Lessons From A Boeing 777 Main Electrical Load Center Fire
On Feb 26, at 1,000 hrs local, a United Airlines 777-222 N768UA pushed back with its APU running and started engines at London’s Heathrow Airport.
After both engines were started, and at about the time the engine driven Integrated Drive Generators (IDGs) would have normally come on-line, the flight deck instrument displays flickered, the crew heard an abnormal noise, and an EICAS message, among other related messages, indicated that the Right Main AC Bus had failed.
A low-pitched intermittent growling noise was coming from the aft right-hand side of the flight-deck. The AC “ELEC BUS RIGHT” EICAS caution light illuminated indicating that the right main AC BUS had failed (EICAS: Engine Indication and Crew Alerting System).
Simultaneously, the right “GEN CTRL OFF” caption illuminated, indicating that the right IDG had lost power. This was explained by another caption on the “R BUS TIE” switch where “ISLN” was showing. This indicated that the right bus tie circuit breaker had tripped open.
From the Flight Data Recorder (FDR), the U.K. Accident Board was able to determine that 40 seconds after the second engine started and stabilized at flight idle, the smoke detector inside the main equipt center (MEC) had detected smoke. The MEC is beneath the flight-deck and contains the vast majority of the aircraft’s avionics and electrical switching gear.
The FDR recorded “EE Bay Smoke Warn”. The Cockpit Voice Recorder contained crew comments from the three pilots that the “whole Right Main Bus had failed”. It also recorded sounds of equipment powering down.
The Flight Deck crew completed the EICAS drill for “ELEC AC BUS R” of which the initial action was to recycle the RH generator control switch. Two and a half minutes later,s they noticed an “EQUIP COOLING OVRD” EICAS message. At that point, the captain shut down the RH engine. Groundcrew alerts to the pilots of smoke pouring from the MEC vent followed.
The aircraft was quickly taxied to a nearby stand and shut down. By that time, smoke was visible on the flight-deck and ATC had alerted the crew of smoke emanating from the forward outflow valve. About 12 minutes after the first failure, the battery power was secured and passengers evacuated via door 2L.
Fire Service personnel checked the MEC which was full of smoke, but didn’t detect any fire. As the smoke slowly cleared in the MEC, obvious signs of an electrical fire were to be seen. See the graphic evidence at tinyurl.com/29jsb7
Incongruously, when there is a fire within the 777’s MEC, no Master Caution, Master Warning or smoke message is triggered. When smoke is
detected, the avionics cooling system transitions to an override mode and triggers the “EQUIP COOLING OVRD” EICAS message. Because this system relies upon a differential pressure between the cabin and ambient to vent smoke, the system is ineffectual on the ground.
The flow of power from Generators to the Left and Right Main AC Busses can be seen at the tinyurl. The current flow is via seven contactors. These include the RGCB (Rt Gen Circuit Breaker) and the RBTB (Rt Bus Tie Breaker). These are components within the P200 ELMS (Electrical Load Management System) located in the starboard forward of the MEC.
Inspection of the P200 panel disclosed that the fire had centered upon the RGCB and RBTB, parts of which had melted and vaporized. Molten metal had dripped down on the insulation blankets beneath the panel, causing extensive fire damage. Fire damage to those blankets can be seen behind and beneath the power panel under the floor (see above tinyurl).
A floor panel, ducting, wire bundles and some structural frames and stringers were also required to be replaced because of fire damage.
The P200 ELMS was removed and found to have suf-fered extreme heating and electrical arcing. Moveable contacts within each had been destroyed. Busbars in the vicinity of the RBTB had insulation burnt off. The failure had obviously occurred within either the RBTB or RGCB.
You may recall that Swissair 111 had had a bus-tie short-out the day prior to its accident flight, after it had been replaced and rewired incorrectly (see tinyurl.com/yt43v8). That SR-111 replacement occurred because of a fire in a bus-tie relay of another Swissair MD-11 in Bangkok a few days prior to that.
The 777’s two components had been in situ since manufacture and had completed 43,519 flight hours (6622 cycles). They have no overhaul or inspection period stipulated.
Prior to this particular accident, Boeing had been investigating 11 similar power panel overheat events involving the P200 and P300 panels. Those 11 events all involved an ELMS II panel (a later version of the ELMS I fitted to N768UA).
Boeing published a suggested preventative action in its 777 Fleet Team Digest. The U.K. AAIB is working with Boeing and its suppliers to try and determine the cause of the failures.
The images at tinyurl.com/29jsb7 speak for themselves. If the fire had broken out mid-Atlantic, a repeat of Swissair 111 would have been a most likely outcome, given the very short period in which the extensive N768UA fire damage occurred.
The MEC has no fire extinguishing system and it is located immediately below the flight-deck. Inflight the compartment is well ventilated for cooling purposes and these flows would have acted as a fire accelerant, as per the Canadian TSB’s findings on SR-111.
Whether or not the heavily burnt thermal-acoustic insulation blankets are the mylar type which propagated the fire aboard the Swissair 111 MD-11 is unknown. However, if they are an FAA mandated replacement type (dubious proposition at this point), then they are evidently also unsuitable. The aircraft was carrying 185 passengers and 20 crew.