Flight Training - August 2021 - 28

TECHNIQUE
BY IAN J. TWOMBLY
ILLUSTRATION BY CHARLES FLOYD
LIGHT THE FIRES
STARTING A TURBINE ENGINE
IN A SENSE IT'S FOLLY TO TRY TO EXPLAIN HOW TO START A TURBINE
engine because each engine and each installation is unique. Starting a Pratt
& Whitney PT6 is different in a Piper M600 than it is in a Daher TBM 930,
and a Boeing 737 is different than a Boeing 757.
There are some similarities, however. Like piston engines, turbine
engines work on the principle of suck, squeeze, burn, blow. They both take
in air, compress it, mix it with fuel to create combustion, and then expel the
exhaust. But where starting a piston engine carries little risk to the aircraft's
components, the temperatures and pressures generated inside a turbine
engine must be closely monitored, lest they quickly cause major damage.
Battery power on a turbine engine
is critical because it must provide
enough power to spool the turbine
fan blade fast enough to not only
draw air in for proper combustion,
but also to keep the whole process
reasonably cool. Most turbines
require around 24 to 25 volts. If the
battery doesn't meet the minimum
starting specification, a ground
power source must be used.
By pressing a button, turning a knob,
or flipping a switch, the start sequence
begins. These actions engage the
battery with the starter to begin
spinning the hot section compressor
and N1
fan. Here, the pilot must check
to ensure N1 or NG is rising fast enough
and ITT, or the interstage turbine
temperature, isn't going too high. If
you're standing outside the aircraft you
may hear ticking. Those are the igniters
waiting to start the burn process.
Once N1 or NG is beyond a minimum
level, fuel is added, usually with a
condition lever. Sort of like a fuel on/
off switch in what looks like a mixture
lever, the condition lever adds fuel,
starting the combustion process.
JARGON
N1
28
FLIGHT TRAINING AUGUST 2021
shows the rotational speed in the section that takes in air and sends it along to the combustion
chamber. The section covers a jet engine's intake fan and several rows of low-speed compressor
blades. In modern jet engines, the fan also sends bypass air around the core of the engine. This
arrangement boosts thrust without burning fuel.
compressor and turbine wheels, and its job is to send air to the combustion chamber, where it's
mixed with fuel and ignited, producing exhaust and sustaining the rotation of the turbine wheels.
This rotation can be as high as 38,000 rpm.
JARGON
NG
N1
NG, gas generator speed, shows the speed in the core of the engine. The core has high-pressure

Flight Training - August 2021

Table of Contents for the Digital Edition of Flight Training - August 2021

Contents
Flight Training - August 2021 - Intro
Flight Training - August 2021 - Cover1
Flight Training - August 2021 - Cover2
Flight Training - August 2021 - Contents
Flight Training - August 2021 - 2
Flight Training - August 2021 - 3
Flight Training - August 2021 - 4
Flight Training - August 2021 - 5
Flight Training - August 2021 - 6
Flight Training - August 2021 - 7
Flight Training - August 2021 - 8
Flight Training - August 2021 - 9
Flight Training - August 2021 - 10
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Flight Training - August 2021 - Cover3
Flight Training - August 2021 - Cover4
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