The year is 1977, Los Rodeos Airport, Tenerife. KLM Flight 4805 (a Boeing 747-206B) initiates takeoff due to a mistake, while Pan Am Flight 1736 (a Boeing 747-121) is still on the runway. They collide. The impact and the resulting fire killed everyone onboard the KLM 747, and the majority of the Pan Am 747: causing more than 500 fatalities… To this day, it’s still the deadliest aviation accident in human history! One of the main contributing factors? Fog. Even today, the dangers of fog still show their face on a regular basis. The aviation industry has gotten a lot better at mitigating the risks, but there’s still plenty to improve on.

This one is just one example of many many accidents that have been catastrophic and caused lots of fatalities. Today, we’re going to have a look at what fog is exactly, how it’s formed, the different types of fog, the threats and accidents they have caused, and how we can mitigate this threat as aviators!

Do you like our articles and want to stay up to date with our latest & most relevant content? Follow Pilots Who Ask Why!

Aviation Accidents Caused by Fog

There have been a wide selection of incidents, serious indidents, accidents, and full on crashes because of fog. The dangers of fog are all highlighted really well in most of the accident reports of the accidents we’ll show you here.

Let’s look at fixed wing first. While most of the serious accidents that were fog related were a longer time ago than helicopter crashes, there have still been quite a few. These are the most catastrophic ones:

Dangers of Fog

Here are the official accident reports:

1977 Tenerife Runway Collision

1983 Madrid Runway Collision

1998 Amsterdam Near Miss

2001 Linate Runway Collision

2002 Air China CFIT

On the rotary wing side, crashes unfortunately still happen very regularly due to fog, low visibility, or low cloud. The main root cause for this is the type of operations that get flown in helicopters, and the lack of a controlled airport environment. Here are the more serious ones of the last 20 years:

Dangers of Fog

Here are the official accident reports (apart from the Danish HEMS accident, which is a link to Aerossurance):

2004 HEMS Terrain Collision

2014 VIP Take-Off Crash

2019 Offshore Loss of Control

2020 HEMS Near Miss

2021 HEMS Ground Impact

Of course, these lists are not exhaustive and there are many more accidents that have happened due to fog, these are just the more serious ones.

What is Fog and how is it Formed?

The formation of fog can have multiple causes. However, they always come back to the same 5 main steps (6 if you include how it dissipates as well), let’s go over what those look like:

Dangers of Fog

1) Moisture in the Air

Fog formation always begins with the presence of moisture in the air. Without enough moisture, you won’t get fog. This moisture can come from lots of different sources, such as nearby bodies of water, the ground, or recent precipitation.

The air mass that currently sits over the airport is the main contributing factor here though. For instance, air masses in the UK are usually of maritime origin. This means that their origin is often the sea. This is a type of airmass with lots of moisture in it, with therefore lots of potential to create fog, depending on the actual temperature.

On average, moist air masses have a higher dew point than dryer air masses. This means that the outside air temperature at which fog can be created is higher than for more dry air masses.

2) Air Cooling Down

Fog forms when an airmass cools down enough. Look at normal cloud formation for example. If the outside air temperature is 15 degrees, and the dew point of the airmass you’re in is 11 degrees, then the air would have to cool down 4º C in order to form cloud. Normal temperature lapse rates are roughly 2º C per 1000 feet. So the cloud base will likely be 2000 feet that day.

The reason this happens is because the higher we go, the lower the atmospheric pressure. This causes the air to expand, which lowers the temperature.

Other than normal atmospheric cooling due to altitude, cooling can occur in a lot of other ways as well:

Radiation: At night, the ground loses heat through radiation, causing the air near the ground to cool. This effect is even more pronounced without clouds keeping the heat within the lower parts of the atmosphere. This can cause a rapid drop in temperature closer to the ground.

Advection: When warm, moist air moves over a cooler surface (like a cold ocean), it can cool down as well. Usually this is a little more gradual than radiation cooling.

The air at the lower levels of the warm mass comes into contact with the cooler surface. Heat naturally flows from warmer objects to cooler objects. In this case, the warm air transfers some of its heat to the cooler surface it touches.

Then, as the warm air loses heat to the cooler surface, its temperature begins to drop. This process continues as long as the warm air remains in contact with the cooler surface.

Frontal Lifting: When warm air is lifted by a cold front, it rises and cools down. Colder air is heavier than warmer air. So the air inside a cold front will try to push the air in the warm sector upwards. For a warm front, the warmer air tends to slide over the already present colder air. Both force air upwards, causing cooling.

Orographic Lifting: Orographic lifting occurs in regions with elevated terrain, such as mountains or hills. When the local winds carrying moist air meet elevated terrain, the air is forced to ascend.

3) Air Reaching its Saturation Point or Dew Point

As the air cools, it reaches a point where it becomes saturated, meaning it can no longer hold all the moisture it contains. In other words, the outside air temperature (OAT) = the dew point. The smaller the difference between OAT and dew point, the higher risk for fog and low cloud.

A good measuring method for this is relative humidity. Relative humidity is a percentage that shows the current amount of moisture content in the air, compared to the maximum amount of moisture the air can hold before reaching its saturation point.

4) Condensation Nuclei Being Present

Tiny particles or dust in the air, known as condensation nuclei, play a crucial role in the formation of fog. They are tiny particles or aerosols that are naturally present in the air. These particles can be made up of lots of different types of material, such as dust, salt, pollution, or natural particles like pollen and sea particles.

Condensation nuclei act as the starting point for the condensation of water vapor in the atmosphere. When air becomes saturated with moisture (reaches its dew point temperature), it needs something to condense onto. This is where condensation nuclei come into play.

Condensation nuclei provide a surface for water vapor to condense onto. Water molecules in the air are attracted to these particles due to their slightly hygroscopic (or water-attracting in plain english) properties.

5) Visible Cloud Formation

As water vapor condenses onto condensation nuclei, it forms tiny water droplets. These droplets are so small that they remain suspended in the air, creating a cloud or fog.

When enough water droplets come together, they become visible as a cloud near the ground. Tadaa, we have fog!

6) Fog Dissipation

For fog to dissipate, one of these 2 things need to happen: either a decrease in moisture content, or an increase in air temperature.

Moisture reduction could be caused by higher winds that mix the fog layer with dryer air next to it, advection of drier air (more on this later), or a situation where fog transitions into higher clouds.

Heating could be caused by wind mixing warmer adjacent air with foggy areas. It could be due to air moving downslope and warming up, or due to turbulent air mixing air from above with the fog layer.

What are the Different Types of Fog?

Fog can take up many different forms and ways it gets created, here are the main ones:

Dangers of Fog

Radiation Fog

Radiation fog is a type of fog created by the rapid cooling of the earth’s surface. Often caused clear and calm nights where surface heat has an easier time radiating into space. In winter, nights last longer which increases the cooldown risk. However, the other side of the coin in moisture content, which tend to be higher within warmer air masses.

Low winds are required, usually 2-7 kts. Radiation fog is associated with high pressure areas, where the airmass is warm and stable, settles down, ready to cool down at night.

Advection Fog

Advection fog happens when a moist, warm air pocket travels across colder surfaces, cooling down, and reaching it’s dewpoint. It requires light winds, like 3-9 kts. If the wind is stronger than this, advection fog tends to transform into low stratus clouds.

Advection fog

Orographic Fog

Ever seen those time-lapses where a hill is causing continuous fog that seems to transition into cloud? That’s Orographic fog. It’s caused by moist warm air moving into an elevated surface, being pushed up, cooled down, and reaching its saturation point.

Orographic fog

Evaporation Fog

Evaporation fog is also called steam for or sea smoke depending on where you are in the world. It forms when cold dry air moves across warm water surfaces. Usually it happens during cold weather days, and is caused by the rapid condensation of water vapour from warm water bodies into water droplets when it comes in contact with cold dry air above the water.

Frontal Fog

When a warm front approaches, it pushes / slides warmer air over already present colder air. The warmer air is forced to rise, which cools it down, usually creating (low) stratus clouds.

The fog itself gets created by the warmer air inside the warm front coming in contact with a colder surface. Due to the higher moisture content in warmer air, there is a higher chance of it reaching it’s dew point.

Freezing Fog

Freezing fog is any fog that includes supercooled water droplets. Supercooled water droplets are droplets that technically have a temperature below freezing point, but have not frozen into ice crystals (yet). The reason for this is because there are no particles they can freeze onto (nuclei).

Shallow Fog

Shallow fog is a type of radiation fog that does not obstruct horizontal visibility higher than 6ft form the surface. If you look at a Meteorological Aerodrome Report (METAR), it shows up as MIFG (MI meaning Shallow, FG meaning FOG).

Basically, it’s fog that stays close to the ground, almost like a soft, misty blanket hugging the surface. It can make it impossible to see the surface at all, but sometimes you can see through it from the air (more on this later).

It’s different from regular fog, which can be thick and cover taller obstacles.

This is what it looks like (although it can be much shallower than in the picture):

Dangers of Fog

What are the Dangers of Fog and Reduced Visibility?

Despite a large part of aviation using Instrument Flight Rules, poor visibility and fog continues to be a big influence on aviation accident statistics.

Dangers of Fog

Increased Risk of Controlled Flight Into Terrain (CFIT)

CFIT continues to be one of the leading causes of crashes in the rotary industry. Of course, it’s never the only cause but part of a bigger picture. However, most CFIT accidents are linked to either fog, reduced visibility, or extremely low cloud bases.

Whichever one you pick, you’re dealing with circumstances that unfold quicker around you than high up with plenty of visibility.

Loss of Control in Flight

Even under IFR, visibility has a lot of influence on pilots. Situations such as Loss of Control In Flight (LOC-I) often have a lack of visibility or fog as one of the causal factors.


A famous one of this AW139 accident in Norwich, where pilots took off in fog from a private site and ended up in a crash that killed everyone on board. We are not designed to deal with fog. It’s up to us to use effective decision-making to mitigate the threats that come with degraded visual environments, more on this down below.

Inadvertent Entry into Instrument Meteorological Conditions

For operations that also operate under VFR such as SAR and HEMS, a reduction in visibility can even be a higher risk factor than a low cloudbase for Inadvertent Entry Into Instrument Meteorological Conditions.

If the visibility is good underneath cloud, it can be much easier to react to changing circumstances. Anything from other traffic, elevation differences, an unexpected change in the aircraft’s flightpath, and many more.

Increased Risk of Collision

VFR flight fundamentally relies on see-and-avoid procedures to ensure there’s enough traffic separation. If you take away visibility, you’re making things significantly riskier for VFR traffic.

Especially operations near airfields, where circuit traffic can be very close together, or funneled via the same incomiing and outbound routes and Visual Reporting Points (VRP’s), can become extra risky.

Increased Risk of Go-Arounds

Low visibility and fog is the main cause of go-arounds globally. A go-around by itself of course isn’t a reason for concern, but paired with other things that could go wrong, it can present some very unique challenges for pilots.

Reduced Situational Awareness for Air Traffic Control

As we’ve seen in the 1977 accident we mentioned earlier, ATC’s Situational Awareness is heavily reduced if the entire airfield is covered in fog. Tower controllers are aided by what they physically see outside to gross error-check their instructions. Taking away this layer of security and you’re one step closer to potential accidents within an airfield boundary.

Sensory Illusions

As humans, one of the main sensory inputs is vision, especially on a dynamic flight deck. Taking away the ability to see ahead of the aircraft flight path, even under IFR, can contribute to a lack of situational awareness and an increased risk for sensory illusions.

Increased Workload

Without visual stimuli ahead of the aircraft, we rely on instruments and workarounds to understand what is happening. Scanning these effectively can increase pilot workload. If you’re flying in a degraded visual environment, you will also have to react to things quicker, as they are happening closer to the location of the aircraft.

Higher Difficulty for Dealing with Emergencies

When things do go wrong, flying in reduced visibility can complicate things. Navigation can be trickier and the decision on where to go could be influenced by visually being able to judge weather ahead of you. If you have a weather radar and access to accurate weather reports, this is obviously slightly different. But even for IFR traffic, flying in IMC can unconsciously impact our minds compared to flying IFR in VMC.

How can we Mitigate the Threats of Fog?

Let’s cover how we can mitigate against these threats. Here are the main things we can do as pilots to ensure we don’t end up being part of accident statistics:

Thorough Pre-Flight Weather Briefings

Taking time for a thorough pre-flight weather briefing can make all the difference. In many of the accidents we’ll highlight at the end, a better brief could have made the difference.

When we’re unprepared to deal with unforeseen circumstances like fog or poor visibility, in combination with deviating from SOP, things go wrong. This is backed up by so many accident reports. It’s not the flights where poor weather is expected and properly briefed. It’s the flights that happened in periods of nice weather and crews that let complacency creep in.

Know your Aircraft’s Limitations and Equipment

What are your aircraft’s limitations when it comes to visibility and cloud base? That doesn’t just mean speeds and weather limits. Can you fly a CAT IIIC autoland, what is the lowest Decision Height you can fly to in the aircraft type you fly in?

Instrument Flying Recency and Proficiency

You can have all the equipment with the most forgiving aircraft limitations in the world, but if you’re not qualified and proficient to use any of them, it’s not going to get you very far.

Recency does not equal proficiency either. If your flight records dictate you’re current, you could still feel not proficient enough. This could definitely impact the way situations in fog and low visibility unfold.

Have a Plan B, and a Plan C

Get home-itis is one of the most sneaky human behavioural issues for aviators. Not having a plan B or even plan C makes this problem worse. If the crew is experiencing tunnel vision in a way that screams ‘we have to get back to airport X’, you’ve already made the first step to ending up in a big fireball.

Situational Awareness, Right Now and in the Future

Situational Awareness (SA) is something that always comes back in accident reports. It’s essentially the difference between actual reality and perceived reality by the flight crew. The smaller the difference, the higher your SA.

It’s not just about actual reality though, but also ‘future reality’. How efficient is the crew at thinking ahead, and expecting the next steps and the potential deviations that could take place.

Utilising Air Traffic Control

In many of the accident highlighted below, the contribution ATC played had a big role as well. If you’re flying around in fog or poor vis, ATC can act as an aid with both situational awareness as well as overall decision making.

The role of ATC becomes even more obvious when an entire airfield is completely covered in fog and no one, including ATC, can make visual contact with anyone else. Communication becomes even more crucial, and the crash in 1977 proves this.

Adhere to Standard Operating Procedures (SOP’s)

Yes yes you’ve probably heard this one a thousand times by now. But let’s be honest, SOP’s are there for a reason. People much smarter than us pilots have sat down at some point and thought about what to do in situation x or y.

Of course there might be moments, in the interest of flight safety, where deviation from SOP’s might be necessary. But do you really think you know better than an entire industry’s best practices?

Degraded Visual Environment Training

Degraded Visual Environment (DVE) Training is hugely beneficial to helicopter pilots. Dealing with white-out or brownout is part of this training, but situations that include fog and low visibility can yield the same difficulties. It’s not just militaries that are starting to invest more in DVE training, civilian operators are starting to do it more and more as well.

Thorough Fuel Management

Fuel = Time = Options. It’s such a basic but overlooked principle. The fact that some airlines even have entire ranking chart of captains who take the least amount of extra fuel doesn’t help..

If you suspect fog or low visibility operations are on the agenda for any flight, taking extra fuel onboard can make a massive difference for when shit does eventually hit the fan.

Diverting to another airfield won’t be possible at a certain point. Delaying this point as much as possible makes it easier to make a decision that isn’t pressured by the lack of options.

Appropriate Use of Crew Resource Management

Use all the information and tools that are available to you. Sounds obvious right? Well, it actually takes a lot of training and practice to do this properly. In the heat of the moment, it can sometimes be difficult to take a step back to make sure all information has been taken into account.

Of course, having a second pilot next to you who is there to not only help you, but also cross check your decisions is extremely valuable. Unfortunately this is not available to pilots who fly single pilot operations, so it becomes even more important for those pilots to cross check that they have taken all information and resources into account.

Efficient Decision Making

The brilliant thing about IFR flight is that we have a set ‘black and white’ decision point for any approach. This makes it easier to decide whether or not we can (legally) land at the destination airport. The problem is that not all airports are equipped with an instrument approach landing system (ILS) or even an approach based on GPS waypoints.

This blurs the lines on whether or not we should continue. If you ask 5 different captains, you might get 5 different answers whether or not they think it’s safe and/or appropriate to continue.

Effective CRM, communication, situational awareness and training all contribute to this with the aim to make the final decision less ambiguous. Ask yourself what the framework is you use when determining whether or not you should continue to a destination that isn’t equipped with IFR approaches, or worse, without weather reporting at all!

Familiarity with Visual Reference Requirements

Right, let’s revise what actually counts as ‘visual’ during an instrument approach. When things get really marginal, it could come down to whether or not you’re just about to see enough visual landmarks or not.

For this (in Europe), we need to look at EASA AMC1 to IR-OPS CAT.OP.MPA.305(e) and Appendix 1 to EU-OPS 1.430. They have this to say:

Non-Precision Approach
A pilot may not continue an approach below MDA/H unless at least one of the following visual references for the intended runway is distinctly visible and identifiable to the pilot:

(i) Elements of the approach light system;
(ii) The threshold;
(iii) The threshold markings;
(iv) The threshold lights;
(v) The threshold identification lights;
(vi) The visual glide slope indicator;
(vii) The touchdown zone or touchdown zone markings;
(viii) The touchdown zone lights;
(ix) Runway edge lights; or
(x) Other visual references accepted by the Authority.

Precision Approach
A pilot may not continue an approach below the Category I decision height … unless at least one of the following visual references for the intended runway is distinctly visible and identifiable to the pilot:

(i) Elements of the approach light system;
(ii) The threshold;
(iii) The threshold markings;
(iv) The threshold lights;
(v) The threshold identification lights;
(vi) The visual glide slope indicator;
(vii) The touchdown zone or touchdown zone markings;
(viii) The touchdown zone lights; or
(ix) Runway edge lights.

Category II Operations
A pilot may not continue an approach below the Category II decision height … unless visual reference containing a segment of at least 3 consecutive lights being the centre line of the approach lights, or touchdown zone lights, or runway centre line lights, or runway edge lights, or a combination of these is attained and can be maintained. This visual reference must include a lateral element of the ground pattern, i.e. an approach lighting crossbar or the landing threshold or a barette of the touchdown zone lighting.

Category IIIA Operations
For Category IIIA operations, and for Category IIIB operations with failpassive flight control systems, a pilot may not continue an approach below the decision height … unless a visual reference containing a segment of at least 3 consecutive lights being the centreline of the approach lights, or touchdown zone lights, or runway centreline lights, or runway edge lights, or a combination of these is attained and can be maintained.

Category IIIB Operations
For Category IIIB operations with fail-operational flight control systems using a decision height a pilot may not continue an approach below the Decision Height … unless a visual reference containing at least one centreline light is attained and can be maintained.

Category IIIC Operations
For Category IIIC operations, there are not visual requirements as it’s flown completely automatically, with NO decision height! Kind of crazy huh?

Efficient Use of the Autopilot

If your aircraft type is equipped with a (reliable) autopilot, making use of it in an efficient way can help massively with the reduction of workload and making sure the flight path is stable and predictable. This frees you up to think about more important matters.

There is still a reluctancy in the helicopter industry to use the resources an autopilot can provide to flight crew. This could be due to developed habits, the types the crew flew over the years, or the culture they’re part of. Either way, making sure you use the tools available to you is crucial for a safe outcome when fog and low visibility make things tougher than usual.

Contributing to a Safety Culture

Finally, the good old safety culture. You might have been bombarded with this word for the last few years, or maybe you’ve never heard of it (we hope you have). If the culture in your organisation is to ‘get the job done no matter what’, you might find it difficult to make safe decisions without feeling pressure by the environment you’re in.

This could result in pushing on when you shouldn’t, deciding to take off when you shouldn’t, or even discounting information that suggests your should change to plan B.


There we have it: Fog, its dangers, how its formed, how we can mitigate the threats it presents, and the accidents that have been largely caused by fog.

It’s up to us in the aviation industry to push forward and to continue mitigating threats that have proven to be a problem for overall safety. Feel free to share this article with anyone you think could benefit from it, and If you have any questions or comments, please leave them bbe

Categories: Weather

Jop Dingemans

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


Toby · September 12, 2023 at 4:02 PM

Hi Jop,

Excellent article, the Norfolk A139 VIP Take-Off Crash accident report links to the Leicester city football club A169 crash though.



Leave a Reply