When you hit the Start/Ignition switch and the engine spools up but fails to light off, what do you do? Is it a fuel problem, an ignition problem or both? Obviously if your engine gauges indicate fuel is flowing, then you’ve got a problem somewhere in the ignition system. I’d have a tidy sum in the bank if I got a dollar for every time someone suspected the ignition box as the reason for ignition failure. I don’t know where it got such a bad rap. In my experience the ignition box is the least culpable of all the components in the ignition system. But I’m getting ahead of myself.
Some would argue that if you don’t hear the igniters snapping, you know it’s an ignition problem. Unfortunately, the noise of other aircraft running nearby or taxiing to and fro makes the snapping hard to hear. Chances are (if Murphy’s law has anything to do with it, and it usually does), you are at an unfamiliar airport, far from your trusted mechanic or a shop that knows King Airs. In the above scenario, you can try the auto-ignition switch to see if you get spark. If you do, the engine starts up and you now know that there is an issue with your Start/Ignition switch or its related relays. If there is still no spark with the auto-ignition switch, then there is an ignition problem at the engine.
Be sure to check your annunciator panel. Go back to the Start/Ignition switch. (Don’t forget to clear the engine of fuel if you’ve already attempted a start.) Turn it on and see if you have an ignition light. It could be yellow or green, depending on the model of your King Air. This light is wired directly to the positive lead at the cannon plug on the ignition box. If your annunciator panel shows the ignition light in start-ignition mode or auto-ignition mode, then you know power is going all the way to the ignition box.
So now what? Your suspects are the ignition box, the igniter leads, and/or the igniter plugs. Believe it or not, most of the time it’s going to be the igniters. They get checked at every phase, but if they are within their designated wear limit, they are not replaced. Igniters are expensive and you want to exhaust their useful life before putting in new ones.
Igniters wear down to a point where the gap is too large and the spark fails to jump across. There is a maximum allowable diameter for the center hole surrounding the electrode. In the photo (right) comparing a new igniter with a worn-out one, you can see the center hole is much larger in the old igniter – the gap is too wide. Additionally, the electrode in the old plug is severely worn.
You might be wondering, since there are two igniters per engine, why would both igniters go bad? I suspect they go out one at a time. Let’s say the igniter box is a tad low on amperage – it’s within limits and doing its job, but the spark doesn’t travel quite as far. As the igniters wear down, the gap widens and one plug fails. At this point maybe the starts are a slightly slower or a tiny bit hotter, but these differences are negligible. So many times, gradual degradation in performance escapes our attention. It’s just a matter of time until the other plug fails and you have a no-start situation. That gets your attention.
Something that accelerates igniter wear is leaving auto-ignition on all the time. The checklists in the POH for pre-takeoff and pre-landing specify when the auto-ignition switch is to be turned on as a safety measure in the event of engine flameout. I’ve met many a King Air pilot that worries they will forget to turn their auto-ignition on before takeoff and before landing, so they turn it on early and leave it on. They know that the auto-ignition system goes off automatically once 400 foot-pounds of torque is reached after takeoff, and likewise, it comes back on when the torque falls below that mark. At shutdown they turn everything off.
There is nothing wrong with doing this, particularly from a safety point of view. The downside to this procedure is that the igniters on each engine are furiously snapping away throughout the whole pre-takeoff routine, the taxi out, the taxi in and the pre-shutdown routine. This is a lot of wear and tear on the igniters.
When I had my shop, I had some customers that could not get 300 hours out of an igniter, and I had others that would get 600-700 hours. I have no problem with anyone that keeps auto-ignition engaged in this manner. I am not being critical and I don’t want to mess with anyone’s routine. All igniters wear out eventually, so it never hurts to have a spare one stashed in your aircraft. When it comes to ignition failure, igniter plugs are the number-one cause in my book.
Worn Igniter Leads
Igniter leads are disconnected at each Phase Inspection in order to inspect the igniter plugs. These leads have an insulator on the end that goes into the igniter plug. The insulator has a tendency to stick to the plug. When your mechanic removes the nut that secures the lead, he or she often must tug and twist the lead to disengage it. The insulator eventually breaks and this allows the high voltage to jump to ground instead of going into the plug and creating spark. Just like your ignitors, you can have one failed lead while the other keeps working. Again, you are down to one igniter on that engine and you might not notice much difference in your starts. But when that second lead fails or its plug wears out, you’ll have a no-start situation.
If the plugs and leads check good, then the igniter box must be the culprit. Remember that your ignition light on the annunciator panel indicates you have power going to the box. Once you exchange that igniter box you should be good to go. Igniter boxes do not fail often. Many vendors don’t keep them on the shelf ready to go; they wait until you need one before they overhaul or repair for exchange. If you order an exchange box and it turns out you don’t need it, you might be stuck with it. So, make darn sure the igniter leads and plugs check out before ordering an exchange ignition box. Years ago, I had a spare igniter box that I kept around for troubleshooting. One of my customers was stranded in a far-flung location with an engine that wouldn’t start. Over the phone, I ascertained the igniters and leads were OK, so I overnighted my spare box to them and it did the trick. Once home, we got a proper exchange box installed, their core unit went to the vendor, and my spare came back to me. That didn’t happen often, but that spare box sure came in handy from time to time.
Here’s to many hassle-free hours of King Air flying. Don’t forget to stash a spare igniter somewhere … and remember where you put it!