Incident Overview
On April 15, 2018, a Boeing 737-800 operated by an unspecified airline was conducting a descent into Ben Gurion Airport (LLBG) in Tel Aviv, Israel. As the aircraft approached flight level 100, the flight crew received a Flight Management Computer (FMC) message indicating 'GPS invalid L/R' and observed erroneous Enhanced Ground Proximity Warning System (EGPWS) terrain indications. At that point, the crew had only Inertial Reference System (IRS) data available.
Navigation and Speed Anomalies
The Required Navigation Performance (RNP) value initially increased to 0.45 nautical miles, but since the approach required RNP 5, the crew decided to continue. After a short time, the RNP decreased to 0.30 nautical miles. Subsequently, the crew encountered an inconsistency in Vref speeds. During FMC setup for approach, the calculated Vref for flaps 30 was 148 knots, which was entered into the Takeoff and Landing (TOLD) chart. However, while conducting an Instrument Landing System (ILS) approach with flaps 30 extended, the FMC displayed a Vref of 135 knots. The pilot flying set an approach speed of 140 knots, but soon noticed that Vref for flaps 40 was 141 knots—higher than Vref for flaps 30, a physically implausible situation. The crew then flew the approach at 148 + 5 knots. After reselecting the landing flap setting in the FMC, the Vref returned to 148 knots.
Crew Awareness and Context
The crew was briefed on a NOTAM related to the situation in Syria and was aware of the possibility of interference with the aircraft's electronic equipment. The incident was logged in the operator's SMS database and reported to EVAIR.
Probable Cause
Interference with aircraft systems related to military operations in Syria.