Final answer:
RNAV substitution is using RNAV to replace conventional navigation aids within existing clearances, while RNAV alternate means refers to using RNAV for planning with alternate airports in the preflight process.
Step-by-step explanation:
The terms RNAV substitution and RNAV alternate means are navigational concepts used in the field of aviation. RNAV substitution refers to the use of RNAV (Area Navigation) systems to replace conventional navigation aids without the need to alter the existing air traffic control clearance. For example, a pilot with an RNAV system can replace the need for a VOR (VHF Omnidirectional Range) to navigate along a specific airway.
RNAV alternate means, on the other hand, involves using RNAV for flight planning purposes when selecting an alternate airport during the preflight planning process. Instead of relying on traditional navigational aids, a pilot can plan to use RNAV procedures at the alternate airport, provided the aircraft and crew are equipped and qualified for RNAV operations. This offers more flexibility and can be beneficial in areas with limited ground-based navigational infrastructure.