Definition
An FAA-published list of acceptable alternatives a pilot may use when a specific navigation component or visual aid required for an instrument approach is out of service or unusable. The substitutions are listed in the Terminal Procedures Publication and allow an approach to remain authorized when a component such as an inner marker, middle marker, or compass locator is inoperative, provided the listed substitute (for example, a suitable RNAV system, DME, surveillance radar, or PAR) is available and approved.
Plain English
If something needed for an approach is broken or unavailable, the FAA publishes a list of approved replacements. If you have one of the listed replacements, you can still legally fly the approach.
Context Anchor
Seen in Standard Terminal Arrival Route discussions and chart notes when a published arrival depends on specific navigation sources, equipment, or instructions.
Derivation
Substitute comes from Latin words meaning “to put in place of.” Inoperative means “not working,” and unusable means “not fit to use.” Together, the phrase points to something approved that is put in place of a required part that is not working or cannot be used.
Why Pilots Care
Allows pilots to complete arrivals safely and legally without canceling or diverting due to equipment failure.
Grounding Statement
If the arrival depends on a navigation source that is down or unavailable in your aircraft, you need an approved replacement before you fly that part of the arrival.
Intuition Check
Do not read “substitute” as “anything that seems close enough.” In this context, a substitute must be an authorized replacement for that specific procedure, equipment requirement, or clearance.
Example Sentence 1
The middle marker was out of service, so the pilot consulted the Substitutes for Inoperative or Unusable Components table and confirmed that a compass locator could be used in its place.
Example Sentence 2
Before flying the arrival, the pilot checked for substitutes for inoperative or unusable components on the chart.