Small Turbines Starting

[His dudeness]

Hi,
we had a discussion on starting our Engines...(our outfit operates Cessna Citations).
My idea was and is, that I like the engine to accelerate as high as possible before putting in fuel. Checklist says 8-10%n2 Minimum to move from cutoff to idle.
Some guys just wait until 8 %, some past 10% and I wait until about 13-14%.
(I don´t wan´t to drain the Battery and stress the starter too much, so I wait til acceleration on starter only begins to slow. Certainly subject to OAT and batt status)

We are talking about JT15D and Williams FJ44 Engines.

Your toughts ?

[Intruder]

(I don´t wan´t to drain the Battery and stress the starter too much, so I wait til acceleration on starter only begins to slow. Certainly subject to OAT and batt status)
Why do you and others believe your method is better than what the approved checklist tells you to do? Why is 10-12% inappropriate for your goals?

[30/30 Green Light]

As you say the checklist says "8-10% minimum---".There is no reason why you shouldn't let the engine accelerate on the starter to 12% or more before opening the start lever.The minimum is set because start fuel flow is (more or less) a fixed figure,and introduction of fuel before the minimum rpm means that with reduced airflow through the combustion chamber you run the risk of hung/hot starts.Letting the engine accelerate higher on the starter means more air flow (including cooling air),faster light off and generally quicker acceleration to stabilised idle.If you stick to the published procedure you can't go wrong.Engineers tend to let the starter do more of the initial work because we are maybe ? more mindful of the internal stresses involved in starts and starters are easier and cheaper to replace than engines! Cheers!

[His dudeness]

Intruder, 30/30´s post sums my thoughts up. I´m always willing to learn - so if there are backdraws to my method, I´d like to know them... If checklist would tell me "open up exactly at 8%" I´d certainly do that. Anyhow I´d feel better about it, if I would knew why...

[CJ Driver]

Essentially you are trading off engine stress against battery (and indirectly, generator) stress. If you always crank the engine until the acceleration starts to sag, then you've hit the battery harder than you needed to. You will shorten the battery life, and since you then hit the generator harder to top it up and cross-start the second engine, you will increase wear and tear on the generators too.

In my experience of small turbine engines they are very robust, particularly compared to their larger brethren. With several thousand starts in my logbook, I have never seen a hot start, and from what I have seen these engines usually make it to overhaul with no problems.

On the other hand, the batteries and generators in smaller turbine airplanes are generally underspecified, even though (unlike larger aircraft with APU's and ground power trucks) all your starts will be on the battery and then cross-started.

In my experience, these batteries very rarely make it to their rated life. Frequently cells, or the whole thing, will need replacing, and you should also expect periodic early replacement of parts in the generators. Unlike the apocryphal "hot start", which you can spend a career on small turbines without ever seeing outside the simulator, generator or battery failure is something you might reasonably see more than once.

So my advice is, once you've met the start criteria, go ahead and put the fuel in. The engine is designed to get hot, whereas the battery is NOT designed to keep cranking away.

[RatherBeFlying]

Ni-Cad batteries (among their many foibles) have a tendency to produce close to full voltage and then drop to nearly nothing. If one cell does this while its neighbors are still near top voltage, it may reverse polarity and a thermal runaway and concommitant hull loss may not be long in the future:uhoh:

I would check the manual for maximum cranking time and stay well inside it.

[high-flyer]

Guys

I have done much research into the subject of NiCd batteries and their application to biz jets, working on one particular large bizjet. One of the key factors in determining battery life and it's time between removal or failure is how it is treated in service, this refers to both maintenance and daily ops by the pilots.

The current draw on initial APU cranking is up to 1800A (on the APU fitted to the aircraft I've been working on). As the APU accelerates, the starter draws less current, but after 15 secs this is still in the hundreds of amps. After a successful start, the APU battery has heated up by around 5 deg C. For certification requirements we look at 2 failed APU starts then one successful start. This is a tough test, especially when we must be able to start the APU at -40 deg C.

Much testing is done to establish the minimum rpm for starting the APU. It indeed will start at somewhat lower rpm than given. I would suggest that your battery reliability would be improved by starting at the lowest possible (per the AFM) APU rpm.

As for thermal runaway, for a NiCd battery this is actually quite difficult to achieve. For the cert test for our battery it required a batt temp of 71 deg C with a very high state of charge being constantly charged.

The point about the rapid fall in voltage is indeed a trait of the NiCd battery. Voltage will fall from around 24 deg C to 16-17 v during an APU start. If one of the battery cells is weaker than the others it may reverse polarity. This is usually reversible. The weakness of this cell will show up during battery maintenance, and may show up as a depressed APU battery voltage during cranking of the APU.

Regards

high-flyer