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

[stilton]

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 ?

[hetfield]

Good for the engine bad for the starter.

[Junkflyer]

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

[SNS3Guppy]

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.

[Angels 60]

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..

[SNS3Guppy]

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.

[sudden Winds]

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.

[groundfloor]

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:

[Beeline]

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

[lomapaseo]

groundfloor
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.


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.

[groundfloor]

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..

[Angels 60]

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

[SNS3Guppy]

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.


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.

[Angels 60]

Guppy +1

Question:

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

[Check Airman]

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

[Mr.Brown]

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

Because as stated earlier
The thing is jets are air cooled so you need to pump air in to cool them
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.

[SNS3Guppy]

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


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.

[Angels 60]

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?

[SNS3Guppy]

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.


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?

[N1 Vibes]

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

[Angels 60]

Guppy..Pratt equiped aircraft that I have flown...either 8% for jets or 12% for Tprops. So as you probably know, not a big push for the starter. Compared to say Garretts...

I guess the way it's taught is you either get a hung start prior to to introducing fuel, or if you have introduced fuel, maybe the igniters didn't light them off, or any number of other factors where a burn didn't happen...once you've disengaged the motor, probably all unignited fuel has been blown through

After a light off, if the starter doesn't push things farther along after then the classic hot start seems to happen, alot of fire and not enough air to blow it through...

Not sure how to differentiate a hung start after light off, and a hot start, if in both cases the temp increases. So it's treated as a hot start.

Classic cases of the NICAD or GPU dropping off in the start sequence right after light off, and no power to run the starter, while rare, gives one pause to consider the solution...fuel cut off, and hope that's enough...

Thanks for the explanations.

[SNS3Guppy]

So any further damage to the combustor will only be exposed in the overhaul shop, on a scheduled visit.


Or a borescope inspection when looking for other damage. It may also show up as an early requirement for a blade or inlet guide vane replacement, as do streaking nozzles.

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.


There's a two-fold impetus for this: one is that the powerplants are being operated by professional crews who are probably running them to max motoring speed, and the other is that the combustor is seldom looked at until other aspects of the engine demand attention, or until a combustor blockage or failure leads to other component irregularities.

Additionally, most of those powerplant installations are FADEC controlled with smart regulation to prevent many anomolies from becoming issues that demand demating or splitting the components.

They're not more robust in the combustor so much as they are simply not failing. This isn't the result of a significant increase in thickness or strength, but the design of the combustor itself, it's cooling properties, the improved bleeds, etc, which improve better airflow at lower RPM's.

A significant different exists in the way overhaul and repair periods are arranged with most business airplanes and their smaller turbines, vs. those on larger transport category airline equipment. Whereas many smaller powerplants such as the TFE-731 have overhaul intervals in the 3,500 hour to 6,000 hour range, with hot section inspections at mid time, airline equipment tends to go much longer with scheduled maintenance and inspections done on the wing. In business aircraft, generally the powerplant is removed and a loaner hung while the work is done. The busines aircraft does ten years or more before coming close to the inspection, and sees more frequent cycles than an accumulation of hours in many cases. The Airline aircraft, particularly those used on longer routes, sees many times the number of hours in a much shorter time, with generally far fewer cyles per hour.

[Old Fella]

Angels 60. A hung start is where the engine "bogs down", or if you prefer, stagnates at a RPM below idle. It may be that a compressor unloading valve has failed closed and the engine cannot accelerate past the RPM at which it stagnates. This may or may not be above starter cut-out speed. A hot start can happen at any phase of the start cycle and does not have to be accompanied by a stagnation in engine RPM. In the case of either a hung start or a hot start the action required is the same. Shut down and either try again or, if a hot start has been allowed to exceed limits, inspect the hot end for any damage i.e. via a borescope inspection.

[Angels 60]

Agreed Old Fella!

[SuperRanger]

what!? no autostart?! :p

[Ka8 Flyer]

I have two follow up questions:

1) Is lubrication any issue at max motoring speed? I.e. is oil pressure high enough to keep everything lubricated for an extended period of time?

2) On FADEC equipped aicraft with autostart capability - will FADEC also wait until max motoring before introducing fuel? If not, why not?

[Angels 60]

1- No

2- No -

For the latter question, some people in here have decided to introduce fuel when the starter rpm has maxed out...to improve cooling, ect..

Ofcourse the checklist says something different..they give a minimum N1/2/3 to add fuel... which I personaly beleive is fine, because the manufacturer thinks it's ok, ....and I believe they don't want people sitting there for 5 minutes 'pre cooling thier engines' with a starter motoring at max, before they add fuel

If people use Max rpm before they add fuel, then it goes that someone will read this, take the cooling argument to it's logical extent and sit there with a motoring starter for much longer then the manufacturer intended.

And when the starter goes, no more flying untill it get's fixed....

Max starter rpm or how fast it accelerates to N1 desired varies just by hooking up a GPU....

[SNS3Guppy]

Ofcourse the checklist says something different..they give a minimum N1/2/3 to add fuel... which I personaly beleive is fine, because the manufacturer thinks it's ok, ....and I believe they don't want people sitting there for 5 minutes 'pre cooling thier engines' with a starter motoring at max, before they add fuel

If people use Max rpm before they add fuel, then it goes that someone will read this, take the cooling argument to it's logical extent and sit there with a motoring starter for much longer then the manufacturer intended.


Max motoring speed is typically 18-20% depending on the type of starter motor, and takes only a few seconds longer than achieving minimum RPM.

You've really no experience with this, have you?

For the latter question, some people in here have decided to introduce fuel when the starter rpm has maxed out...to improve cooling, ect..


No, some people here havent' decided this. Every turbine engine manufacturer recommends this, save for automatic start processes.

Lubrication not an issue. FADEC and auto starting systems utilize a computer to schedule not only the starter function, but also the fuel, obviating the need for the operator to intervene in a normal start process.

[SNS3Guppy]

Incorrect Guppy, starters are varied, so are electrcal systems, so are engines..start up a Garret Dash 10 and let me know what your start speed is vs a Pratt PT6, vs a JT15D, then tell me all starters max at 18-20% N1/2/3


The Garrett TPE-331-10 is best run to max motoring RPM prior to the introduction of fuel. Garrett (Honeywell) strongly recommends it.

You've probably not had the opportunity to use the rapid start system which is an option on many TPE-331 installations. This system increases the starter voltage, spinning up the engine in half the time and to a higher speed. The reduction in start temperatures is dramatic.

Without the fast start function or with it, Honeywell still recommends that the TPE-331 be motored to max max motoring speed on the starter before introducing the fuel. The faster you spin it, the better.

What the actual start speed each powerplant achieves is really irrelevant; take it to whatever the start motor will deliver. If it's on external power or has a very good battery, it may be higher than a normal battery start. Regardless, your start will be cooler and the engine life increased, the higher the motoring speed before introducing fuel.

You may not have bothered to read past your checklist or look into the manufacturer recommendations. You should try it.

If you flew a variety of Jets and turboprops, with and without GPUS/APUs...24 volts vs 28 volts, you would know the difference..


What are you attempting to say?

[Old Fella]

Angels 60. This thread is not a place for personal attacks or fanciful posts. If you pose a question and you do not get the answer you hope for, at least be gracious in your responses. By the way, starter cut-out for the Allison T56 is above 50% RPM with fuel being introduced 16% RPM. All starters have a Duty Cycle and none I have seen have been the 5 minutes you mentioned in one of your recent posts, whether they be electric or pneumatic starters.

[stilton]

Max motoring does not mean, motor for '5 minutes' or to starter time limits, it means motoring to maximum N3 / N2 rotation speed prior to introducing fuel.On the 75/67 this is not even remotely close to the starter limit time.In 11 years on the type I have not seen a starter or engine problem.

I am not familiar with light business jet / turboprop type engines but imagine that they would probably start cooler using the same procedure.

[Old Fella]

I read and understood your post Angels 60. I am sure Guppy knows that all starters are not the same and as I read his post he says it is typically 18-20% N1/2/3, not exclusively. That is not really my popint though, I just want to say nothing is gained by endeavouring to stir people. We all have different experience levels and ideas about what is correct. You can't rub me the wrong way old son, I've been around too long to let my feathers get ruffled.

[Denti]

I go with the checklist, they say introduce fuel at this number, that's what I do, and I feel uncomfortable letting the motor run out there, any longer then it has to...that saves my starter, which I might need, up in the air for a start..


You see, the thing is in professional aviation, as opposed to google guessing work, checklists change. Quite often in fact. Based on prior experiences.

We had such a change regarding engine start half a year ago. Before we would introduce fuel at 25% N2 or max motoring (max motoring defined as an acceleration of less than 1% N2 in 5 seconds) but not below 20% N2. Now it got changed to wait for max motoring in any case. Starter duty cycle never represented a problem since that change, not even on our old engines, but all starts generate a much lower max EGT.

[tankdriver45]

I use the checklist, I suggest posters do the same. When it says to introduce fuel, I think the manufacturer did the testing, assessed the risk.

I mean motoring a starter harder is kind of like using max take off..doesn't it make sense the using a lower start speed, given it's enough to keep your engine from getting warm on start is akin to not running your engine hard...?

[barit1]

Digging way back in the brain cells -

Boeing always called for 15% N2(/N3) for the 747, because that's what the JT9D people required. The CF6 started very nicely at 10%, which was a good thing because the APU in the DC-10 had to be derated to prevent bad things in that department. Often DC-10 max motoring was no more than 11-12%. :rolleyes:

But Boeing still called for 15% for standardization between JT9D/RB211/CF6 donks. :=

[SNS3Guppy]

Our checklist on the JT9D (B747-200) simply says "max motoring," which the FE calls out during the start process. The FO moves the start lever to idle, (or rich) at max motoring.

[SNS3Guppy]

Max motoring means just what it says; until the maximum RPM's are reached that are deliverable by the starter under a given voltage or air pressure; typically only a few seconds longer than the minimum specified RPM for introducing fuel.

There's no "max" position to hold; the starter is either on or off. Maximum motoring refers to the maximum speed achievable by starter power alone.

[RAT 5]

Hi Guys, Lots of interesting comments. May I digress slightly, but in a similar field.

B738 Engine Inflgiht Start. X-bleed using GND start switch. You're at lowish level below Fl100. QRH says minimum N2% for fuel on is 11%. Normal for engine start is 25% or max motoring. On the GND 25% is first.

I teach to wait for 25% or max motoring for inflight start at low levels. The envelope goes to FL270. If you can get 25% why not wait. Faster = cooler and better chance of a successful start IMHO. At high levels you may max motor belwo 25%, even down to 11% I presume. I see other SFI's have been teaching slam it in at 11% and it doesn't matter if you can reach higher. QRH says 11% is Ok so do it.

Thoughts?

[SNS3Guppy]

You're always best lighting off at the maximum speed you can get.

Think about it this way; there's never an airstart checklist which tells you to reduce airspeed to the minimum possible to slow the engine down before lighting off.

The faster you can spin that engine prior to introducing fuel, the better, more positive, and cooler the lightoff and subsequent spool up. There are no detrimental effects from spinning it faster, either to the engine or the starter, and only benifits.

[SNS3Guppy]

Okey dokey. Cattleflyer/ssg/strickle451/tankdriver45/onpercenter is being added to the ignore list, until he gets banned too.

After his "which side" thread comments in this forum, the folks in the helicopter forum have been wiping the floor with him. He's been preaching there, and knows less about helicopters than he knows about fixed wing (didn't think it was possible, did you??).

But I try to apply 'Flex Logic' to my starter...the easier I am on it, the longer it will last.


Whereas the starter will normally stay engaged to about 50 percent on most turbine starts, regardless of when you introduce the fuel...one saves nothing. The starter motor is powered and engaged anyway. Early introduction of fuel doesn't do a thing but increase start temperatures. The starter will be engaged for the duration of the start process anyway.

If you bother to reply, of course, I won't see you...your latest name goes on the ignore pile with the rest of your banned personna non-gratas.