The course ends where the plan starts working. Every number computed on the ground (the leg times, the fuel remaining at each waypoint, the reserves) becomes a yardstick the moment the wheels leave, and this lesson is about holding the flight against that yardstick: watching track and time, keeping the fuel log, and changing the plan in the air when the comparison says it no longer holds.
Watching track and time
Flights deviate from plan for reasons the briefing already hinted at:
The wind — the actual wind is a measurement, the planned one a forecast.
ATC — assigned levels, speeds and routings other than those filed.
The air — outside air temperature moving TAS at a fixed Mach or CAS.
The aircraft — this airframe's performance differing from the book.
The tools against the drift are the plan's own: the actual ground speed is the distance between two fixes divided by the time between them, the expected time for the next leg is its distance over that measured ground speed, and the flight log is kept alive by entering, at every checkpoint or turning point, the actual time over and the estimated time over the next. A revised ETA is the same arithmetic run to the destination: any change of wind, level, temperature, Mach or CAS enters through the ground speed, and the ground speed re-prices every remaining minute.
Before the descent
One monitoring rule stands alone because it protects the ground clearance the whole IFR module was built on: the aircraft's position must be accurately determined before the descent is begun. Descent planning trades altitude for distance, and every safe-clearance figure along the way down was computed for a particular piece of ground; an aircraft that starts down from the wrong place takes those figures with it to ground that may be higher.
The fuel log
Fuel is checked at regular intervals and every check is recorded, for a reason beyond tidiness: a single reading proves little, but a written series exposes a trend, and a trend is how abnormal consumption (a leak, a stuck valve, a thirstier-than-book engine) is caught while there is still fuel to do something with. The arithmetic at each waypoint is the ground school's: fuel used is the before minus the now, the consumption rate is used over elapsed time, and the remaining figure is compared with the planned remaining the navigation plan printed for that waypoint. From the remaining fuel come the two survival numbers: endurance, the usable fuel less the final reserve divided by the current flow, and range, that endurance at the current ground speed. A change of wind, level, temperature, Mach or CAS revises the consumption exactly as it revised the ETA: through the new flow and the new ground speed.
The commander's floor
All monitoring serves one regulatory sentence: the commander must ensure that the fuel remaining is never less than what is needed to reach an aerodrome where a safe landing can be made with the final reserve still on board — diversion included. When the comparison of actual against planned says the destination can no longer be reached above that floor, the plan changes in the air, and an in-flight update is checked like a small pre-flight:
Suitability — the new destination or alternate must be adequate and available for the aircraft and the arrival time.
Weather — the conditions on the revised routing and at the revised destination and alternate must permit the operation.
The floor again — the aircraft must be able to land with the prescribed final reserve fuel intact.
The revised plan also reprices the arrival itself: the expected landing mass is the current mass less the fuel that will burn on the way to the new destination, checked against that aerodrome's landing-mass limit before the diversion is committed.