May 28, 2019
Weather conditions are a huge factor in aviation safety, and learning about cloud formations and their potential dangers when flying is a vital part of pilot training in New Zealand. Sometimes flying through clouds is just like driving through fog – you really can’t see much but nothing bad happens – but sometimes they can represent extreme danger for pilots, planes and passengers.
Across the history of aircraft flight around the world planes have been damaged and even broken up when encountering severe thunderstorm clouds in-flight. Some clouds such as the cumulonimbus are definitely a direct danger to aircraft; some clouds simply indicate a potential problem and others have no effect at all. During flight and in pre-flight planning it is the pilot’s job to assess and evaluate weather conditions as to whether they are a threat to the aircraft or not.
Types Of Clouds
In order to accurately assess threat levels as a pilot it is important to know the different types of clouds and how they affect weather and flight conditions. While there are officially hundreds of types of clouds, here in New Zealand the Civil Aviation Authority (CAA) categorises NZ cloud types in the following 3 ways.
1. Low Level Clouds that form below 6500 feet
- Stratocumulus (Sc) – flattened heaped cloud
- Stratus (St) – low level dull grey layer cloud
- Cumulus (Cu) – heaped or piled cloud
- Towering Cumulus (TCu) – towering heaped cloud
- Cumulonimbus (Cb) – rainy thundery heaped cloud
2. Middle Clouds that usually form between 6500 feet and 20,000 feet
(Nimbostratus clouds may form at lower altitudes in some parts of NZ)
- Altocumulus (Ac) – billowy mid-level cloud
- Altostratus (As) – mid level layer cloud
- Nimbostratus (Ns) – rainy layer cloud
3. High Level Clouds that usually form above 20,000 feet over New Zealand
- Cirrus (Ci) – thin wispy hair-like high level cloud
- Cirrocumulus (Cc) – billowy or rippled wispy cloud
- Cirrostratus (Cs) – veil like high layer cloud
Turbulence When Flying Through Clouds
Why do aeroplanes jump up and down and lurch around when flying through clouds? And can severe turbulence damage the plane? To understand the answer to these questions we must first look at exactly what a cloud is and the also understand the forces acting on an aeroplane during flight.
Clouds are made up of tiny water droplets formed by rising water vapour as it cools. These tiny water droplets form clusters in which the air swirls about unpredictably due to the denser internal conditions compared to the surrounding external air. As a result complex updrafts and downdrafts are created.
These cloud-borne updrafts and downdrafts result in rapid and unpredictable changes to the lift force on the wings of an aircraft. More or less lift and the difference between these changes is what causes the aircraft to lurch and jump about during flight, or turbulence as it is called within the industry.
While most encounters with turbulence are fairly routine (due to careful planning and skill), and aeroplanes are designed to cope with the forces and pressures placed upon them during these minor occurrences, it is the unpredictable nature combined with the low visibility that makes them a danger to pilots, aircraft and passengers.
What Happens In Severe Turbulence?
If a pilot is not skilled enough or experiences extreme levels of turbulence, this can cause them to lose control of the aircraft during flight. This is a particular concern around Cumulonimbus clouds (thunderstorm clouds) which can be concealing severe turbulence, strong vertical motions, severe icing, thunderstorms and hail.
This is why pilots do not choose to fly through clouds if they are avoidable and plan their flight path in order to avoid certain weather patterns involving large and dangerous cloud formations (for larger passenger aircraft this means flying up above the cloud base).
How Do Planes Fly Through Clouds?
Even though clouds are dangerous it is not always possible to avoid them during flight, so how do planes fly through clouds? First we need to understand the difference between flying using visual versus instrument based skills.
When training to be a pilot there are two sets of regulations that determine how and when pilots are able to fly their aircraft. These are using Visual Flight Rules (VFR) and using Instrument Flight Rules (IFR).
Visual flight rules allow pilots to basically look out the window and fly according to what they see. Flying IFR only means you have to rely solely on the aircrafts on-board instruments to maintain flight as there are little or no external visual references.
Not all pilots are IFR rated (able to fly IFR only) as this requires extra training and skills. The low visibility encountered when flying through clouds means using Instrument Flight Rules are necessary. This explains the need, not only legally but also practically to avoid clouds during flight if you are not IFR rated.
What Makes Flying Through Clouds So Dangerous?
Other than turbulence and having the right skills to navigate low visibility flight conditions there is one major thing that makes flying through clouds extremely dangerous. This is the possibility of mid-air collision, mostly likely with other aircraft but also with protruding buildings or landmarks taking up the combined airspace.
For aircraft which are able to navigate using IFR like large passenger planes, it is impossible to avoid passing through clouds during their daily flight paths, so how do they avoid collisions with other aircraft?
The pilots are kept informed by ground based Air Traffic Control (ATC) which keep track of all aircraft in the area using on-board transponders and radio communications. Should this fail some aircraft are equipped with on-board emergency Traffic Collision Avoidance Systems (TCAS).