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Early aircraft took off from any sufficiently large field that allowed take-off into the wind.
As aircraft became large and faster, runways were built in multiple directions. Nowadays,
new airports mostly have parallel runways. However, there are some airports use crossing
runways for take-off and landings. The question is why some airports still have intersecting runways
operations?, if you are curious on the answer, stay tuned and watch this video till the end!
Between the first and second world war, aircraft grew larger and flew faster.
These aircraft required longer runways. Few fields would be large enough to allow landing
in any direction. This was exacerbated through the need for a hardened surface,
at which point the real “runway” was born. To ensure that aircraft could largely operate
against the wind. With heavier aircraft, higher speeds, and longer runways most airports expanded
into parallel operations after WWII. Cross-wind tolerance was increasing as aircraft speed,
mass, navigation, and control improved. While operation into the wind was preferable,
an airport with only parallel runways was no longer an impossibility.
All of new airports have parallel runways since they have some advantages. Simultaneous use of
converging runways requires extra monitoring, so it counts on the factor of safety. It is also
expandable. Most airport expansions also increase the number of runways by adding parallel runways
and the expansion of through further parallel runways seems
to be less complex. One of a good example for this is Chicago O’Hare.
The following scene shows the evolution of Chicago O’Hare runway layout from time to time.
This airport changed from a system of three pairs in three directions before 2000,
followed by an additional parallel runway in 2008 and some additional improvement in 2015,
into a system with six parallel runways and only two intersecting runways in 2021.
However, some airports also have intersecting runways, often as a consequence of expansion
but are often used to provide a minimal crosswind option where wind direction is changing variably.
Although the use of both runways simultaneously may serve to increase flight efficiency
especially for shorter approach tracks and taxi routes for example,
there are considerable risks associated with simultaneous operation of intersecting runways,
rigorous protocols must be in place to avoid a runway incursion.
But then, why do crossing runways still in use?
The following are some of the reasons. First, it seems to have more sustainable capacity.
While crosswind limits have risen over the years, they still exist and do occasionally close down
airports with only parallel runways. This benefit only realises however, when multiple runways in a
different direction are available as crossing runways limit capacity through interaction.
Furthermore, the benefit depends on the amount of variation in wind direction at the airport.
Second, it has a historic growth. Once such an operation exists,
and the surrounding towns and cities expand, the interaction of aircraft noise
and building permissions make it difficult to significantly alter the operation without large
impact to the now-established surrounding community. Perhaps this aspect extends
further in that the varying direction allows for much more spread of the impact of noise.
Third, it demonstrates safety. Years of operations at large airports like Amsterdam,
JFK, Boston, Chicago O’Hare and San Francisco have proven that such runways can be operated safely.
This leads us to the last question, are the benefits really that significant?
Although the benefit of a more sustainable capacity appears to be significant, but adapting
to shifting winds comes at a cost. Wind conditions become more volatile as more
options are available. That is, having more options increases the risk of the
runway combination being used. Furthermore, the reconfiguration procedure is costly,
because the old and new aircraft streams on the ground and in the air may not be compatible,
there will be a temporary reduction in capacity. Additionally, the reconfiguration
is a high-workload process for ground and TMA air traffic control. Finally, a change
in runway orientation may have a significant impact on taxi times and consequent delays.
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