When using dead reckoning, which of the following is essential for accurate navigation?

Constant speed is essential for accurate navigation when using dead reckoning because this method involves calculating the aircraft's position by using a previously determined position and advancing that position based on known speeds, headings, and elapsed time. When the speed is constant, it simplifies the calculations, allowing for more precise estimations of where the aircraft will be at any given time. Any variation in speed could lead to inaccuracies in the navigation calculations, resulting in potentially significant errors in positioning.

In contrast, visual landmarks, weather observations, and flight plan adjustments can provide useful information or context during navigation, but they do not directly contribute to the fundamental requirement of dead reckoning calculations. Visual landmarks can help verify position but are not necessary for dead reckoning, which relies on mathematical calculations rather than visual cues. Weather observations can inform the pilot about flying conditions but do not influence the basic mechanics of dead reckoning itself. Likewise, while flight plan adjustments are often necessary in response to changing circumstances, they do not directly affect the accuracy of the dead reckoning navigation method when constant speed is maintained.