airlift.gms : Airlift operations schedule

**Description**

In scheduling monthly airlift operations, demands for specific routes can be predicted. Actual requirements will be known in the future, and they may not agree with predicted requirements. Recourse actions are then required to meet the actual requirements. The actual requirements are expressed in tons, or any other appropriate measure, and they can be represented by a random variable. Aircraft of several different types are available for service. Each of these types of aircraft has its own restriction on number of flight hours available during the month. The recourse actions available include allowing available flight time to go unused, switching aircraft from one route to another, and buying commercial flights. Each of these has its associated cost, depending on the type(s) of aircraft involved.

**Small Model of Type : ** SP

**Category :** GAMS EMP library

**Main file :** airlift.gms

```
$Title Airlift operations schedule (AIRLIFT,SEQ=86)
$Ontext
In scheduling monthly airlift operations, demands for specific routes
can be predicted. Actual requirements will be known in the future, and
they may not agree with predicted requirements. Recourse actions are
then required to meet the actual requirements. The actual requirements
are expressed in tons, or any other appropriate measure, and they can
be represented by a random variable. Aircraft of several different
types are available for service. Each of these types of aircraft has
its own restriction on number of flight hours available during the
month.
The recourse actions available include allowing available flight time
to go unused, switching aircraft from one route to another, and buying
commercial flights. Each of these has its associated cost, depending
on the type(s) of aircraft involved.
Midler, J L, and Wollmer, R D, Stochastic programming models
for airlift operations, Naval Research Logistics Quarterly 16,
315-330, 1969.
Ariyawansa, K A, and Felt, A J, On a New Collection of Stochastic
Linear Programming Test Problems, INFORMS Journal on Computing 16(3),
291-299, 2004
$Offtext
Sets
i aircraft type / f1*f2 /
j routes / r1*r2 /
Alias (j,k);
Parameter
* Felt has 7200 but that's never binding ...
F(i) maximum number of flight hours for aircraft type / #i 7200 /
a(i,j) flying hours per round trip
b(i,j) carrying capacity in tons
c(i,j) cost per flight in $
d(j) demand / r1 1000, r2 1500 /
as(i,j,k) flying hours per round trip - switched flights
cs(i,j,k) cost per flight in $ - switched flights
pplus(j) cost for commerically contracted flight in $ / r1 500, r2 250 /
pminus(j) cost for unused capacity in $ / #j 0 /;
Table data(i,j,*,*)
a.'' b.'' c.'' as.r1 as.r2 cs.r1 cs.r2
f1.r1 24 50 7200 19 7000
f2.r1 49 20 7200 36 5500
f1.r2 14 75 6000 29 8200
f2.r2 29 20 4000 56 8700
;
a(i,j) = data(i,j,'a' ,'');
b(i,j) = data(i,j,'b' ,'');
c(i,j) = data(i,j,'c' ,'');
as(i,j,k) = data(i,j,'as',k);
cs(i,j,k) = data(i,j,'cs',k);
Variables
obj objective
x(i,j) number of flights originally planned
xs(i,j,k) increase in number of flights on k switched from j
slackp(j) contracted flights
slackn(j) unused capacity
Positive variable x, xs, slackp, slackn;
Equations
defobj objective
defcap(i) capacity
defcaps(i,j) limit the number of switches by original assignment
defdem(j) demand fullfilment;
defobj.. sum((i,j), c(i,j)*x(i,j) + sum(k$(not sameas(j,k)),
(cs(i,j,k)-c(i,j)*as(i,j,k)/a(i,j))*xs(i,j,k)))
+ sum(j, (pplus(j)*slackp(j) + pminus(j)*slackn(j))) =e= obj;
defcap(i).. sum(j, a(i,j)*x(i,j)) =l= F(i);
defcaps(i,j).. sum(k$(not sameas(j,k)), as(i,j,k)*xs(i,j,k)) =l= a(i,j)*x(i,j);
defdem(j).. sum(i, b(i,j)*x(i,j) - sum(k$(not sameas(j,k)),
b(i,j)*as(i,j,k)/a(i,j)*xs(i,j,k) - b(i,j)*xs(i,k,j)))
+ slackp(j) - slackn(j) =e= d(j);
model airlift /all/;
file emp / '%emp.info%' /; put emp '* problem %gams.i%'/;
$onput
randvar d('r1') lognormal 1000 50
randvar d('r2') lognormal 1500 300
stage 2 d xs slackp slackn defcaps defdem
$offput
putclose emp;
Set s scenarios / s1*s36 /;
Parameter
s_d(s,j) demand by scenario
s_x(s,i,j), s_xs(s,i,j,k) flights planned and switched;
Set dict / s .scenario.''
d .randvar. s_d
x .level. s_x
xs .level. s_xs /;
$ifi %gams.emp%==lindo
solve airlift min obj using emp scenario dict;
* Stochastic demand as Independent random variables
Set r realization / x1*x5 /
Table dRV(r,*) stochastic demand
r1 pr1 r2 pr2
x1 988.16 0.0668 1428.94 0.0668
x2 989.98 0.2417 1439.86 0.2417
x3 994.92 0.3830 1469.54 0.3830
x4 1008.37 0.2417 1550.22 0.2417
x5 1044.92 0.0668 1769.54 0.0668;
put emp '* problem %gams.i%';
put / "randvar d('r1') discrete "; loop(r, put dRV(r,'pr1') dRV(r,'r1'));
put / "randvar d('r2') discrete "; loop(r, put dRV(r,'pr2') dRV(r,'r2'));
put / 'stage 2 d xs slackp slackn defcaps defdem'
putclose emp;
solve airlift min obj using emp scenario dict;
```