One of the nagging worries with any aircraft flight is
turbulence. It sometimes strikes without warning and can produce a frightening
experience for even the most seasoned of fliers.
Mostly turbulence is only minor – a series of “bumps” that
might last for a few seconds - but at the other end of the scale it can be far
more serious. Major and abrupt vertical and horizontal changes in the position
of the aircraft can throw loose objects and even people, around the cabin with
injuries sometimes resulting. And at the extreme end of the spectrum turbulence
can produce structural damage to the aircraft and in some very rare cases
result in an aircraft accident.
Turbulence has four broad categories:
(1) Light: slight “bumpiness” but with little of no
discomfort for passengers.
(2) Moderate: Passengers walking down the aisle will be
moved off track; loose objects in the cabin will move about.
(3) Severe: Any standing passengers will be thrown off
balance. Loose objects will pitch onto the floor.
(4) Extreme: Unsecured passengers thrown violently about – they
may strike the roof or lateral areas of the cabin. Loose objects turn into interior
missiles.
Cases three and four are quite rare but when they do occur
can cause injury to passengers or cabin staff. In Australian airspace we see
about between 2 and 3 reported significant turbulence encounters per month, or
24 to 36 per year. In terms of the number of flights and people carried during
this time this represents only a very small percentage of flights.
In flight turbulence is caused by small scale currents or
vortices in the air that produce “bumpiness” when the aircraft flies through
it, similar in some ways to a car driving over a series of corrugations in the
road. There are several ways turbulence is produced in the atmosphere.
Thermals: These are rising parcels of air generated by the
atmosphere being in contact with a warm surface below (land or ocean). If there
is sufficient moisture in the air, cloud will form over these thermals and form
billowing cotton wool formations called cumulus cloud. An aircraft flying
through this type of cloud will usually experience light to moderate
turbulence.
Aircraft flying through cumulus clouds will normally experience turbulence because of the thermal activity. (Click to enlarge)
Thunderstorms: If rising air continues to be generated in a
cumulus cloud it can grow to prodigious heights before it finally flattens out,
typically at heights of 10 to 15 km above the ground. When the cloud grows to
this height it contains regions of powerful “up and down” draughts in which
there is a mixture of rain and hail, with lightning and thunder also added to
the mix. This is a giant cumulus cloud, more properly called a cumulonimbus and
is popularly known as a thunderstorm.
Pilots will avoid penetrating a cumulonimbus cloud
because turbulence inside can be severe to even extreme. Fortunately these
clouds are highly visible by day and also easily detectable on radar both day
and night so are readily avoidable in a modern passenger aircraft.
Moderate to severe turbulence is often encountered within cumulonimbus clouds because of the violent mix of updrafts and downdrafts within. This diagram is not drawn to scale - in reality the aircraft would be less than the size of a pin head here. (Cloud image from Wikipedia Commons, click to enlarge)
An infamous case of thunderstorm penetration involving the
loss of an aircraft occurred in November 1961 at Mascot airport in Sydney. See
Mechanical turbulence: As wind passes over the ground, air
in contact with the surface is slowed by friction, while the air above
continues at the unimpeded speed. This creates many thousands of rolling
vortices, and depending on the wind speed and roughness of the terrain, these
can extend upwards for a considerable height. Aircraft flying in these regions
will encounter “bumpiness” called mechanical turbulence, that is usually only
in the light to moderate range.
However in some cases, when strong winds blow over a
mountain range, the air can organise itself into large organised waves that
extend several kilometres downstream of the mountains. Very powerful rising and
falling air currents are often present in these waves and organised bands of
clouds sometimes form near the wave peaks. These waves are called lee waves or
mountain waves and the clouds are called lenticular because of the
characteristic “lens” shape they often adopt. Just as we see with waves on the
ocean, mountain waves sometimes topple and break producing circulations called
rotors.
Mountain waves (or lee waves) can produce severe turbulence. Clouds sometimes form at the peak of the waves. (Click to enlarge)
Turbulence produced by mountain waves can be moderate to
severe, and in extreme cases, the actual destruction of an aircraft has been
recorded. Perhaps the most infamous of these was the disintegration of a Boeing
707 jet, Flight 911 that was flying near Mt. Fuji in Japan on February 4th
1966. It is believed that the aircraft flew into an area of powerful mountain
waves that produced extreme turbulence and failure of the air-frame. A detailed
analysis of this incident can be found at
Wind shear:
Put simply, wind shear is the change of wind speed and direction
with height or over a horizontal distance. There is always a degree of wind
shear in the atmosphere, but when big changes take place over short distances, significant
turbulence can result.
Wind shear turbulence can form when one layer of air slides
across another, at a different speed or from a different direction, or both. This
can generate wave like disturbances at the interface, called Kelvin Helmholtz
waves, and if there is cloud present in this zone, the waves become visible.
Kelvin Helmholtz waves form when the winds above a certain layer (the dotted line here), are stronger than those below. (Image from Wikipedia Commons, click to enlarge)
But in many cases there is no cloud to alert the pilot that
unusual atmospheric motion may be present and in these cases the phenomenon is
referred to as “clear air turbulence” or CAT.
Jet streams, that are high-speed rivers of air that
circulate the globe at approximately 10 to 15 km above the surface, are areas
where CAT is often encountered. There are two main jet streams in each
hemisphere, the polar jet and the subtropical jet and these meander and change
direction as they circle the earth.
The main jet streams of the Earth. (Image from Wikipedia Commons)
Because jet
streams are zones of high-speed winds running within the broad-scale atmospheric
flow, there are surrounding regions where the wind speeds change quickly with
both height and horizontal distance, and this can generate considerable wind
shear and therefore turbulence. For more information on jet streams see
To get the
issue of in flight turbulence into perspective it should be noted that even the
most frequent fliers will never experience severe or extreme turbulence, and
that the modern passenger jet is fantastically strong and well able to handle in
flight turbulence in all its forms.
But for the
passenger, by far the best protection against turbulence is to fasten your
seatbelt. When not moving about the cabin keep the belt buckled up, and even if
severe turbulence is encountered you will safely ride it out if properly
secured.
