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Flying high

Question: I was recently aboard a transatlantic flight which had television
monitors displaying information about the flight and a map of our position. The
altitude of the flight puzzled me: we were flying higher than Mount Everest but
there are no mountains in the way between the US and Europe. What is the point
of flying at this height?

Answer: Drag on an aircraft is greatly reduced at altitude, so it can fly
faster. In essence, that is because there are fewer air molecules to get in the
way.

Engineers express the way altitude affects speed by calculating two speeds
for an aircraft: the indicated air speed (IAS) and the true air speed (TAS). IAS
is measured by air molecules entering a forward-facing, open-ended
intake—the air pressure this creates moves the speedometer needle.

At sea level, IAS equals TAS. If you fly higher, the air is less dense. So,
to get the same number of molecules down the intake, you fly faster. Now TAS is
no longer the same as IAS, and at cruising level TAS is about twice IAS.

Lift and drag both depend on IAS, not TAS. The higher you go with the same
speed on your IAS speedometer, the faster you will actually go over the
ground.

Engines for commercial airliners are also designed to operate better at
altitude. Optimum fuel performance is achieved at about 80 to 90 per cent of the
engine’s maximum revolutions per minute (RPM). Air density decreases with
altitude and, at fixed RPM, so does thrust. Only at high altitude will the drag
be low enough to allow maximum engine efficiency. To push the aircraft along at
the same speed at a lower altitude, where the air is denser, the engines would
have to operate at a lower percentage of maximum RPM.

Ian MacKenzie

Salisbury, Wiltshire

While reduced drag is the primary reason for high-altitude flight, there are
a other factors which benefit from flying at such levels—Ed

Answer: One reason why airliners fly at great heights is that jet engines are
far more efficient at those heights than at the minimum altitude needed to
provide safe terrain clearance. A jet engine produces thrust primarily by
heating air. Because the air at high altitude is much colder than at, say, 5000
feet, the temperature rise is greater and more thrust is produced from a given
mass of air.

Peter Elliot

Senior Keeper, Hendon Royal Air Force Museum

London

Answer: One advantage is that at high altitude the aircraft is more or less
clear of the troposphere, where all the bad weather, mixing of airstreams,
storms and turbulence take place. Being above the weather means a safer,
smoother flight for passengers, easier working conditions for the cabin crew and
less stress on the airframe.

Tom Croskery

Coleraine, County Londonderry

Readers will have noticed that the above answer leads naturally to
another question—Ed

Answer: Why don’t airliners keep on climbing until they are tickling the edge
of outer space so that they can maximise their performance? Well, there are
limitations. The first is an aircraft’s stalling speed. If an aircraft continues
to slow down in straight flight, there will eventually not be enough airflow
over the wings for them to do their job properly, and the aircraft will stall.
As the airliner climbs, and the air gets thinner, the stall speed increases,
which means that the aircraft’s minimum speed has to increase.

The second problem as the airliner goes higher is connected with the speed of
sound. The speed of sound in the air is related to air temperature and, as the
temperature gets cooler with altitude, the speed of sound decreases. An
airliner’s maximum speed is limited to a certain fraction of the speed of sound
(see below), so the airliner’s maximum speed also decreases as altitude
increases. As the aircraft climbs, its minimum speed and its maximum speed get
closer together. The maximum sensible altitude is one where there is a
reasonable range of speeds for the aircraft to operate in. For most airliners
this is normally around 35,000 feet.

Additionally, as the air gets thinner, there is less oxygen to burn the fuel,
so the engines lose power, but this is less important, because the aircraft
doesn’t need to dodge hard things, such as hills, at that altitude.

Mike Watson

Australian Transport Safety Bureau

Canberra

Answer: An aircraft’s speed is referred to in terms of a Mach number, the
ratio of its speed to the speed of sound. Above a height of about 28,000 feet,
the maximum TAS becomes constrained by the maximum Mach number. Most modern
aircraft have a maximum Mach number between 0.8 and 0.9, and normally cruise at
0.78 to 0.85. If the maximum Mach number is exceeded, the airflow over parts of
the aircraft becomes supersonic and this gives a rapid rise in drag and can
upset the aircraft’s handling.

Captain Richard King

Pershore, Worcestershire

This week’s question

Boxing clever: How do box kites fly?

Mike Taylor

Borough Green, Kent