Electrical System 1 (AC Power System)

Question 1

Two, three phase AC generators normally provide all AC power to the T-38C when the engines are operating above what RPM?

Answer 1

43-48% RPM

Question 2

What is the generator cut-in range/speed?

Answer 2

When the T38C engines are operating at or above 43-48% RPM and able to supply all AC electrical power to the T38

Question 3

What are the rated outputs of each generator?

Answer 3

115 to 200 Volts
Three phase
320-480 cycles per second

Question 4

How does the T-38 keep each generator’s RPM and frequency output within the rated limits?

Answer 4

This is accomplished by placing the generators on an airframe mounted gearbox that shifts at 65 to 75% engine RPM.

Question 5

When pacing the Generator Control Switch into the off position, what happens?

Answer 5

It does not stop the generator from spinning, but it disconnects the generator output from its bus.

Question 6

Through what component does each electrical item receive power from the generators?

Answer 6

The AC bus

Question 7

What does each bus contactors(relays) do for the T-38C electrical system?

Answer 7

They connect or disconnect the generator to and from the bus

Question 8

When a generator goes off-line, what does the transfer relay do?

Answer 8

It automatically transfers the electrical bus to the opposite generator and illuminates the affected generator caution light, the MASTER CAUTION light, CAUTION alert displays on the HUD/MFD and activates the “CAUTION, CAUTION” VM

Question 9

What does the protection panel do?

Answer 9

It protects the AC electrical system from a faulty generator and protects the generator form a faulty bus

Question 10

How many protection panels are there and how do they work?

Answer 10

There are two protection panels in the T-38C. Each protection panel will disconnect its generator from the bus when there is a bus fault or generator malfunction.

Question 11

What does the protection panel do in the case of a bus fault?

Answer 11

When the affected generator drops offline due to a system malfunction, the protection panel will first analyze the situation to determine if the malfunction is caused by the bus. In the case of a bus fault, the protection panel will not allow for a generator reset or bus transfer to the opposite generator; thus preventing the remaining generator from being subject to the same bus fault and preventing a complete AC failure.

Question 12

The APU connects power directly to what?

Answer 12

The APU connects power directly to the AC buses, which power the TRU to generate DC power.

Question 13

If the aircraft is not accepting power from the APU, what may help?

Answer 13

Cycling the battery switching OFF then back ON

Question 14

What position should the battery switch be in during APU operation?

Answer 14

The ON position

Question 15

What is the static inverter powered by and what does it do?

Answer 15

It is powered by the battery BUS and converts DC power to AC power.

Question 16

What are the three ways the static inverter can be activated?

Answer 16

1. By pushing either start button
2. Moving either throttle into the afterburner range (during flight only)
3. Holding the fuel/oxygen check switch in either GAGE TEST or QTY CHECK position

Question 17

The static inverter, powered by the battery bus, converts the DC BUS voltage to _____ Volts AC.

Answer 17

115 Volts AC

Question 18

When activated, the inverter provides an alternate source of AC power to what?

Answer 18

1. The first engine start on the ground or during flight
2. Operation of right engine autosyn instruments (fuel flow, oil pressure) and left and right fuel quantity sensors
3. Oxygen quantity sensors

Question 19

How can an operational check of the static inverter be accomplished with normal AC/DC power or DC power only?

Answer 19

By positioning the OXY/FUEL Check switch to GAGE TEST and observing a counterclockwise movement of oxygen quantity indicator pointers.

Question 20

What is DC power supplied by?

Answer 20

Two Transformer-Rectifier Units (TRUs) that convert AC to DC power

Question 21

Is it possible to stop the AC generators from turning in flight, even if you shut down the engine?

Answer 21

No, they will windmill unless the engine seizes.

Question 22

If one generator fails or is turned OFF, the other generator automatically picks up the load and supplies electrical power to both systems through the relays and protection panels? (T/F?)

Answer 22

True

Question 23

The power protection systems allow sustained operation of either bus with the respective generator operating at what minimum voltage?

Answer 23

90 Volts Minimum

Question 24

Each AC buss carries about how much of the electrical load?

Answer 24

About half of the electrical load

Question 25

Does the left or right VAC BUS carry more items?

Answer 25

The right VAC BUS

Question 26

At what rate does the generator turn to maintain 420 to 480 cps?

Answer 26

6,400 to 9,600 RPM

Question 27

Why is the gearbox shift neccesary?

Answer 27

Due to the generator needing to maintain 320 to 480 cps, the high RPM of the engines would overload the generator unit without a “shift”. The engine operates at a range from 7,920 RPM to 16,500 RPM. At 65-75% engine RPM range, the generator shift occurs to keep the AC generator output within the acceptable frequency range.

Question 28

What operates the AC generator on each engine, as well as the hydraulic pump?

Answer 28

An airframe mounted gearbox

Question 29

IS the static inverter part of the normal AC electrical system?

Answer 29

No

Question 30

What is the primary purpose of the static inverter?

Answer 30

To provide an alternate source of AC power for certain components when only DC power is available.

Question 31

What voltage (and range) do both generators operate at?

Answer 31

115 V +/- 2V

Question 32

Automatic transfer of electrical load through the interaction of both power protection panels may take up to how many seconds?

Answer 32

2.5 Seconds

Question 33

The AC power supply system consists of what?

Answer 33

Two engine driven AC Generators and the External Power Receptacle

Question 34

What bus powers the instrument and console lights?

Answer 34

The Left AC bus

Question 35

What bus powers the floodlights?

Answer 35

The right AC bus

Question 36

If AC power fails, what condition must be met for the two floodlights in each cockpit to revert to left essential DC power?

Answer 36

The floodlight switch on the lightning control panel is out of the OFF position.

Question 37

If the AC power fails and the floodlights are using DC power, can their intensity be changed?

Answer 37

No, their intensity is constant

Question 38

Which bus powers the rotating beacons and position lights?

Answer 38

The left AC bus

Question 39

How many down positions does the landing light have and what are they?

Answer 39

Two
The first down position is when the weight of the aircraft is off the main gear, the light retracts slightly to the landing position and both filaments are illuminated.
The second down position is when the weight of the aircraft is on the main gear, the light is nearly vertical with only one filament illuminated

Question 40

What controls the extension and retraction of the landing light?

Answer 40

A combination of the landing gear movement and the position light switch.

Question 41

How many position lights are there on the T-38?

Answer 41

4

Question 42

Landing light extension uses the left AC bus only (T/F?)

Answer 42

False, extension requires both the left AC bus and the right essential DC bus power

Landing light retraction uses only left AC bus power

Question 43

The landing/taxi light will extend when what two conditions are met?

Answer 43

1. Position light switch is in the Bright or DIM position

2. Landing Gear is down