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Hot and High Starts - When High Idle is Not Enough (by Tom Clements)


Hot and High Starts - When High Idle is Not Enough (by Tom Clements)

Now that summertime is arriving again, I thought it would be timely to present a simple PT6 starting modification that you should probably be making during some of your upcoming starts. As I have often said, when a Ground Power Unit (GPU) is conveniently available, that’s the way to go, especially on very hot days. It saves workload on a generator and allows both engines to receive the benefit of a 28-volt assisted start. N1 will likely stabilize at least three to four percent higher than with a battery start alone and the resultant ITT peak may be as much as 100°F cooler. However, since GPUs are not always conveniently available, you’ll be doing plenty of battery and generator-assisted starts, which is where a problem often surfaces.

Before continuing to address hot and high starts, I should mention that it is also a great idea to use a conveniently available GPU in the dead of winter, too. Now it’s not cooler ITTs we are after, but instead the goal is to supply help for our poor little frigid battery. Also, realize that most King Airs allow for preheating the cabin with a GPU … nice!

The actual power available from a PT6 to drive both the propeller and various accessories depends not solely on N1 but also on pressure altitude and OAT (Outside Air Temperature). Just as power decreases as you climb without moving the power lever – and thereby maintaining a constant compressor speed, N1, in that climb – so also does power decrease at idle when moving from Sea Level to higher altitudes. Even the best of PT6s does not have sufficient power at Sea Level, when spinning near a 50 percent minimum Low Idle N1 speed, to handle the load demanded when using the generator to assist with the opposite side’s start. In fact, even a 60 percent Low Idle setting – the Low Idle speed associated with all four-blade propeller King Airs – rarely supplies sufficient power to drive the generator during an assisted start at Sea Level. That’s why Beech specifies going to High Idle on the operating engine before activating its generator.

As we move to the thinner air associated with higher altitudes and hotter OATs, power decreases so dramatically we find that even our 70 percent, High Idle setting is oftentimes not enough to handle the opposite side’s starting task without experiencing difficulty.

What is this “difficulty?” It is compressor bog-down, accompanied by highly elevated ITTs. When the generator is turned on to assist with the opposite start, the large electrical demand makes it harder to spin the generator. The additional drag on the compressor shaft causes it to slow down. Since the Fuel Control Unit (FCU) is merely a governor for compressor speed, N1, it should respond to the slow down by adding more fuel and returning the speed to its original value. But on the hot, high situation, when the power at 70 percent is not enough, the FCU will be incapable of restoring the lost compressor speed and it will remain below the 70 percent target. In fact, it is likely to keep decreasing almost without end, since each bit of slowing down leads to less available power. In extreme cases, the Low Idle ITT limit will be exceeded.

If the pilot notices the decreasing N1 speed and decides to remedy the situation by pushing the power lever forward to accelerate the engine and to get more power with which to run the generator, he finds that it is impossible – the engine will not accelerate. You see, if there is insufficient power to even maintain the selected High Idle, N1, there certainly is a lack of the additional power needed to spin the shaft even faster.

Sadly, this is usually a case of needing to see it to believe it. Although you have read this article, to remember to do what I am about to advise before you see the evidence of the bog-down, is the exception to the rule. And if serious bog-down is observed – and since advancing the power lever won’t work, it’s too late – the only solution is to turn off the operating generator and finish the second start as a battery start only … with the associated higher-than-desirable ITT peak, but nevertheless a peak well within limits.

So what’s the advice? I know that’s what you are asking about now. Here it is: Before initiating the second start, take the power lever and push it up until you see between 75 and 80 percent N1.

Noisier? Yes. More fuel consumption? Yes. More prop wash blowing that Bonanza tied down behind you? Yes. But now there is enough power to handle the electrical demand without excessive bog-down. Oh sure, the N1 may fall back one or two percent before the FCU catches it, but the ITT rise will be nearly insignificant compared to rolling back well under 70 percent.

Here’s the bottom line: Consider High Idle to be the minimum speed you want when conducting a generator-assisted start. In those situations, when you know that power will be significantly down – in round numbers, above a Density Altitude of 5,000 feet – then use the power lever to set the operating engine speed up to 75 percent or a bit more. You will be amazed at how much cooler the operating engine remains!

Author: Tom Clements, King Air Academy

About Tom - Tom Clements has been flying and instructing in King Airs for over 46 years, and is the author of “The King Air Book.” Tom is actively mentoring the instructors at King Air Academy in Phoenix.

Banner image by Calvin Pinder Jr., check his King Air photo spotlight gallery here