What happened
On October 16, 1993, a McDonnell Douglas MD-81 operating a scheduled flight from Munich to Zurich experienced a significant inflight disturbance. Approximately ten minutes after departing Munich at flight level 180, the flight crew detected an unusual odor followed by visible smoke appearing above the landing light switches.
As the smoke intensified, the crew attempted to follow the "Electrical Smoke in Cockpit" checklist. During the process of managing the electrical load, the crew deactivated the right generator, which resulted in the near-total failure of the left-side flight instruments. Due to the worsening conditions, the crew declared an emergency and initiated a return to Munich.
During the approach, visibility in the cockpit was severely compromised by thick smoke, making it difficult for the pilots to read instruments or see the runway. The pilot flying reported being unable to clearly discern the airspeed indicator. Upon landing, the aircraft performed a heavy braking maneuver due to a total loss of external visibility during the rollout. Following the landing, an emergency evacuation was ordered. During the evacuation, 14 passengers and one crew member sustained minor injuries while exiting via the wings or slides.
The investigation
The investigation, involving authorities from Germany, the USA, and Switzerland, focused on the source of the smoke and the failure of the aircraft's electrical systems. Investigators examined the overhead panel, the flight data recorder (DFDR), the cockpit voice recorder (CVR), and the aircraft's electrical wiring.
Technical analysis revealed that the smoke originated from a severe smoldering fire within the Emergency Power Switch. The investigation also looked into the evacuation process, specifically why a rear emergency slide failed to deploy.
Findings
- The primary cause of the smoke and electrical malfunction was a massive smoldering fire within the Emergency Power Switch.
- The switch failed due to a combination of mechanical and electrical overload.
- The fire caused extensive damage to the electrical wiring, including the 115V AC bus distribution and various relays and circuit breakers.
- The rear emergency slide failed to deploy because a safety pin required for slide assembly had not been removed during maintenance/installation.
Safety action
- The investigation noted that the electrical damage was so extensive that nearly all wiring in the affected area of the overhead panel required replacement.