Navigating Thunderstorms: Threats Every Pilot Needs to Know

In 2006, the pilot of a PA-34 Seneca attempted to fly through a line of convective activity over Tafton, Pennsylvania ⛈️

The extreme turbulence from a nearby thunderstorm ripped the aircraft apart, and killed the pilot and his two passengers.

Unfortunately this is just one of many examples. Delta Flight 191 crashed a mile short of the runway due to a thunderstorm-related microburst, killing 134 people.

There are many more…

Mother nature and aviation are not a good mix if we just leave them to it⚡️

What are the threats of thunderstorms, and how can (or can’t) they be mitigated?

From SHITWEL (yea that does sound ridiculous) to avoidance strategies: We cover it all in just a few minutes ⏱️

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How Does a Thunderstorm Form?

For a thunderstorm to develop, we need a well developed Cumulonimbus Cloud (CB). These are more likely to develop if we have:

1) A Lifting Force Trigger

Something will need to trigger the rising of air first ⬆️

There are a few ways this can happen, but mainly:

• Convective Uplift (air being warmed up)
• Orographic Uplift (air going up rising terrain)
• Convergence Uplift (air masses meeting together)
• Frontal Uplift (air being pushed up by a front)

But if dry air rises, nothing will happen. So we also need:

2) A High Humidity

No cloud without water vapour! ☁️

Moisture in the air will eventually turn into cloud when the dew point is reached. The higher the moisture content, the easier this point is reached.

3) A High Atmospheric Lapse Rate

When our parcel of air is rising, it will only keep rising if it remains warmer than the air around it, right?

This means that if the environment around this parcel cools down quicker than the air parcel, it will keep rising ⬆️

So, for a thunderstorm, we need a high temperature lapse rate in the atmosphere: i.e the air in the atmosphere cools down quickly.

What are the 3 Thunderstorm Stages?

A thunderstorm forms and develops through 3 main stages 📈

The Initial Stage

This is where warm air, with moisture, starts rising ⬆️

Air from the sides and below is drawn in to replace the lifted air. It can grow to a height of 25,000 feet at this stage.

The initial stage lasts about 20 minutes.

The Mature Stage

When rain starts falling, we’re in the mature stage.

The falling rain (or hail) creates downdrafts, which can reach up to 2400 feet per minute! The air going downwards is colder than the warmer layers below, so it often sinks even further than the bottom of the CB ⬇️

If the rain evaporates in this process, it makes the air even cooler, and therefore the downdrafts more intense.

These updrafts and downdrafts within the CB create a lot of friction, which builds up static electricity. Guess what that means? Lightning!⚡️

At this point, the CB can reach about 40,000 feet.

This stage lasts around another 15-20 minutes.

The Dissipating Stage:

Then finally, after about 40 minutes, the CB usually reaches the dissipating stage.

The turbulance here can be extreme, precipiation is heavy, and thunder and lightning is very common.

During this stage, you’ll see the CB grow as far as the tropopause. At this height the cloud is spread out by the upper winds, which can create these anvil looking tops:

The anvil can reach heights of 45,000 to 55,000 feet, and this dissipating stage lasts for another 1.5 – 2.5 hours! ⏱️

What are the Signs of Thunderstorms?

Rather than us rambling on about all the different signs of thunderstorms, here are all of the main ones you should look out for in one handy image ⤵️

What are the Threats of Thunderstorms?

Here’s a ridiculous sounding acronym to memorise all the threats of thunderstorms: SHITWEL

Let’s have a look:

Squalls

Squalls involve very sudden and intense changes in wind direction and speed (rising by at least 16 knots, and up to 22 knots or more!). They last for at least one minute (unlike the few seconds of a gust).

This can cause some challenging conditions for pilots to maintain control of the aircraft, especially during an approach where airspeed is lower, or during a confined area landing for helicopters 🚁

Hail

Hail can cause severe damage to wings, windows, antennas, and even the fuselage of a plane or helicopters.

If a large amount of hail gets ingested into the engine, the ice will melt and could cause an engine flameout💧

Pitot tubes can be damaged, sensors can get obstructed, and control surfaces can be disrupted by trapped ice.

Icing

Icing is always a risk for aircraft, but during thunderstorms, this risk is significantly increased.

Icing changes the aerodynamic function of wings and blades. It can cause damage to compressor blades, engine flameout, freeze control surfaces, and block pitot tubes.

It doesn’t stop there though!

Windscreens can lose their transparency if covered by ice, fuel vent systems can no longer balance pressure differences inside and outside fuel tanks, and overall aircraft weight can increase a lot ⚖️

Turbulence

While most aircraft have no issue dealing with even severe turbulence, it’s still a threat.

Why? 💡

A few things! Loss of control is very much a threat, especially in the mountains, at night, or during rescue missions. Turbulence has been known to (in severe cases) cause a complete loss of control.

The last few months have also demonstrated that turbulence is still a massive threat to passenger safety as well (wear those seatbelts!).

While airframes are designed to withstand turbulence, airframe fatigue and stress does increase over time if turbulence isn’t manage over the aircraft’s lifespan.

Wind Shear

Wind shear is a sudden large change in windspeed and direction over a short distance.

Wind shear is especially tricky for fixed wing pilots. While helicopters are affected by it, it generally has more impact on planes, especially during approach and takeoff.

However, helicopters also have performance penalties for having a tailwind component. In fact, having a headwind component is often a requirement for CAT A profiles ✅

On top of this, sudden changes in airspeed that wind shear creates also make an aircraft approach less stable overall, which is a massive cause of incidents in fixed wing aviation.

Electrical Interference

Onboard avionics can be badly affected by electrical interference. Nav systems, attitude and heading reference systems, magnetic compasses, and many others can be affected.

Antennas also become less effective if static electricity builds up on the aircraft’s fuselage. This can result in both poor reception and transmission quality.

Lightning

And then the next hazard: lightning ⚡️

We covered lightning, as well as triggered lightning extensively in this article.

For helicopters, disappating electrical charge is harder than for fixed wing aircraft. Something like a tail rotor or main rotor assembly is often the biggest victim if the aircraft gets hit by lightning.

For fixed wing aircraft, while strikes can cause damage, it’s usually quite manageable. Static discharge wicks and conductive pathways in the fuselage aim to provide a way for the electricity to leave the aircraft without causing damage.

How to Avoid Thunderstorms?

If you get caught in a thunderstorm, you’ve probably allowed for a few circumstances to develop without actively mitigating the threats. This, or you might not have had a plan B.

The cliche of prevention is better than a cure applies here. So what can you do to avoid thunderstorms?

Here’s an overview ⤵️

See and Avoid

On the days where thunderstorms are surrounded by clear skies, your eyes are a great tool. You can see them and then change course to avoid them.

This becomes trickier if thunderstorms are embedded in cloud layers. When you’re flying at night or in instrument meteorological conditions (IMC), you’ll need to use another tool in your bag…

Use Airborne Weather Radar

For those fortunate enough to fly with an airborne weather radar, this is the most obvious and effective tool to avoid CB’s. ⛈️

Weather radar technology is still improving a lot every day, and it’s becoming more and more effective at providing us with an accurate picture of where to expect bad weather.

Use Ground Based Weather Radar

If your aircraft does not have an airborne weather radar, there are still plenty of tools available to avoid thunderstorms and CB’s.

First of all, consult your own OPS manual and revise your company SOP on thunderstorm management.

Depending on where you are in the world, tools like Windy are an amazing resource for forecasts, but there are many other services.

Carefully Analyse Weather Forecasts

Weather forecasts include TAFs, SIGMETs, low altitude weather charts, high altitude weather charts, and many more. Especially for pilots who are used to flying in ‘fair weather’, it’s important to reflect on how thorough your pre-flight planning really is.

In the UK for instance, the transition from summer to winter often brings situations where pilots get caught off-guard due to the months of ‘fair weather’ (despite it being the UK).

We’ve covered vigilance vs complacency here. It can be beneficial to a think about where you can improve your own attitude towards pre-flight weather analysis.

Consider Flight Path Diversions

Having a thorough pre-flight planning phase will automatically set you up for having plan B’s and C’s. This should include what-if’s regarding the encounter of CB’s en-route.

Your OPS manual will specify what your horizontal and vertical clearance from CB’s should be, which usually depends on whether a weather radar is onboard or not.

Utilise Air Traffic Control

Air traffic control has access to ground based weather radar, and can inform pilots of developing weather situations. If you’re not sure, ask!

Listen out for Pilot Reports

Whether it’s pilots requesting a heading change due to weather, or pilots reporting weather conditions that aren’t forecast: Listening out for what others are up to can be extremely valuable data to build a mental picture of planned and unplanned situations.

Manage Your Altitude

Similar to diverting from your planned flight path, changing your flight altitude can be just as effective. The problem with CB’s though is that they can (as we’ve mentioned earlier) extend as high as 55,000 feet.

However, for CB’s that are still developing, or are confined to lower altitudes, climbing above them could be helpful. Usually though, horizontal separation is what’s advised by the literature and OPS manuals.

What if you fly into a Thunderstorm?

Hopefully with all of the awareness and tools we have today you will never find yourself anywhere near a thunderstorm, let alone in one. But, as it’s always better to be prepared:

• Fly straight and level: both you and your aircraft will already be under a lot of stress. To avoid further disorientation and aircraft stresses, focus on flying straight ahead and keeping wings level. Accept the inevitable changes in altitude and speed.
• Slow down: reduce your power requirement for helicopters, or use turbulence speed for fixed wing.
• Inform ATC: they are there to help you. Let them know that you’re in extreme turbulence and cannot maintain your assigned altitude. They can manage other aircraft in the area, and be ready with any other support you may need.
• Turn on your icing equipment: if it isn’t already on! You may well be in severe icing conditions, which as discussed above, can make this bad situation even worse.

Conclusion

Thunderstorms are one of the most dangerous threats in aviation. Understanding their formation, identifying their stages, and recognizing the hazards they come with, such as squalls, hail, icing, turbulence, wind shear, electrical interference, and lightning, is part of our threat error management as pilots.

Feel free to use this information and infographics amongst your workplace, but please reference Pilots Who Ask Why.

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