Air transportation safety investigation report A98H0003
Supporting technical information (STI)
The environmental systems are powered by pneumatic air pressure, which is supplied by either the engines, the APU, or ground sources. This pressurized air is used to operate the following systems:
- Air conditioning and pressurization
- Engine starting and engine anti-ice
- Wing and stabilizer anti-icing
- Cargo heating
- Potable water pressurization
- Avionics cooling
Air conditioning system
The MD-11 is equipped with three air conditioning packs (air packs).Footnote 1 Air packs 1 and 2 are co-located in the air conditioning/wheel well compartment at the forward end of the aircraft, to the left of the nosewheel well. Air Pack 3 is located on the right side, across from air packs 1 and 2. All three air packs supply a common, conditioned air manifold, which is located below the cabin floor. The air packs feed this manifold through three check valves. Air from the conditioned air manifold is directed through ducts, upward along the left and right side walls of the aircraft (behind the cockpit rear walls) and then through duct work to the five individually controlled zones in the aircraft. One duct from the manifold is dedicated to supply the cockpit only.
Hot pneumatic bleed air to each air pack is passed through a pneumatically actuated flow control valve. The flow control valves are spring-loaded to the closed position; that is, with a loss of pneumatic supply pressure, the valve will close. The flow control valve normally operates at 35 psig, but the ACC,Footnote 2 which regulates the air pack temperature and flow functions, can reduce the pressure to meet system demands through the use of an electrically operated torque motor located in the valve. If the ACC cannot control flow rate with the flow control valve (because of a failure), it sends a signal to the related pneumatic system controller. This causes the pressure regulator valve to modulate, and act as a backup means to regulate the air pack temperature and flow functions.
With a loss of electrical power, the flow control valves will modulate pneumatically to regulate air pack operation. To shut down an air pack with an operating pneumatic source, the torque motor has to be electrically driven to close the valve. The torque motor on Air Pack 3 flow control valve is electrically powered by the battery bus, through CB B1-347, at location C-11 on the overhead CB panel.
Air Pack 3 can be shut down manually, through push-button selection on the ASCP or by rotation of the SMOKE ELEC/AIR selector (to the 2/3 position). The air pack can also be shut down automatically by the ACC, as a result of a manifold failure or the air pack overheating. In the event of a pack overheat condition shutting down the air pack, the pack ram air doors will remain open.
There are two pneumatic system isolation valves, identified as Isolation Valve 1-2 and Isolation Valve 1-3. They are installed in the pneumatic supply duct work, upstream of the air packs, in the centre accessory compartment. In case of a pneumatic supply system problem or an engine failure, these valves provide for an alternate supply of air to the air packs, from an active pneumatic system to an inactive one. The valves are normally kept closed by the ESC and will open when the ESC senses a need to bleed air from one system to another.
Operation of the air conditioning system is controlled automatically by the ESC in the auto mode or manually by push-button selection on the ASCP with the ESC in manual mode.
Ram air cooling
The ram air cooling system supplies metered outside air to each of the three air pack heat exchangers and composite plenum assemblies. The air is then dumped overboard through an exhaust door. The flow of cooling air to the heat exchangers is controlled by the positioning of three ram air inlet and exhaust doors. The efficiency of the heat exchanger is affected by the amount of cooling ram air that passes across the bleed air tubes in the heat exchanger. The ram air inlet door assemblies are located on the lower side of the forward fuselage, forward of the air packs. Two inlet doors are located on the left side of the fuselage to supply air packs 1 and 2, and one inlet door is located on the right side of the fuselage to supply Air Pack 3.
A ram air door actuator is connected to each of the exhaust doors, which in turn are interconnected to their respective inlet doors by a push/pull cable arrangement. The ram air door actuators are electrically operated jack screws that move in response to signals from their respective ACCs and the ESC. The actuators extend to close the doors and retract to open the doors. When an air pack is shut down by operation of the SMOKE ELEC/AIR selector or by push-button selection on the ASCP (in manual mode), the ACC will close the air pack's corresponding ram air inlet door. If the air pack is shut down by the ACC because of a fault (i.e., if the pack temperature exceeds 180°C), the ram air door will remain open. The ram air door actuators are electrically powered by the left emergency 115 V AC bus through CBs B1-351, B1-352, and B1-353 at locations F-07, F-08, and F-09 on the overhead CB panel.
All three ram air inlet door assemblies were recovered; however, they could not be matched to specific air packs. An examination of the door assemblies exhibited water impact marks on the interior of all three flap assembly housings. Of the three ram air door actuators, only the actuator from Air Pack 3 was recovered. The actuator was captured in the retracted, or door-open, position.
The water impact marks on the interior of all three flap assembly housings were caused by contact with their respective ram air inlet doors. The location of the water impact marks placed two of the ram air inlet doors near the fully open position, and one near the half-open position.
Conditioned air manifold
During the reconstruction and examination of the ram air ducting and the air conditioning ducting–downstream of the air conditioning units to the conditioned air manifold–a black soot-like material was noted on the inner surface of the duct walls. The outer surface of the walls showed no signs of heat discolouration. Another Swissair MD-11 aircraft, HB-IWA, was inspected during its second "D check;" it too exhibited the same material adhering to the inner surfaces of the ducts. Swissair was asked to provide photographs and swab samples of the interior of the air conditioning duct work downstream of the air packs from one of their MD-11 aircraft in service.
No signs of fire or overheat damage were observed within the conditioned air duct work below the cabin floor.
The photographs and swab samples provided by Swissair were consistent with the material noted on the duct work of the accident aircraft. As a result, the manufacturer of the air pack component, Allied Signal, was consulted. The soot-like material was determined to be a normal bi-product of dust particles that impinged on the duct walls after coalescing with water from condensation within the air conditioning units.
Fan air (Heat exchanger) plenums
Ram air that has been heated by passing through the heat exchangers is directed overboard by the fan air plenums. The plenums are constructed from a phenolic resin/fibreglass composite material that is black when assembled.
The phenolic resin fibreglass composite material had whitened on some of the recovered pieces of the plenums, leaving only the fibreglass cloth, without the phenolic resin. No charring of the plenum material was found. These pieces were examined by the manufacturer of the component, Allied Signal, who found a change in colour and resin loss in the plenum samples.
The mating heat exchangers from the three air packs were also examined for exposure to heat.
The change in colour and resin loss in the plenum was consistent with the normal "baking out" or evaporative loss of the resins over the long service life of the plenum (in this instance more than 30 000 hours in service). Allied Signal conducted oven and burner tests on samples of new composite plenum material. The results of the oven test showed that when the samples were heated to 232°C (450°F) in a circulating oven for 500 hours (a temperature consistent with the normal operating temperatures associated with air pack operation), there was a 7% weight loss in the composite samples. The loss in weight was determined to be from the evaporative loss of the cured phenolic resin.
During testing, the composite material burned when subjected to an open flame; however, combustion stopped once the flame was removed. The burned sample appeared charred, unlike the material that was recovered from the crash site, indicating that the recovered material had not been exposed to fire.
The heat exchangers are made of aluminum, and would exhibit some evidence of a fire, had it been present. It was determined that the heat exchangers were not exposed to excessive heat.
Pneumatic system isolation valves
The pneumatic system isolation valves are a electrically actuated, butterfly-type shut-off valves. The electrical actuator is attached to the body assembly and contains two independent motor and planetary reduction gears. Either of the motor/gear sets can drive the differential planetary reduction gears to rotate the butterfly shaft. When the butterfly shaft rotates, it turns the butterfly plate and operates the position indicator and limit switches. If voltage is removed during valve operation, the valve stops and stays in that position. The valves can be manually positioned (during maintenance) by rotating an override knob on the electrical actuator. The override knob requires approximately 20 rotations to move the valve through the full range.
The ASCP has two switch-lights for control of the isolation valves. The ESC illuminates the correct light to indicate the following valve conditions: ON, to show that the ESC issued a valve-open command; and DISAG, to show that the valve's position disagrees with the commanded position. When the ESC is in auto mode, the switches on the ASCP are not active. When the ESC is in manual mode, pushing the switches will command the valves alternately open or closed.
One of the two isolation valve electrical actuators was recovered and was identified by a data plate as AiResearch PN 544964-1, SN 49-1957. The actuator had broken free of the valve body assembly, which was not found. The actuator was heavily damaged; both electrical drive motors had been broken off and were not found. The actuator indicator, which is driven by the internal planetary gears, was aligned slightly beyond the closed position mark on the actuator body. The manual drive override knob had been broken away from its mounting shaft, and the shaft appeared to have been rotated from the closed position to slightly beyond the closed position at the time of impact.
The second isolation valve electrical actuator was recovered; however, its data plate was not found. The actuator had broken free from the body assembly; the valve body assembly was not found. The actuator was heavily damaged; both electrical drive motors had been broken off and were not found. The actuator indicator, which is driven by the internal planetary gears, was aligned with the closed position mark on the actuator body. The manual drive override knob was heavily damaged. A scratch on the actuator body aligned with a sheared edge of the override knob.
Since the serial numbers of the isolation valve actuators were not recorded in the aircraft's technical records, it could not be determined whether the first isolation valve electrical actuator recovered corresponded to Isolation Valve 1-2 or Isolation Valve 1-3. The valve was determined to be in the "valve closed" position at the time of impact.
The alignment of the marks on the actuator body of the manual drive override knob on the second isolation valve electrical actuator indicates that the knob was likely in the "valve closed" position when the damage occurred and had not moved subsequently.
All three air packs were recovered.
The units were heavily damaged and contaminated with silt and salt corrosion products. The air packs were an item of interest since the crew had indicated that they were, initially, using the Air Conditioning Smoke checklist. A team that included representatives from the manufacturers of the aircraft (Boeing) and components (Allied Signal), the airline company (Swissair), and the TSB was assembled to examine the units. The packs were examined for evidence of rotor burst, rupture, or signs of fire or heat that may have been associated with the origins of the on-board fire. The packs were also examined for signs of rotational damage that would indicate whether they had been rotating at the time of impact.
Air pack 1
Scratches were observed on the turbine and compressor shrouds and wheel assemblies; the flow control valve was found in the partially open position. The turbine exducer blades were folded and the fan blades were sheared. The bearings did not show any signs of having been overloaded and did not exhibit any pre-impact damage. Air Pack 1 did not exhibit fire damage or any internal failure that might have occurred before impact with the water.
Air pack 2
Scratch marks were observed on the compressor; a rub mark was found on the flow control valve. The turbine exducer blades were bent and fractured, and the fan blades were sheared. The bearings did not show any signs of having been overloaded; however, the material coating on four of the turbine thrust bearing pads exhibited discolouration. Air Pack 2 did not exhibit fire damage or any internal failure that might have occurred before impact with the water.
Air pack 3
No witness rub marks were noted on the compressor or turbine shroud and wheel assemblies. No physical evidence was found to suggest that the flow control valve was in other than the recovered, closed position. The lock-out blocks were found in the partially engaged position. The turbine exducer blades were folded and the fan blades were sheared. The bearings did not show any signs of having been overloaded and did not exhibit any pre-impact damage. The Air Pack 3 ram air doors and door actuator were found in the open position. Air Pack 3 did not exhibit fire damage or any sign of internal failure that might have occurred before impact with the water.
Air pack 1
Scratches on the turbine and compressor shrouds and wheel assemblies indicated that the air pack was most likely operating at the time of impact, which is consistent with the partially open position of the flow control valve. Folding of the turbine exducer blades suggest a high blade loading, possibly from water impingement, as there was no crush damage to cause the deformation. Shearing of the fan blades was most likely a result of a water impact load during separation of the fan inlet.
Air pack 2
Scratch marks on the compressor indicate that the air pack was most likely operating at the time of impact, but with low rotational energy. This is consistent with the rub mark on the flow control valve indicating that the valve was partially open at the time of impact. Bending and fracture of the turbine exducer blades was most likely a result of high-impact loads caused by the collapse of the turbine shroud during the sudden stoppage. Shearing of the fan blades was most likely the result of an impact load during separation of the fan inlet. Discolouration of the material coating on four of the turbine thrust bearing pads indicated that the air pack was likely subjected to a high operating temperature condition at some time during the life of the unit.
Since Air Pack 2 was running (to a lesser degree than Air Pack 1), and both isolation valves were closed, Air Pack 2 was probably in the stages of spooling down at the time of impact. With the shutdown of Engine 2, there would have been a loss of pneumatic pressure or bleed air from the engine to Air Pack 2. This loss of pneumatic system or duct pressure would have been sensed by the pneumatic system controller and a signal would have been sent to the ESC. With the ESC operating in auto mode, Isolation Valve 1-2 should have opened automatically to allow Engine 1 bleed air to drive Air Pack 2.
The failure of Isolation Valve 1-2 to open in this case implies a loss or interruption of electrical power to the valve. The valve is powered by the right emergency 115 V AC bus through CB B1-311 at position G-25 on the overhead CB panel. The overhead CB panel was damaged by heat and fire, which could have caused a thermal tripping of the CB. The CB was not identified; however, a CB at position F-24 (one row up and one to the left of G-25) was recovered with no soot accumulation on the white insulator (indicating that CB F-24 had not been tripped before impact). The control circuitry for the isolation valve was also routed through an area of high heat and known electrical arcing. If the control circuitry was compromised by the fire, the valve would have remained closed.
If, prior to the engine shutdown, the ESC had been selected to manual mode (as in the case of the Air Pack 3 manual shutdown scenario), Isolation Valve 1-2 would not have opened automatically and would only be commanded open by crew selection. Because the shutdown of Engine 2 occurred in the latter stages of flight (just prior to impact), it is unlikely that the selection of the isolation valve would have been a high priority for the crew, and may not have been possible because of the overhead fire.
Without an alternate supply of bleed air through Isolation Valve 1-2, a spring in the flow control valve would eventually overcome the decreasing pneumatic pressure in the supply duct and close the flow control valve, thereby shutting down the air pack.
Air pack 3
The lack of witness rub marks on the compressor and turbine shroud indicates that the air pack was not operating at the time of impact. This is consistent with the fully closed position of the flow control valve. The partial engagement of the lock-out block suggests that the valve was in the closed position when the valve body was struck. The folded turbine exducer blades suggests a high blade loading, possibly from water impingement, as there was no crush damage to cause the deformation. Shearing of the fan blades was most likely the result of an impact load during separation of the fan inlet.
The fact that the pneumatic supply source for Air Pack 3 (Engine 3) was operating suggests that the air pack was shut down by an electrical signal to the flow control valve's torque motor from the ACC rather than by a loss of the pneumatic source pressure.
The Air Pack 3 ram air doors and door actuator were found in the open position, which could indicate that the pack was shut down automatically because of overheating, but examination of the air pack revealed no signs of such a condition. The pack temperature outlet sensor (which would sense the overheating) is located in the ductwork in the Air Pack 3 compartment and would not have been influenced by the heat generated by the on-board fire. The possibility of a manifold failure cannot be ruled out; however, because it would require a mechanical failure of the ductwork (below the floor), it is considered unlikely.
The manual shutdown of the air pack is a likely event, as the flight crew were trying to isolate a potential source of smoke (as per the emergency checklist procedures for Air Conditioning Smoke). The shutdown of the air pack would have to have occurred after the loss of the FDR (as no pack shutdowns were recorded). To shut down Air Pack 3, the crew could have used the SMOKE ELEC/AIR selector (in the 2/3 position); however, as the 115 V AC Bus 2 is considered to have been powered at the time of impact, it is unlikely that the air pack was shut down for this reason (since power to the 115 V AC Bus 2 indicates that the SMOKE ELEC/AIR selector was not in the 2/3 position at the time of impact).
The crew could have shut down the air pack manually by push-button selection on the ASCP. The Air Conditioning Smoke checklist calls for the shutting down of Air Pack 1 (as the first event), and then, if the smoke does not abate, the sequential shutting down of Air Pack 3 and Air Pack 2 (while turning on the air pack previously shut down). To activate the pack push buttons, the crew would first have to select the ESC to manual mode. With a manual shutdown of an air pack, the ram air doors would close. For the ram air doors to remain open (as in the case of Air Pack 3), a loss of electrical power to the ram air door actuator would have to occur. The ram air door actuators are powered by the left emergency 115 V AC bus, which was found to have heavy arcing damage to the bus feed wire. If this arcing damage had interrupted electrical power to the door actuator prior to the shutdown of the air pack, then the door would have remained open, its last-selected position.
Battery bus voltage would have been available to the flow control valve to close the valve. The fact that Air Pack 3 had spooled down and was off at the time of impact indicates that the shutdown of the air pack likely occurred as an earlier event within the last six minutes, after the loss of the FDR.
Air duct temperature sensors
The duct temperature dual sensors are paired, thermistor-type sensors in the air distribution ducts of each zone. The cockpit duct temperature sensor is installed in the flight compartment conditioned air distribution duct, on the left side of the avionics compartment at STA 390. The signal wiring between the cockpit duct temperature sensor and ACC 1 and ACC 2 is routed below the floor. The other four sensors are mounted in the forward cabin above the ceiling, with one sensor in each conditioned air distribution duct.
The cockpit zone temperature sensor is located in the temperature ejector, mounted in the ceiling above the cockpit door. The zone temperature sensor signal wire is routed from the ejector, across the forward side of the cockpit wall, to wire run ADD, which originates from receptacle R5-423 on the overhead disconnect panel. The signal wire is attached to this wire bundle and is then routed down the right side of the fuselage, behind the avionics CB panels, to the avionics compartment.
An increase in the duct air temperature will be directly shown on the Air Page. If an overheat condition exists (when the temperature exceeds the overheat setpoint of 87°C for 320 seconds), the digital duct temperature readout changes from white to amber and is boxed amber. If the trim air is turned off, the digits are replaced by a cyan OFF display. This will also generate a Level 1 alert "ZONE TEMP SEL OFF" shown on the EAD, but will not illuminate the AIR cue key on the SDCP. If no valid duct temperature is available, the digits are replaced by an amber X.
For a fire to affect the duct temperature sensors, it would have to impinge directly on the sensor or raise the temperature of the air in the duct. If a fire were to damage and open the duct temperature signal wires, then the duct temperature would be replaced by an amber X.
A fire in the cockpit attic area could affect the zone temperature readout if it caused the signal wires to create an open circuit or if the ejector was damaged by fire enough to cause the separation of the signal wires. If the signal wires were opened, the zone temperature readout on the Air Page would have been replaced by an amber X.
There was no indication of a fire in the avionics compartment, nor of heating of the conditioned air ducts below the floor.
Part of the temperature ejector was identified; it exhibited heat damage.
It was determined that fire did not affect the duct temperature sensors.
As the signal wires are all routed below the cockpit floor, it was determined that the fire did not damage the duct temperature signal wires.