Major Electrical System Modifications Slated for MD-11 Aircraft

Major Electrical System Modifications Slated for MD-11

Actions intended to prevent a repeat of the Swissair Flight 111 crash
Without waiting for the investigation to be completed, Swissair is stepping forward with a significant program to separate key functions and to harden the electrical systems on its MD-11's to prevent the catastrophic loss of another aircraft.

The action is a direct outgrowth of the 1998 fatal crash of Flight 111 at Halifax. The accident airplane, an MD-11, is believed to have been brought down by a massive and uncontrollable electrical fire. Even though completion of the crash investigation by the Transportation Safety Board (TSB) of Canada is at least a year away, Swissair officials, in concert with system engineering support from Boeing [BA] (more), already have decided to modify significantly the electrical systems on the carrier's remaining fleet of 19 MD-11's. The cause-effect relationship to the Flight 111 crash is clear, as the Jan. 3 SR Technics memorandum outlining the action categorizes the modification program as an "SR 111 Action".

If pushed to the sticking point, the memorandum raises discomforting questions about the safety of the MD-11's original electrical system design and the rigor of Federal Aviation Administration (FAA) certification. The SR Technics memorandum declares bluntly that "wire separation, routing and protection have been proven to be insufficient" and contributed to the loss of essential functions on the accident airplane. Inadequate separation of critical wiring raises the specter of a single point failure, which the certification process is supposed to prevent. It is believed that the six flat-panel displays in the "glass cockpit" of the accident airplane failed, leaving Capt. Urs Zimmermann and First Officer Stefan Low staring at dark screens. Is it possible they did not even have a standby horizon, given that the wiring to the standby horizon is to be re-routed? In fact, the standby horizon is one of 17 wires to be re-routed and, of note, TSB investigators early in the crash postmortem identified 17 fire-damaged wires.

The modifications are slated to begin in August, at a cost of about $20,000 per aircraft. Most of the cost involves labor for work estimated to take 3.5 days per aircraft. The cost of materials is estimated at about $1,300 per aircraft, raising the tantalizing question: could the Swissair Flight 111 tragedy have been prevented by such a trivial cost in materials beforehand?

Proposed Changes to MD-11 Electrical Configuration
(Extracts of SR Technics action memorandum)

Wire separation, routing and protection have been proven to be insufficient, especially in the cockpit area. In the case of sr111 this has contributed to the loss of flight essential functions, including display functions. An assessment...of the present electrical system layout in the cockpit area...has led to the following modification package:

Re-route battery direct bus, battery bus and the left emergency AC & DC bus power feeders between the avionics compartment and the overhead circuit breaker panel in the cockpit. The power feeders shall be routed from the avionics compartment up to the cockpit, between the main circuit breaker panel and galley G2. From the top of the main circuit breaker panel they shall run in a conduit directly into the left-hand side of the overhead circuit breaker panel.
Where applicable, the Tefzel wire type will be replaced...(Editor's note: Tefzel wire insulation is comparatively soft and can be cut or sawn through by the chafing action of harder wire insulation types)
Re-route 17 single wires from selected 'get home' systems, between the avionics compartment and the overhead circuit breaker panel. The wires shall be routed from various locations in the avionics compartment along existing wire bundles to the left hand side, up to the cabin behind galley G1 into the cockpit, and finally into the left hand side of the overhead circuit breaker panel.
Relocate two circuit breakers from the avionics circuit breaker panel respective overhead circuit breaker panel to the main circuit breaker panel (which is located behind the first officer).
Rewire the audio management unit (AMU) power circuit to the left and right emergency DC bus sensing circuit. This will allow redundant powering of the AMU's, independent of the position of the smoke switch (Ed. Note: the AMU provides radio and intercom).
Rewire the left emergency control circuit, to be additionally fed by the battery. This will allow powering of the left emergency bus after complete loss of the right emergency bus.

Changing the Electrical Layout/Concept
System wires to be rerouted (from the overhead circuit breaker panel to the left-hand side):

Standby horizon
Battery charger control
Left emergency AC & DC Bus Control
Engine Ignition A Ignitors ENG 1
Engine Ignition A Ignitors ENG 2
Engine Ignition A Ignitors ENG 3
Left Emergency AC & DC Bus Control
Battery Bus Feed TR3
Left Emergency DC Bus Sensing
Left Emergency AC Bus Sensing
Battery Relay
Emergency DC Bus Feed LH
Battery Direct Bus Feed
Emergency Inverter Power
Left Emergency AC Bus Sensing
Flow Control Valve Pack 1
Captain Pitot Heat

Power Feeders to be rerouted:

Battery Direct Bus
Battery Bus
28VDC Left Hand (LH) Emergency Bus
115VAC LH Emergency Bus

Circuit Breakers to be relocated:

TR3 Input
Left Emergency DC Bus Feed ADG (air driven generator)

Source: SR Technics

Courtesy of David Evans, Managing Editor, Air Safety Week