Re: Engine start process questions

Date:         16 Jun 97 21:35:39 
From:         graham bowden <graham@ats.com.au>
Organization: Customer of Access One Pty Ltd, Melbourne, Australia
References:   1
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Rich Nute wrote:
> A jet engine is started by:
>
> 1.  Spin the turbine up to some rpm.
>
>     How many rpm?

Depends on the engine.  Gas turbine tacho's are usually in % RPM ie
engine is at 100% or 50% or whatever.  A CFM56 as on a B737-300 will
spin to aprroximately 25% on the starter .

>     The spin is either by an electric motor (for small
>     engines) or compressed air (for larger engines).
>
>     Does the electric motor also serve as the generator?

Depends on the manufacturer.  As I recall DHC-6 Twin Otters had a
starter/generator and they wouldn't be the only ones thus fitted.

>     What is the approximate gear ratio between the
>     electric motor and the turbine?

Don't know

>     How is the spin air injected into the engine?

	Bleed air from the aircraft's APU (auxiliary power unit) or ground air
supply drives a turbine driven starter usually on the gearbox and this
turns the motor.  Gearbox to motor gear ratio (very roughly) 8 to7 ie
gearbox spins faster than engine.

> 2.  Inject fuel.
>
>     How is this done?

	Electric boost pumps in the tanks deliver fuel to engine driven pumps
which in turn deliver the fuel to an FCU (Fuel Control Unit) or MEC
(Main Engine Control) which then delivers programmed amounts of fuel to
the engine's fuel injectors.

> 3.  Ignite fuel.
>
>     How is this done?

	By ignitors, essentially very large spark plugs.  Once the engine is
running the ignitors can (and usually are) switched off as gas turbines
have a continuous and self sustaining combustion process.  Flight
through rain though, often calls for the ignition to be on.

> In the cockpit, what are the engine start controls,
> and what are the pilots' jobs during engine start?
> Doesn't one pilot keep a hand on the engine fire
> extinguisher?

	Start controls generally are a start selector switch and start lever
(fuel cut-off lever if you like).

The crew would perform a fire warning test prior to start then, if a
turbine starter is fitted they'e check pneumatic pressure, make sure the
fuel boost pumps are on, anti-collision lights on, start clearance then
select the start switch for the engine being started.
	The N2 tacho is observed for engine rotation, oil pressure should be
seen rising.  When the engine reaches its light off speed the start
lever is raised.  This turns the fuel and ignition on. Then the fuel
flow and EGT (Exhaust Gas Temperature) are monitored.  As the engine
accelerates the starter is still engaged and this also is monitored to
ensure it drops out when it should.  If it doesn't the starter can spin
to destruction.
	With the engine accelerating on its own after starter cut out the N2,
N1, EGT and Fuel low are continually monitored until the engine
stabilises at ground idle.

N1 and N2 by the way, are tachos reading %RPM for the Fan & low pressure
compressor (N1) and High pressure compressor (N2) which can and do spin
at different RPMs.

> Thanks,
> Richard Nute
> San Diego

Hope that satisfies you Richard
Graham Bowden