Aviation Investigation Report A98H0003

1.6.2  Environmental (Air) System

  1. - General
  2. - Air Distribution System – Cockpit and Cabin
  3. - Passenger Cabin Air System
  4. - Air Conditioning – Smoke Isolation System  General (STI1-5)

Outside air is pressurized by each of the three engines. This pressurized air is bled off the engines to provide a source of heated and pressurized air to operate the various environmental subsystems, including the air conditioning packs and pressurization systems (see Figure 8). The three air packs are contained in compartments located to the left and right of the nosewheel well area. Each air pack supplies conditioned air to a common manifold located below the cabin floor.

Air from the common manifold travels through a self-contained distribution system of lines and ducts, and enters the cockpit and passenger areas via outlets located throughout the aircraft. Anomalies, such as leaking engine oil seals, can sometimes introduce contaminants, such as engine lubricating oil, into the bleed air system. Pyrolysis of these contaminants can give rise to potential smoke and odours in the conditioned air supply. Incidents where smoke or odours have entered the cockpit and passenger cabin through the bleed air system of commercial aircraft as a result of contamination have been reported frequently.

Air from the cockpit, passenger cabin, and the remainder of the pressure vessel[17] is vented overboard through an outflow valve located on the left side of the aircraft slightly ahead of the wing.

For normal operations, the air conditioning system is automatically controlled by the environmental system controller (ESC). The air system can also be operated manually by the pilots using the air systems control panel (ASCP) located in the overhead switch panel in the cockpit (see Figure 8 and Figure 11).

Insulation blankets are used extensively throughout the aircraft to wrap the air distribution ducts to provide a thermal barrier. They are also installed between all fuselage frames; in some areas a second layer is installed over the frames. These insulation blankets provide a barrier against hot or cold exterior temperatures, and noise that could otherwise enter the passenger cabin and cockpit.  Air Distribution System – Cockpit and Cabin

In the Swissair MD-11 configuration, conditioned air from the common air manifold located below the cabin floor is distributed to five zones through lines and ducts; Zone 1 is the cockpit and zones 2 to 5 are areas within the cabin (see Figure 8).

The ducts and lines continuously supply the cockpit with 500 cubic feet per minute (cfm) of conditioned fresh air regardless of the flow setting selected for the passenger cabin. The air enters the cockpit from numerous vents, including three outlets from the overhead diffuser assembly, window diffusers, overhead individual air outlets, and foot-warmer outlets (see Figure 8, Figure 9, and Figure 10). All of these cockpit vents can be fully closed with the exception of the centre overhead diffuser, which has a minimum fixed opening. Manually operated controls are used to regulate the airflow from the overhead diffuser assembly and the window diffusers. Three rotary controls for the overhead diffuser assembly are located at the rear of the overhead ceiling liner. The right window diffuser slide control is located in the right ceiling liner, above the first officer's position aft of the windscreen. The left window diffuser slide control is located in the left ceiling liner behind the captain's position, just inboard of the left aft window.

Air in the cockpit generally flows from the diffusers down and around the flight crew seats, then forward past the rudder pedals and into the avionics compartment below the cockpit floor. (See Figure 10.)

Although the incoming conditioned air from all three air packs is mixed in the common manifold before the air enters the distribution ducts, the proximity within the manifold of the Air Pack 1 inlet and the cockpit and Zone 5 outlets is such that an odour from Air Pack 1 could reach the cockpit and Zone 5 before reaching the other zones.

Conditioned air for the passenger cabin areas is ducted to overhead plenums and directed down toward the floor. This air circulates around the passenger seats, then migrates to airflow vent boxes located along both sides of the passenger cabin floor. Air from the airflow vent boxes is directed through under-floor tunnels to the outflow valve. The outflow valve consists of two small doors located on the lower left side of the fuselage at STA 920. These doors are regulated open or closed to control cabin pressurization.  Passenger Cabin Air System

The passenger cabin air system in the MD-11 is equipped with an economy (ECON) mode[18] that mixes fresh conditioned air with recirculated cabin air and distributes it to the cabin zones (see Figure 8). The cabin air system consists of four recirculation fans and one individual air fan, called a "gasper" fan, which are all located above the ceiling in the forward and centre cabin area. In the ECON mode, the recirculation fans draw air from above the ceiling. This air is then mixed with the fresh conditioned air supply before being distributed back into the passenger cabin. Normally, the four recirculation fans operate continuously, but can be manually turned off by selection in the cockpit of either the ECON switch, the CABIN BUS switch, or the SMOKE ELEC/AIR selector. The ESC will automatically shut off the recirculation fans when there is a demand for a lower cabin temperature or when a generator overload occurs.

The gasper fan provides a constant supply of air to the passengers' individual air outlets, and operates independently of both the ECON mode and the temperature selection. The gasper fan is turned off by selecting the CABIN BUS switch to the OFF position, or by selecting the SMOKE ELEC/AIR selector to the 3/1 OFF position.

There is a thumbwheel PAX LOAD selector on the ASCP to allow the pilots to input the number of passengers on board to the nearest 10. The ESC schedules the flow of conditioned air to the cabin based on this input. In the ECON ON configuration, the MD-11 air conditioning schedule is determined by combining 10 cfm of fresh air for each of the passengers, with 700 cfm from each of the four recirculation fans. Swissair chose to use a default setting of 260 passengers with all four recirculation fans operating. This default setting results in a mixed airflow of 5 400 cfm of fresh and recirculated air to the passenger cabin. In the ECON OFF configuration, the air conditioning schedule is set to 5 500 cfm to the passenger cabin.

Each of the recirculation fans and the gasper fan incorporates a high-efficiency particulate air filter (Donaldson Company PN AB0467286) constructed of pleated microglass fibre media with aluminum separators to maintain pleat spacing. The filter was life tested to the American Society of Heating, Refrigeration and Air Conditioning Engineers[19] Standard 52.1, meets military standard (MIL-STD)-282,[20] and is rated by its ability to capture and retain oil particles that are 0.3 micrometres (microns) in size.[21]

The filter is rated to remove 95 per cent of all 0.3 micron–size particles, and various capture mechanisms within the filter result in a higher efficiency in removing particles both smaller than, and larger than, 0.3 microns. For example, most tobacco smoke particulates, which are typically 0.01 to 1.0 micron in size, would be removed, as would larger particles, such as those produced when thermal acoustic insulation cover material burns.

During the initial stages of the fire on board the occurrence aircraft, the filter efficiency would have increased over time as particulates became entrapped in the filter. It would be expected that the filters would remove most of the smoke[22] particulates from the recirculated air during the initial stages of the in-flight fire. Although this filter is not classified as an odour-removing type, some odours associated with particulate contaminants would also be expected to be removed or diminished, while gaseous odours would be expected to pass through the filter.  Air Conditioning – Smoke Isolation System

If smoke or fumes are identified as coming from the air conditioning system, the flight crew are trained to use the Air Conditioning Smoke Checklist (see Appendix B). The checklist directs the flight crew to isolate the smoke source by selecting ECON OFF. If this does not isolate the smoke source, the next action on the checklist, after pushing the AIR SYSTEM push button to MANUAL, is to re-select ECON ON and select one of the air conditioning packs off. If this does not isolate the smoke source, the pack is selected back on and another pack is selected off. Each of the three air conditioning packs can be individually shut down to determine which of the three is the origin of the smoke. Air conditioning packs are shut down by selecting the air system to MANUAL, and then turning the appropriate air conditioning pack off on the ASCP; in turn, this closes the respective pack flow control valve. If the smoke decreases, the bleed air source for the air conditioning pack can be turned off, and the respective isolation valve can be opened.

[17]    For the purposes of this report, the pressure vessel or pressurized portion of the aircraft includes cockpit, cabin, avionics compartments, cargo compartments, and the various accessory spaces between the passenger compartment and the pressure hull.

[18]    The ECON mode is the default mode selected under normal operating conditions as it has an associated fuel saving.

[19]    An internationally recognized American organization specializing in assessing and recommending air quality standards in air conditioned and ventilated environments, including those in aircraft.

[20]    MIL-STD-282 refers to filter units, protective clothing, gas mask components, and related products: performance test methods.

[21]    To establish perspective relative to a more familiar item, the size of a human hair is about 70 microns in diameter.

[22]    For the purposes of this report, smoke is defined by the American Society of Heating, Refrigeration and Air Conditioning Engineers as small solid or liquid particles, or both, produced by incomplete combustion of organic substances such as tobacco, wood, coal, oil, and other carbonaceous materials. The term "smoke" is applied to a mixture of solid, liquid, and gaseous products, although technical literature distinguishes between such components as soot or carbon particles, fly ash, cinders, tarry matter, unburned gases, and gaseous combustion products. Smoke particles vary in size, the smallest being smaller than 1 micron. The average often ranges between 0.1 and 0.3 microns.

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