FAA Instrument Rating Airplane (IRA)Regulations and CurrencyHard
A pilot is planning an approach that requires DME per the chart notes, but the aircraft has no DME installed. The aircraft does have an IFR-approved GPS with a current database. According to AC 90-108 guidance, may the pilot substitute GPS for the required DME on this approach?
- AYes, but only if ATC issues a specific waiver for that flight
- BNo, GPS may never substitute for required DME under any circumstances
- CYes, GPS may generally substitute for DME requirements when using an IFR-approved GPS, except where explicitly prohibited
- DNo, substitution is allowed only for ADF, never for DME
Show answer & explanationAnswer & explanation
Correct answer: C. Yes, GPS may generally substitute for DME requirements when using an IFR-approved GPS, except where explicitly prohibited
AC 90-108 permits IFR-approved GPS systems to substitute for ADF and DME functions (including DME required for procedures, position fixing, and holding) except where the chart or regulation specifically prohibits substitution (such as certain precision approach final segments).
Why the other options are wrong
- A. Incorrect — no per-flight ATC waiver is required; the authorization is standing guidance.
- B. Incorrect — GPS substitution for DME is broadly authorized, not prohibited.
- D. Incorrect — substitution applies to both ADF and DME functions, not ADF alone.
GPS Substitution for ADF/DME (AC 90-108)
An IFR-approved GPS with a current database may generally substitute for ADF and DME equipment requirements, except where specifically prohibited by chart or regulation.
- Applies to en route, terminal, and approach procedures
- Exceptions exist for certain final approach segments
- Requires IFR-approved GPS, not just any GPS unit
Memory trick: GPS steps in for ADF and DME — unless the chart says 'not here.'