Re: Engine start process questions

Date:         16 Jun 97 21:35:40 
From:         Seth Dillon <sdillon@ilinks.net>
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Rich Nute wrote:
> A jet engine is started by:
>
> 1.  Spin the turbine up to some rpm.
>
>     How many rpm?

It varies from engine type to type but roughly anywhere from 15-25% N2.
I always go to MAX Motor RPM ( the highest that the APU/GRND AIR) will
spin it.  MAX Motor depends on the health of your APU or Grnd air source.

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

On some early and or smaller engines it does.  I have no idea what the
gear ratio is.

>     How is the spin air injected into the engine?

It isn't.  The air used to start is usually at around 40psi.  It is ducted
through a "starter control valve" to a small pneumatic turbine monted on
the engine gear box.  This turns the gearbox, which in turns drives the
high pressure spool (N2 or N3 on Rolls Royce RB211) of the engine the low
pressure spoolstarts turning on its own due to the airflow being generated
by the N2.

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

When the engine reaches max motor speed the pilot opens a valve in the
fuel control of the engine by advancing the fuel shut off lever usually
located on the pedestal just behind the throttle or moving a switch on
the newer types which does the same thing.

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

Each engine has two (usually) igniters located in the burner cans.  These
are similar to the spark plugs in your car but much more powerful.  Each
aircraft type is a little different, and differences exist on similiar
type depending on customer options.  Here are the ones I am familiar with:

DC9/MD88 - Select "A", "B", or "BOTH" ignitors on the overhead panel via
a rotary switch.  This arms the ignition switch.  Press down and hold a
guarded "Start" switch on the overhead this opens the starter control
valve and turns on the ignition.  At max motor advance the fuel lever to
on.  Continue to hold the starter switch to help accelerate the engine
and keep the ignition going untill 35% N2, while monitoring the EGT, N1,
Fuel Flow.  At 35% N2 release the starter switch and monitor gauges untill
the engine stabilizes at idle.

L1011 - Select "A", "B", or BOTH on the overhead, depress the Start button
on the overhead and make sure the greenlight indicating the start valve
has opened.  Allow engine to spool up to 18% N3, make sure the N2 is
turning (Second Officers panel), make sure N1 is turning, then advance
the fuel switch, watch EGT and N3 as the engine accelerates.  At 45% N3
the start valve green light should go out indicating the starter has shut
off, and at 50% the button should release indicating that ignition is off.
Monitor gauges to ensure a stabilized idle.

757/767 - Turn the start/ign control for the engine you want to start to
the "GND START" position, at max motor make sure the N1 is turning, and
turn on the fuel, monitor EGT etc.  At 50% N2 the starter switch will pop
back to the off position. Make sure the engine stabilizes at idle.

On all aircraft you should be sure you have some oil pressure indication
before putting fuel to the engine.  At idle oil pressure should be within
limits.

> 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?

No, the fire one would likely encounter during engine start is in the
tailpipe.  The fire extinguishing system has no effect on tailpipe fires
as it discharges into the nacelle.  In the event of a tailpipe fire you
have two options, one is to shut off fuel and continue to motor the engine
to prevent overtemping and blow out the fire, the other is to continue
the start and blow out the fire.  Tailpipe fires are a pain because you
usually have no cockpit indication of the fire and are dependent on the
ground crew to notify you.  The L1011 has a nasty habit of rolling back
at idle, causing the fuel control to dump more fuel in an attempt to
accelerate to engine to idle speed.  This excess fuel can sometimes start
a tailpipe fire.

Hot starts are more of a problem, that is when the engine will reach the
max start EGT.  Then you have to abort the start and motor the engine
untill it cools enough to try again.

> Thanks,
> Richard Nute
> San Diego

Your welcome,
Seth