GPS Failure and Unreliable Glideslope
You are flying a Cessna 172S IFR from Haifa (LLHA) to Ben Gurion (LLBG) at 4,000 ft. Ten minutes after departure, the GPS/navigation display fails and cannot be restarted. You are in IMC with moderate turbulence, and ATC clears you to hold at the BGN VOR, then expect the VOR/DME approach to runway 26. You have a serviceable VOR/ILS receiver, DME, and standby instruments. The ILS glideslope for runway 26 is NOTAMed unreliable. You have 2 h of fuel and the weather at LLBG is 800 ft ceiling, 4 km visibility. Your alternate is 25 min away. The aircraft has no GNSS approach capability. You must decide how to navigate, what approach to fly, and when to divert.
1. How do you determine your position and navigate to the BGN VOR without GPS?
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Use the VOR/ILS receiver and DME. Tune the BGN VOR, identify it, and centre the CDI to determine the radial you are on; the DME gives distance from the station. If you are not on a published route, request ATC vectors and then proceed to the VOR using the displayed bearing and distance. Cross-check with dead reckoning using heading, time, and TAS, and monitor the standby instruments in turbulence. Do not rely on the failed GPS or attempt to restart it in flight unless the checklist permits it. The key is to re-establish a known position using ground-based navigation and ATC before entering the hold.
2. The ILS glideslope is NOTAMed unreliable. Can you fly the ILS? What approach should you fly?
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You must not use the ILS glideslope because it is NOTAMed unreliable. You may fly the localizer only if a localizer-only procedure with published step-down fixes and minima is authorised; otherwise, fly the VOR/DME approach to runway 26. Use DME distances and published step-down altitudes, and do not descend below each segment minimum until the required fix is identified. In turbulence and IMC, configure early and fly a stable approach. If the aircraft is not established or the required visual reference is not in sight at the minima, go around. The presence of an ILS receiver does not make the glideslope usable when it is NOTAMed unreliable.
3. When and how do you decide to divert?
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Set a clear diversion trigger before starting the approach. If you are not established on the final approach course by the published fix, if you cannot identify the required DME step-down fixes, if the approach becomes unstable, or if you reach the minima without the required visual reference, go around and divert. Also divert if fuel remaining falls below the planned reserve plus alternate fuel. With 2 hours of fuel and a 25-minute alternate, you have time for one approach, not repeated attempts. Tell ATC early: 'Cessna 172, going around, request diversion to the alternate.' The decision must be made on position, stability, weather, and fuel, not on a hope that the weather will improve.