Aviation Investigation Report A98H0003

1.6.1  General

  1. - MD-11 Design and Configuration
  2. - Weight and Balance
  3. - Aircraft Coordinate System
  4. - Cockpit Attic and Forward Cabin Drop-Ceiling Areas – Description

Table 5: General Information about the Occurrence Aircraft (HB-IWF)

Manufacturer McDonnell Douglas Corporation
Type and Model MD-11
Year of Manufacture 1991
Serial Number (SN) 48448
Certificate of Airworthiness Issued 28 July 1991
Total Airframe Time (hours) 36 041
Engine Type (number of) Pratt & Whitney 4462 (3)
Maximum Take-Off Weight 285 990 kilograms (kg)
Recommended Fuel Types Jet A, Jet A-1, JP-5, JP-8, Jet B
Fuel Type Used Jet A  MD-11 Design and Configuration

The McDonnell Douglas MD-11 design project began in 1986. The MD-11 design is structurally based on the McDonnell Douglas DC-10 design (see Figure 1 and Figure 2). The MD-11 was designed for more economical and efficient operation than the DC-10, by incorporating modern, automated systems. The redesign automated most of the functions that were performed by the flight engineer in the DC-10, thereby allowing for a two-crew cockpit. The first MD-11 flight was on 10 January 1990 and delivery of the aircraft to the first customer was on 7 December 1990. The occurrence aircraft was manufactured in 1991 and was put directly into service by Swissair.

As the MD-11 was manufactured and certified in the United States (US) in accordance with applicable Federal Aviation Regulations (FAR), the regulatory focus of this report is directed toward the Federal Aviation Administration (FAA). Many civil aviation authorities (CAA) have drafted or harmonized their respective certification and continuing airworthiness regulations based on the FAA model; therefore, the issues in this report may also apply to other regulatory authorities.

The occurrence aircraft was configured with 241 passenger seats: 12 first class, 49 business class, and 180 economy class. The first- and business-class seats were equipped with an in-flight entertainment system,[11] certified and installed in accordance with a US FAA Supplemental Type Certificate (STC).  Weight and Balance

Weight and balance calculations completed after the occurrence determined that the actual take-off weight for SR 111 was approximately 241 100 kg. The centre of gravity (C of G) was calculated to be 20 per cent mean aerodynamic chord (MAC). Other than very small differences, the post-occurrence calculations confirmed that the weight and balance calculations used for dispatch were accurate. (STI1-4) The aircraft's weight was within limits, and throughout the flight the C of G was within the normal range (15 to 32 per cent MAC). The maximum allowable landing weight for the aircraft was 199 580 kg; the maximum overweight landing weight, allowable under certain conditions, was 218 400 kg. In an emergency, from an aircraft structural limit perspective, the aircraft can land at any weight; however, operational aspects, such as required stopping distance versus available runway distance, must be considered.  Aircraft Coordinate System

The MD-11 fuselage comprises six major sections and two minor sections (see Figure 2). The major sections extend from Section B, the nose/cockpit area of the aircraft, to Section G, the aft fuselage section. The two minor sections, sections 6 and 5, were inserted fore and aft of Section E to extend the length of the original DC-10 fuselage. Each fuselage section consists of the external skin, internal circumferential frames, and longitudinal stiffening members (longerons and intercostals). Figure 2 also shows the locations of numerous manufacturing stations (STA), fuselage sections, the forward doors, lavatories (LAV), and galleys.

An X, Y, Z Cartesian coordinate system is used to identify any point within the aircraft.

  • The X-axis extends laterally across the width of the aircraft. Lateral coordinates are measured in inches left or right of the fuselage longitudinal centre line. From the centre line toward the left wing, locations are positive coordinates (e.g., X= 80); locations toward the right wing are negative coordinates (e.g., X= –80).
  • The Y-axis extends longitudinally from the nose to tail, is expressed in STAs, and is measured in inches aft of a designated point in front of the aircraft. For the MD-11, the tip of the nose of the aircraft is located at STA 239 and the cockpit door is located at STA 383.
  • The Z-axis extends vertically through the aircraft. Vertical coordinates are measured in inches above or below the waterline (Z= 0), which, in the MD-11, is located 18 inches above the cabin floor. The cabin floor is therefore located at Z= –18.  Cockpit Attic and Forward Cabin Drop-Ceiling Areas – Description

The following section describes the cockpit attic and forward cabin drop-ceiling areas (see Figure 2, Figure 3, Figure 4, Figure 5, Figure 6, and Figure 7); the fire damage[12] and fire propagation in these areas is discussed in other sections of this report.

The space above the cockpit ceiling liner and the passenger cabin ceiling is referred to as the "attic" (see Figure 2). In Swissair MD-11 aircraft, the attic was divided at the cockpit rear wall. On the right side, the aluminum cockpit wall extended vertically to provide the division. On the left side, a single vertical smoke barrier was installed. (See Figure 3.)

The smoke barrier assembly above the left half of the cockpit rear wall consisted of a curtain made of nylon elastomer-coated cloth that was suspended from a curved aluminum alloy curtain rod. Hook-and-loop fastener[13] was used around most of the outer periphery of the cloth to attach it to the curtain rod, as well as to attach it to the adjacent aircraft structure along the bottom and right side. Thermal acoustic insulation blanket (insulation blanket) splicing tape was installed along the entire top edge of the smoke barrier to close gaps between the rod and the adjacent insulation blankets. The smoke barrier was designed with the following openings: three near the top of the curtain to permit the engine fire shut-off cables to pass through and two near the centre of the curtain to accommodate the installation of the cockpit air ducts.

Regulations require the installation of a smoke barrier between the cockpit and the rear of the aircraft in cargo and combination cargo/passenger configurations. However, there is no regulatory requirement to install smoke barriers in passenger aircraft, nor is there a requirement for the smoke barrier to meet a fire rating or fire blocking standard specific to a passenger aircraft application. Regardless, the barrier was certified to meet general aircraft material requirements and was installed in the aircraft during manufacture.

Examination of other Swissair MD-11 aircraft in the Swissair fleet disclosed that openings existed in the smoke barriers, and in areas adjacent to the barrier. Some of these openings were located at conduit and wire run locations that pass through or above the cockpit rear wall. The top edge of the rear, right cockpit wall near STA 383 has a cut-out in it to permit the passage of wire bundles and conduits. (See Figure 4 and Figure 5.)

Three 102-centimetre (cm) (40-inch) long conduits[14] and five wire bundles pass over the cockpit rear wall at this point, and continue aft over the top of Galley 2 between STA 383 and STA 420. (See Figure 3, Figure 4, Figure 5, and Figure 7.) The ends of the conduits were not required to be sealed and were found unsealed in other MD-11 aircraft that were examined. These conduits and wire bundles are attached by straps to a series of wire support brackets located at STA 383, 392, 401, 410, and 420. The wire bracket positioned at STA 383 is at a slight angle relative to the cockpit wall, which is directly below it. The top edge of this bracket, and attached wire bundles, are in contact with the metallized polyethylene terephthalate (MPET)–covered insulation blanket. Each of the conduits protrude forward of the cockpit wall by varying amounts because of the angle of the wall to the bracket.

Typically, the forward protrusion of the outboard conduit is the shortest of the three and the forward protrusion of the inboard conduit is the longest. These lengths, as measured from the bracket, vary from approximately 2.5 to 8 cm (1 to 3 inches) for the outboard and middle conduits. The inboard conduit was not used for any of the in-flight entertainment network (IFEN) installations. The cut-out extends downward approximately 8 cm (3 inches) from the top of the wall and is approximately 48 cm (19 inches) wide. A piece of closed-cell polyethylene foam containing fire retardant additives (i.e., part number (PN) NBN6718-83; Douglas Material Specification (DMS) 1954, Class 1, Grade 4101) is installed at this location to act as filler material for the cut-out.

Between STA 366 and STA 383 there are a number of wire support brackets installed in the fore-aft direction. These brackets are used to support wire bundles routed from behind the observer's station down into the avionics compartment; this area is commonly referred to as the "ladder area."[15] The aft end of the top bracket in the "ladder" is located near the outboard end of the cut-out in the cockpit wall (see Figure 3 and Figure 5). The brackets, and many of the wire bundles, are pressed up against, and closely follow, the curved contour of the fuselage over-frame MPET-covered insulation blankets.

Just aft of the right side of the cockpit rear wall, above Galley 2, a sound-suppression muff assembly (muff assembly) was installed around a splice junction of the conditioned air riser duct assembly (see Figure 6). The muff assembly uses an MPET-covered insulation blanket secured at both ends by hook-and-loop fasteners.

A second type of closed-cell polyethylene foam (PN ABE7049-41) was used around the windshield defog terminal blocks on the left side of the cockpit. A sample of the second type of foam (PN ABE7049-41) was removed from a Swissair MD-11 aircraft and tested. When the sample specimen was exposed to a small flame, the specimen ignited easily and burned.

Both of these foam materials were specified to DMS 1954, Class 1, Grade 4101, which states that the foam should possess fire-retardant additives and be certified to pass a 12-second vertical burn test as required in FAR 25.853, Appendix F. Literature indicates that both foams met FAR 25.853, Appendix F for commercial aircraft interior compartment components.

The manufacturer's material safety data sheet product code 37076 for the Dow Chemical Ethafoam® 4101, PN NBN6718-83, dated 23 August 1993, and current product information indicate that this polyethylene foam is combustible[16] and should not be exposed to flame or other ignition sources.

No foam was identified from the cockpit area of the occurrence aircraft.

In the Swissair MD-11s, the forward end of the muff assembly comes into close proximity to the lower right edge of the smoke barrier, and to the vent duct assembly for Galley 2. The galley vent duct, which is designed to exhaust odours and hot air from the galley when in operation, was not connected to the top of Galley 2, as Galley 2 was not electrically powered and not in service. A silicone elastomeric end cap was placed over the vent duct to close it off. The cap was located between the aft side of the cockpit rear wall and the forward side of one of the three riser ducts (see Figure 4 and Figure 6).

Five wire bundles and three conduits run aft from the cockpit and over the top of the riser duct assembly. The majority of the wire bundles descend from the wire support bracket at STA 420 to pass under the R1 door, flapper door ramp deflector. This drop in the wire bundles is generally referred to as the "waterfall" area (see Figure 7). Two of the wire runs, namely FDC and FBC, are clamped together and attached to a ceiling support tube located at approximately STA 427. This clamping arrangement is referred to in this report as a "marriage clamp." The ramp deflector is used to minimize the possibility of the forward right passenger door flapper panel from damaging adjacent wire assemblies if the flapper panel torsion spring should fail. The door flapper panel moves with the passenger cabin door when the door is raised or lowered.

[11]    The in-flight entertainment system installed in the occurrence aircraft was referred to as the in-flight entertainment network.

[12]    Fire damage is defined as heat and smoke damage as caused by a fire.

[13]    Velcro® is a commonly known brand of hook-and-loop fastener.

[14]    The conduits were identified by part number ABP7646-39 as 1.0 (inside diameter) x .020 (thick) x 40 inches long. Measurements of similar conduits on other MD-11 aircraft showed they could be as long as 108 centimetres (cm) (42.5 inches).

[15]    The wire brackets and the frames to which they are mounted are similar in appearance to a ladder. This area is commonly referred to as the "ladder area."

[16]    A material that will ignite and burn when sufficient heat is applied to it.

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