The primary effect of operating below optimum cruise altitude is?

Operating below the optimum cruise altitude primarily affects the aircraft's aerodynamic efficiency and engine performance, leading to a notable loss of range capability. At lower altitudes, the air density is higher, which increases drag on the aircraft. As drag increases, more engine thrust is required to maintain the same speed and altitude, leading to higher fuel consumption.

Moreover, flying at an altitude that is not optimal means that the aircraft may not be able to take full advantage of the thinner air profile, which typically allows for more efficient engine operation and better overall fuel efficiency. The excess fuel consumption directly translates to a reduced range, as the aircraft will deplete its fuel reserves more quickly operating outside the ideal altitude band.

While there may be instances where engine performance might degrade marginally below optimal flight levels and passenger comfort could improve due to lower cabin pressures at altitude, these factors are secondary to the significant impact on range when cruising at less efficient altitudes.