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					Science in the News: Planes slow down to cut costs

Title: Planes slow down to cut costs


This activity contains an article about planes flying slower in order to cut fuel consumption.
The article is followed by questions to test students’ comprehension, linking it to their
knowledge of forces and motion. Finally there is a challenge to encourage discussion about
how this relates to other issues such as lifestyle and the environment.

Links to the curriculum:

This activity looks at the factors affecting fuel consumption of an aeroplane, linking to:
Sc4      Physical processes – forces and motion
       2c          that unbalanced forces change the speed or direction of movement of objects
                   and that balanced forces produce no change in the movement of an object
       2d          ways in which frictional forces, including air resistance, affect motion [for
                   example, streamlining cars, friction between tyre and road].

Sc4      Physical processes – forces and motion
       2e          that balanced forces do not alter the velocity of a moving object.
Science in the News: Planes slow down to cut costs

                    Planes slow down to cut costs

Brussels Airlines in Belgium has announced that some of its
planes are flying slower to reduce costs, and that reducing the
speed of their planes by just 10 km/h will cut their annual fuel
bill by £800 000, adding only one or two minutes to the flying
time. An airline spokesperson said that only the 30 planes
flying on short routes within Europe are affected. The 19 planes
that fly longer routes to Russia, the Middle East and Africa fly at
pre-set speeds so the speed cannot be reduced.
The airline points out that using less aviation fuel is good for               (iStockphoto)
the environment, as well as being good for the airline. And using less fuel
means that the planes emit less global warming greenhouse gases and less pollution of
other kinds.
The airline is also looking at nearly 100 other ways to reduce fuel use, for example, using
the fuel more efficiently, reducing the weight of their aeroplanes and angling the tip of the
aircraft wings so that they have less drag on take-off. Some of the ways of reducing weight
include carrying less fuel, carrying less water, and removing the built-in stairways on the
planes. The airline also wants the European Union to change the way flights around Europe
are organised. This is because planes often end up flying a zig-zag route from one country
to another in order to avoid flying over other countries’ airspace.


1   Describe the main forces that act on an aeroplane when it is flying. You may wish to do
    this using a clearly labelled diagram.

2   Use your knowledge of forces to explain as fully as you can why flying at a slower speed
    uses less fuel.

3   Explain, again as fully as you can, why reducing the weight of the aircraft means less
    fuel is needed.

4   You are probably aware that the passenger cabins of aircraft are pressurised. This is
    because aircraft, especially those flying long-haul routes, usually fly very high. Use
    forces to explain the main benefit of flying at a higher altitude.

Discussion and challenge
Environmentalists criticise air travel, saying that air travel is one of the areas where
emissions of greenhouse gases have increased hugely in recent years. Split into a few
groups, representing different people who benefit from air travel, and ask each group to think
of ways in which their use of air travel could be changed so that it has less impact on the
environment. Hold a debate.
Science in the News: Planes slow down to cut costs

                  Answers and useful information

Answers to questions

1   Main vertical forces: gravity acting downwards, upthrust or lift acting upwards.

2   Main horizontal forces: thrust from the engine acting forwards, air resistance acting
    backwards, wind acting in any direction.

3   When the aircraft is flying at a constant speed the forward and backward forces on it are
    balanced. As the speed of the aircraft decreases, the air resistance decreases (as the
    aircraft does not have to push so many air molecules out of the way each second),
    therefore the forward force needed to balance the air resistance and so ensure the
    constant speed also decreases. It takes less energy to provide a smaller forward force,
    so the engine does not have to use as much fuel.

4   If the weight of the aircraft is less, the upthrust or lift from the aircraft wings needed to
    prevent it falling downwards is also less. The lift from the wings is provided by airflow
    over the wings. The faster the airflow, the greater the lift, so if less lift is needed the
    aircraft can fly at a lower speed, reducing air resistance and so reducing fuel

5   As the altitude increases, the air density decreases (thus the pressure falls, causing the
    need for the passenger cabin to be pressurised). This means that at any given speed the
    number of air molecules to be pushed out of the way each second is less, so the air
    resistance is less. Less air resistance requires a smaller forward force to maintain a
    given constant speed. Therefore it is more economical to fly at a higher altitude.

Discussion and challenge
Groups benefiting from air travel could include, for example, holidaymakers, business
travellers, food producers transporting perishable goods.
Ways to reduce the impact on the environment could include: reducing the weight of luggage
each passenger is allowed (thus reducing the weight of the aircraft), ensuring that aircraft fly
with a full complement of passengers instead of with empty seats, ensuring that cargo
aircraft are fully loaded in each direction, flying at higher altitudes.

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