Systems Guide

Aerosoft
Twin Otter E
XTENDED
Systems
Vol
2
2-1-3
19 November 2013
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 PT6A-
20 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 de-
icing boots. Note that the "Low Pneu" warning light will be lit when engines are at idle.