Q-AFM KEY NOTES Flashcards
Secondary Bus Failures
In the case of a secondary bus short, the overcurrent condition will immediately trip the associated TRU pri-mary circuit breaker. 7 s later, with the EPCU declaring TRU failed the contactor L to R Secondary bus tie will close, transferring the short circuit to the opposite side TRU. At that moment, the cross tie fuse is blown isolate ing the fault. This is indicated by a L TRU or R TRU caution light and loss of services on the associated sec-ondary bus.
Main Bus Failures
If a main bus fault occurs, the EPCU prevents the upper horizonal and 2 vertical bus ties from closing, isolating the bus. The DC BUS caution light comes on to warn of the fault impending condition. If the fault persists after approximately 5 s, the EPCU sends a TRIP signal to the GCU, isolating the affected generator. The EPCU will also open and lock-out the contactors connecting the batteries to the affected main bus. The MAIN BATTERY or AUX and STBY BATTERY caution light(s) and related DC GEN caution light will come on as a result. The EPCU continues to monitor the operating buses. All main DC services on the faulted bus side will not function.
Variable-Frequency AC Power
Two 115 V, 45 KVA AC generators (mounted on the propeller reduction gearbox) supply variable frequency (340 to 560 Hz) AC power. The AC power is supplied to the left and right AC buses. AC power sources are pre-vented from being operated in parallel.AC power is available once the condition levers are out of START & FEATHER in the MIN /850 to MAX / 1020 range and the GEN 1 and GEN 2 switches on the AC CONTROL panel are on.If one AC generator fails, the # 1 AC GEN or # 2 AC GEN caution light comes and the remaining generator is
capable of carrying the airplane’s AC electrical load except galley power. An automatic cross tie function, controlled by the AC GCU logic circuits, ensures that all variable-frequency buses are powered when only one AC generator is on line. Whenever a fault condition exists, the GCU of the inoperative generator issues a transfer request signal to the operational side AC GCU. The operational side AC GCU will issue a CLOSE command to the failed side line contactor. In this configuration, the remaining generator will power both AC buses. In this situation the load shedding relays will not allow power to the galley buses.The AC generators are protected from bus faults by the GCUs that detect any excessive load that might result from a short circuit on a bus. Once a heavy load is detected, the GCU isolates the bus and turns on the appropriate L AC BUS or R AC BUS caution lights.The # 1 AC GEN HOT or # 2 AC GEN HOT caution lights come on whenever an AC generator overheats. The AC generator must be switched off.
Transformer Rectifier Units (TRU)
Located in the nose, the 2 Transformer Rectifier Units change 3 phases, 115 V, variable frequency AC input power into 28 VDC (300 A max) nominal output. The TRUs are unregulated but provide DC power in the range of 26 to 29 VDC during operation. Under normal conditions, each TRU powers its respective secondary bus. The L TRU or R TRU caution light comes on if either TRU is off line or failed. The L TRU HOT or R TRU HOT caution light comes on if the sensor in either detects an overheat condition. The light will go out when the temperature drops below the overheat condition. The 2 TRUs alone are capable of powering the entire DC system.
Air Conditioning System
The Air Conditioning System (Figure 6.2-5) receives bleed air when the BLEED switches on the AIR CONDITIONING control panel (Figure 6.2-3) or the BL AIR switchlight on the APU CONTROL panel are selected on.The Air Conditioning System is controlled by selecting the CABIN and FLT COMP PACKS switches (Figure6.2-2) to the MAN or AUTO positions and then adjusting the temperature using the TEMP CONTROL knobs.These switch settings determine the bleed air source, manual or automatic Environmental Control System
(ECS) operation, and the air flow temperatures for the flight and passenger compartments. The ECS Electronic Control Unit (ECU) (Figure 6.2-6) controls the two Nacelle Shut-Off Valves (NSOV) to reg-
ulate the air flow to the air conditioning packs. The ECU receives bleed air pressure and temperature data from the pack inlet absolute pressure and inlet temperature sensors. The ECU uses these data to control bleed air flow through the pack Flow Control Shut-Off Valve (FCSOV). The ECU also uses this data to control bleed air
flow rate when APU bleed air is selected on
Automatic Mode ( CPC )
When electrical power is first supplied to the system, a full power up self test is done. The FAULT alert light, on the Cabin Pressure Control (CPC) panel comes on momentarily during the power up test mode. If there is a failure in the system, the light will stay on. The system operation is fully automatic with the data programmed
into the controller (Figure 6.2-17).With the system in AUTO mode, a pre-programmed cabin pressure controller does all pressure scheduling from take-off to landing with minimal crew input. The computer receives inputs from the crew and various airplane systems, and modulates the aft outflow valve. This keeps a fixed schedule of cabin altitude versus airplane altitude for complete regulation of cabin pressure.
On Ground ( CPC )
When the airplane is on the ground and the engine power lever angles are set at less than 60°, the aft outflow valve is positioned at the fully open position to prevent airplane pressurization. The aft safety valve located on the aft pressure bulkhead, also opens on the ground when at least one engine is running at idle, or the APU is operating.
Take-off ( CPC )
When the engine power levers angles are set to greater than 60° the controller sends a signal to the aft outflow valve to modulate, as necessary, to provide two take-off sequences:
• Pre-pressurization
• Take-off abort
The aft outflow valve moves from the fully open position and starts to modulate to control the pressure changes that occur during take-off. After take-off (as sensed by the PSEU), the aft outflow valve modulates to keep the set airplane pressure.
Pre-Pressurization ( CPC )
The purpose of automatic pre-pressurization is to avoid a cabin pressure “bump” at take-off. During this sequence the cabin is pressurized to 400 ft below the take-off altitude at a rate of 300 ft/min.
In the case of a take-off without bleed air selected, this sequence leads to both the aft outflow valve and the aft safety valve closing.
Take-off Abort ( CPC )
The Cabin Pressure Controller (CPC) is in take-off mode for at most 10 minutes after lift off. This avoids the requirement to reselect the landing altitude in case of an aborted flight and emergency return to the departure airport. During 10 minutes after the take-off the pre-pressurization remains in effect as long as:
• The scheduled cabin altitude is higher than the theoretical cabin altitude, or
• The airplane altitude is less than the take-off altitude + 5000 ft (valid only for take-off altitude over 8000 ft)
Once one of the above conditions is met, the CPC begins flight scheduling.
Fire Detection
When a fire overheat condition occurs, the alarm signals are processed by the Control Amplifier then sent to the Fire Protection Panel in the flight compartment. If a fire or overheat condition occurs in either engine, this will cause the gas within the APD to expand and turn on the following lights in the flight compartment:
- Applicable PULL FUEL / HYD OFF T-handle light (red) comes on
- Both ENGINE FIRE Warning Press to Reset lights (red) flash
- CHECK FIRE DET warning light (red) flashes
- Fire tone (optional)
Either ENGINE FIRE PRESS TO RESET indicator is pushed to turn the audible tone (optional) warning off and/or cancel the flashing engine fire lights. The ENGINE FIRE PRESS TO RESET stay on steady for the
duration of the alarm condition.
Fire Extinguishing
The forward and aft bottle squibs are armed by pulling the PULL FUEL / HYD OFF handle. After arming, the extinguisher bottle is
discharged by selecting the EXTG switch on the fire protection panel to FWD or AFT position. An electrical signal is sent which ignites the Electro-Explosive Device (EED). When the EED explodes it ruptures a burst disc and the pressurized bottle then discharges the suppressant into the engine zones.
Baggage Compartments - Smoke Detection and Fire Extinguishing
Fire extinguishing for the baggage compartments is performed by two High Rate (HR) fire extinguisher bottles and one Low Rate (LR) fire extinguisher bottle. Each baggage compartment has one high rate fire extinguisher bottle. The Low Rate fire extinguisher bottle is shared between the FWD and AFT baggage compartments but is located in the AFT equipment bay (rear fuselage).
Fire Extinguishing-Baggage
Pushing the SMOKE / EXTG switchlight activates the High Rate fire extinguisher bottle into the aft baggage area. The AFT ARM light will go out and the AFT LOW light turn on. After a seven minute delay, the Low Rate fire extinguisher bottle will automatically discharge into the aft baggage area and the FWD LOW light will turn on when the LRD bottle has depleted.
Engine Fuel Feed
Fuel to each engine is fed from the collector tank, from a primary ejector pump or an AC driven auxiliary pump and delivered to the engine driven pump (Figure 6.9-12). If the engine driven pump inlet pressure drops below a preset limit, the related # 1 or # 2 ENG FUEL PRESS caution light comes on. An AC (Variable Frequency) auxiliary pump in each collector bay serves as a back up source of fuel boost
pressure for take-off and landing and in case the related primary ejector pump does not supply the necessary fuel pressure. Related TANK 1 or TANK 2 AUX PUMP switchlights on the FUEL CONTROL TRANSFER panel control the auxiliary pumps manually (Figure 6.9-13).
A TANK 1 or TANK 2 AUX PUMP switch indicator on the MFD Fuel Page shows the position of the switchlight. When the pump is supplying sufficient boost pressure, the TANK 1 or TANK 2 AUX PUMP light on the Fuel Page will turn green and the related ON switchlight segment turns green. The engine feed shutoff valve closes when the related PULL FUEL / HYD OFF handle, on the Fire Protection Panel (FPP), is pulled (Figure 6.9-13). Advisory lights on the FPP show when the valve is open or closed. The fuel is filtered and heated by Fuel Oil Heat Exchanger (FOHE) before entering the FMU. If the fuel filter becomes blocked, fuel bypasses the filter. The # 1 or # 2 FUEL FLTR BYPASS caution light will comes on if a related bypass is impending.