1
Airport Congestion Modelling
Airport surface congestion has several undesirable impacts, the most noticeable of which is an
increase in taxi-out times. The first step to mitigating the impacts of surface congestion is
characterizing the relationship between airport congestion and taxi-out delays. Understanding this
relationship would help us develop congestion control mechanisms that will ultimately reduce
excessive taxi-out times and the associated costs, including fuel burn, emissions and controller
workload.
In the context of this model, a pushback is defined as the moment in time when the aircraft is
physically detached from its gate and the taxi-out time is defined as the time between the pushback
and the take-off. The proposed model, shown in the figure above, consists of two modules, namely,
the process between aircraft pushing back from their gates and traveling to the runway, and the
queuing process at the departure runway.
Aircraft pushback from their gates according to the pushback schedule. They enter the ramp and then
the taxiway system, and taxi to the departure queue which is formed at the threshold of the departure
runway.
In order for an aircraft to pushback from the gate, a pushback tug is required. These pushback tugs
are special low profile vehicles that push the aircraft back from the gate.
The airport currently plans to have three such tugs to service departing aircraft. It takes a tug on an
average 8 minutes to push an aircraft back from the gate. Once the aircraft has been pushed back, it
will taxi forward to the runway. During this traveling phase, aircraft interact with each other. For
example, aircraft queue to get access to a confined part of the ramp, to cross an active runway, or to
enter a taxiway segment on which another aircraft is taxiing, or they get rerouted to avoid passing
through already congested spots. The magnitude of this delay will depend on the exact nature of
interactions among the taxiing aircraft, that is, the level and location of congestion in the ramp and
the taxiways. It has been estimated that it takes an aircraft an average of 10 minutes to taxi to the
runway.
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The departure process is a probabilistic process, impacted by factors, such as controllers’ and pilots’
decisions, aircraft performance, aborted take-offs, runway closures, etc. Aircraft arrive at the queue
for take-off, the service rate is assumed to follow an exponential distribution. There is finite queuing
space for the departing aircraft to wait in. Aircraft in the departure queue are served on a First-Come-
First-Served (FCFS) basis. There are two runways planned for the airport. Once an aircraft reaches the
runway, it waits behind other aircraft for its turn to take off.
The planners feel that the number of tugs deployed is insufficient. The airport planners would also
like to determine the space to be allocated for aircraft to wait for their turn to take off.
Table 1 shows a summary of some operational information.
Questions
1. Determine the time from requesting pushback to take-off in the current scenario.
2. Draw a graph of total cost (aircraft fuel burn for pushback + tug cost) vs. the number of tugs
deployed. How many tugs should be deployed?
3. If the airport decides to allocate space for three times the average number of aircraft waiting for
take-off, how much space should be should be allocated?
Table 1
Aircraft fuel burn rate during taxi 30 litres/min.
Cost of aviation turbine fuel Rs. 43,450 per kilolitre
Number of tugs available 3
Runways 2
Average number of departing flights per hour 20
Average take-off + separation time on the runway 5 min.
Operating cost + amortized capital cost for a tug Rs. 5,000/hour
You may make suitable assumptions if required. Clearly state your assumptions and their
justification.
Answers
Answer:
Airport Congestion Modelling
Airport surface congestion has several undesirable impacts, the most noticeable of which is an
increase in taxi-out times. The first step to mitigating the impacts of surface congestion is
characterizing the relationship between airport congestion and taxi-out delays. Understanding this
relationship would help us develop congestion control mechanisms that will ultimately reduce
excessive taxi-out times and the associated costs, including fuel burn, emissions and controller
workload.
In the context of this model, a pushback is defined as the moment in time when the aircraft is
physically detached from its gate and the taxi-out time is defined as the time between the pushback
and the take-off. The proposed model, shown in the figure above, consists of two modules, namely,
the process between aircraft pushing back from their gates and traveling to the runway, and the
queuing process at the departure runway.
Aircraft pushback from their gates according to the pushback schedule. They enter the ramp and then
the taxiway system, and taxi to the departure queue which is formed at the threshold of the departure
runway.
In order for an aircraft to pushback from the gate, a pushback tug is required. These pushback tugs
are special low profile vehicles that push the aircraft back from the gate.
The airport currently plans to have three such tugs to service departing aircraft. It takes a tug on an
average 8 minutes to push an aircraft back from the gate. Once the aircraft has been pushed back, it
will taxi forward to the runway. During this traveling phase, aircraft interact with each other. For
example, aircraft queue to get access to a confined part of the ramp, to cross an active runway, or to
enter a taxiway segment on which another aircraft is taxiing, or they get rerouted to avoid passing
through already congested spots. The magnitude of this delay will depend on the exact nature of
interactions among the taxiing aircraft, that is, the level and location of congestion in the ramp and
the taxiways. It has been estimated that it takes an aircraft an average of 10 minutes to taxi to the
runway.
2
The departure process is a probabilistic process, impacted by factors, such as controllers’ and pilots’
decisions, aircraft performance, aborted take-offs, runway closures, etc. Aircraft arrive at the queue
for take-off, the service rate is assumed to follow an exponential distribution. There is finite queuing
space for the departing aircraft to wait in. Aircraft in the departure queue are served on a First-Come-
First-Served (FCFS) basis. There are two runways planned for the airport. Once an aircraft reaches the
runway, it waits behind other aircraft for its turn to take off.
The planners feel that the number of tugs deployed is insufficient. The airport planners would also
like to determine the space to be allocated for aircraft to wait for their turn to take off.
Table 1 shows a summary of some operational information.
Questions
1. Determine the time from requesting pushback to take-off in the current scenario.
2. Draw a graph of total cost (aircraft fuel burn for pushback + tug cost) vs. the number of tugs
deployed. How many tugs should be deployed?
3. If the airport decides to allocate space for three times the average number of aircraft waiting for
take-off, how much space should be should be allocated?
Table 1
Aircraft fuel burn rate during taxi 30 litres/min.
Cost of aviation turbine fuel Rs. 43,450 per kilolitre
Number of tugs available 3
Runways 2
Average number of departing flights per hour 20
Average take-off + separation time on the runway 5 min.
Operating cost + amortized capital cost for a tug Rs. 5,000/hour
You may make suitable assumptions if required. Clearly state your assumptions and their
justification.