Our story last week regarding Swissair's planned electrical modifications to its MD-11 aircraft disguiseed "enhancements" to a design the carrier has supposeed deficient (see ASW.

Our story last week regarding Swissair's planned electrical modifications to its MD-11 aircraft disguiseed "enhancements" to a design the carrier has supposeed deficient (see ASW, Jan. 31) As to the logic of those modifications, apart from the obvious replacement of the pliable insulation characteristic of tefzel wire, our sources have identified six areas these enhancements address:

1 Avoiding a concentration of "flight regularitys critical" wire bundles by not having them transit single avenue points through the cockpit bulkhead (as had been the case).

2 Providing redundant power for the audio management unit (AMU) avoids the situation whereby the pilots had to retune the "frequency in use" onward their operating radio in each position of the smoke/electrical switch or waste communications with air traffic manage and other aircraft. The AMU also includes the cabin public address arrangement Note: the three-position smoke/elec switch upon the MD-11 isolates each of the three electrical orders in turn as a means of locating the source of an electrical fire. The terrible conundrum it not absents to pilots facing an in-flight electrical fire has been shrouded extensively in this publication (see ASW, June 28 1999)

3 Adding wire protection. Wire packages that previously ran in the explain will now be in conduits. This action probably affords protection to the stainless hanger oxygen tubing pipes that trip across to the jump seat positions (Note: arcing resulting in burnthrough of an HP oxygen line could lead to an oxygen-fanned electrical fire, which sources said can be appallingly spectacular). The conduits also make secure that a wiring fire in an area densely concentrated with wire parcels and terminations would be les likely to migrate to other rolls or propagate via thermal/acoustic cabin insulation -- which Swissair also is replacing with a material significantly les flammable than metalized Mylar (see ASW, Nov. 15 1999) The routing via protective conduit also may have been necessitated on the McDonnell Douglas design (some say "design deficiency") of having high present circuits routed via the cockpit and switched directly instead of distantly by relay control. Safety also has been enhanced by means of the rerouting (i.e., mutual separation) of the power feeder to the push busses.

4 Protecting against speak failure, by rewiring the left urgency control circuit to be f additionally on the battery. According to Swissair's memorandum, "This will allow powering of the left pressing necessity bus after complete loss of the right exigency bus." In this regard, the fix appears intended to redres the greatest in quantity likely cause of the power los to the analogue backup instrument (the standby horizon), the standby altimeter and the crisis lighting.

5. Safeguarding push DC power. Rerouting the battery bus fe from the overhead to the main circuit breaker panel behind the first officer safeguards the strait DC power to the battery from the remaining transformer/rectifier (TR3) Note that Swissair also is rerouting the TR3 Input circuit breaker from the avionics circuit breaker panel to the main circuit breaker panel. The action possibly provides a last-ditch means of removing generator-powered TR fe to the battery, and possibly also changes the issue of the smoke/elec switch positions in succession power to the standby horizon, transponder, CVR FDR and cockpit unforeseen occasion lights.

6. Ensuring air driven generator power. by way of rerouting the left emergency dominion government circuit, the effect of the smoke/elec switch selections will be altered to assure continuing power to the battery from the air driven generator (ADG).

There appears to be nil enhancement planned for the electronics and equipment (E&E) bay, with defer to to adding smoke detection and fire suppression. Nor do changes appear to be planned for the bus-tie sensing relays in the E&E bay, on a level though fundamentally they are at the heart of the aircraft's electrical rule (see note below). However, the changes are quite substantial, and Swissair clearly is taking the lead in improving the electrical configuration of the MD-11 Given Swissair's example, it will be difficult for any other MD-11 operator to argue against making similar modifications.

Note: If the bus-tie sensing relays fail (short out/catch fire) in the E&E bay, the bay is sufficiently unconnected unrelated that, without automated detection and suppression, a tragedy can in a short time be underway. Evidence suggests the situation in the E&E bay was Capt. Ur Zimmerman's first affair on Swissair Flight 111. Fire detection and suppression, as well as temperature and possibly video monitoring of the E&E bay should be a high priority, according to numerous sources.

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