Anonymous · November 6, 2023 at 7:13 PM

Thanks for sharing this information

Anonymous · November 6, 2023 at 5:56 PM

Great article sir. Thank you!

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