What happened
On 13 October 1997, a Fokker F28 Mark 0100, registration G-UKFK, was operating a public transport flight departing from Amsterdam Schiphol Airport. While climbing to 6,000 feet in instrument meteorological conditions, the aircraft encountered small hail. During this period, the aircraft sustained a lightning strike.
Following the strike, cabin crew reported seeing fire-balls moving through the cabin from the front toward the overwing emergency exits. Simultaneously, the pilot observed master caution lights and warnings on the Multi-Function Display indicating that hydraulic pressure had dropped in both the left and right elevator hydraulic control units.
Following established procedures, the crew operated the hydraulic bypass buttons on the overhead panel. This action intentionally deactivated the hydraulic boost for the elevators, reverting the flight controls to manual operation. The crew decided to return to Schiphel, where they completed an uneventful landing. There were no fatalities and no injuries among the 5 crew members or 100 passengers on board.
The investigation
Investigators examined the aircraft and found various types of damage, including dents in the upper fuselage, damaged rivets on the left side of the nose, and missing components from the right-hand stabiliser. Small metal fragments were also found welded to the pitot head.
Technical analysis revealed that the lightning strike had traveled through the airframe between the nose and the elevators. This caused the functional failure of both elevator boost system Sequence Valve Proximity Switches. These switches are located on the manifold assemblies near the junction of the fin and tailplane.
Findings
- The lightning strike caused the proximity switches to send signals to the Flight Warning Computer that mimicked a loss of hydraulic pressure.
- The crew's correct application of the checklist actually caused the loss of hydraulic boost, as the manual bypass action closed the pilot valves.
- Although the switches indicated a failure, hydraulic pressure had actually remained available to both boost units.
- Ground-based lightning detection equipment did not record any major discharges at the time of the event.