Skip to main content

​
CPM Engine Fundamentals

GlidePath uses Critical Path Method (CPM) scheduling to automatically calculate task dates, identify the longest path through your project, and quantify scheduling flexibility for every task.

​
Feature Spec Reference

Module 3.1 — Forward Pass
Module 3.2 — Backward Pass

​
What CPM Does

CPM answers three questions for every task in your schedule:
  1. What is the earliest this task can start and finish? (Forward Pass)
  2. What is the latest this task can start and finish without delaying the project? (Backward Pass)
  3. How much scheduling flexibility does this task have? (Float Calculation)
GlidePath runs CPM automatically every time you make a change — add a task, edit a date, modify a dependency, or change a constraint.

​
The Forward Pass

The forward pass starts at the project start date and calculates Early Start (ES) and Early Finish (EF) for every task:
For the first task (no predecessors):
  • Early Start = Project Start Date
  • Early Finish = Project Start Date + Duration
Effect of ASAP scheduling: Most construction tasks default to As-Soon-As-Possible (ASAP), which means GlidePath schedules them at their earliest possible date. This produces the shortest overall project duration.

​
The Backward Pass

The backward pass starts at the project end date (the latest Early Finish calculated in the forward pass) and calculates Late Start (LS) and Late Finish (LF) for every task:
For the last task (no successors):
  • Late Finish = Project Early Finish
  • Late Start = Project Early Finish - Duration

​
Float (Total Slack)

Float represents how many days a task can slip before it delays the project end date:

​
Recalculation Triggers

GlidePath recalculates CPM immediately after any of the following:
  • A task’s duration changes
  • A task’s start or finish date is directly edited
  • A dependency is added, removed, or modified
  • A constraint is applied, changed, or removed
  • A calendar change affects working days
  • A task is added, deleted, indented, or outdented

​
ALAP (As-Late-As-Possible)

Non-critical tasks have float that gives them scheduling flexibility. If you want a task scheduled as late as possible without delaying the project, you can apply an ALAP constraint. GlidePath will schedule it at its Late Start date. Most tasks should remain ASAP unless there is a specific reason to delay them (e.g., deferring material procurement until the last possible moment to preserve cash flow).

​
Free Float vs. Total Float

Total Float measures how much a task can slip before affecting the project end date. Free Float measures how much a task can slip before affecting its immediate successors. A task can have total float but zero free float if its successors have no float themselves. GlidePath displays Total Float in the Float column. Free float is not displayed separately but is implied by the schedule structure.

​
Next Steps