For pilots that are exposed to seasons, going from summer to winter often requires a mental flip of a switch. Just as you were getting used to flying in warm weather, you now have to get used to flying in winter! Say hi to freezing temperatures, icy runways, more unpredictable weather patterns, freezing fog, and queuing up for anti-icing, yay!
In a 19 year period, airframe icing resulted in 583 accidents and more than 800 fatalities in the US alone. More than 80% of these took place between October and March. Winter always comes with unique challenges for pilots. Challenges that often require more preparation, a higher level of vigilance and attention to detail. It presents threats for both cadets as well as experienced aviators. So today, we’ll go over exactly what the threats are, how we can prep in the best ways possible, and what accidents have already happened because of ice. Grab a hot drink…
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What are the Biggest Threats of Flying in Winter?
Winter flying presents a lot of unique threats and challenges for pilots. Let’s talk about the most important ones.
Icing can cause all sorts of issues for aircraft. When ice forms on aircraft it can cause:
As pilots, it’s up to us to make sure we manage each and every one of these factors, which we’ll talk about later. They can sneak up on anyone, and if left unchecked they can have horrible outcomes. Some of these will be covered in future articles.
Cold Weather Start-Ups
Cold weather affects the way we start jet engines in many ways. Let’s go over the main negative effects of cold weather on jet engine starts.
1) Battery Efficiency: First of all, if you fly an aircraft that starts from batteries instead of an APU or GPU, know that batteries are massively influenced by outside air temperature. Colder temperatures reduce battery efficiency.
When it’s very cold, the batteries might struggle providing enough current to initiate the start cycle at all.
2) Frozen Engine Components: This might sound crazy, but various reports have shown control surfaces, engine components, and even landing gears to be frozen after being left outside in freezing conditions. This can cause limited movement in various parts. In regards to the engine start cycle, this might mean that more power is required to get things going.
3) Low Fuel Temperature: Jet fuel can become thicker when it’s really cold. When this happens, it could present problems with atomising the fuel, which is required for ignition and proper combustion.
4) Higher Oil Viscocity: When oil becomes thicker due to colder temperatures, it reduces the amount of lubrication in the engines. This can affect the pilot’s (or FADEC’s) ability to start an engine properly.
5) Longer Warm-up times: Depending on the aircraft type, engines usually need to be at a certain temperature before you can continue with the rest of the checklists. During cold weather, this will often require extra time.
6) Fuel System Icing: Moisture in fuel tanks that is caused by condensation could freeze in the fuel lines. This could cause blockages and disrupt an aircraft start cycle.
7) Thermal Stresses: When engines go from very cold to very hot in a short period, it causes stress in components that would otherwise not be stressed as much. Over time, this can reduce life cycles of many different engine components.
Snow can have a big impact on visibility, and tends to reduce visibility quicker than rain does. This can cause surprising situations where the amount of precipitation isn’t forecast to be heavy, but still reduces the visibility a lot.
Double-check if your OPS manual considers snow as ‘icing conditions’. There are differences in where operators draw the line between icing and non-icing conditions. Often, you’ll find that a certain visibility threshold is used to determine this.
Yes, inversions can also form in summer. However, winter inversions tend to form quicker, and be stronger as well.
Rime and clear ice that form on the ground can create a layer of air above the surface that’s cooler than air higher up. This makes inversions quite common during winter. The problem is that this can cause two main threats:
1) Turbulance and strong winds: The sharp change in temperature creates layers of air with different temperatures that push and pull each other around, creating strong winds and turbulence.
2) Freezing Rain: When a warm layer of air is surrounded by two colder layers, things get a little interesting. The snow that is created in the upper cold layer travels through the warmer layer and starts melting. Upon entering the bottom layer, it starts freezing again, and once it hits the ground, can turn into ice. This is called freezing rain and can be a nightmare for airports to deal with.
Increased Required Landing Distance
While it’s the airport’s responsibility to keep the runway in an acceptable condition, winter will inevitably cause unfavourable runway conditions. This can increase the required landing distance.
Freezing rain, freezing fog, snow, hail and anything else that combines moisture and freezing temperatures can all result in various things. Mainly ice, snow and slush on the runway, which all increase the landing distance required.
In addition to these, an effect called frost heaving can cause runways to crack and reduce in quality. During cold weather, when moisture in the runway tarmac turns into ice, it expands and pushes the tarmac away.
When it then warms up again, the opposite happens, and the tarmac shifts again. This constant movement inside the tarmac can cause cracks and elevation differences. This reduces the quality of the runway over time.
Risks of White-Out
This one is for the helicopter pilots here, but it can also affect fixed wing pilots. White-out is a situation where the downwash or wake of an aircraft stirs up dust, or in this case snow.
This can massively reduce visibility in a very short time-frame, to the point where pilots can completely lose visual references, depth perception, or any sort of horizon.
This has caused many accidents in the past, and are almost always due to the above listed factors causing crew disorientation.
Reduced Visual References
This is slightly different than visibility. The vis could be perfect, but it might still be hard to make out shapes, buildings, masts, and even runways. When everything is covered in snow, depth perception can be tricky as well. It’s almost like flying with night vision goggles (although a little better than that).
Reduced Reliability of Equipment
Freezing and ice formation on equipment can have all sorts of effects. Sensors, inlets, membranes, pitot tubes, engine inlets, can all be covered, frozen, blocked, or completely broken if left unchecked.
Wheel fairings can trap water, which can freeze and jam the wheels or brakes. The same counts for the raising and lowering mechanisms, depending on the type you fly.
Anti-Icing Holdover Times
Holdover Time (or HOT) is the length of time that any type of anti-icing fluid will be able to do its actual job: prevent ice building up on your aircraft.
The published HOT is almost always a guideline and can never be guaranteed. There are a few factors that reduce the HOT:
Keep these in mind when planning your flight with anti-icing.
Airport Closures and Delays
Delays and airport closures are almost inevitable during winter. From snow and ice buildup on the runway that need clearing, to aircraft queueing up to get de-iced.
Once one airport gets behind schedule, it can create a domino effect of constant streams of traffic being diverted or otherwise not able to land. This can spiral out of control really quickly, affecting surrounding airports and diversion options.
Pilot and Passenger Comfort Challenges
With colder temperatures comes the need for adequate protection. Whether we choose to dress warmer, or make sure that Environmental Control Systems work properly. Please find EASA’s Carbon Monoxide checklist at the bottom of this article, for those who fly piston engine aircraft.
Something as simple as a pre-flight walkaround can be less straightforward when it’s icy, slippery, and cold outside.
Fog can be a challenge by itself, for both landing and taking off. Combine it with temperatures below zero and things get a little trickier.
Ice accumulation happens very quickly in freezing fog due to the many small supercooled water droplets within the fog coming in contact with the airframe.
The other considerations are a reduced braking efficiency due to potential icy moisture in the brakes, slippery runways, poor visibility due the combination of fog and the chance of icy windscreens. But probably the most sinister one: a weight increase due to the ice formation, which will impact the take-off performance of any aircraft.
How to Manage the Threats of Flying in Winter
So what can you do to mitigate all of these threats? There are many ways luckily, let’s cover the most important ones:
The Clean Aircraft Concept
The clean aircraft concept was developed by the FAA in FAR 121.629 and states:
This same regulation also prohibits taking off when you can expect ice deposits to form on the aircraft, unless you are part of an approved deicing/anti-icing program, which should publish HOT values (the ones we described earlier).
Plan for De-Icing and Anti-Icing
To achieve the clean aircraft concept we described above, you could either elect to keep the aircraft hangared, or use de-icing / anti-icing from the airport, if your OPS manual and aircraft flight manual allow for that.
However, for helicopters, most operators specifically mention that the use of de-icing or anti-icing is actually prohibited. In this case, the only way to keep it clean from icing is putting it in a warm hangar, no hairdryers please!
Check Aircraft Heating Systems
Engine inlet heating, blade heating, windshield heating, pitot heating, and the Environmental Control System should all be in perfect condition before flying in icy conditions.
If there are issues with any of these, you are taking on more risks than already present during winter OPS.
Cross-check your flight manual to see what parts are officially part of your icing-clearance. Try to be on top of how they work, so that if things go south, you are able to understand what is going on as early as possible.
Check for Snow and Ice Deposits
Many of the accidents mentioned in the introduction (and the ones we are going to show you at the end of this article), were caused by ice being dislodged and causing an engine flame-out.
Check wings, check inlet barrier filters, check engine inlets, and know how long your anti-icing fluid protects you for.
Ice being dislodged during take-off is a massive safety risk. For helicopters, knowing how your inlet barrier filters work will help in understanding what the risks are. See the accident reports at the end of this article for more information on this.
Consider the Risk of Fuel Tank Condensation
Fuel tank condensation is a common issue with many aircraft types. This becomes more of an issue during winter when that same condensation can potentially freeze, get into the fuel, and potentially block lines or eventually get into the engines.
Keeping an aircraft in a warm hangar when not in use is the best way to avoid this.
Check Battery Health
Maintenance departments should preventively check battery health of aircraft when winter is approaching. For you pilots lucky enough to always start off an APU or GPU, this is less of an issue.
However, there are still many fixed wing and rotary aircraft that sometimes have to rely on batteries.
Check Tyre Pressure
Tyre pressure can fall more quickly during colder periods. Make sure you inspect this, whether you have an electronic readout, or check it during a visual pre-flight check.
Include Extra Equipment on the Weight and Balance
For those in operations where dispatch sorts out your weight and balance, this is less of an issue. However, for those who are completely responsible for their own weight and balance calculation make sure those extra winter clothes, equipment, spare aircraft batteries, and all the other stuff operators tend to ‘throw’ into the aircraft during winter, are actually on the weight and balance.
Consider Extra Engine Warm-Up Time
This is an item particularly relevant for operations where time is critical. Operations such as SAR or HEMS come to mind.
It can be hard to just sit there and wait for the engines to warm up. But this is critical, especially during winter. Don’t let impatience get the better of you, give the engines enough time to get to a normal operating range.
Obtain Accurate Weather Updates
Weather can be even more of an issue during winter. This is mainly due to weather being more variable. Things can change rapidly, and after months of summery sunshine, we might have gotten slightly more complacent!
Carry and Know about Available Survival Equipment
What equipment do you actually carry on board? Are you still familiar with all the different items. Are those items regularly checked, and if so, are they still in date?
Thorough Alternate Selections
When selecting alternates, pay extra attention to your required landing distance based on different runway conditions. Consider the services available at the airport, and make sure you have access to updated weather forecast for each alternate option.
Have a Plan B, and C
When there are so many different variables that are trying to ruin your day, having your backup plans ready to go can make such a huge difference. Don’t settle for just having a plan B, have a plan C as well. As soon as plan B comes around the corner, line up your plan D!
Carry Out Temperature Altitude Corrections
With cold weather, it’s even more important to check the approach plate for the minimum temperature, if you’re relying on barometric altitude for your vertical guidance. After months of summer flying, this might not have been an issue, which makes it harder to get in the habit of checking this thoroughly before every approach briefing.
Know the Effects of a Blocked Pitot Tube
Aviation Accidents Caused by Ice
Many many accidents have happened due to winter (or simply cold) weather. We’ve selected a few of the most relevant and notable ones, and will cover the main lessons for each.
Air France Flight 447
An Air France A330 suffered from inconsistent airspeed indications, which led the pilots to stall the aircraft. They failed to recover from it and crashed into the Atlantic Ocean. All 228 people on board were killed.
The investigation revealed that ice formation in the pitot tubes caused blockages, which caused the autopilot to disconnect. While the pilots did not correctly deal with the malfunction, the root cause was blocked pitot tubes due to ice formation.
The investigation notes:
Scandinavian Airlines System Flight 751
An MD-81 on a scheduled flight from Stockholm to Warsaw, had to make an emergency landing. Ice had been collecting on the root of the inner wings, and had broken off during takeoff and was ingested into the engines while becoming airborne.
Both engines failed, resulting in the emergency landing. No one was injured.
The investigation report concludes:
Norwegian Air Ambulance Engine Flame-Outs
A BK-117 D2 experienced a single engine flame-out while in the cruise at night using Night Vision Goggles. The crew decided to land. Shortly after landing, the other engine unexpectedly flamed-out as well.
Air Methods Helicopter Engine Flame-Out
In 2013, an Air Methods Medical EC130 Helicopter experienced a hard landing due to a loss of power from its main engine. All people on board were seriously injured.
The investigation revealed:
British Airways Flight 38
A Boeing 777-200ER crashed just short of the runway at London Heathrow Airport. It was the first B777 that was declared a hull loss. There were no fatalities, but 47 people were injured.
The investigation revealed that ice crystals in the fuel were the root cause of the accident. The ice clogged the fuel/oil heat exchanger of both engines, restricting the fuel flow into each engine.
Winter Flying Resources
We’ve gathered a list of winter flying resources. Click on any of the links below to check them out!
It’s up to us to raise our vigilance during winter, and doublecheck items that you could otherwise get away with in more fair weather.
If this article helped you refresh some items, please consider leaving us some feedback, and share it with your colleagues. For any questions, please feel free to reach out to us!