Descent...

hakansokmen
Apr 27
2 min read

When Should We Start the Descent?

Although it may vary from airline to airline, we generally request and initiate descent 20 NM prior to the Top of Descent (T/D).The reason for this is twofold: to allow the aircraft to transition smoothly into descent and to avoid overshooting the T/D and remaining unnecessarily high.

How Do We Initiate the Descent?

Set the target descent altitude in the MCP ALTITUDE window.
On the FMC DESCENT page, select DESCEND NOW.
Confirm the action with the Pilot Monitoring (PM) and then EXECUTE.
The aircraft will start descending.

How Does the Aircraft Descend?

When descent is initiated before the T/D point, the B738 systems initially command a shallow descent at approximately 1000 feet per minute.

The goal is to intercept the original descent path from below.
Once the aircraft captures the descent path, thrust is reduced to Flight Idle to maintain the descent profile efficiently.
If the aircraft falls below the descent path, thrust is increased — something we want to avoid in order to maximize fuel efficiency.
If the aircraft cannot maintain the planned descent speed, the FMC will display an UNABLE message.
In such a case:
- Use SPEED BRAKE to assist the descent.
- If this is insufficient, VNAV can no longer be used, and you must switch to LEVEL CHANGE mode.
- Adjust descent with a combination of Speed Brake and airspeed management.

What Is the Effect of Wind?

Strong tailwind:
- Increases the risk of staying too high.
- It is advisable to start the descent even earlier.
- Practical descent calculations assume no wind unless corrected manually.
- Although winds aloft can be entered into the FMC, we cannot always input winds for the STAR portion; if the FORECAST page is not completed, the FMC may miscalculate the T/D point, initiating descent too late.
- Tailwind increases ground speed, reducing the available time for descent — be prepared for this.
Headwind:
- Headwinds are generally less critical.
- At worst, you might start descent slightly early, but it is not as problematic as tailwinds.

Note for Simulator Operations:

While flying in the simulator environment, please apply descent management exactly as described above to practice proper energy management and descent planning.