BdKoss, on your A320s, do you have one or more the packs running during start?

Getting a good airflow through the engines (all rotors turning in the right directions) may help reduce fuel fumes in the cabin.

As engineers, we mostly use manual starts unless there is something specifically wrong with the Autostart System. We have more control over the start: The process is slower, giving us time to monitor specific parameters at specific times. We get to choose when the fuel/ignition goes on. If fuel has pooled in the combustion chamber, a relatively long dry spin ensures that all the excess fuel is blasted out the back before ignition is turned on. EECs are not that smart.

My P/P remarks are directed to colleague TurbineD - but here's the elucidation: P/P is simply the total pressure ratio, in this case the tailwind upstream pressure divided by downstream pressure. In the tailwind case the number is very low, close to unity, and when applied to the rear end of the LPT won't do much.

But applied to the much larger fan blades, a substantial amount of torque can be had even at very low P/P.

Compris?

And deploying the reverser places the blocker doors in the way of this P/P, closing the valve, spoiling the driving force of the fan rotor.

NSEU,on the ones that I fly, once the Engine mode selector is switched to START, the FADEC powers up and the PACK valves are closed. The Pack valves are opened 30 secs after the second engine start . I don't know if this logic is engine specific or common across the 320 fleet. This is the part which baffles me. When you do a MAN start , apart from the dual igniters firing as rightly pointed out by Meikleour in one of the replies, I find no difference in the start procedure as listed in the procedure . The procedure says , place the master switch to ON position, when the N2 reaches 22% which is what the FADEC actually does in an automatic start . So in effect you are just mimicking the actions a FADEC would do. If not, can you please throw light on the other cues that help you in timing the start sequence better than what a FADEC would do ?

BKDOSS: Re-read your SUP PRO's again!

For a manual start the fuel MAY BE put in at 22%N2 or MAX MOTORING.

ie. if it is possible to achieve a higher N2 speed on the starter then do that before the fuel is added. This may help with an engine that is prone to over temp. By the same token with an air source that is a bit weak you may not even achieve 22%N2

Do you understand now why MAN STARTs may be used?

Thanks a lot for your explanation Meikleour
All the while I have been switching on masters at N2 of 22% even though the engine was accelerating,as the SUP procedures does not clearly enlist the factors that would help me decide whether to let the N2 flatten out above 22% or start the Masters at 22%. So your suggestion is if the engine has a history of high EGTs during start, then delay opening the fuel flow. The weak bleed source is easier to identify as N2 wouldnt build up sufficiently, so you might want to reduce the load on the APU by isolating the consumers such as fuel pumps etc.

Bkdoss
I think you did not read it carefully. SUP PRO for CFM clearly mentions:
The maximum motoring speed is defined as the speed at which N2 acceleration is less than 1 % in approximately 5 s.
and for IAE
Maximum motoring speed is reached when a significant decrease in N2 acceleration is observed. Do not attempt start unless N2 is at least 15 %

As "Meikleour" has pointed out Min motoring "Before" introduction of fuel (22%)and as a lot of engineers look for a positive turning of N1..Fadec during a auto start doesn't care if its turning backwards or forwards and will introduce fuel. The starter limits for operation are fairly tolerant (An example is when doing "Low Pressure starts" with a dodgy start unit..or with a worn out APU..

fruitloop:This bit may not apply to either CFM or V2500, but is a bit of history from earlier high-bypass engines: A new engine, or one with a fresh LPT module, may have too-tight clearances, and the N1 rotor may not turn freely - it may not even turn at ground idle.

The solution applied was to shut down after perhaps 30 seconds, wait a bit for the LPT case to expand from the heat, then re-start. By then the blade-to-shroud clearance should be opened up a bit (the rotor discs are massive and don't heat up as fast).

Your mileage may vary! :uhoh:

Quote: " ....... and the N1 rotor may not turn freely - it may not even turn at ground idle."

Trent engines have logic within the EEC that prevents a start if all 3 shafts aren't rotating so the above scenario isn't even an option.

If doing a manual start (CFM`s again) and you decide to abort start (due to whatever reason you decided to) - and if you wanted to then Crank - (for whatever reason you decided to Crank) - then is there a lower speed (N2) that you would wait for like on the L1011 - you had to wait for the turbine speed to get below a certain . . .speed or the starter would crash start the engine at a higher turbine speed = not good. I do not see this written anywhere.

20% - Normal Procedures, Engine Start.

There's no problem crash re-engaging the starter on the old RB 211-22b (L-1011) as long as the N3 speed is at, or below 10% :ok:

Nats
From FCOM for CFM


Dry crank the engine for 30 s. The start valve automatically reopens when N2 is below 20 %. After the starter cools, and for any subsequent attempt to start the engine, the flight crew must perform a manual engine, or must report the “no start condition” to maintenance for appropriate action.

I’d have thought it best to do what it says in your manuals, rather than inventing a procedure or copying one from another type.

If you carried out a non-SOP engine start and it spat some of the hot section out onto the tarmac, how would the subsequent discussion of the incident go, especially when asked why you didn’t follow SOPs? The engine was probably going to do that whatever but now you’ve included yourself in the blame loop...

I had this happen to an engine (not CFM) in the test cell a few decades back. The engine had preservative oil in the fuel system, and the test crew did not flush it properly. In fact there was some residual oil pooled in the LPT section. First start attempt he had a puff of flame in the tailpipe; this panicked him and he chopped fuel and hit the starter at the same time.

Not too smart.

As the starter was winding up, N2 was unwinding, and when the starter clutch caught, it overstressed the "shear point" on the tower shaft. We had to cannibalize another engine to resume the test. :yuk:

Barit1, a 30 second dry crank followed by a normal start would have sufficed. There's no need to flush out inhibiting fluid as the engine will start and run quite happily on it. :ok:

Agreed,DevX. This crew panicked when they saw the flame. This engine design had a trap - a small pooling area in the LPT case that could hold a few ounces of fuel or preservative oil that could feed a tailpipe fire.

Back to the OP.

Do any of you, er, more mature, ladies & gents remember the chap on the headset delivering a "positive N1" remark during start?

I still do it occassionally when starting engines in to a tail wind. The big fan engines will light off and N2/N3 rotors will happily accelerate while the N1 is still going the wrong way. CF6 seems to be the most common. Autostart doesn't seem to worry about negative N1, however, a smaller fan engine such as the V2500, may be more susceptible hence the FCOM requirement.

Oh, and by the way, the fan drives the rotor/LPT when windmilling in either direction.

As BOAC states, the old trick of deploying the reverser is to block the airflow through the cold stream duct therefore removing the driving force on the fan.

My company's policy for engineers is to max motor the engines during start due to an embarrasing incident on a 747 some years ago when 3 donks got trashed during engines tests.