A320 engine start
 

Why does the fadec stop monitoring engine start above 50% N2 ?
I've been told that this is the reason why it is recommended to wait for stabilization before startig other engine.
Thanks!

Hi,

I found nothing in my FCOM saying that "the fadec stops monitoring eng start > 50% N2", so what exactly do you mean? What is that assumption based on? Please elaborate!

Sabenaboy

Above 50% N2 the engine is self sustaining as is the case with nearly every other turbine engine.

@MD83FO-I've seen it somewhere. Airbus did mention it. Need to dig out my old notes ( its not in the FCOM ).

@sabenaboy- I don't think its in the FCOM.

Hi MD83FO,

During an automatic engine start, there is not such an issue. The FADEC has continous monitoring of the engine during the whole start process. During a manual engine start, the FADEC will make a passive monitoring of the engine but if the EGT exceeds the starting limit before 50%N2, it will abort the start. Is that what you meant ?

Cheers.

It is mentioned in fcom 1 powerplant.

But it doesnt say what will happen if the n2 is higher than 50% In case of say.. A hot start.

AUTOMATIC STARTING

This sequence is under the full authority of the FADEC, which controls: - the start valve - the igniters - the fuel HP valves The FADEC : - detects a hot start, a hung start, a stall, or no light up - announces FAULT and identifies the fault in an ECAM message - runs an abort sequence if a start aborts on the ground when N2 < 50 % · closes the HP valve · closes the start valve · turns off ignition · cranks the engine after the start abort in order to clear out fuel vapors - ensures an automatic dry crank sequence of 30 seconds at maximum motoring speed.

does not imply that FADEC stops monitoring above 50% N2. N2 has a grey background until stable - provided by FADEC doing the monitoring!

Aah, it's about a V2500 engine...
 

I found the passage in the FCOM for V2500 equipped engines. It says that the FADEC runs an abort sequence if a start aborts on the ground when N2 < 50 %. That doesn't mean that the FADEC doesn't monitor the eng start any more.

For the CFM56 engine the FCOM says that the Fadec will abort the start in case of start problems without mentioning a 50 % N2 or other N2 limit.

So apparently this is strictly a V2500 thing. I guess that if a problem occurs after 50 % N2, it will not be considered as a start problem any more and that you will get an ecam message, but no automatic abort. I suppose however that it's unlikely for a start problem not to be detected before N2 reaches 50 %.

Last year a crew in my outfit reported an eng failure just after the start. (don't remember if it was on a CFM56 or V2500) Investigation revealed that it wasn't actually a real eng failure, but rather a (late) eng start problem. After that we received a reminder not to start the second engine before it was fully stabilized (grey background on N2 indication disappears) (ref FCOM 3.03.08 P2)

I don't think there's a technical reason why you couldn't start the second engine right after the first start valve closed but it's simply believed to be a good operating practice to wait for the first start to be fully completed as per sop. (It's annoying because it appears to take ages before that grey background disappears when starting an old V2500)

Should anybody know a technical reason why one shouldn't start the second engine right after the first start valve closure, I'd be very interested to hear about it.

Smooth flights,
Sabenaboy

My Co policy is to maintain hand on Eng Master till grey background disappears (V2500). There has been an incident in the past... rapid EGT increase during late stage of start, FADEC did nothing, crew reacted too late... engine total write-off.

My CFM engines grey background lasts till well passed the 50%.

Last ditch

so from your experience we can conclude that the grey background means start up sequence (and monitoring). No grey background, the engine is considered already started.

Hi,
 

TECHNICAL no, i think to prevent a double tailpipe.

I saw a few captains turn on both at same time when we are in a hurry.

Logic within most aircraft systems take 50% N2/N3 as the figure for the engine having been started. High heat on probes etc.

Airbus has sufficient engine oil pressure at this speed as a lot of systems require this info to enter the next phase, simulating engines running on ground requires LOP CBs to be pulled.

On v2500s The auto starting phase is controlled by the Engine Interface units EIUs commanding the EEC, the EEC monitors and protects. With the Starter cutting out at 50% the EIU passes to the EEC and the FADEC system to assume control of the stabilisation phase and operating ranges. Hence if an EIU is U/s you have no auto start capability.

Would never attempt a double engine start on the bus; you are asking for a hot/hung start, doubt the APU would have enough puff anyway.

Not a 320 guy but logic dictates that most engine starters cut out at approx 50% n2, thus the end of the start cycle. If you had egt run away it would have happened prior to this. Then understand that Full Authority Digital Engine Control is just that, allways.