EXTERNAL AC POWER SOURCE

 

There are two main generators and two TR units of an external ac power source. If engines are operating and hydraulic pressure is available, the emergency generator is available as a source of power. Switching between power supply systems is automatic without pilot action. However, the pilot can selectively isolate power sources and the distribution system in emergency situations. When operating normally, the buses receive power through a series of contacts and logic situations, depending on the power source(s) in use. Normal power sources include external power and the left and right generators.

The external electrical power systempermits application of three-phase, ac power to the aircraft electrical power distribution system. External power goes to the ground power monitor (GPM) and de-energized contacts of the external power contactor.

The GPM prevents application of external power not within tolerances. If an undervoltage, overvoltage, underfrequency, overfrequency, or a phase reversal fault occurs, the GPM disconnects external power from the power distribution system. When all the power parameters are within tolerance, the GPM relay energizes, supplying 28 V dc from the GPM transformer-rectifier to pin F of the external power panel.Pins E and F are jumpered in the external power plug.

The power then runs through pin E and energizes the external power contactor. Three-phase power at pin one of the external power contactor then runs to the left main ac bus and right main ac bus through their respective contactors. Power from the left main ac bus goes to the left TR unit. It also goes through the de-energized contacts of both ac essential power transfer relays to all essential ac buses and to the 26-volt ac transformer. This transformer, in turn, feeds power to the 26 Vac instrument and navigational buses. The right main ac bus supplies power to the right TR unit. Both left and right TR units provide power to all dc buses through their respective power contactors and power transfer relays.

External electrical power is automatically inhibited from some systems when external air conditioning is not being supplied to the aircraft. After aircraft engines start and the left generator comes on line, the left main contactor automatically disconnects external power. Some aircraft have a light on the caution /advisory panel that is illuminated by a switch, mounted on the external power receptacle, when the door is open. However, there is no cockpit indication of external power application. The only control the pilot has over external power being applied or removed is the hand signals between the pilot and the plane captain.

With external power connected and engine start initiated, the left generator comes on line when all parameters are within tolerance. With the left generator on line, the left main ac power contactor energizes. The left generator now supplies power through the left main ac power contactor (K1) pins one and two to the left and right main ac buses. These two buses, in turn, provide power to both TR units, all other 115-volt ac buses, and the 26-volt ac transformer. The TR units and the 26-volt ac transformer provide power to all other respective buses.

When the left main contactor energizes, it removes external power from the aircraft bus system. The external power unit is now shut down and cable removal may be done safely.

With the right generator on line, the right main contactor energizes, allowing the right generator to power the right main bus. The energized contacts of the right main ac power contactor (K2) prevent the left generator from powering the right main bus. Likewise, the energized left main contactor prevents the right generator from applying power to the left main bus.

When both generators are operating, the left generator powers the left main bus. The left main bus powers all 115-volt ac essential and instrument buses through the ac essential power transfer relays and through the 26-volt ac transformer. The transformer powers the instrument and navigation buses.

The left TR unit receives power from the left main bus. It provides power to the left main dc bus through contact Al of the energized left main dc power contactor. The left main dc bus provides power to the essential dc buses through the contacts of de-energized dc essential power transfer relays. In addition, the left main dc bus powers the interruption-free dc bus. This bus, in turn, powers the 28-volt dc AFCS bus through the energized autopilot power transfer relay.

The right generator provides power to the right main bus, which through the ac monitor bus relay powers the monitor ac bus. The 28-volt dc right main bus receives power from the TR unit through the energized contacts of the right main dc power contactor. The right main dc bus energizes the monitor dc bus through the dc monitor bus relay. It also energizes the 28-volt dc interruption-free bus. Because the interruption-free bus receives power from both TR units, the bus is not affected by a loss of either main generator or either TR unit.

The power distribution system design ensures power is available to operate all aircraft equipment. This includes all equipment essential to accomplish the assigned mission and ensure safety of flight. If a component or engine should fail, power must be available for continued safe operation. The distribution system design provides a continuous power source under all adverse conditions. Either of the generators is capable of supplying the entire load of the aircraft. Likewise, either of the TR units is able to supply the entire dc load. Adverse conditions that could occur in the aircraft’s electrical systems include the following:

Left generator failure

Right generator failure

Left TR unit failure

Right TR unit failure

Emergency Generator Operation. The emergency generator system automatically activates when the left main dc bus loses power. The emergency generator solenoid de-energizes, activating the hydraulic motor that drives the emergency generator. The emergency generator provides ac and dc power to the essential bus circuits when normal power fails. The emergency system is completely self-sufficient and independent of the primary and secondary power sources.

The emergency generator control unit monitors ac output and regulates the output at 115 volts. This voltage is available as long as the hydraulic motor is operating under full system pressure. If both engines flame out, the hydraulic pressure produced by windmilling engines cannot operate the hydraulic motor at optimum output. This causes the left and right generator outputs to drop below tolerance. The emergency generator control unit disconnects the essential ac and dc power transfer number two relays.

Equipment on essential ac and dc number one buses must be capable of operating on below normal voltage in an emergency.

When the emergency generator activates (both generators or both TR units failed), ac power routes through both transfer relays to essential ac buses. Dc power goes through the dc essential power transfer relays to the dc essential and AFCS buses. Not all ac and dc main and monitored buses receive power during emergency generator operation.

If both engines fail, the windmilling effect drives the emergency generator below normal performance levels. The ac essential and dc essential power transfer number two relays now de-energize. Only the ac essential and dc essential number one buses receive power. This configuration enables the flight crew to jettison aircraft stores, communicate, and destroy classified equipment before taking emergency escape procedures.

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