Aerosoft Twin Otter Extended Systems Guide
TWIN OTTER EXTENDED
Volume 2
Systems
Version 19/11/2013 14:32:51
revision n°
Issue date
Release
Description
1.00
06 July 2013
1.00
Online release
1.01
12 July 2013
1.01
Change in AP text
1.02
31 July 2013
1.03
Changed text about AP
1.03
14 Aug 2013
1.04
Changed paragraph on MDA
1.04
18 Nov. 13
1.10
Added text on MSG button (GPS) and Clock
1.05
19 Nov. 13
1.10
Final version, added text on ski/amphib versions
RECORD OF REVISIONS
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CONTENTS
GENERAL ................................................................................................................................................................. 3
POWER PLANTS ...................................................................................................................................................... 3
ENGINE FAILURES ............................................................................................................................................... 4
STARTING SYSTEM .............................................................................................................................................. 4
START SWITCH ................................................................................................................................................ 4
ENGINE INSTRUMENTS ....................................................................................................................................... 4
TORQUE PRESSURE ......................................................................................................................................... 4
PROPELLER RPM ............................................................................................................................................. 4
ENGINE TEMPERATURE (T5) ............................................................................................................................ 5
GAS GENERATOR RPM (NG)............................................................................................................................ 5
FUEL FLOW ..................................................................................................................................................... 5
OIL PRESSURE / OIL TEMPERATURES .............................................................................................................. 5
PROPELLERS ............................................................................................................................................................ 5
PROPELLER BETA RANGE .................................................................................................................................... 5
PROPELLER GOVERNOR ...................................................................................................................................... 6
POWER / PROPELLER / FUEL CONTROL LEVERS...................................................................................................... 6
POWER LEVERS ................................................................................................................................................... 6
PROPELLER LEVERS ............................................................................................................................................. 7
FUEL LEVERS ....................................................................................................................................................... 7
OIL SYSTEM ............................................................................................................................................................. 7
LOW OIL PRESSURE CAUTION LIGHTS ................................................................................................................ 7
OIL TEMPERATURE AND PRESSURE INDICATORS ............................................................................................... 7
FUEL SYSTEM .......................................................................................................................................................... 7
FUEL TANKS .................................................................................................................................................... 7
FUEL TANK SELECTOR ..................................................................................................................................... 8
BOOSTER PUMPS SWITCHES. ......................................................................................................................... 8
BOOSTER PUMP PRESSURE CAUTION LIGHTS ................................................................................................ 8
STANDBY BOOSTER PUMP EMERGENCY SWITCHES ....................................................................................... 8
FUEL EMERGENCY SHUTOFF SWITCHES ......................................................................................................... 8
FUEL LOW CAUTION LIGHTS ........................................................................................................................... 8
FUEL QUANTITY INDICATORS ......................................................................................................................... 8
FUEL QUANTITY INDICATOR TEST SWITCH ..................................................................................................... 8
FUEL HEATERS ................................................................................................................................................. 8
ELECTRICAL SYSTEM ............................................................................................................................................... 9
STARTER-GENERATORS ................................................................................................................................... 9
DC MASTER SWITCH ....................................................................................................................................... 9
EXTERNAL / BATTERY SWITCH ........................................................................................................................ 9
BUS TIE SWITCH .............................................................................................................................................. 9
GENERATOR SWITCHES .................................................................................................................................. 9
GENERATOR CAUTION LIGHTS ...................................................................................................................... 10
GENERATOR OVERHEAT CAUTION LIGHTS ................................................................................................... 10
BATTERY ........................................................................................................................................................ 10
BATTERY TEMPERATURE MONITOR ............................................................................................................. 10
DC VOLTMETER & DC LOADMETER .............................................................................................................. 10
INVERTERS .................................................................................................................................................... 10
CAUTION LIGHTS PANEL ............................................................................................................................... 10
HYDRAULIC SYSTEM .............................................................................................................................................. 11
HYDRAULIC PRESSURE INDICATION .................................................................................................................. 11
PNEUMATIC / BLEED SYSTEM ............................................................................................................................... 11
BLEED AIR SWITCHES ........................................................................................................................................ 11
HEATING ........................................................................................................................................................... 11
OPERATING ....................................................................................................................................................... 11
FIRE DETECTING AND EXTINGUISHING SYSTEMS ................................................................................................. 11
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EXTERNAL LIGHTS ................................................................................................................................................. 12
POSITION LIGHTS .......................................................................................................................................... 12
ANTI-COLLISION LIGHTS ................................................................................................................................ 12
ANTI-ICING / DE-ICING .......................................................................................................................................... 12
ICING EFFECT SIMULATED ............................................................................................................................ 12
TRIM ..................................................................................................................................................................... 12
WING FLAP SYSTEM .............................................................................................................................................. 12
FLAP SELECTOR LEVER ...................................................................................................................................... 13
LANDING GEAR ..................................................................................................................................................... 13
NOSE WHEEL STEERING .................................................................................................................................... 13
BRAKE SYSTEM .................................................................................................................................................. 13
PARKING BRAKE ................................................................................................................................................ 13
AMPHIBIAN RETRACTABLE GEAR / SKI ............................................................................................................. 13
AVIONICS .............................................................................................................................................................. 13
COLLINS AP-106 AUTOPILOT............................................................................................................................. 14
FD-112V FLIGHT DIRECTOR ............................................................................................................................... 14
UNITED INSTRUMENTS 5506L ALTITUDE ALERTER ........................................................................................... 15
GARMIN GNS530 GPS ....................................................................................................................................... 15
BENDIX KING KR 87 ADF RECEIVER ................................................................................................................... 17
BENDIX KING KN62A DME DISPLAY .................................................................................................................. 17
CLOCK ............................................................................................................................................................... 17
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GENERAL
The de Havilland DHC-6 Twin Otter was developed by de Havilland Canada as STOL aircraft. It is still produced
by Viking air. It has been successful as a cargo, passenger, firefighting and Medevac aircraft as well as being
popular for sky diving operations. The Twin Otter comes with floats, skis and tricycle landing gear as well as
amphibian versions and many are used as bush planes in remote places like Canada, Alaska, Africa, Australia
and Antarctica.
It was developed as a replacement for the single engine Otter, also made by de Havilland Canada.
Development started back in 1964 and first flight was in 1965. The Twin Otter features double slotted flaps as
well as using ailerons for STOL performance. The engines selected where the Pratt and Whitney 550 shp PT6A20 turboprop engines. The first version was the 100 series and later the 200 series with a longer nose and
larger baggage compartment was introduced, still using the 550 Shp PW PT6A-20 engines. In 1969 the 300
series was developed with the 620 shp PT6A-27 engines.
Production of the 300 series ended in 1988 thereafter Viking air took over production. In 2010 the first 400
series was delivered with modern avionics and use of composite in some of the structures.
POWER PLANTS
The Twin Otter airplanes are powered by wing-mounted PT6A turboprop engines constructed by Pratt and
Whitney. These light weight gas turbine feature twin independent turbines that are only aerodynamically
coupled. The first stage (compressor) drives the gas generator section while the second turbine drives the
propellers through a reduction gearbox. It might surprise you that air is actually entering the engine at the
back and exhausts at the front. Inside the engine the airflow reverses three times. Although more complex
than a straight jet engine the PT6A is highly reliable as long as temperatures are controlled. Any operation in
the beta ranges cause the engine to heat up and should be monitored.
The engines used on the Twin Otter are the Pratt & Whitney PT-6A range of free turbine turboprop engines. In
a free turbine there is a core engine called the gas producer. It works like a normal jet engine, where air is
compressed by a couple of compressor stages, then entering the combustion chambers where fuel is mixed
with the air and ignited. The air then expands due to the temperature increase and can then be used to drive a
turbine, which again is connected to the compressor section of the engine. Another turbine is then driving the
propeller via a reduction gear. Since this turbine isn´t mechanical connected to the core engine, it´s called a
free turbine.
On the PT6A the air inlet is at the rear of the engine nacelle and exhaust is in front. This way the free turbine
that drives the propeller gets closer to the gearbox and propeller. An air scoop runs at the bottom of the
nacelle and contains the inertial separator. The inertial separator includes an electrical controlled,
pneumatically operated deflector as well as a diffuser.
The deflector can be retracted or extended via a switch in the cockpit. The diffuser is a fine mesh screen
projecting into the air path at the upswept and of the engine air inlet. An exit duct door in the aft portion of
the nacelle operates in synchronisation with the deflector. When the inertial separator is extended, debris or
ice will be deflected through the exit duct, due to the inertia of the particles. With the inertial separator
extended, maximum continuous torque from the engines will be lower, due to the loss of air through the exit
door. The compressor part of the engines also supplies bleed air for cabin heating and air pressure for the deicing boots. Note that the "Low Pneu" warning light will be lit when engines are at idle.
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ENGINE FAILURES
The following failures are simulated:
• Excessive T5 temperature: Whenever the T5 temperature gauges goes into the red area, an over
temperature situation exist. Depending on the amount of over temperature, the engines will fail after
some time.
• Ice ingestion: When icing conditions exist, ice might build up near the engine air-intakes which might
break off and get ingested to the engines, thus causing a flameout. To prevent flameouts, turn on
intake heaters and extend the inertial separators.
• Hot-start: Hot-starts occur if fuel is introduced before Ng (Gas-producer RPM) has reached 12%. The
combustion chambers gets flooded with fuel with insufficient airflow. You will see that T5
temperature gets very hot.
• Engine over torque: Engine over torque occurs when torque is set higher than maximum allowed
continuous torque. The torque gauges has a red mark at approximately 49% for the PT6A-27. This
only marks the maximum to be reached under ISA conditions (15°C, 1013 mbar) at sea level. When
outside temperature increases or as you climb, this value will decrease. In this chart the actual
maximum allowable torque can be found: (Chart to be inserted for PT6A-27). You can also open the
checklist and look on page 1 where a blue bug indicates actually maximum torque on the torque
gauges found there. The engines will fail after a certain time as a function of over torque amount.
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Failure due to over torque and over temperature will trigger an engine fire. Shut off fuel and pull the fire
handle to activate the fire extinguisher.
STARTING SYSTEM
The starting system for each engine combines a starter/generator, start switch and starter relays. With the DC
Master switch on and the starter switch held to the start position, the starter motor will rotate the gas
generator turbine. When it reaches sufficient RPM the engine will light-up. The starter switch can then be
released and the starter motor will start to function as generator..
START SWITCH
A three positioned start switch is located on the left overhead panel. The spring loaded LEFT and RIGHT
positions energize the left and right engine starter systems.
ENGINE INSTRUMENTS
TORQUE PRESSURE
The main thrust indicator as it shows the portion of total thrust to the propeller. This gauge requires 26 volts
to operate. It is your primary trust indicator as it indicates how much of the total power is delivered to the
propeller.
PROPELLER RPM
This mechanical gauge (does not require electrical power) indicates the revolution per minute of the propeller.
It indicates in percentage of maximum RPM (around 2200).
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ENGINE TEMPERATURE (T5)
Engine temperature is measured at 8 locations in the gas stream between the gas generator turbine and the
power turbine. An average of the temperature is send to the instrument panel and the gauge requires 26 volts
to operate.
GAS GENERATOR RPM (NG)
Gas Generator RPM is measured at the accessory gear and is send to the instrument panel in percentage of
maximum design RPM.
FUEL FLOW
See Fuel System section.
OIL PRESSURE / OIL TEMPERATURES
See Oil System section.
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PROPELLERS
The Twin Otter comes with either 3 bladed or a 4 bladed propellers with similar performance. The 4 bladed
propellers make less noise and are therefore better suited in some conditions. The propellers are of the
constant speed type, which means that blade pitch is set to keep a certain propeller RPM.
Normally when the engines are stopped, the propeller blades will feather due to loss of oil pressure. On some
floatplanes there are devices called “start locks” which prevents the propeller blades from feathering. This is
done to prevent any thrust to be produced during startup, which can be troublesome on floatplanes.
The Twin Otter has an auto feather system, which will automatically will feather the propeller if an engine
should fail. It senses if there is too large a difference of torque between the two engines.
Below the power levers flight idle range the propellers are operated in ‘beta range’. In beta range the blades
are controlled directly by the power lever and can also be set to zero pitch as well as reverse. While not
certified to be used when flying, reverse power can still be applied.
During the run-up check certain functions of the auto feather and beta range controls are checked to ensure
their proper function. The testing of the beta control system include the beta range backup system, which
prevents the propeller blades to run into an unacceptable low pitch if the beta control system should fail. A
micro switch that senses when reverse power is applied is tested during run-up.
The two constant speed propellers (3 or 4 bladed) have a propeller reverse cam mechanism to control the
blades between its 17° and -15° (beta) range. It features an overspeed protection, auto feathering and an
optional blade latch system. The rotational speed is indicated on a propeller tachometer on the engine
instrument panel
PROPELLER BETA RANGE
The propeller beta range is the pitch range that is controlled by the power levers (+17° to -15°). Propeller
speed in the beta range is controlled by the propeller governor fuel section together with the engine fuel
controllers.
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