What happened
On 0/3 December 1999, a Skyways Saab SF340A, registration SE-KPE, was performing a scheduled flight from Örebro to Malmö/Sturup when it experienced a significant electrical failure. While established on the ILS approach to runway 17, the aircraft was struck by lightning. This strike caused both engine-driven electrical generators to trip off simultaneously.
Following the strike, the aircraft's primary instruments and communication systems ceased to function, leaving the crew dependent on standby instruments and emergency lighting. The crew attempted to reset the generators by cycling the switches, but were unsuccessful. They managed to re-establish contact with Air Traffic Control by using a similar cycling procedure on the radio, eventually requesting radar vectors for the approach.
During the descent, the crew encountered further difficulties when attempting to extend the landing gear. Despite the lamps testing correctly, they could not obtain the required three green down-and-locked indications through normal means. The crew eventually utilized the emergency gear extension procedure, which successfully provided the necessary indications at approximately 2000 feet.
After making visual contact with the runway at 1000 feet, the crew completed an uneventful landing. Once at the parking stand, the generators were successfully reset, though the EFIS screens remained with a pale color scheme. A subsequent inspection revealed a 40–50 cm crack in the upper part of the nose radome.
The investigation
The investigation focused on the cause of the simultaneous generator disconnection and the failure of the landing gear indications. Investigators noted that the aircraft's electrical systems are designed to be independent, connected via a Bus Tie Relay that automatically disconnects systems during a fault to protect the intact side. However, it was established that lightning strikes had previously caused the overvoltage protectors on this specific aircraft type to disconnect both generators.
Regarding the landing gear, the investigation found no mechanical fault in the gear system itself. It was determined that the lack of green indications during the initial extension attempts was most likely due to low hydraulic pressure, resulting from the loss of power to the electrically driven hydraulic pump following the generator failure.
Findings
- The primary cause of the incident was the automatic disconnection of both generators following a lightning strike.
- The difficulty in obtaining landing gear indications was likely caused by insufficient hydraulic pressure due to the loss of the electric hydraulic pump.
- The crew's decision to continue the approach rather than performing the full malfunction checklist was deemed understandable, given the limited battery life and the uncertainty of the checklist's success in that moment.