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

[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]

Hung Start v's Hot Start

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?!

[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]

Starter Duty Cycle

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]

Starters

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%.

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.