Information Technology Reference
In-Depth Information
Some gates cannot be used simultaneously, because usage of one gate blocks
usage of another one, this constraint is called a 'shadow restriction' [9]. Moreover,
sometimes one large gate can be treated as either two smaller gates (for smaller
aircraft) or one large gate (for a single large aircraft). These large gates are
modelled as three separate gates with a shadowing constraint between them.
Figure 3 shows a large gate (
G
3) which would be modelled as three gates:
G
3
F
(full size gate),
G
3
L
(the left half of
G
3) and
G
3
R
(the right half of
G
3).
G
3
L
and
G
3
R
could be used at the same time by two small aircraft (
F
2,
F
3) but if
there is a large aircraft (
F
1) on
G
3
F
then neither
G
3
L
nor
G
3
R
can be used.
Let
Sh
denote the set of pairs of gates which have to be used exclusively (pairs
(
G
3
F,G
3
L
)and(
G
3
F,G
3
R
) would belong to this set). The 'shadow restriction'
can then be described as follows:
X
f
1
,g
1
+
X
f
2
,g
2
≤
1
,
∀
f
1
∈
F
(
g
1
)
,
∀
f
2
∈
F
(
g
2
)
,
∀
(
g
1
,g
2
)
Sh,f
1
=
f
2
,
(
e
f
2
≥
e
f
1
)
∧
(
e
f
2
−
l
f
1
≤
10)
.
(6)
Fig. 3.
Large gates can be used as two small gates by small aircraft
When an aircraft is at the airport for a long time, it is often towed from a gate
to a remote stand so that it is not occupying a gate that could be used by other
aircraft at that time. The aircraft is then usually towed back to the same or a
different gate some time before the scheduled departure time. Although having
many towing operations is not desired by airports (towed aircraft may cause
additional conflicts on the taxiways) there is often no other option if a terminal
does not have enough gates to serve all aircraft. It is assumed in this model that
an aircraft is towed away from a gate if it is going to stay at the airport for more
than 4 hours. This time can be changed according to airport preferences and
has been chosen based upon the data set that has been used in the experiments.
These long flights are modelled as two one-hour long flights. The first flight
represents the arrival and lasts one hour starting from the scheduled arrival
time. The second flight starts one hour before the scheduled departure time.
As previously discussed, the middle part is not modelled and sucient remote
stands are assumed, which is usually the case for most airports. According to
the data used in our experiments, an aircraft needs no more than an hour to do
all servicing after it arrives and no more than one hour before it departs, the
towing procedures are considered to be included in this hour. An exact time of
