Airbus 320manual start
 

It is the Air Turbine starter that powers the engine for rotation during a ground start (both auto or manual start)
It is a small motor that takes bleed air as input and rotates the engine drive shaft as its output.https://cimg6.ibsrv.net/gimg/www.gmf...8eababd99e.jpg

Basically you lose protections

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Manual Start:

Aircraft configuration:
- APU is running and APU bleed air is available.
When the mode selector is set to IGN START, the
Electronic Engine Control is armed for the start
sequence.

Action on the engine MAN START pushbutton provides
opening of the start valve, via the EEC.

On the upper ECAM display, check N2 increasing and
when it reaches 18%, set the MASTER lever to ON.

As soon as the MASTER lever is in the ON position,
both ignition systems are energized, LP, and PR Shut
Off Valves are opened and fuel flow increases.
At 18% of N2 with the MASTER lever ON:
- Dual ignition and fuel flow.

The MAN START pushbutton stays latched and is normally
left alone during the start sequence.

Action of the MAN START pushbutton has no effect on
the start sequence as long as the MASTER lever is set
in the ON position.

The MAN START pushbutton is normally released at the
end when the engine parameters are stabilized.

In case of malfunction, set the MASTER lever in the
OFF position to abort the start.
Max EGT:
- Starting, for up to 2 minutes, 590 degrees C.
- Transient 635 degrees C.

NOTE: There is no automatic shut down function.

The only mechanical difference are that both igniters are used instead of just one.

If you use Manual Start you dont get the following protections that would cause the start sequence to be automatically aborted.

Start valve failure.
Ignition failure.
Pressure Raising Shut Off Valve failure.
Hot start.
Hung start.
No N1 rotation.

So if you do a Manual Start in high OAT then you should be more successful at avoiding a hot start (I used to do that in the Middle East when carrying out multiple runs on the same engine) BUT you have to watch all the parameters and abort the start before any exceedence - if you get it wrong then the blame falls on you.

The main advantage is you can motor the engine longer and dual igniters giving better combustion. That brings in more air also lowers the existing EGT giving more margin for EGT to stabilize. Since FADEC is not going to shut it you may allow the EGT to go closer to the limit but I am not sure whether it's wise. FADEC being a computer can simultaniously monitor multiple parameters for a start abort which humans may not be able. In CFM engine in manual start if EGT reaches limit before N2 reaches 50% auto shut down will happen. Dual ignition is also provided in an auto starts after a start abort. Auto start is very smart. When EGT rises rapidly it cuts ignition for 6 seconds and attempts two more starts with 7% less fuel flow on each occasion before aborting the start.

Tiny reword of vilas': "The main advantage is you can motor the engine into higher N2 before introducing fuel, as well as dual igniters giving better combustion"

FD
Thanks. That's what I meant. Longer motoring will allow the N2 to increase till max motoring.

The other option being to disable the defective ignition system to force FADEC into using the working one.
It's in the MEL Ops procedure.

safelife: I found the same in our book, but upon closer inspection discovered some MEL ITEMS & PROC where MAN start was the only advised option.

Maybe a CFM / IAE thing.

Interesting. We only use IAE engines and in the MEL the only operational procedure available is to do a manual engine start.

that's what our MEL OPS procedure is suggesting as well (CFM). pull the respective (defective) igniter CB before automatic engine start, push it back in after start. alternatively do a manual engine start without the CB work-around.