The GRD PROX TERR INHIB button is used to inhibit predictive EGPWS function during which condition?

The GRD PROX TERR INHIB button is used to inhibit predictive EGPWS function during which condition?

• A. QFE operations with GPS data unavailable or inop
• B. Enroute with no GPS
• C. VFR flight
• D. Approach in CAT II

Answer: (A)

The key idea is that predictive EGPWS terrain relies on accurate navigation data and an altitude reference to forecast potential terrain conflicts. When GPS data is unavailable or inoperative, the position information the system uses can become unreliable, which can cause false or confusing terrain warnings during certain operating modes that depend on a local elevation reference. During QFE operations you’re using the field elevation as the altitude reference, which can further complicate the predictive calculations if the nav data isn’t current. In these conditions, the GRD PROX TERR INHIB switch is used to mute the predictive terrain alerts so you’re not bombarded with warnings that may not reflect the actual situation, while you manage the flight with the available cues. Once GPS is back up, you would restore normal EGPWS operation. In other scenarios—enroute with no GPS, VFR flight, or an approach in CAT II—the navigation data and flight conditions don’t present the same risk of misleading predictive terrain logic, so there’s no need to inhibit the system in those cases.

The key idea is that predictive EGPWS terrain relies on accurate navigation data and an altitude reference to forecast potential terrain conflicts. When GPS data is unavailable or inoperative, the position information the system uses can become unreliable, which can cause false or confusing terrain warnings during certain operating modes that depend on a local elevation reference. During QFE operations you’re using the field elevation as the altitude reference, which can further complicate the predictive calculations if the nav data isn’t current. In these conditions, the GRD PROX TERR INHIB switch is used to mute the predictive terrain alerts so you’re not bombarded with warnings that may not reflect the actual situation, while you manage the flight with the available cues. Once GPS is back up, you would restore normal EGPWS operation.

In other scenarios—enroute with no GPS, VFR flight, or an approach in CAT II—the navigation data and flight conditions don’t present the same risk of misleading predictive terrain logic, so there’s no need to inhibit the system in those cases.