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 Post subject: Effect of Start Duration on Start ITT
PostPosted: Sat Oct 24, 2009 8:06 pm 
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Joined: Tue Sep 16, 2008 4:56 pm
Posts: 6
Hi all,
Was looking through some ADAs engine data from a couple of aircraft and seem to notice a trend. A longer start duration seems to more often than not lead to a higher start ITT when compared with the start ITT attained with a shorter start duration.
This brings to mind a question; Is placing the condition lever to low idle shortly after attaining 12% Ng going to result in a cooler start than waiting for maximum stabilzed Ng?


 
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 Post subject: Re: Effect of Start Duration on Start ITT
PostPosted: Sun Oct 25, 2009 9:39 am 
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Joined: Sat Oct 13, 2007 5:29 am
Posts: 8
Our higher start ITT's are associated with a quick turnaround. Whilst not advocated by the POH, our pilots wait for the ITT to drop to below about 150 before introducing fuel - this results in a longer start cycle.

Which comes first - the chicken or the egg?

Edit to add - Longer start cycles could also be an indicator of a flatter battery or lower voltage from an APU - again leading to higher start temperatures.


 
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 Post subject: Re: Effect of Start Duration on Start ITT
PostPosted: Thu Oct 29, 2009 10:05 am 
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Joined: Fri Aug 15, 2008 10:43 am
Posts: 14
Location: Thailand
Wanted to throw this in:

After much debate and consultation with Pratt engineers, it became obvious to me that a lower peak ITT during start (and engine operation) means longer turbine blade life. This translates to long term lower operating cost, which can be achieved by causing longer start cycles intentionally before introducing fuel (means a higher Ng as well), thus having more airflow in the combustion chamber to envelope the flame and keep a lower peak ITT . It is important to note that in terms of when to introduce fuel during the start sequence, 12% is the minimum in order to allow sufficient airflow to envelope the flame during start. 12% Ng may not always be the best value for fuel introduction depending on Density Altitude and residual ITT temp (from quick turnarounds etc). This is most likely why the POH mentions a min value of 12%Ng for a "safe" start, while Section 2 of the POH allows up to 30 seconds for the start cycle in terms of starter/generator limitations. Our average starts in hot and humid environments were between 7-14 seconds. A longer start cycle than that may be caused intentionally by the pilot, or due to a weak battery or damaged starter generator/GCU, which takes significantly longer to achieve a Min 12%Ng. If that is the case, the system should be checked and repaired as a hot start is probably just around the corner.

Another thing to mention is that the fuel metering (quantity of fuel being supplied to the nozzles by the FCU) is a geared fixed ratio on the accessory side of the engine. This means that the amount of fuel spray at 12% Ng is less than 16%Ng etc. The higher the Ng during introduction of fuel during start, the bigger the intial flame (due to more fuel) will be BUT it will have additional airflow to envelope that flame.

Another thing to consider is air density and wind direction during start during the turnaround. If you are facing a headwind during start then you naturally get a better airflow with lower Ng readings than I mentioned above. If the air is nice and cool (dense) you will also get a slightly lower peak Ng, but that depends on the condition of your battery or APU.

Bottom line for us was keeping the turbine blades "happy" with lower operating temps over the full spectrum of engine opertaions. The cost of battery replacements and startor generators are significantly cheaper than blade fatigue due to hotter temps (especially considering this is a single engine application of the PT6).

Hope this helps present some more food for thought.


 
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 Post subject: Re: Effect of Start Duration on Start ITT
PostPosted: Thu Oct 29, 2009 3:10 pm 
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Joined: Tue Sep 16, 2008 4:56 pm
Posts: 6
Well put bassman, thanks. The minimum fuel flow setting on the FCU might also be responsible for hot starts too.


 
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 Post subject: Re: Effect of Start Duration on Start ITT
PostPosted: Thu Oct 29, 2009 9:36 pm 
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Joined: Fri Aug 15, 2008 10:43 am
Posts: 14
Location: Thailand
Always glad to share. I don't know much about the FCU adjustments as I took a slightly active interest regarding the starting procedure from a pilot perspective. There was word of a book called something along the lines of "power management for pilots", which apparently was put out by P&W a long time ago. I haven't seen the book and don't know where to get it, but apparently it deals with these topics. I would love a copy if (especially PDF) if anyone has it. Please PM me if you have info on how to get it.

Thanks!


 
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 Post subject: Re: Effect of Start Duration on Start ITT
PostPosted: Fri Oct 30, 2009 5:22 pm 
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Joined: Tue Sep 16, 2008 4:56 pm
Posts: 6
bassman wrote:
Always glad to share. I don't know much about the FCU adjustments as I took a slightly active interest regarding the starting procedure from a pilot perspective. There was word of a book called something along the lines of "power management for pilots", which apparently was put out by P&W a long time ago. I haven't seen the book and don't know where to get it, but apparently it deals with these topics. I would love a copy if (especially PDF) if anyone has it. Please PM me if you have info on how to get it.

Thanks!


Might this be the book? http://www.aircraftmanuals.aero/list/sh ... p?mID=7120 Will try get a copy for our library here, have a great weekend.


 
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 Post subject: Re: Effect of Start Duration on Start ITT
PostPosted: Sun Nov 01, 2009 4:41 am 
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Joined: Wed Nov 24, 2004 2:25 am
Posts: 18
Location: Central Texas, U.S.A.
Another tip! We have had hotter than normal starts until we fixed it with WD-40! Keep your linkage under the cowl of the Fuel Condition Lever oiled with WD-40 or similar. When it starts sticking it tends to add too much fuel even at the low idle position.

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