Autostart program details

[John H Watson]

Here's an interesting answer to my question on PPRuNe... from a gentleman from Everett WA... 

http://www.pprune.org/tech-log/589831-747-400-rb211-autostart-windmilling-start.html

"Autostart" is not the same as "Auto-relight" - different logic paths in the FADEC. If the engine flames out or runs down uncommanded, it goes into Auto-relight. In Auto-relight it will obviously turn on ignition and adjust fuel flow for the burner conditions, and if there is evidence of a surge/stall take appropriate action to attempt to clear the surge/stall as well. If the FADEC sees the fuel switch transition it will enter Autostart which will then schedule fuel, ignition, etc. for an in-flight start (potentially with starter assist if the start valve is enabled). While the details vary between engine types (the 747-400/RB211 was a very early autostart implementation - the newer systems are much more sophisticated and complex), there is a major difference between auto-relight and autostart:
Autostart can turn off fuel and will respect EGT limits - auto-relight won't (although the inflight start EGT limit is higher than on-ground, autostart will still turn off fuel to prevent exceeding it).
BTW, the RB211/747-400 isn't really a FADEC - it's an FAFC - "Full Authority Fuel Control". It doesn't control the vanes and bleeds like a true FADEC - the vanes and bleeds are controlled by a fluidic (pneumatic) system that is little changed from what was on the early RB211s.

Of course, the CF6 is full FADEC, but I'm still not sure it will go into Autostart automatically without cycling the cutoff switches.

Cheers
JHW

[Hardy Heinlin]

Hi all,

does anybody know how low N2 must be on the PW4000 before autostart opens the start valve?

CF6 and RB211 values are known.

CF6: N2 must be below 20% when on the ground, or below 30% when in flight.

RB211: N3 must be below 10% when on the ground, or below 30% when in flight.

PW4000: N2 must be below 15%(?) when on the ground; below 15% when in flight.


Regards,

|-|ardy


Edit: I just noticed the PW4000 tries a windmill start if N2 is not above 15% within 20 seconds. I think it uses this 15% limit on the ground as well. There's a note re hung start on the ground below 16% N2 due to a poor cranking system; that's not necessarily related to the start valve limit, but it makes sense.

[Hardy Heinlin]

Hi all,

I've learned that the PW4000 autostart program for the ground start includes 4 categories of anomaly with respective automatic responses:

1. No light: Autostart will retry after a 30 sec dry spin if N2 <50%, else "stops the start procedure".

2. Starter duty cycle time (5 minutes) exceeded: Autostart "stops the start procedure".

3. Hung start: Autostart "stops the start procedure".

4. Hot start: Autostart "stops fuel supply and ignition".


I'm not sure about the last one (hot start), whether "stops fuel supply and ignition" really means just that and suggests that a retry will follow. Or whether it actually means "stops the start procedure" (closes the start valve).

Does anybody know whether the PW4000 ground autostart will retry further starts in case of hot starts (EGT>535°C) as long as 5 minutes are not exceeded?


Thanks!

|-|ardy

[John H Watson]

I get the impression that the PW seems to err on the side of caution on the ground when it comes to things which it cannot control:

High EGT (doesn't know what caused it)
Poor cranking health (doesn't know what caused it.. or assumes low duct pressure/faulty starter... in which case, it cannot control it)
Locked N1 (doesn't know what caused it)

It does allow a second attempt after no lightup/lightoff, but that's because it can double the number of ignitors used. It simply thinks that the first ignitor used has failed or is weak.

[Hardy Heinlin]

Good thoughts! So I'll stop it after a hot start as well -- for now, until a PW expert says it shouldn't stop.


|-|

[Hardy Heinlin]

I wonder why they explicitly say that both ignitors will be used on the next attempt. Both ignitors are always used when the AUTO IGN selector is set to BOTH, and that's the normal setting on PW aircraft.

Do PW pilots select "1" or "2" during engine start, not "BOTH"?

[John H Watson]

According to a well-known PW airline checklist, 1 is selected on "odd" flights.. and 2 on "even" flights.

There was no mention of Autostart in this manual. I don't know if that makes a difference.

[Hardy Heinlin]

Ah, right, on the PW the 1/2 swapping is not automatic. Wonderful, so it makes sense: I programmed it so that both ignitors* will be used after the first attempt even when 1 or 2 is selected.

(* Why do I always write "ignitor" if the Oxford Dictionary says "igniter"?)

With no light-up, with a 20 second timeout for light-up and a 30 sec dry spin and a starter duty cycle timeout of 300 seconds, the PW can make up to six start attempts. Bizarre, isn't it?


P.S.: I just learned from my own code that I wrote 7 years ago based on engineering data that the PW too autoswaps the igniter after every engine shutdown; however, the automatically selected igniter is only used during autostart. If it is used during autostart, and the automation selects, say, igniter 1, that igniter will be used even if the AUTO IGN selector on the overhead panel is set to 2. When the autostart is completed, or a manual start is in progress, the AUTO IGN selector on the overhead panel has the command again. -- On aircraft without autostart there is obviously no autoswap effect, and the pilot has to swap the igniters.

[John H Watson]

I just learned from my own code that I wrote 7 years ago based on engineering data that the PW...

All correct. The Supplemental Control Unit (SCU/Autostart controller) only knows if "single" or "both" has been selected. It doesn't know if 1 or 2 has been selected, so it swaps between the two ignitors* igniters.

Now we hit trouble again...

From the manual:

If the engine does not start, the SCU will select both ignitions for the second try of the engine
start if you have all of the conditions listed below:

The AUTO switch is not set to "BOTH"

The engine start is on the ground

A hung start or a locked rotor is not detected during the first engine start

If the second engine start fails, EICAS status message(s) ENG X IGNITION 1 OR ENG X IGNITION
2 (where 'X' denotes engine 1,2,3, or 4) shows on the main EICAS display

On the ground, is the text saying there is no "second try of the engine start" if the igniters are in BOTH? Trying to start the engine up to 5 times doesn't sound good.. and if you get the EICAS messages, it's basically telling you to give up. Your ignitors are not working. I don't see the point of the 5 minute timer though. Also, I don't see why "OR" is used (regarding the messages). If the engine doesn't ignite and both have been used, then both have failed.
More confusion... The Fault Isolation Manual (PW) says these Status messages appear if an engine fails to light using Ignitor 1 or ignitor 2

ENGINE X IGNITOR 1/2

The FIM only says regarding failed autostarts...

ENG X AUTOSTART (CAUTION): Autostart system has failed to start the engine and has aborted all start attempts.

I think we need some PW experts 

Cheers
JHW




*re Ignitors... Bad habit of mine, too (even though some dictionaries...and the maintenance manuals offer it as an alternative). Perhaps "ignitor" is the human version (tailor, sailor, author)... although there are lots of -er type humans, too (preacher, teacher, carpenter... ) 

[Hardy Heinlin]

I think that text just carries over strict logical language into human language, i.e. it sounds bizarre but it's not wrong:

"If the AUTO switch is not set to BOTH" -- Means: The SCU doesn't need to select both internally because there are already both selected externally. Both are used in any case.

The "OR" operator is not an "exclusive OR" operator (not an XOR). The text may mean to say this:

ENG () IGNITION 1 -- Is shown when 1 fails on the first or second attempt.

ENG () IGNITION 2 -- Is shown when 2 fails on the first or second attempt.

Both messages are shown when both fail; this dual failure cannot happen before the second attempt.


I also read that the PW has an AUTOSTART status messsage, not just the caution message, and that the status and caution appears after the second failed start. The text doesn't say that autostart is then aborted; it just says the messages will appear. The status message is not mentioned in other cases of AUTOSTART cautions. The text also omits some commas in the failure tables; I'm having the impression I shouldn't take everything literally in this text, and before I spend too much work with vague features, I just don't implement them until they are 100% confirmed; i.e. I won't implement those status messages.


Cheers,

|-|ardy


ENG () IGNITOR () status message logic is already implemented for all engine models. That's nothing new. However, if the status message should remain after the start is aborted and the ignition is turned off, then the status message should be latched. In PSX this message is not in the latched type category. When the ignition power is turned off and the ignition isn't sparking, there is no disagreement anymore and the message is removed.

Edit: In the next PSX update the ENG () IGNITOR () status message is now semi-latched; it can be erased by the "Repair" button of the ignition malfunction on the Malfunctions page, not by the "Erase latched messages" button on the Service page. Like before, it can also be erased by a fuel valve disagree message because that will inhibit it.

[John H Watson]

Still not sure about the number of starts... I'll keep investigating. I just don't see the point of trying to start more than 2 or 3 times. If you reach the duty cycle and it still fails... You're limiting your number of subsequent manual starts you can do. If engineering come up with a 5 minute repair... and then they do their start tests...  and then the pilots restart the engine for departure, you could be doing 7 or more starts in 15 minutes. Of course, the pilots may be told by their manuals to shut down after they notice the engine starting twice with Autostart.

I can't remember if PSX models faulty HP fuel valve scenarios (other than having circuit breakers out). I just noticed that the igniter messages in PSX have EIU message screening. The EIU logic uses an input from the HP fuel valve. For example, if there is no lightup during Autostart, it could be due to a faulty HP valve or an igniter. The EIU monitors the position of the HP valve, and if that's ok, it assumes the problem was due to igniters and puts up the message/s.

Cheers
JHW 

[Hardy Heinlin]

Yes, in PSX a HP fuel valve disagreement with the commanded position will inhibit igniter messages. It makes sense to blame the faulty fuel valve instead of the igniter if the start fails due to missing fuel flow.


Cheers,

|-|ardy

[John H Watson]

It seems the complexity of Autostart never ends. The Maintenance Manual says that if N2<16% after 20 seconds results in an aborted autostart on the ground. However, in the Fault Isolation Manual, wisely, the value varies according to altitude.

However, in the fault isolation manual...

"On the ground, N2<16% within 20, 25, 35 and 55 seconds at 0 feet, 5000', 10,000' and 15,000' respectively. OR... on the ground, the engine got to 16%, but did not get to the "on-ground ignition speed "in 16 seconds or 22 seconds (at 0 feet and 15,000' repectively). It seems the SCU delays the ignition command depending on altitude.

I don't know if the first part ties in with the "N2<0.2% in 10 second intervals" statement in the Maintenance Manual.

[John H Watson]

The faults which can cause the Autostart Caution message are endless.... However, according to the Fault Isolation manual, the list is a little shorter for the Status message and includes:

1) Engine X EEC failures (Channel fault)

2) Engine X Supplemental Control Unit failures (including circuit breakers "A", "B" or "Backup" out on P180... i.e. main bus power with battery bus backup CBs (the backup is used in the air).  SSM 80-11-01. Can also be caused by SCU bus failures (usually accompanied by ENG X SCU Status message)

3) Engine X start system fail (low N2) i.e. 20%N2 in air within 20 seconds. On the ground, N2<16% within 20, 25, 35, 55 seconds at 0 feet, 5000', 10,000' and 15,000' respectively. OR... on the ground, the engine got to 16%, but did not get to the on-ground ignition speed in 16 seconds, 22 seconds (at 0 feet and 15,000' repectively).

4) Engine X dedicated generator/alternator failures detected by the SCU.

[Hardy Heinlin]

"On the ground, N2<16% within 20, 25, 35 and 55 seconds at 0 feet, 5000', 10,000' and 15,000' respectively. OR... on the ground, the engine got to 16%, but did not get to the "on-ground ignition speed "in 16 seconds or 22 seconds (at 0 feet and 15,000' repectively).

Thanks! I was just about to upload beta 7, but I'll implement these timer variations first. I'll set these altitude blocks:

"0 ft" : < 2500 ft pressure altitude
"5000 ft" : 2500 - 7500 ft pressure altitude
"10000 ft" : 7500 - 12500 ft pressure altitude
"15000 ft" : > 12500 ft pressure altitude

No ground is higher than La Paz at ca. 13300 ft.


Cheers,

|-|ardy

[John H Watson]

Excellent 

Speaking of "IGNITORS"  If it's good enough for Boeing.... it's good enough for the forum...

Ignitors

[Hardy Heinlin]

I just discarded the idea of the altitude dependancy. The phase with fuel & ignition on and the light-up timer doesn't even begin before N2 is above 15%. There is no chance to see those altitude related timeouts. When PSX models a hung start, N2 is always at or above max motoring speed. The only way to see fault effects below 15% N2 is by activating malfunctions of the start valve or the igniters, i.e. the timer will wait for fuel-on conditions, and not for altitude related light-up detections.

New autostart programs in PSX 10.1.1-beta7: http://aerowinx.com/board/index.php?topic=4020.0

[Avi]

Great work on the Autostart Hardy.
I just played with the GE engines and my flow charts. Beautiful.
RR and PW will have to wait until I catch up with the posts reading.

Cheers,

[Hardy Heinlin]

Glad to hear! Thanks :-)


|-|ardy

[John H Watson]

Quick questions (perhaps long answers)...

In PSX on the RB211, what controls the time between fuel off and fuel back on again (for the second start attempt)?

Should the ignitor messages appear on the RB211s if you pull the 1, 2 and Standby CBs for a particular engine prior to start?

Thanks.

Cheers
JHW