747-400 RR Manual Engine Start

[QAVION]

I've noticed that some airline's AOM's manual engine start procedures use the wording....

"At 25% N3 or max motoring put the fuel levers to RUN"

How does the average 747-400 pilot interpret the "or"?

Maintenance engineers define "max motoring" as when the N3's stop increasing. If the pilots have the same interpretation, what's to stop the pilots selecting run at less than 25% if the N3's stop increasing at less than 25%? (If a lot less, putting fuel on may result in damage to the engine)

If you are meant to select RUN at 25% or greater, why don't they say that in the manuals?

Thanks.
Regards.
Q.

[HotDog]

25% N3 is the minimum acceptable for start. The company I flew with prefered max motoring N3 which was above 25% and thereby improved the start characteristics of the engine.

[QAVION]

...and just to throw a spanner in the works... I've just read that the Autostart uses this "OR" logic.

Doesn't make much sense. 25% always comes before max motoring (if max motoring is higher than 25%). Why not just say fuel is automatically turned on at 25%? :confused:

Cheers.
Q.

P.S. Thanks, Hot Dog.

[jtr]

My mob (which I thought was The Revs old mob?) use the same philosophy as you state. 25% or max motoring.

We only have manual start (tight pr1cks), and I have only had one occaision where the fuel was thrown in at mm (23%) Lit off a little hotter than normal, but seemed to run OK. I imagine places like JNB in summer et al you may have a struggle to reach 25%.

$.02

[Waste Gate]

QAVION:

For max. motoring we use less than 1% N2 (N3 for RR) rise in 5 seconds.

WG.

[LEM]

I thought everybody interpreted 25% or max motoring whichever comes first .
OK, I see your point, you say "if max motoring is much lower than 25%, then damage could result". You are right! They should specify what is the minimum value to turn the fuel on.
But actually, unless you have some damage, if the rotors are free, you won't get an N2 that low.
I've had bad APUs, unable to reach 25%: in that case the trick in a turnaround was to motor until the EGT dropped below 180°C.
That worked to keep the peak within limits.
Of course start by removing all the electrical load possible from the APU gen.

[HotDog]

Which engine are you talking about WG?

For the RR we always used min N3 of 25% (RR recomended).

On the PW JT-9 20% or higher N2 is desired. If unable to obtain 20%, use of max motoring to a minimum of 15% was the acceptable RPM for selecting start lever to IDLE/Rich position. With an unserviceable N2 gauge, start lever to idle was positioned at 3% N1.

The CF6-50 min N2 is also 15%

LEM,

But actually, unless you have some damage, if the rotors are free, you won't get an N2 that low.

You can get a low N2 for various reasons but mainly due to low pneumatic pressure, i.e. APU U/S and having to use ground air cart for starting, start valve partially opening or defective starter.

Motoring engine to below 100 deg, EGT before start lever to idle, was SOP in our operation even if we achieved the required start RPM.

[VnV2178B]

HI Q,

I recall that there was a statement about low starter air pressure may cause max motoring to be below 25%.

Certainly the autostart logic has a minimum N3 for starting threshold and a max motoring detector based on the rate of change of N3, but for a manual start the decision must be more difficult.

VnV

by the way the early engines didn't have autostart because no-one had ordered it. When a customer wanted it there was near panic in the systems team trying to cram it into the box.

[TopBunk]

Just looking through my old 744RR manual (BA).

Procedure given as:
1. Pull engine start switch, confirm N3 rise, if necessary motor to reduce EGT below 100deg C
2. When N3 reaches 25% or max motoring if less and provided N2 is 10% or more
3. Position fuel control switch to run, start stopwatch
4. Observe initial EGT rise within 20 seconds
5. Check N1 rotation by 50% N3 (starter cutout)
6. Check oil pressure by idle N3.

Warning - not flown it for a couple of years.

[Waste Gate]

HotDog: I'm referring to the -524H (&HT).

We position the FCS to run when:
a) EGT less than 100 deg C
b) N3 reaches 25%, or if 25% is not achievable, allow N3 to stabilise at max. motoring at or above 15% and . . .
c) N2 reaches 10%, or if 10% not achievable, allow N2 to stabilise at max. motoring

For the JT9, we use 5 seconds at max. motoring and a minimum of 15% N2.

For the CF6 (-80), we use max. motoring and a minimum of 15% N2.

Across all 3 of the engine types in our fleet, most pilots look for around 4% N1 before introducing the fuel. I doubt whether we can dispatch with inoperative N2 indications (EICAS), but your procedure for an inoperative gauge sounds good.

WG.

[QAVION]

Lots of interesting answers, thanks, guys.

VnV...

"Certainly the autostart logic has a minimum N3 for starting threshold and a max motoring detector based on the rate of change of N3,..."

I apologize. I just realized you answered a similar question I had about Autostart logic a few years ago (I guess I must have had brain fade). :O

You said (regarding Autostart) ....

" the N3dot threshold is 0.275 %/sec and strictly the IP shaft (N2) has a variable threshold for fuel on between 7 and 10 %, depends on oil temperature and also a max. motoring restraint too, determined in the same way as N3."

Regarding this.... Are you saying that the N3 or N2 can be lower as long as they've reached max motoring? (How about both being lower? Are there absolute no-go rpms even though both rotors are max motoring?).

I ask because we had a failed Autostart recently... and for the CMC message generated, the Fault Isolation Manual simply said that the engine failed to meet the start requirements. Less than 10kts tailwind was mentioned in the logbook, but we would also liked to have known things like how many engines were being started at the time, was there a pack on and what was the bleed pressure at the time. Of course, the flight crew cannot be expected to look at and report all these things.

Thanks.
Cheers.
Q.