5 The tasks involved in turning around an "AirGB" aircraft for its return flight are listed in the table. The table gives the durations of the tasks and their immediate predecessors.
| Activity | Duration (mins) | Immediate Predecessors |
| A Refuel | 30 | - |
| B Clean cabin | 25 | - |
| C Pre-flight technical check | 15 | A |
| D Load luggage | 20 | - |
| E Load passengers | 25 | A, B |
| F Safety demonstration | 5 | E |
| G Obtain air traffic clearance | 10 | C |
| H Taxi to runway | 5 | G, D |
- Draw an activity on arc network for these activities.
- Mark on your diagram the early time and the late time for each event. Give the minimum completion time and the critical activities.
Because of delays on the outbound flight the aircraft has to be turned around within 50 minutes, so as not to lose its air traffic slot for the return journey. There are four tasks on which time can be saved. These, together with associated costs, are listed below.
| Task | A | B | D | E |
| New time (mins) | 20 | 20 | 15 | 15 |
| Extra cost | 250 | 50 | 50 | 100 |
- List the activities which need to be speeded up in order to turn the aircraft around within 50 minutes at minimum extra cost. Give the extra cost and the new set of critical activities.