What happens if the N1 does not shift from flight idle to ground idle upon landing?

When the N1 does not shift from flight idle to ground idle upon landing, the primary effect is that the thrust of the aircraft remains at a higher level than it should during the landing phase. This can lead to a longer landing distance because the engine continues to produce thrust rather than reducing power to allow for deceleration. As a result, the aircraft may require a greater distance on the runway to come to a complete stop, which can be critical depending on the runway length and conditions.

The other options suggest drastic measures or outcomes that are not directly influenced by the failure of N1 to shift properly. Aborting the flight, grounding the aircraft, or automatic reset by the PMU are not immediate consequences of the failure to transition to ground idle. Thus, recognizing the implications of the N1 situation informs pilots of the importance of engine behavior during landing and the potential operational adjustments they may need to make.