FMC Forecast Page

How can the FMC FORECAST page be accessed? Where can the required input values be obtained? What is the purpose of entering these values, when should they be entered, and what is the operational significance of the FORECAST page?

The FORECAST page can be accessed by pressing the FORECAST prompt located at the lower left corner of the DESCENT page

Before explaining the purpose of the FORECAST page, let’s first review how the FMC plans a descent.

The FMC creates a descent profile starting from the highest descent constraint altitude along the route to the destination airport, extending up to the current cruise altitude. For example, if there is a restriction at YALOVA requiring 6000 feet, the FMC plans the descent to meet that constraint. This descent path is calculated assuming Flight Idle thrust — meaning the autopilot pulls the thrust levers back and the aircraft attempts to glide down the profile similar to a glider. On the Navigation Display (ND), in the lower right corner, a glide slope–like vertical deviation indicator shows whether the aircraft is above, on, or below the descent path.

What Factors Can Cause Deviation from the Descent Path?

1. Winds (the most significant factor)

2. Altimeter settings (if descent calculations are based on altitude rather than flight level)

3. Thermal Anti-Ice (TAI) usage:

   • If icing conditions are present, the engines must maintain a higher thrust setting than Flight Idle due to TAI being activated, which impacts the descent profile.

Although wind forecasts can be entered into the FMC, wind corrections for STAR waypoints cannot be input directly into the LEGS page.

How Do We Prepare for Descent Winds?

Before starting the descent, we try to obtain the ATIS for the destination airport.If ATIS is not available due to distance, we use the wind and weather information provided on the Operational Flight Plan (OFP).

When Should We Complete the FORECAST Page?

We complete the FORECAST page during descent preparations and prior to starting the descent.

What Information Do We Enter on the FORECAST Page?

First, we enter three sets of wind and altitude data:

1. Current wind and altitude:

   • This can be read directly from the Navigation Display (ND) and entered into the FMC.

2. Mid-level wind:

   • This is a wind report at an altitude halfway between cruise level and the destination airport elevation.

   • The data can be obtained from the Operational Flight Plan (OFP).

3. Final approach wind (approximately 1000 feet above ground level at the destination):

   • This also comes from the OFP or destination weather report.

The entry order does not matter — the FMC automatically arranges the winds in the correct sequence.

Next, we input the following:

• Destination airport altimeter setting:

  • This ensures that altitude constraints in feet are displayed correctly during descent.

• Transition Level (TL):

  • This tells the system when to switch from flight levels to altitudes, and triggers an alert on the Primary Flight Display (PFD) when passing through it.

• Thermal Anti-Ice (TAI) Status:

  • Indicate whether TAI will be used during descent (since TAI affects thrust and descent planning).

• ISA Deviation:

  • If available, input the deviation from standard temperature (ISA) for more precise descent calculations.

All of these inputs allow the FMC to calculate an accurate descent profile.

The Key Question:

When does the FORECAST page really make a difference?

Answer: When encountering tailwinds!

The primary risk during descent is staying too high.

• With headwinds, ground speed decreases, making descent easier.

• With tailwinds, ground speed increases, leaving less time and distance to descend, and significantly raising the risk of staying too high.

Thus:

• If experiencing strong headwinds, completing the FORECAST page is less critical.

• But if facing strong tailwinds, filling out the FORECAST page properly can save your day.

Pilot Tip:

"A headwind might just make you a little early — no big deal.

But a strong tailwind can leave you too high, too fast, and too far from making a stable approach. Plan your descent early and use the FORECAST page wisely!"