RR 250 starting characteristics

[EN48]

All,

I recently took delivery of a new Enstrom 480B with a RR 250-C20W engine. This engine seems to start differently from the other 480's I have flown. On the first start of the day, the engine accelerates normally until about 40% N1; from 40% to 60% N1, the engine accelerates much more slowly than others I have experience with. On the second and later (warm) starts, the engine accelerates rapidly through the 40% to 60% zone, and stabilizes at about 62% N1. Just wondering if this slow acceleration on the first start is an issue, and if so, is there a fix for the problem? (The engine has 25 hours TTSN.)

EN48

[IHL]

I was not aware that RR made a C-20W. What is its power rating.

On any RR C20 engine I operated there were usually 2 settings on the fuel control: start derich and acceleration. Maintenance engineers usually adjust them as per seasonal changes. It sounds like you may need a maintenance engineer have a look at it to set it up properly.

[EN48]

The C20W is rated at 420 HP, and has a down firing exhaust system.

[relyon]

The compressor bleed valve mechanism is stiff and slow to open when it's cold?

Bob

[Heli-phile]

Usually a heater valve/bleed has stuck or not closing as it should, Best sign would be higher than usual temperature on start, Also stick a good ground power source in and see if that improves matters. Might have a stuffed battery, not delivering enough "cranking amps"

Also next time engine running pull full antice and check temp rise ties in with switch position. repeat for cabin heat. sometime fully closed actually is not!!:)

[Chris P Bacon]

During the start sequence, the electrical starter fitted to the engine will provide power to accelerate N1 until the mass airflow is sufficiently high to permit the introduction of fuel into the combustion liner and light off is achieved.
The additional power required to continue accelerating N1 up towards GI must be generated by the Gas Producer Turbine. The only indication of this power is the temperature produced and indicated on the TOT Gauge. Thus it is important for you to carefully monitor TOT throughout the start.
The main criteria for achieving a good start is the correct setting of the basic fuel flow schedule. The best indication that this is correctly set is the light off temperature achieved.
The figure quoted for the different RR 250 engines vary slightly, but basically lie between 760°C and 820°C and ideally should be towards the middle to the top end of the temperature range.
The start time should be between 20 and 25 seconds if an high capacity starter generator is fitted.

Temperature measurement is sensed after the second stage turbine wheel, i.e. between the Gas Producer and Power Turbines

HIGH TOT
The turbine inlet temperature at the first stage nozzle will be some 200° - 250°C above the reading on the TOT gauge. Excessively high inlet temperature will result in cracking and burning of the first stage and possibly second stage nozzles, plus blade tip rub. This could result in seizure of the first and second stage turbine wheels. In addition, a surge condition may occur with the likelihood of further damage.
High TOT results in a faster, hotter start.

LOW TOT
A low TOT indicates a weak air/fuel ratio, therefore less power is generated in the Gas Producer Turbine and the start time will be longer.
The weaker mixture will result in an elongated combustion flame reaching closer to the turbine inlet. This reduces the time that the cooling air flow has to mix with the combustion gases in order to provide an acceptable and even temperature at the turbine inlet.

[EN48]

CPB,

Thanks for the technical details. Start TOT is towards the top of the yellow band but never exceeds 810 C on either a cold or warm start, and stays at the highest temp for less than 1 second. A warm start is well within 25 seconds total; a cold start looks to be longer than this, but I havent timed it yet.

EN48

[407TX]

EN48,

Try to find the following information letter from Bell Helicopters, that one will give you excelent information about the Starting characteristics of the RR 250engines(Allison at that time).

November 4, 1983
INFORMATION LETTER
206-83-36
206L-83-23

[nodrama]

If your engine is starting fine till 40% N1, the battery should be ok.

Start derichment isn't relevant, fuel derichment takes place between 25-40% on this engine.

Acceleration adjustment is relevant. Your engine maybe 25 hrs TSN, but how old is the fuel control unit?

If it is a CECO system, there are two 'Max Pc stop' settings, '+' and '-'. It is basically an acceleration fuel flow setting. This is normally set before the fuel control unit is fitted to the engine and depends on what aircraft type the engine is fitted to. Just a thought, and might be worth checking if a dirty compressor, faulty fuel nozzle, sticking bleed valve aren't the problem.

Probably telling you something that you already know, but this engine prefers to be started hot and quick (in limits of course).

[EN48]

Nodrama,

The engine was delivered in a new helicopter on April 30, so I am assuming that it and its various components are also "new." Fuel control is Bendix.

EN48

[spinwing]

Mmmmmm .....

Just 'cos the engine is new .... does not mean the FCU cannot be faulty ....

Sounds like the acceleration bellows might be sticky when everything is COLD ..... once you have done a start and the engine components have expanded a mite the acceleration schedule works as designed ...

Go see a LAME ....it should be a warranty fix anyhow!

Cheers :E

[that chinese fella]

Reylon,

The bleed valve should definately be open during all stage of the start and unless you have left the bleed valve wedge in after a compressor rinse/wash it has absolutley no bearing on the start.

It wont start closing until around 90-92% N1 in normal ops.

TCF

[Heli-phile]

I forgot to ask at the end of the last post, whats the 480b like?
It looks like a good alternative to the MD500/B206 niche. Keen to know, might consider one in my operation?

[206 jock]

Interesting thread (seems a rarity these days...)

EN48, what does your TOT settle at? My 250 has a similar start characteristic to yours (it seems like it has a 'hump' to get over at c40% on first start), and seems to settle at around 630-650 degrees at 75% torque in the cruise with no anti-ice or bleed air heat. This is a little higher than I'm used to (206 with no particle separator), I wonder if there's a connection?

[EN48]

Heli-phile:

IMHO, the Enstrom 480B is an impressive acft. See my posts in the Enstrom Corner section of Rotorheads for more details.

206jock:

I am new to this acft and havent yet looked closely at the temp in cruise except to note that is is about "mid green." (I have made only two flights since delivery.) Will check this on the next flight.

407TX has provided a copy of the Bell Information Letter referenced in an earlier post. THis is very helpful to understanding what is happening. It looks like one click of the "tweaker" CW should improve accelereation through the 40% to 60% N1 zone.

EN48

[nodrama]

EN48
The tweaker is called the 'start-acceleration flow schedule' adjuster. Small adjustments at a time to avoid scarey starts! The fact that your problem only seems to happen on the first start of the day, leans me to agree with Spinwing, but an adjustment is worth a try.

206jock
Your acceleration ‘hump’ could be due to the same FCU acceleration adjustment problem.

The higher TOT in the cruise is a different matter.
In my experience, on this type of engine, the main contributing factors to a higher TOT than normal are an inefficient compressor and/ or air leaks.

The engine may just need something as straight forward as a good compressor chemical wash. Is oil being ingested from an oil leak somewhere? Another common fault with Allison 250s is that the internal plastic liner of the compressor casing is beginning to break up and is disrupting the airflow.

You’ve said that there is no anti-ice or customer bleeds fitted. Could possibly be that the compressor discharge tube seals are leaking excessively (they tend to leak a little anyway). Check that the circlips holding the seals in at the back end haven’t popped out. Anther common fault is that the air sensing line to the bleed valve is leaking at one of the unions. This could also mean that the bleed valve isn’t operating correctly.

If none of the above are causing the problem, get the TOT gauge check calibrated. Another possible cause is that the starter/ generator is getting tired and causing more work for the compressor through the accessory drive train (= more work for turbine = more fuel). I would personally start with the compressor and air leaks first though.
:ok:

[RVDT]

Is there a difference between Battery start and GPU start? This will eliminate the problem of a "weak" battery. On the warm start the battery will be at a higher state of charge as compared to cold.

One obscure thing to check is that the excitation of the GEN side of the Starter/Generator is isolated during the start i.e. no field voltage. The way this is done varies between airframes. The generator is always generating and is normally isolated from the battery and main bus by the RCR. During the start though the field excitation needs to be isolated so the SG is not trying to start and generate at the same time.

The other quick test of course would be to throw another FCU at it and see if there is a difference.

[IHL]

EN48:
A word of caution: unless you have a maintenance background, stay away from the tweaker for the following reasons:
1) The regulatory authority that you fly under probably does not allow pilots to adjust fuel controls.
2) Should you screw it up it could end up costing you big $$ /LBS/Euros for a hot end overhaul.

Those are just my thoughts of course.

[EN48]

IHL,

Not to worry. I am not going to tweak anything myself. I have close at hand an excellent Enstrom/Bell/RR shop and they are on the case.

EN48

[Shawn Coyle]

This is a very strange, but not completely unusual problem. I saw first in a 206 that Bell Helicopter had as a taxi between facilities. The engine was glacial in spool up from 48 to 62%- it would actually stop accelerating when the starter switch was let off, and needed the starter to re-accelerate to idle.
Next saw something similar on an OH-58C. Exactly the same symptoms you describe. Cold start is very slow from 45-62%, warm start OK.
Talk to an Rolls Royce rep - it's something in the fuel control - either a diaphragm that's got a hole in it, or something else leaking in the fuel control.
Definitely not normal, and someone in Rolls should know how to fix it.

[relyon]

that chinese fella,

The bleed valve should definately be open during all stage of the start and unless you have left the bleed valve wedge in after a compressor rinse/wash it has absolutley no bearing on the start.

It wont start closing until around 90-92% N1 in normal ops.

Clearly the OP asked the question since cold starts on this engine are obviously not normal. Yes, it should definately be open, but it may not be for whatever reason. It's because I've seen the results of a bleed valve wedge being left in that I mentioned the possibility of the valve being closed. It was a guess and I stated such, free advice being worth every cent.

I'm not interested in a technical p:mad:g contest.

Bob

[that chinese fella]

Hey Bob,

Good onya mate for being so detailed with your initial guess. Had you elaborated, as you subsequently did, it would not have warranted a peep from me.

But screw it, lets start a real humdinger of a p:mad:g match and suggest that it was such a half witted guess that any monkey worth his salt would know that a chock left in would have posed more problems than a cold first start.

And while I'm at it, do yourself a favour and review all the other posters suggestions who talk about FCU's, battery condition etc etc. But no sir-re-Bob, you tap away merrily and submit some left field bulls:mad:it bleed valve theory/guess.

It wasnt even worth a cent that one.

Spare me please. :ugh:

[nodrama]

Here we go again.......http://www.clicksmilies.com/s1106/aktion/action-smiley-070.gif

[nodrama]

Not to worry. I am not going to tweak anything myself. I have close at hand an excellent Enstrom/Bell/RR shop and they are on the case.

EN48

Did they sort out the problem? What was the cause/ solution in the end?

[EN48]

nodrama,

Their initial thinking is that the fuel control is not set up quite right. They are regarded as quite expert in this area, so I am inclined to accept this notion. I havent had time yet to get the acft to the shop to do a proper diagnosis.

After doing some further research, I find that RR states that if the start is complete within 1 minute, then no problem, and this engine is certainly under 1 minute from light off to 58% N1. However, as I originally stated, it does require considerably longer than any other I have experience with to accelerate from 40% to 60%.

EN48

[nodrama]

EN48

Thanks. Just nice to know the outcome/ solution of the problem.

[diethelm]

EN48,

when you say slower, how much slower? Although it does sound like the bellows in the FCU might have a potential problem, it would depend on how cold and cold soaked the engine is and how much slower it actually accelerates. Has it ever hung up and completely stopped accelerating? Also watch your TOT on the first start, does it drop off more quickly with the first start versus a start after being warm?

In either event, given it is under warranty, get a loaner FCU from your RR AMC and see if this solves the problem. I have seen many series II and series III engines which accelerated slowly on first start in winter. However, these starts were within the limits.

[EN48]

deithelm,

The cold start is within the 1 minute time allowed by RR/Enstrom and TOT never exceeds top of the yellow (just under 810 deg C) and stays at peak temp for less than 1 sec. Three other 480's I fly with the C20W engine accelerate much more quickly through the 40% to 60% zone. My engine never stops accelerating through this zone. I am in the process of collecting more definitive data.

EN48

[that chinese fella]

Hey EN,

Confirm that there are no starter time limits on the Enstrom? 60 seconds is a long time for the poor little thing.

I never have had anything to do with them thats all just curious, promise.

TCF

[EN48]

TCF,

The RFM starter limitation is 60 sec provided that the engine lights off. The limitation is 40 sec if it does not light off. The RFM goes on to state that if the engine is accelerating when 60 seconds is up it is permissable to continue the start. Apparently if the engine is running and accelerating, the starter is not working as hard.

EN48

[EN48]

Helo to the shop today. Some data:

Start time (cold) before FCU adjustments: 38 sec light off to 58% N1; peak TOT 800 deg
Start time (warm) before FCU adjustments: 27 sec; peak TOT 810 deg

FCU adjustments: one increment "down" on start derich, one increment "up" on start acceleration

Start time (cold): TBD when next cold start occurs
Start time (warm) after FCU adjustments: 20 sec; peak TOT 840 deg; shop tells me that TOT in this case is can vary a quite bit depending on when fuel is introduced, and advise cranking a bit (2 sec) longer before cracking the throttle.

TOT in cruise (55 psi torque, 110 KIAS): 630 deg before and after FCU adjustments

My guess is that the "problem" is solved; we'll find out soon.

EN48