Aviation Investigation Report A98H0003

1.6.10  Electrical System

  1. - General
  2. - Air-Driven Generator
  3. - Emergency Electrical Power Isolation
  4. - Cockpit Circuit Breaker Panels
  5. - Overhead Circuit Breaker Panel Bus Feed Wires
  6. - Upper and Lower Avionics Circuit Breaker Panel Bus Feed Wires
  7. - Upper and Lower Main CB Panel Bus Feeds
  8. - Wire Identification, Location, and Routing
  9. - Wire Description – MD-11 Aircraft  General

Normal primary electrical power is generated by three, engine-driven, integrated-drive generators (IDG). An auxiliary power unit (APU) generator is also available as a back-up source of electrical power in certain ground or flight phases. The three IDGs supply electrical power to their respective generator buses,[29] which in turn supply electrical power to several sub-buses located throughout the aircraft. Electrical power distribution is normally automatic; however, if necessary, the pilots can control the electrical system manually with controls located on the overhead switch panel. (STI1-14)

The following definitions are used throughout the report. They are based on the Society of Automotive Engineers' (SAE) Aerospace Standard AS50881, Rev. A, entitled Wiring, Aerospace Vehicle:

  • Wire: A single metallic conductor of solid, stranded, or tinsel construction designed to carry current in an electric circuit, but not having a metallic covering, sheath, or shield. For the purpose of this report, "wire" refers to "insulated electric conductor."
  • Cable: Two or more wires contained in a common covering, or two or more wires twisted or moulded together without a common covering, or one wire with a metallic covering shield or outer conductor.
  • Wire bundle: Any number of wires or cables routed and supported together along some distance within the aircraft.
  • American Wire Gauge (AWG): A standard set of non-ferrous wire conductor sizes. "Gauge" is based on diameter. The higher the gauge number, the smaller the diameter and the thinner the wire.  Air-Driven Generator (STI1-15)

The ADG is an air-powered turbine that drives an electrical generator. The ADG is manually deployed via a lever in the cockpit; once deployed, it cannot be retracted in flight. The ADG is located on the lower right-hand side of the fuselage to the right of the nose gear doors.

When deployed, the ADG automatically supplies hydraulic power for the flight controls by electrically powering auxiliary Hydraulic Pump 1. With a switch on the electrical system control panel (SCP), the pilots can switch the ADG to an electrical mode of operation. In doing so, the ADG supplies emergency electrical power that operates instruments and communication equipment. In this configuration, electrical power is no longer supplied to auxiliary Hydraulic Pump 1; in the absence of primary power, the pump will cease to operate.

On the occurrence aircraft, the ADG was stowed at the time of impact. There would have been no requirement to deploy the ADG unless electrical or hydraulic power or both were unavailable from other sources. Information derived from the examination of various system components indicates that, at the time of impact, electrical and hydraulic power were available from sources other than the ADG.  Emergency Electrical Power Isolation

For the purpose of isolating a source of smoke, electrical power can be shed in sequence from the electrical buses through the four-position SMOKE ELEC/AIR selector located on the overhead electrical control switch panel (see Figure 11). This selector allows for the isolation of electrical or air conditioning systems that could be the source of fumes or smoke.

The selector must be pushed in and rotated clockwise to move it to the next position. The selector cannot be turned counter-clockwise. As the selector is rotated, electrical power is returned to the systems associated with the previous position prior to shutting off electrical power associated with the new selector position. If the selector is rotated through to the NORM position, all electrical power from the three generator systems is returned, and the three air systems are restored.  Cockpit Circuit Breaker Panels

There are nine separate CB panels in the cockpit; the five most pertinent to this investigation are the overhead CB panel, the upper and lower avionics CB panels, and the upper and lower main CB panels (see Figure 12). The remaining four are the captain's and first officer's console CB panels, the centre overhead integral lighting CB panel, and the lower maintenance CB panel.

The overhead CB panel contains wiring from the following six separate buses:

  • 28 V DC battery bus;
  • 28 V DC battery direct bus;
  • left and right emergency AC buses; and
  • left and right emergency DC buses.[30]

The upper avionics CB panel contains wiring from the following seven separate buses:

  • 115 V AC buses 1 and 3;
  • 28 V DC buses 1 and 3; and
  • 28 V AC 1, 2, and 3 instrument buses.

The lower avionics CB panel contains wiring from the following two separate buses:

  • 28 V DC Bus 2; and
  • 28 V DC ground bus system.

The 28 V DC ground bus system CBs, installed on the lower avionics CB panel, were all 0.5 ampere (A) CBs used for indication and control of their respective remote control CBs. The 28 V DC Bus 2 consisted of three, 3 A and one, 5 A CBs. A jumper wire from the line side of the "SLAT CONTROL PWR B" CB, which was a 3 A CB, was used to provide 28 V DC to a 1 A CB used to power the IFEN control relays. The four, 115 V AC three-phase power supply 15 A CBs for the IFEN were installed in the lower avionics CB panel.

The upper and lower main CB panels contain wiring from both the 115 V AC and 28 V DC buses 1, 2, and 3.

The standard used by the aircraft manufacturer for CB identification was to identify each row by a letter, and each column by a number. This methodology was used to identify the location of individual CBs on the panel.  Overhead Circuit Breaker Panel Bus Feed Wires

These bus feed wires were routed through five conduits that were installed along the right side of the fuselage, from the avionics compartment to approximately halfway up the fuselage side wall. In the cockpit, outside of the conduits, the bus feed wires were individually clamped to wire support brackets that were attached to the aircraft structure by nylon standoffs. The individual wires were bundled together, just prior to entering the right side of the overhead CB panel. Table 7 describes the bus feeds.

Table 7: Overhead CB Panel Bus Feeds

Bus Feed Right Emergency AC Bus, Phases A, B, and C 28 V DC Battery Direct and Battery Buses Left Emergency AC Bus Right Emergency DC Bus Left Emergency DC Bus
Wire harness number ABS9208 ABS9206 ABS9205 ABS9206 ABS9205
Wire run letter ALB ALN ALC ALP ALE
Wire size 3 – #8AWG 1 – #8AWG 1 – #6AWG 1 – #10AWG 1 – #6AWG 1 – #6AWG
Function 115 V right emergency AC bus, Phases A, B, and C 28 V DC battery direct and battery buses 115 V left emergency AC bus 28 V right emergency DC bus 28 V left emergency DC bus  Upper and Lower Avionics Circuit Breaker Panel Bus Feed Wires

The 115 V AC bus feeds originate in the Centre Accessory Compartment and the 28 V DC bus feeds originate from the avionics compartment. The three 28 V AC instrument bus feed wires originate from instrument transformers that are mounted on the aft face of the cockpit wall. Primary electrical power to these transformers is supplied from the lower main CB panel 115 V AC buses 1, 2, and 3 respectively.

All of the bus feed wires supplying the avionics CB panel are routed from the right aft side of the cockpit wall, forward through a hole behind Galley 2, and then inboard to the avionics CB bus bars.

The HF Comm 1 requires a three-phase electrical power source to operate. As a result, two additional 115 V AC Bus 1 feed wires (phases B and C) are routed to the HF Comm 1 CB. Similarly, an additional DC Bus 2 feed wire is routed to two CBs: the AFCS MISC PNL LIGHTS and the PRIMARY HOR STAB TRIM. Table 8 describes the main bus feed wires and their run letters.

Table 8: Avionics Bus Feed Wires and Run Letters

Function 115 V AC
Bus 1
115 V AC
Bus 3
28 V DC
Bus 1
28 V DC
Bus 3
28 V AC
Instr – 1
28 V AC
Instr – 2
28 V AC
Instr – 3
Wire Number B110-7-8A B110-22-8A B117-3-8 B117-1-8 B108-5-16 B108-7-16 B107-9-16
Wire Run Letter AEU AEV ASD ASE ASC ASC ASC  Upper and Lower Main CB Panel Bus Feeds

The upper and lower main CB panels receive electrical power from bus feed wires that are routed from the avionics compartment located below the floor; these bus feed wires were not routed through any area where heat damage was observed.  Wire Identification, Location, and Routing

All McDonnell Douglas–installed wires in the MD-11 are identified by a wire number consisting of an alpha character followed by a numeric string (e.g., B203-974-24). The alpha character designates the aircraft section in which the wire is installed (see Section The six digits that follow identify the individual wire number; the final two digits identify the wire gauge. Therefore, wire B203-974-24 indicates that the wire is installed in the B section, that its individual wire number is 203-974, and that it is a 24 AWG wire. An N suffix indicates a ground wire.

Typically, wires that are installed in aircraft are tied together in bundles called wire runs. Therefore, individual wires can be further referenced by identifying the wire run in which they are included.

In the MD-11, every wire run is identified by a three-letter designator, such as "FBC," which provides information about where and how the wire run is routed through the aircraft.

  • The first letter indicates the location of the wire run in the aircraft. Letters A, B, C, R, Q, and S are used for the cockpit and nose area. Letters D, E, and H refer to the fuselage section below the floor. Letters F, G, and J refer to the cabin above the floor. The letter K identifies the right wing. The letter L identifies the left wing. Letters T and V identify the tail location.
  • The second letter indicates whether the wire run is enclosed in a conduit or in an open wire bundle. The letters V, W, Y, and Z are used if the wire run is in a conduit; all other letters indicate an open wire bundle. The second letter also indicates the applicable RF interference category of the wires in the bundle.
  • The third letter identifies the specific run.

Where practical, the wire number is directly marked on the outer insulation of each wire; otherwise, the wire number is affixed to the wire by tags at both the start and termination points. A wire may need multiple sets of run letters to completely describe its routing through the aircraft.

Once a wire number is known, it is possible to use the manufacturer's wire list to determine where the wire is installed in the aircraft. The wire list also provides information about the wire's composition, length, to-and-from termination points, circuit function, and wire run affiliation.  Wire Description – MD-11 Aircraft  Selection Criteria for Wires – Douglas Aircraft Company

In accordance with FAR 25.869, the only certification test required for aircraft wires is the 60-degree Bunsen burner test (see Section Aircraft manufacturers typically perform additional wire tests to meet manufacturing and customer requirements, and select wire types based on a balance between the characteristics of the wire types available and the required application.

In 1976, Douglas Aircraft Company (Douglas) was informed by its wire supplier that the general purpose wire they were providing for the wide body aircraft program was going to be discontinued. Douglas initiated a wire evaluation program to select a new general purpose wire. The review included an assessment of various wire insulation types with respect to their electrical, mechanical, chemical, and thermal properties, along with their inherent flame resistance and smoke production characteristics. The evaluation resulted in two types of insulation being selected: a modified cross-linked ethylene-tetrafluoroethylene (XL-ETFE)[31] in accordance with Douglas specification BXS7008 and an aromatic polyimide,[32] hereinafter referred to as polyimide, in accordance with Douglas specification BXS7007.

Polyimide insulation was viewed as having favourable weight and volume characteristics. Also, it offers superior resistance to abrasion, cut-through, and fire. Polyimide does not flame or support combustion. The limitations of polyimide included less resistance to arc tracking[33) and less flexibility than other insulation types. Polyimide insulation is an amber-coloured film that is wrapped on the wire. In some cases, a modified aromatic polyimide resin coating was applied over the polyimide film to provide a suitable topcoat surface to allow the wire identification number to be directly marked on the wire. This topcoat appears dull yellow in colour.

In 1975, the FAA issued a Notice of Proposed Rulemaking (NPRM)[34] stating that for wire, the specific optical density[35] requirement for smoke emission would be a value of 15 (maximum) within 20 minutes after the start of the test. Although this NPRM was expected to be adopted, it was terminated without affecting the existing rules. However, before the NPRM was terminated, Douglas testing showed that the polyimide insulation would pass the specific optical density test requirements and that the XL-ETFE would not.

Based on cost and other considerations, Douglas chose XL-ETFE for its BXS7008 general purpose wire insulation, and used XL-ETFE in the DC-10. At the same time, polyimide insulation in accordance with BXS7007 was selected for the pressurized passenger section primarily because it produced less smoke when exposed to heat or flame compared to XL-ETFE. Polyimide could also be used in special applications, such as in locations where the temperature exceeded 150°C (302°F), whereas XL-ETFE was not rated for such temperatures.

In the early 1980s, a crimping problem was discovered with wires that had XL-ETFE insulation and tin-coated copper conductors. Because of this, Douglas decided to switch to nickel-coated conductors, even though they were more expensive. Subsequently, XL-ETFE lost its cost advantage, and Douglas switched to polyimide-insulated, nickel-coated conductors for all its general purpose wire.

In 1991, a US Air Force wire evaluation program identified a suitable general purpose wire replacement. It was a composite insulation made from polytetrafluoroethylene-polyimide-polytetrafluoroethylene (PTFE-PI-PTFE). That same year, Douglas initiated another wire evaluation program, using the polyimide general purpose wire as its baseline for comparison testing of other wire insulations types. The testing showed that the PTFE-PI-PTFE insulation performed as well as or exceeded the polyimide insulation; Douglas selected the PTFE-PI-PTFE insulation, in accordance with DMS 2426, for its general purpose wire in 1995.

Table 9 shows the comparative properties of four wire insulations.

Table 9: Comparative Properties of Wire Insulation Systems[36]

Relative Ranking Most Desirable -------- to -------- Least Desirable
1 2 3 4
Abrasion resistance PI ETFE COMP PTFE
Cut-through resistance PI COMP ETFE PTFE
Chemical resistance PTFE ETFE COMP PI
Flammability PTFE COMP PI ETFE
Smoke generation PI COMP PTFE ETFE
Creep[37] (at temperature) PI COMP PTFE ETFE
Arc propagation (arc tracking) resistance PTFE ETFE COMP PI
  1. PI – MIL-W-81381/7 (aromatic polyimide)
  2. ETFE – MIL-W-22759/16
  3. COMP – MIL-W-22759/80-92 (PTFE-PI-PTFE)
  4. PTFE – MIL-W-22759/80-92  MD-11 Wire Specification

Douglas identified the following two general purpose wire specifications for the MD-11: BXS7007 and BXS7008 (see Figure 13). These wire specifications adopt by reference, unless otherwise indicated, certain government-furnished documents, including Military Specifications and Standards and Federal and Industry Standards, as well as certain Douglas Material and Process Specifications. BXS7007 and BXS7008 also establish performance and test requirements that the wires must meet in addition to those adopted from the referenced documents, including, for example, the 60-degree burn test required by FAR 25.869.

BXS7007 specification is entitled "Wire, Electric, Copper & Copper Alloy, Polyimide Tape Insulated, 600 Volt." This specification covers wires and cables that must pass all the applicable performance and test requirements for the specified gauges as defined in MIL-W-81381, MIL-W-81381/12, and MIL-W-81381/14, as well as MIL-W-27500 and other referenced documents, unless otherwise indicated in the specification. Douglas started using the BXS7007 wire in production aircraft in 1980.

Wires that conform to BXS7007[38] are polyimide insulated with nickel-plated conductors. All BXS7007 wire conforms to the requirements of MIL-W-81381/12 (in addition to other applicable requirements), except for 24 AWG wire, which is high-strength alloy that conforms to the requirements of MIL-W-81381/14 (in addition to other applicable requirements). All BXS7007 wire is rated at 200C, 600 volts. The temperature rating refers to the maximum temperature in which the wire may be used, and is derived by combining ambient and wire-generated heating.

BXS7008 specification is entitled "Wire, Electric, General Purpose, Copper & Copper Alloy, Fluoropolymer Insulated."[39] This specification covers wires and cables that must pass all the applicable performance and test requirements for the specified gauges as defined in MIL-W-22759, MIL-W-22759/34, and MIL-W-22759/42, as well as MIL-W-27500 and other referenced documents, unless otherwise indicated in the specification (see Figure 13). Douglas started using the BXS7008 wires in production aircraft in 1977.

Wires that conform to BXS7008 are insulated with modified, XL-ETFE. BXS7008 requires that the insulation be applied or extruded on the conductor in two layers of contrasting colour to aid in the identification of insulation damage. Wires 22-00 gauges are tin-coated copper. These wires must conform to MIL-W-22759/34 and are rated at 150°C, 600 volts. BXS7008 24 gauge wire is nickel-coated high-strength copper alloy. This wire must conform to MIL-W-22759/42 and is rated at 200°C, and 600 volts.

MIL-W-27500, which is one of the documents adopted in both BXS7007 and BXS7008 unless otherwise indicated, covers requirements for special purpose cables and electrical power cables, including the basic wire size and type, number of wires, and shield and jacket styles. BXS7007 and BXS7008 also adopt documents requiring identification coding of wires.

In the areas of SR 111 where the fire occurred, it is estimated that more than 95 per cent of the wiring installed at the time of manufacture was BXS7007 (i.e., polyimide insulated wire). Douglas also used various other types of wire, in small amounts, where a specific requirement existed.

[29]    An electrical bus is a power distribution point to which a number of circuits may be connected. It can consist of a solid metal strip in which a number of terminals are installed, or a section of wire.

[30]    The report refers to the left and right emergency 28 V direct current (DC) buses as left and right emergency DC buses respectively.

[31]    XL-ETFE is produced by first modifying and then irradiating ethylene-tetrafluoroethylene.

[32]    Polyimide-type insulation is frequently known by the trade name Kapton®, when manufactured by the DuPont Company; or Apical®, when manufactured by Kaneka High Tech Materials Inc.

[33]    The Federal Aviation Administration's (FAA) Advisory Circular (AC) 25-16 Electrical Fault and Fire Prevention and Protection dated 5 April 1991 defines arc tracking as a phenomenon in which a conductive carbon path is formed across an insulating surface. This carbon path provides a short-circuit path through which current can flow. This phenomenon normally occurs as a result of electrical arcing and is known variously as carbon, wet, or dry arc tracking.

[34]    Federal Register, Vol. 40, No. 30, 12 February 1975.

[35]    Specific optical density is a dimensionless measure of the amount of smoke produced per unit area by a material when burned.

[36]    Table originally created by DuPont.

[37]    Creep occurs over time when a plastic part or object is subjected to a load. High temperature can accentuate creep.

[38]    In this report, BXS7007 specification wires are referred to as polyimide.

[39]    The fluoropolymer insulation under MIL-W-22759 suffixes other than "/34" and "/42" may be polytetrafluoroethylene, fluorinated ethylene-propylene, polyvinylidene fluoride (PVF2), unmodified ethylene-tetrafluoroethylene, or other fluoropolymer resin. The fluoropolymer may be used alone or in combination with other insulation materials.

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