What is turbulence? Is turbulence dangerous for commercial airliners?
Is turbulence dangerous? Can turbulence crash a plane?
Turbulence…. Annoying but rarely dangerous
Turbulence is probably the single most common cause of anxiety for airline passengers, yet it rarely causes the pilots any concerns about the safety of the aircraft. In all but the most extreme cases, turbulence is not a danger to the safety of the flight, rather more of an uncomfortable inconvenience .
There are a number of different types of turbulence, some of which can be detected by most commercial aircraft, whilst other types are invisible.
Turbulence comprises of the following types:
- Convective Turbulence. Convection is what causes clouds and convective turbulence is often associated with clouds (and is also responsible for low level thermal turbulence). The greater the vertical extent of the cloud, the greater the updrafts and therefore the worse the turbulence is likely to be. It is particularly bad when flying through Cumulonimbus clouds (which are associated with very heavy rain showers / hail or thunderstorms).
- Clear Air Turbulence. This is caused by jet streams which are very strong corridors of wind found at high altitudes. Jet Streams form between the boundaries of warm and cold air and therefore vary in position throughout the year.
- Low Level Thermals. This is hot air rising from the ground which is prevellent on hot days at low altitudes.
- Wake Turbulence. This is not weather related, rather a phenominom caused by other aircraft. As there is low pressure at the top of the wing and high pressure at the bottom (needed to produce lift), air moves from the bottom to the top of the wing at the wing tip. This causes wing tip vortices which are the cause of Wake Turbulence. Wake turbulence occurs for a few seconds and can be very violent. Air traffic control apply minimum spacing between aircraft to ensure adequate wake turbulence separation. Very strong wake turbulence has caused aircraft to crash in the past – hence there is now minimum distance or time separation between aircraft.
- Wind. Generally speaking, when closer to the ground, the wind is subject to friction and it’s flow is effected by the Earth’s surface and anything on it (like buildings etc). Therefore near, the ground, the wind is less laminar (straight and steady) and more turbulent (varying in direction and speed). Rapid variations in direction and speed can cause turbulence at low level, particularly when the aircraft is on final approach coming into land.
Can Pilots Detect Turbulence?
Pilots can detect certain types of turbulence using an onboard weather radar. The radar highlights where there are large quantities of water droplets (rain) on the pilot’s instruments. Generally speaking the bigger the rain drops, the bigger the cloud, and the more turbulent it will be inside that cloud. The pilots make a judgement on if they can fly through the cloud or avoid it by flying around it.
Can Pilots Avoid Turbulence?
Sometimes. There are some types of turbulence which can’t be detected, like Clear Air Turbulence or CAT which is associated with a Jet Steam. Sometimes it’s forecast and the pilots can do their best to avoid it by avoiding certain flight levels, but sometimes the forecasts are wrong and the pilots fly into it without knowing it’s there. That’s why they ALWAYS recommend that passengers keep their seat belts fastened whenever seated, regardless of the status of the fasten belt sign.
Prior and during flight, the pilots will be studying various weather charts which predict where any areas of Clear Air Turbulence is located, or where any thunderstorm (cumulonimbus) clouds mights appear during the flight. It provides them with a rough location and the altitudes that they are likely to be prevalent. Flight crew can then take action at the pre-flight planning stage to adjust their planned flight level or routing if required.
When pilots find themselves in turbulence they are almost always doing their best to get out of it. Not because it’s dangerous, but because it’s uncomfortable for both the passengers and crew. The pilots don’t like it any more than you do. To get out of it, pilots are regularly speaking to air traffic control and other aircraft to see what levels are free of turbulence. Unfortunately on some days, turbulence is prevalent at all levels and is impossible to avoid. In extremely rare cases where there is severe to extremely turbulence, the pilots may decide to divert the aircraft and land, but this is very, very rare.
On most occasions, what the general public would consider to be severe turbulence, is rarely more than moderate from the pilots point of view.
Turbulence is categorised into Light, Moderate and Severe. The official definitions from IATA are as follows:
Slight, erratic changes in altitude and/or attitude (pitch, roll, yaw).
- Liquids are shaking but not splashing out of cups
- Carts can be maneuvered with little difficulty
- Passengers may feel a light strain against seat belts
Changes is altitude and/or attitude occur but with more intensity than light turbulence. Aircraft remains in control at all times.
- Liquids are splashing out of cups
- Difficulties to walk or stand without balancing or holding on to something. Carts are difficult to maneuver
- Passengers feel definite strain against seat belt
Large, abrupt changes in altitude and/or attitude. Usually causes large variations in airspeed.
- Items are falling over unsecured objects are tossed about.
- Walking is impossible
- Passengers are forced violently against seat belts