Navigation System Failure During a Charter Flight
A twin-engine charter aircraft is 40 minutes into a 90-minute flight when the primary GNSS receiver fails. The aircraft carries a secondary GNSS unit and a VOR/DME set. The destination has a VOR/DME approach and a GNSS-only approach. Weather at destination is 900 m visibility and a 300 m cloud base. The pilot's last position fix was 12 minutes ago, and the aircraft is in controlled airspace under IFR. Company standard operating procedures require two independent navigation sources for IFR flight in controlled airspace. The first officer suggests continuing to the GNSS-only approach because the secondary GNSS is still working. The captain must decide whether to continue, divert, or declare an emergency.
1. Determine whether the aircraft still meets the SOP requirement for two independent navigation sources, and explain the reasoning.
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The SOP requires two independent navigation sources. After the primary GNSS fails, the remaining sources are the secondary GNSS and the VOR/DME. These are independent in the sense that they rely on different infrastructure (satellite versus ground-based), so the two-source requirement can still be met if both are serviceable and the crew can use them.
However, independence must be genuine. If the secondary GNSS shares an antenna, power bus, or processing path with the failed primary, it is not truly independent. The captain should confirm the secondary unit is on a separate feed. Assuming it is, the aircraft retains two independent sources and the SOP condition is satisfied. The decision then turns on approach suitability rather than raw source count, and the captain should not treat the secondary GNSS as automatically equivalent to the failed primary without that check.
2. Recommend a course of action given the weather and the available approaches, justifying the choice.
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The destination weather (900 m visibility, 300 m cloud base) is marginal for the GNSS-only approach, and the crew's position confidence is degraded because the last fix was 12 minutes ago. The safer plan is to use the VOR/DME to re-establish position and track, then plan the VOR/DME approach, which does not depend on the failed GNSS at all.
Continuing toward the GNSS-only approach leans on a single remaining satellite source in poor weather, which is a poor risk trade. The captain should advise ATC of the equipment failure, request the VOR/DME approach, and hold diversion as the fallback if position or fuel margins erode. Declaring an emergency is not yet warranted because two sources remain and the aircraft is controllable; the correct action is a conservative re-route with ATC notification, not an escalation that overstates the situation.