Final answer:
The yellow line on the FUEL indicator of the PFD indicates the caution zone for the fuel quantity. It warns the pilot that the fuel level is approaching a low level. The pilot should monitor the fuel level and take action if necessary.
Step-by-step explanation:
The yellow line on the Fuel indicator of the Primary Flight Display (PFD) in an aircraft indicates the caution zone for the fuel quantity. It serves as a visual warning to the pilot that the fuel level is approaching a low level and may need attention soon. The line is usually located below the redline, which represents the minimum safe fuel level.
For example, let's say a PFD has a fuel gauge that ranges from 0 to 100. The yellow line may be set at 25, indicating that when the fuel quantity reaches this level or falls below it, the pilot should be cautious and consider refueling. In conclusion, the yellow line on the FUEL indicator indicates the caution zone for the fuel quantity, serving as a warning for the pilot to monitor the fuel level and take appropriate action if necessary.