Occurrence
On 28 March 2016 at 05:35 UTC, the air traffic controller from the Approach Control at Szczecin-Goleniów Airport received a report of abnormal operation of the Precision Approach Path Indicator (PAPI) lights for Runway 31. The report came from the crew of a Ryanair Boeing 737-800 operating a flight from Oslo Rygge, which was on approach to land at Szczecin-Goleniów. The crew described the abnormality as indications above the glide path compared to the Instrument Landing System (ILS) signal. At the time of the report, the aircraft was approximately 16 kilometers from the runway threshold. Subsequent aircraft approaching to land did not confirm such indications.
Weather conditions at the time of the event included 3/8 cloud cover from high-level Cirrus clouds and visibility exceeding 10 kilometers. Sunrise was at 06:19 local time.
Investigation
The investigation was conducted by the relevant authority. No aircraft registration or operator details were specified in the report. The investigation examined the PAPI system for Runway 31. It was determined that the probable cause of the event was fogging and frosting of the light units.
Probable Cause
The probable cause of the incident was fogging and frosting of the light units.
Actions Taken
Preventive actions undertaken by the investigating entity included:
- Reviewing similar reports of PAPI RWY31 indication irregularities received between 15 August 2015 and 28 March 2016.
- Disabling the PAPI system.
- Conducting inspections of the light units, including factory seals, internal and external optics, light source functionality, reflector condition, heating elements, electrical connections, support structure, and paint coatings.
- Checking the constant current regulators.
- Verifying the angular settings of each light unit in the horizontal plane and relative to the runway axis.
- Inspecting the condition of the light unit foundations.
- Checking the validity of the equipment certificate and consulting with the manufacturer.
- Discussing the event at an Operational Safety Analysis Team meeting.
- Developing two risk assessment and corrective program variants: pessimistic and optimistic. The operator implemented the optimistic variant with monitoring of PAPI indication problem reports until a subsequent flight check.
- Deeming it appropriate to inform airport operators about the alignment principles of the PAPI RWY31 for the reference parameter 8<EWH<14.
- Redesigning the heating elements in the units (installing larger heaters) and monitoring unit cleanliness to prevent fogging and frosting.
- Inspecting the light units several times a day.
- Performing an airborne check of the PAPI system, which found no deficiencies.