Max motoring or min N3 / N2 for fuel on at start ?
 

Staying well within starter limitations I have always delayed fuel on until max
rotation speed N3 (RB 211 / 757) and N2 (CF6-80 / 767)

This procedure has always given me the coolest possible starts and I think it is a good technique for maximum engine life.

Thoughts, opinions ?

Good for the engine bad for the starter.

The starter has a long duty cycle and is much cheaper than turbine blades.

It's always been the policy in any turbine I've flown to introduce fuel at max motoring speed. It's not bad for the starter in the least, and the start temp is of far greater interest and importance than starter life. Starter life won't be harmed by taking it to max motoring speed prior to introducing fuel and spark.

All turbine manufacturers post a minimum motoring speed prior to introducing fuel, but I don't know of any that insist fuel be introduced at the minimum speed.

The faster you can spin the engine prior to start, in general the cooler the start.

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Some planes seem to start warm, no matter what you do, so getting N up is a solid idea, along with an apu/gpu...even pointing into the wind for some..

Fortunately, the planes I fly always start cool on batteries, hot or cold days, so I tend to introduce fuel as soon as checklist allows to save the starter..

I agree with the posters in here that saving your engine before the starter is the right logic..

Seeing as your starter stays engaged and running typically up to 45 or 50% N1...how do you imagine the starter might be stressed allowing it to drive the engine to it's maximum motoring RPM?

It isn't. The starter is going to stay engaged for the duration of the start, anyway, still being driven until it's cut-out point. That's the case regardless of when the fuel is introduced. If you happen to be flying an aircraft with a starter-generator, then it's not an issue at all, because that starter is going to stop driving by function of speed anyway, and it's always connected the powerplant.

A little research will find that your engine manufacturer will recommend allowing the starter to drive the engine to max motoring RPM in order to achieve coolest start temps and maximum engine life. The RPM specified in the start procedure will be a minimum speed before introducing fuel.

not a bad idea for domestic flights where you´re starting the engine after 30 or 40 minutes of last shutdown and residual EGT is normally high. Being a First officer and having attempted to do that (and briefed so) I was told to stick to what the manual or airline policy says... :rolleyes: seems that they wanted to go home quickly and couldn´t wait 20 more secs total.:yuk:
What I was allowed to do a few times was to reposition the a/c for a no tailwind start.

Max motoring is the way to go! Especially on the smaller turbines. Be careful however to become fixated on "cool starts". A perfect start is where there is the correct balance between; start temp, eng rpm and time taken to ground idle from when the fuel goes in.

Eg: at 20 sec 600 deg C 30% N1 and a couple of other checkpoints. On the older turbines you could find these values in the maint manual. If I remember correctly the TURMO IVC on the Puma helicopter had a flt man limit of 600 deg C min for a cold start.

The thing is jets are air cooled so you need to pump air in to cool them - a cold slow start can be very detrimental. Your temp indication might be low but as there is insufficient airflow (low comp rpm) certain components will be cooking.

Trouble free starts to all..:ok:

With regards to 757 (RB211) our EICAS has a magenta fuel on bug on the N3 @ max motoring.

groundfloor
Thanks for that:ok:

I was having trouble getting my head arround the suggestion of increased TIT or EGT with low RPM starts.

What I believe that you are saying is that for a nominal TIT (turbine inlet temp) the actual blade and vane temps may be much hotter due to reduced cooling air flow from the compressor.

Long cold starts are the worst, worst case scenario is a "hung" start, typically on PT6`s. It stops accelerating at say 35% and you can then watch the temp slowly climbing.

Another example of this effect is when you load at small turbine (King Air etc) with generator and aircon. The compressor rpm goes down and the temp goes up as the engine is running on metered fuel - solution on Kingair push up idle on others crack the thrust lever/throttle to increase the Compressor rpm and cool the motor down..

Can't remember anyone tearing down an engine because of a hung start, but they have for a hot start.

This is not correct. While thermal damage can occur in the burner cans due to a hung start or introduction of fuel at too low a speed, the problem which occuring isn't fuel burning beyond the cans and causing thermal damage in the turbine wheels. When damage like that occurs, it's the introduction of too much fuel and fuel streaking (bad nozzles). In such cases where excess fuel is causing excess temperatures, it's going to show up as EGT or ITT/TIT. When the turbine wheels get hot, you're going to see it on the engine instrumentation.

Long cold starts aren't the problem. Long hot starts are. Hung starts are...which is part of the reason that most manufacturers put a five second cutoff for a hung start (or less).

The most fragile part of the engine, and the part that requires and uses the most air cooling and is most critical for air cooling, is the burner can. The burner is very thin. When I have handled them or removed them during hot section inspections, I have often seen them full of cracks; this is common due to thermald damage. A burner can is very thin, beer can thin, and very fragile. Holding one and turning it over in your hands sounds like a tinkling christmas tree ornament. The burner can employs vents and louvers to use airflow to keep the flame pattern off the burner can walls; the walls are actually protected by a layer of air during the start process and beyond. The majority of the airflow through the engine core is for cooling, not burning, and it's primary cooling duty is the burner can. It's the can that suffers the most damage during a hung start, but it's the after-components such as turbine wheels and the nozzle that suffer during a hot start.

The reason for having a minimum start RPM prior to introducing fuel is to protect the burner can, not the turbine wheels. While the turbine wheels can be damaged easily with excess temperature, operating at too low an RPM is a threat to the burner can first and foremost because it takes a minimum RPM to put enough air through the can to create a layer of insulating air inside the burner itself. Additionally, fuel doesn't atomize properly in the center of the can, leading to hot spotting and burning on the can itself, and the carriage of unburned fuel beyond..which can do spot damage to the turbine wheel(s).

Hung starts at higher RPM's, such as 35% (too high to be a max motoring speed for the starter alone) will be doing burner can damage if an excess or stalled EGT tempeature is seen, but more typically will see a stalled RPM with a rapidly climbing EGT. In this case, the threat is to both the burner can and the turbine wheels, stators, nozzle, etc.

An engine may certainly merit an inspection or even teardown following a hung start, depending on what has occured.

Start RPM's that are provided are minimum numbers, not maximum numbers with respect to motoring speeds.

A cooler start with a higher max motoring speed is NOT bad for the engine, and excess airflow beyond that provided at minimum motoring RPM does NOT harm the engine or it's components. A cool start is not the same as a hung start. A cool start is not detrimental to the engine, though too low a motoring speed certainly is. Regardless, you are NOT going to harm the engine or the starter by taking it to max motoring speed prior to introducing fuel and ignition.

Guppy +1

Question:

Isn't the reason to cut off a hung start to save the starter not the engine?

Why does introducing the fuel at lower speed produce higher EGT's?

Because as stated earlier
Not enough air to cool while combustion is taking place causing a rise in EGT. Air going to the combuster after passing through the compressor core is used in different ways, some is mixed with the fuel and burnt some is used to position the flame and some is used for cooling the combuster case, with not enough air, although the fuel will burn the flame is not positioned right and the combuster case losses the cooling effect from the cooling air causing an increased temp exiting the combustor to the turbine which is getting cooked as the turbine section has less cooling air causing the indication to pick up the increased EGT.

Not at all. The starter is insignificant in comparison to the engine in terms of cost or importance. The starter is just an accessory.

Starters have duty limitations which prevent excess use or heat. These duty limitations are provided in terms of start cycles or attempts, and time. Electric starters will have duty cycles expressed in time on and off for cooling, such as one minute on, one off, one on, one off, one on, fifteen off, etc. These are established to provide maximum starter engagement times before cooling is allowed, and the specific cooling periods.

Air driven starters have similiar limitations, usually provided as maximum start attempts, and a max starter time...generally about five minutes to fifteen minutes.

A hung start isn't hard on the starter. It's hard on internal engine components, primarily the burner can.

Part of the procedure following a hung start is generally to cut off fuel to the engine, and to continue motoring the engine to clear any fuel, fire,or vapors that may be there. If the issue were to protect the starter, then certainly we wouldn't keep motoring the engine with the starter.

In fact, if one were to place an interest in the starter rather than the engine, and didn't motor the engine, one could see significant engine damage as a result. Engine damage that may not be confined to just the burner can, but may spread in both directions (toward the diffuser and compressor, as well as the burner cans) as airflow stops with starter disengagement.

Starter use throughout the hung start process is very important. A hung start may result from the introduction of fuel at too low an engine RPM during the start procedure...which may be the result of a low battery or weak starter, but in any event, the procedure for handling a hung start isn't to save the starter. It's to save the engine.

When a hung start occurs, the engine cannot accelerate; it's stuck in a condition of stasis with out the ability to increase RPM. Ideally fuel is being scheduled for that speed, but fuel may continue to feed the engine at an increasing rate, causing other problems...while the RPM doesn't increase. Hung starts can result from a variety of problems, most commonly a failure of an acceleration bleed to operate properly (also for a variety of reasons)...but the central problem is that the engine RPM isn't increasing. There's only one thing to do, and that's shut it down by removing the fuel (and in most cases the ignition, simultaneously). In all cases, you want to keep motoring that engine with the starter...clear evidence that the shutdown isn't anything at all to do with protecting the starter, but the engine.

Every moment that engine stays hung is one more moment for internal components to cook and damage to occur.

When I experience a hung start, I am required to continue to motor the engine for at least 30 seconds. I am also required to motor it until I see a maximum temperature indicated, in my particular case either 100 degrees C or 180 degrees C, depending on which aircraft I'm flying. If no overtemperature has occured, I can continue motoring, reintroduce fuel, and proceed with the second start attempt. I am allowed two start attempts. This occured several days ago during an engine start in Liege. A simple hung start, fuel chop, and reintroduce fuel. It worked fine the second time. At that point, we stopped to review the hung start checklist, and then proceeded to start the other engines.

Motoring to max motoring speed is a good thing for the engine, won't hurt the starter, and produces cooler starts, better engine start protection, longer engine life,and less start abnormal or emergency mishaps.

I think everything you said is correct..in how to handle the hung start...but

Here is the scenario I get in the sim all the time...

Hit starter button, wait for N1 8%(for instance) it sits at 4%, you never introduce fuel, hence no engine issues,.now your just motoring a starter. Hit starter disengage...

That's a hung start.

Hit starter button, at N1 8%, you introduce fuel....temps increase ... go way up...fuel cut off, motor starter to reduce temps..then disengage starter

That's a hot start

Hit the starter, normal start but the starter light is on, never disengaged...so you disengage...button, CB, battery...

Am I missing something here?

That's not a hung start. That's either a weak battery or a starter problem.

A hung start occurs typically when the engine has experienced a lightoff. What you're describing is a failure to lightoff.

If you're motoring the engnine at such a low speed, starter longevity or life just isn't an issue. You're simply looking at a start attempt. If it fails to motor to a minimum speed, then yes, you're shutting down the starter (or closing the start valve, as the case may be) simply because the something isn't allowing it to come up to speed. You may have a bad relay, you may have an internal engine problem, you may have a weak battery (nearly always the case). The start isn't hung, it just never got started.

A hung start will typically begin a normal or possibly slow acceleration, but begin to slow and then stop at some point...often around 30-50%. It may simply stall at that point, or it may slow with an increasing engine temperature. A hung start may go hot, or it may not. A hot start may or may not hang. Often the procedure for a hung or hot start is the same such that they're listed on the checklist as Hot/Hung Start. It could go either way, but neither one is good for the engine, either one could destroy the engine if left unchecked.

What specific aircraft is it to which you are refererring?

Some good points here, to reinforce some of the commercial/design issues with what component needs to be protected and which are sacrificial in the hot/hung start scenario:

- Starter = $80k
- Engine = $7million
- Combustion Chamber = $250k
(and you have to remove the engine to fix it of course!)

On the subject of longevity due to cooler starts, depending on your engine type. If you have an engine type that is usually removed unscheduled/scheduled for combustor damage as the prime cause (instead of cyc limited, EGT limited, turbine/comprssor deterioration), then a cooler start policy is a good thing.

Basically all of the engines that I deal with - Trent, RB211, PW4000, JT9, CFM, V2500, GE90 etc are not driven off for combustor damage, since the designs/modifications make them so much more robust in the combustor area. In fact the only engine I ever removed for combustor damage was at least 6 years ago.

So any further damage to the combustor will only be exposed in the overhaul shop, on a scheduled visit. And since all our engines are power-by-the-hour then the beancounters don't care about the extra damage.

Regards,

N1 Vibes