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Twin Otter > 7. Powerplant/Propellor (pt6a-27) > Flashcards

7. Powerplant/Propellor (pt6a-27) Flashcards

1
Q

Where is T5 (engine interstage turbine temperature) measured?

A

Station 5

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2
Q

When will RESET PROPS annunciator illuminate?

A

When the power levers are retarded to approximately 75% NG or less while the propeller are at 91% NP or less

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3
Q

Why do turbines rotate in opposite direction?

A

To eliminate torsion load buildup/P-factor

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4
Q

What ratio of energy drives compressor and power turbines?

A

2/3 of energy drives compressor turbine and 1/3 drives power turbine

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5
Q

What is the reduction gear ratio?

A

15:1

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6
Q

What is the oil tank capacity and useable?

A

2.3US gal (1.5 usable)

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7
Q

When should oil be checked and why?

A

Within 10 minutes of shutdown. If longer, oil can migrate from the oil tank rearwards to the accessory gearbox

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8
Q

What temperature must engine oil be to heat fuel to above freezing?

A

55 deg

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9
Q

What is the propellor governing range?

A

75-96%

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10
Q

What is the pitch range of the propellor?

A

-15° (reverse) to +87° (feather)

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11
Q

What is the function of the beta reverse valve?

A

Limits the propeller blade angle from decreasing below preset value when the propeller governor is unable to maintain selected rpm

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12
Q

What angle range is the beta range? (Controlled by power levers)

A

+17 to -15deg

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13
Q

What are the functions of the beta reverse valve and fuel topping (NF governor) in the beta range?

A

In beta range, the beta reverse valve

controls blade angle while the fuel topping governor (NF governor) limits the propeller speed in the reverse range

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14
Q

What is the propellor blade angle at IDLE?

A

Approximately 11 deg

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15
Q

What pitch angle is the null range? (Power levers only control bade angle)
What happens if power levers moved further aft?

A

11° to -2°
Further aft movement of power levers. decreases prop pitch while increasing engine speed, resulting in increase in torque. Engine speed will start to increase from approximately -2° propeller blade angle until full reverse thrust is reached at -15°.

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16
Q

What does the overspeed governor do?

A

Restricts oil flow to maintain blade angle for 101.5% NP

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17
Q

What is the function of the beta backup system? How is it disabled?

A

Prevents prop going into reverse if beta reverse valve fails. At 9deg (just past idle) beta backup solenoid is energized to stop oil supply
Twisting power lever for reverse opens circuit to disable

18
Q

How does the NF governor limit NP?

A

If the NP is greater than 6% higher than speed selected, fuel supply to FCU is limited. ie. 102% for max rpm

19
Q

What happens to the NF governor when power levers are moved aft of idle position?

A

NF governor is mechanically reset to 5% below selected rpm (only possible selection in reverse is max) thus 91%

20
Q

What are the 3 conditions for autofeather to arm?

A
  1. System selected on
  2. Power levers reach an angle that contacts the microswitches (normally at 86-88% NG)
  3. Torque,reaches the specific value of pressure switch (varies 18psi or 30psi)
21
Q

When does autofeather activate?

A

When torque drops BELOW 12 psi for 2 seconds on one engine (provided system is armed)

22
Q

When autofeather system is activated on one engine, what happens to other engine?

A

Torque switch on opposite engine is de-energized

23
Q

What does illumination of the blue beta range lights indicate (if installed)?

A

That the microswitches in the propeller dome have closed

24
Q

Max ITT takeoff/max continuous?

A

725c

25
Q

Max torque for takeoff, climb, cruise and max continuous?

A

42.5psi

26
Q

Max acceleration torque, ITT, Ng and Np, and time limit for these?

A

48.5psi, 825c, 102.6%, 110%, for 2 seconds.

27
Q

Max climb and cruise ITT?

A

695c

28
Q

Max starting ITT and time limit?

A

1090c for 2 seconds.

29
Q

Max low idle ITT?

A

660c. Increase power levers to maintain.

30
Q

Max reverse torque/ITT, and time limit?

A

42.5psi and 725c, for 1 minute.

31
Q

Max takeoff and reverse Ng?

A

101.5%

32
Q

Max Np for takeoff, climb, cruise and max continuous?

A

100% (2200rpm)

33
Q

Oil pressure range?

A

80-100psi. Minimum 40psi for idle.

34
Q

Oil temp range?

A

0-99c (10 required for takeoff/max continuous)

-40 min for start

35
Q

Shaft horsepower output of PT6A-27?

A

680 SHP

36
Q

The compressor bleed valve will be fully closed at what power?

A

When the engine has stabilized at or above about 80 to 85% NG

37
Q

What happens to the bleed valve when the engine is shut down?

A

Bleed valve falls fully open due to gravity

38
Q

Why is the compressor bleed valve open at lower RPMs?

A

Axial compressor is more efficient at lower RPM so can supply more air than the centrifugal compressor can accept.
The back pressure that would be created between the axial and centrifugal compressors would disrupt smooth flow of air over the axial compressor blades causing them to stall

39
Q

What will be the effect on takeoff on Ng, of a compressor bleed valve stuck open?

A

NG will be higher than normal because the compressor is being unloaded due to the air being discharged

40
Q

What will be the effect of a stuck open compressor bleed valve on takeoff, on torque and ITT?

A

Inability to achieve calculated torque due to continued dumping of compressor air before it reaches combustion chamber.
As further power is applied to achieve desired torque, ITT will rise above normal values in response to increased fuel flow

41
Q

Below what pressure is the high pressure fuel pump likely to cavitate?

A

5psi (above 8000ft)

If operated below 5psi for greater than 10 hours, overhaul is required.

Twin Otter (14 decks)

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