Starter Time Limits

You memorized this during your Initial King Air training program, right? “Use of the starter is limited

to 40 seconds ON, 60 seconds OFF, 40 seconds ON, 60 seconds OFF, 40 seconds ON, then 30 minutes OFF.” For the 300-series of King Airs, the limits are more restrictive: “Use of the starter is limited to 30 seconds ON, 5 minutes OFF, 30 seconds ON, 5 minutes OFF, 30 seconds ON, then 30 minutes OFF.” In addition to merely reminding you of these important limits, the intent of this article is to explain the reasoning behind the limits and to emphasize the real-world method of observance.

Engine starting subjects the electrical system to the highest current flow it ever experiences during normal operation. A peak flow in excess of 800 amperes is typically experienced in the fraction of a second that the starter motor receives power before it begins to rotate. The current demand rapidly decreases as the starter and engine increase speed, but when stabilized N1 or Ng speed is attained, usually there is still over 200 amps flowing to the starter motor.

Current flow generates heat energy in a squared relationship. That means that when current is tripled, heat production goes up nine-fold. Comparing the heating effect of 800 amps to that of 200 amps, this four-fold reduction reduces heat generation by a factor of sixteen! It is quite apparent then that the starter is building up a lot of thermal energy when it is being used, especially in the early stages of a start. Does this mean it is getting hot? Usually, yes, but not always.

The other factor that determines how hot the starter becomes is its cooling mechanism … how, if at all, the thermal energy is being removed. Although it would be ridiculous to submerge the starter in a vat of ice water in the cowling, it could stay quite cool throughout the starting process if such a setup were available. Since it is not available, however, the only mechanism that is designed to help remove the thermal energy buildup is a fan that pushes air across the starter. This four-blade fan is attached to the rear of the starter’s armature shaft and rotates with it. Hence, the faster the starter turns, the more thermal energy dissipation capacity it has. That explains why when this starter device is being used as a generator, limits exist requiring us to use higher N1 speeds when experiencing higher electrical loads.

During the initial stages of engine starting, when the starter is providing all of the power that is providing compressor rotation – not yet helped by any exhaust gas flow – we are faced with a double-whammy: The most heat production and the worst heat dissipation. The heat production comes from the high current flow at this time and the poor heat dissipation comes from the relatively low N1 – and cooling fan – speed.

The bottom line? We can burn up the starter by getting it much too hot if we do not abide by the starter time limits. To state the reasoning very simply, “Don’t use the starter for such a long time that it gets too hot and also give it enough time at rest to dissipate the heat before using it again.”

The 300-series engines are physically larger than those on previous King Airs and their starters generate more heat while spinning the bigger compressors. This explains why their limits are so much more conservative, requiring both shorter working times and longer cooling off times.

The starter will get hotter during a particular period of use when the ramp temperature is over 100° Fahrenheit on an August afternoon as compared to its operating cooler on a January ramp with the temperatures below zero. I have been told that the actual starter tests that Beech conducted aimed for the worst possible set of extenuating circumstances, hot OATs and higher density altitudes. Furthermore, they did not actually start the engine but rather merely motored it with the starter while monitoring Starter/Generator temperatures.

More than once I have been asked about the typically-sluggish engine starting acceleration experienced on cold days, and the question involves the necessary exceedance of a starter time limit. Namely, starting the stopwatch when the starter switch was activated and stopping the timer when the starter switch was turned off at Low Idle (or above) speed, led to the realization that more than 40 seconds had elapsed. Uh-oh!

Don’t lose any sleep over this one. You see, once the exhaust gases start to flow, the workload of the starter is decreasing in synchronization with the workload of the exhaust gases increasing. At some point – usually in the mid-40 percent N1 range – the engine becomes self-sustaining due to its own gas flow. The starter is no longer expending power at all, but merely going along for the ride. The exact point at which the starter’s heat generation becomes so small as to be immaterial is impossible to locate due to the variety of conditions that may be experienced on a particular start, but it is my strong belief that all concerns about starter time limits are rendered null and void after we see N1 leave its stabilized starter-only speed and continue its acceleration, now being aided by exhaust flow.

I have also heard worries along this line: “My mechanic is not being careful enough about the starter time limits. I’ve seen him start the engine, run it up to make a check, shut it down, make an adjustment, and then do the whole thing again and again. I know he’s not letting the 30-minute cooling time go by, even after the third or fourth start!”

Again, no worries. With the engine actually running, now the starter has been turned off and the generator has been switched on. Yes, the generator also develops heat – the more so as load increases – but not as much as the starter. More significantly, now we have enough compressor speed that the cooling fan is doing good work in helping dissipate the thermal energy. If you’re still worried, just ask the mechanic – after making his engine adjustment and before reclosing the cowling – to simply place his hand on the Starter/Generator to gauge its temperature. If he comes back with blisters on his palm – which he won’t! – then have him wait a little longer to let that sucker cool.

A situation in which starter time limits can indeed be exceeded due to maintenance procedures is while doing motoring compressor or turbine washes and/or rinses. Fuel is never introduced so all rotational energy is provided by the starter. A competent mechanic will know this already, but you may want to remind a new helper that he cannot run the starter too long, nor wait too short of a time between uses, when doing this important task.

Finally, what about us, the pilots? When is it most probable that we may overheat and ruin a starter? This most likely may happen when faced with the aftermath of an unsuccessful starting attempt.

As we know, three ingredients are needed for the fire to begin in the combustion chamber: air, fuel, and ignition. In the early days of King Airs, the only one of these three that the pilot could truly verify was happening was air, as shown by the compressor spinning up to a typical stabilized speed well above the minimum 12 percent required. As for fuel? In those days, the fuel flow gauges were AC-powered and did not operate during the start since no inverter was yet on. Ignition? For all King Airs, the ignition annunciator does not verify actual ignitor operation but instead just shows that the appropriate relay has activated to send power to the system. Well, the glow plug-type of ignitors that early King Airs used made no sound, unlike the rather loud snapping that can be heard from the newer spark-type of ignitor. Thus, if a start was unsuccessful with no light-off in an earlier King Air, the pilot really could not tell whether the problem was ignition or fuel related.

This has not been the case for over three decades. Now in most cases, except on a very noisy ramp, the sparking sound of the ignitors may be heard to support the ignition annunciator. The fuel flow gauges are now DC-powered, meaning they are working during the start and the pilot can observe the expected fuel flow when the condition lever is advanced forward from Cut-Off.

What I am saying is that there is less-than-stellar airmanship being demonstrated if the pilot of a more modern King Air does not recognize that something is amiss with ignition or fuel quite early in the starting attempt and does not immediately terminate the attempt to analyze and remedy the cause. For the pilot to actually run the condition lever forward and then wait the full ten seconds before deciding to terminate the attempt should not be nearly as likely now as it was back in the ‘60s and ‘70s, during the days of AC-powered fuel flow gauges and glow-plug ignitors.

Nevertheless, let’s say we have attempted a start, it was unsuccessful with no light off, and we’re going to try it again. Perhaps we found the ignitor circuit breakers still pulled, an action mechanics have been known to take to prevent accidental engine starts in the shop. So we have reset them and suspect that the next attempt will be successful. (Drat! We should have done a more thorough cockpit prestart check and caught this before the starting attempt, eh?!)

Wait! We cannot do another start attempt yet. What about the unburned fuel that has likely accumulated in the engine during that first attempt? We need to conduct an engine clearing procedure first, to avoid dramatic torching of fire out the exhaust stacks on the next start. This involves using the “Starter Only,” bottom, position of the “Ignition and Engine Start” switch. We keep the condition lever in Cut-Off and then depress and keep holding the Starter Only switch position for a time as long as the starter time limit, although anything more than 20 seconds is probably sufficient to clear most of the residual fuel that did not already exit the engine via the forward and aft case drains. By the way, these case drains dump right onto the ramp. They aren’t involved with any EPA-mandated collector/purge system. Also, if the engine did in fact receive fuel during the first start attempt, there will be a distinct kerosene odor prevalent outside near the engine and the exhaust stacks will likely be damp with kerosene residue. If none of these signs are present, it probably was not an ignition problem after all, but a lack-of-fuel problem.

Since the clearing procedure used some or all of our second allowed starter usage time, now we need to wait for another mandated cooling period, then try again, presuming we have a good handle on knowing why the first attempt was unsuccessful and with the correct solution now having been applied.

You see, therefore, that we cannot do three starting attempts and comply with the starter time limits. No, it’s two attempts with an engine clearing procedure in the middle. If there’s no luck on that second attempt, you’ll need the 30-minute cooling period anyway while you get on the phone and find a mechanic, or perhaps replace the ignitors yourself. (You do carry a spare ignitor or two with you, and the simple tools needed, right? Yes, this is something a pilot may legally do.)

In some future articles I will be discussing other important airplane limitations to remind you of their importance and to present some background information to aid in your understanding. I hope you’ll find it worthwhile. Meanwhile, remember to send any ques-tions or topics that you’d like me to address to Kim Blonigen, our editor.