Definition
An FAA Advisory Circular titled 'Guidelines for Using Global Positioning System Equipment for IFR En Route and Terminal Operations and for Nonprecision Instrument Approaches in the U.S. National Airspace System.' It provides guidance on how pilots may use approved GPS equipment as a substitute for traditional ground-based navigation aids during instrument flight operations.
Plain English
A formal FAA guidance document that explains when and how pilots can use GPS instead of older ground-based navigation equipment when flying on instruments.
Context Anchor
Seen in instrument flying discussions about using GPS when a navigation component, such as certain ground-based navigation equipment, is inoperative or unusable.
Derivation
AC stands for Advisory Circular, the FAA's standard format for issuing non-regulatory guidance. The number 90-94 identifies its subject series (90 = Air Traffic and General Operating Rules) and its sequence within that series.
Why Pilots Care
Allows continued safe IFR flight by documenting acceptable substitutes that meet regulatory requirements without violating equipment rules.
Intuition Check
Do not read AC 90-94 as a regulation by itself. It is FAA guidance that helps explain acceptable practices; the actual authority still comes from the regulations, the aircraft’s approved equipment, and the procedure being flown.
Example Sentence 1
Because the aircraft's ADF was inoperative, the pilot used GPS as a substitute in accordance with AC 90-94.
Example Sentence 2
AC 90-94 lists the conditions under which a pilot may continue an IFR flight after losing a required navigation signal.