Up in the air
Question: While discussing holiday flying arrangements, a friend and I began
to speculate on how many people throughout the world, on average, are flying in
an aircraft at any one time. What proportion of the world鈥檚 population does this
represent?
Answer: According to the International Air Transport Association in Geneva,
1.557 billion people were carried on flights throughout the world in 1999. If
this figure is divided by the number of days in a year, we can calculate there
are 4,265,753 passengers in the air throughout the course of a day. This is
0.071 per cent of the world鈥檚 population of 6 billion people.
Thomas Rendall
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Hereford
This does not answer the question of how many people are in the air at any
one time, a calculation which Thomas Rendall attempted but did not have all the
necessary data for. Here鈥檚 our attempt . . .
To find out how many people are flying at any one time, we first had to know
how long each traveller is airborne. In 1999, 2,764,629 million passenger
kilometres were flown worldwide. If this figure is divided by the 1.557 billion
journeys flown each year, we can calculate that the average distance flown on
each journey is 1753 kilometres. The average airliner flies at a speed of 851
kilometres per hour, including take-off and landing. This means the average
passenger is airborne for 2.06 hours per journey.
If the figure of 4,265,753 passengers in the air over a given 24-hour period
is divided by the number of 2.06-hour periods in 24 hours, we can calculate the
average number of people in the air at any one time. This is 366,144 or 0.0061
per cent of the world鈥檚 population.
Of course, figures like this are often too mind-bogglingly large or small to
put into any kind of mental context and, while we are told by Britain鈥檚 Civil
Aviation Authority that the number of planes over London varies from minute to
minute, it is possible to calculate an average figure for the number of people
circling over London waiting to land at any time.
London鈥檚 airports鈥擧eathrow, Gatwick, Stansted, London City and
Luton鈥攈andle 1000 incoming flights every 24 hours. Obviously, aircraft are
not evenly spaced throughout the 24 hours but for the purposes of the
calculation here, they have been. We have assumed that an airliner circles
London for an average of 15 minutes before landing. Because the average airliner
is carrying 180 passengers, the number of people circulating over London in any
given 15-minute period is 1000 (the number of incoming flights) multiplied by
180 (the average number of passengers per flight) divided by 96 (the number of
15-minute periods in each day). This tells any London-based reader that, at the
moment, 1875 people are in the air above their city鈥擡d
Words in the sky
Question: Pilots who write advertising messages in the sky using smoke from
their aircraft are a dying breed here in the US. Nonetheless, I鈥檝e often
wondered how they do it. How does a pilot manage to orient the plane in three
dimensions to write a message in the sky that can be seen clearly by observers
on the ground?
Answer: David Feldman鈥檚 book How Do Astronauts Scratch an Itch?
(Berkley, 1997) devotes five pages to a discussion of skywriting. The key is
meticulous planning, making a detailed flight plan and knowing when to turn the
smoke on and off.
Straight lines come from attention to the compass, smooth curves from skilful
banking, and equal-sized letters from good timing. And letters are often written
at slightly different altitudes, both so pilots can see what has already been
written, and so that they do not fly through a previous letter.
Skywriting isn鈥檛 written with real smoke, incidentally. A non-polluting oil
is pumped into the exhaust stream of the plane and vaporises in the heat,
producing a white cloud. Furthermore, skywriting is typically done at 2500 to
4000 metres, with letters up to 1000 metres high. The plane is flying at between
125 and 250 kilometres per hour, so it takes less than 20 seconds to draw a
stroke in a letter.
Joseph Mockus
Columbus, Ohio
Answer: Traditional skywriting requires highly specialised skills. But new
techniques and equipment make the job easier by emulating the performance of a
dot-matrix printer.
Smoke generators are used to produce short, sharp puffs that expand into
spherical balls. Continuous operation of the equipment leaves a line of dots
across the sky. By interrupting the sequence, gaps may be left to form
letters.
Seven aircraft are flown abreast with the formation leader in the centre. The
leader has programmed radio equipment which synchronises the smoke generators of
the other six aircraft so that the formation works like a gigantic printer and
the letters appear very square.
Compared with the old method, longer messages can be produced in a short time
in a smaller space. Also, much less skill is required to fly abreast in a line
than to produce individual letters with one plane.
Fred Grisley
Barry, Glamorgan
We contacted the Pepsi team of skywriters in the US who provided the
following additional information on skywriting the old-fashioned way.
Because skywriters work inside their lettering, it is impossible for them to
see the words they are writing. Additionally, they are sometimes working upside
down and looping-the-loop so their detailed flight plan, created before they fly
and pinned to their windscreens, often has to contain letters produced in
reverse. If you were holding a pen you would produce the letters in a constant
stream from left to right. However, skywriters sometimes have to backtrack to
produce letters. For example, an 鈥淎鈥� may be written by producing the horizontal
bar in the normal direction from left to right, but the inverted V used to
finish the rest of the letter may be written from bottom right, to top, to
bottom left because it is quicker that way. Letters may also be produced
鈥渂ackwards鈥� to prevent the wind blowing letters already produced into the flight
path. So skywriters may start at the end of a word and work towards the
beginning.
Often a pilot will use a road or rail track to make sure the writing follows
a straight line. The job has to be finished in as short a time as possible
because letters start to dissipate as they are produced, especially if there is
a strong breeze. Obviously, a blue sky and no wind is a skywriter鈥檚
诲谤别补尘鈥抬诲
This week鈥檚 questions
Wonderspace: I am endowed with a large bust and consequently always have to
wear a bra for support. If I was flying in space in a zero-gravity environment,
would I no longer require my undergarment? And, if not, what shape would my
breasts assume? So many female astronauts have gone into space that I feel sure
there is an answer.
J. Blake
Harrogate, North Yorkshire
The hole story: As I crossed the road yesterday it struck me that most
manhole covers in the street are round. Yet the ones covering the drains in my
backyard are rectangular. Is there a reason for the different shapes?
Nikki Steele
London