Behold the humble aviation spark plug. It's not much to look at, and is hidden from view when at work within a piston engine. Yet when spark plugs start to malfunction, the result can be rough engine performance at best, and loss of power and life at worse. This is why aircraft technicians and pilots alike take spark plug care and maintenance extremely seriously.
So how does one do this? To find out, AMT spoke to manufacturers and technicians alike, to reveal the proper care and maintenance of aviation spark plugs.
A quick refresher
To understand how best to maintain spark plugs, one has to remember how they work. The spark plug's job is to ignite aerosolized fuel within the engine cylinder, by sending a high voltage spark across the gap between its core and ground electrodes. The gap distance between these sides is absolutely critical; too narrow, and the spark will discharge before it has built up to the correct voltage. Too far apart, and the voltage will have to exceed the desired amount to overcome resistance and jump the gap. In turn, the buildup of this excess voltage can damage the aircraft's ignition system.
Every time a spark plug fires, a small amount of metal is ionized on its electrodes. This is why spark plug gaps grow wider over time: With each spark, a trace amount of metal is consumed. Eventually, this consumption erodes the electrodes to the point where the plug must be replaced.
As for working conditions? "Spark plugs operate in a very hostile environment," says Tim Hansen, chief inspector at Penn Yan Aero in Penn Yan, NY. "In most instances the tip temperature of the plug operates at around 1,200 F." Add the fact that aircraft fuel contains lead, a substance that plays havoc with spark plug electrodes, and one can understand just how tough a spark plug's life can be. This is why spark plug manufacturers Champion Aerospace and Unison Industries recommend that spark plugs be serviced every 100 hours.
Fundamentally, there are two types of spark plugs. The first is the common ‘massive electrode' spark plug, with a typical life span of 300 to 500 hours. "Massive electrodes account for about 90 percent of all aviation spark plug sales," says Steve Staudt, Champion Aerospace's regional manager.
The second and more expensive option is the "Fine Wire" or "Iridium" spark plug, which has thin lengths of Iridium wire welded to the electrode tips. "Iridium is the hardest wire available, which is why it is extremely expensive," says Fred Sontag, Unison's distribution manager. "Because it is so hard, it erodes three times slower than conventional massive electrodes."
"The most important indicator for spark plug wear is the electrodes," says Hansen. "The threads, insulator, and case corrosion should also be inspected each time the spark plugs are serviced. A good rule of thumb is if the ground electrode is worn 50 percent or the center electrode is shaped like a football, the plug probably should be replaced."
Worn electrodes are just one element that technicians need to deal with during a spark plug's 100-hour checkup. They also should look for carbon deposits that can cause reduced sparking (due to increased resistance), plus contamination from leaded fuels. Unfortunately, lead loves to adhere to electrodes, ruining their rated resistance and subsequent performance. Other troubles to look for are bent electrodes (due to foreign objects getting into the cylinders), damaged ceramic insulators (due to excessive spark voltage), and oil fouling.
"Spark plugs should be rotated periodically to even out the electrode wear," Hansen notes. "Magnetos fire the spark plugs with negative and positive charges. The negative charge will wear the center electrode and a positive charge will wear the ground electrode. Just like rotating your car's tires, rotating your spark plugs will make them last longer by using up both electrodes."
Finally, the gap between the electrodes needs to be checked with a precise, industry-quality gapping tool. Champion specifies a gap of 16/1000th of an inch, while Unison recommends anywhere from 16/1000th to 21/1000th of an inch. "Personally, I prefer to gap my plugs down to 16/1000th," Sontag says. "This leaves room for the plugs to erode up to 21/1000th of an inch, without compromising their sparking ability."
There are many, many things that people do wrong when maintaining or replacing spark plugs. Of these, dropping them is the most obvious, and the most final. "Even a drop of a few inches onto a hard surface can damage the fragile ceramic insulator of the plug," says Hansen. "The plug may not work at all or a small hairline crack may fire OK when installed. It can take a few minutes to many hours for the ceramic to fail, but eventually it will. So remember the saying, if you drop a plug, drop it twice; the second time being in the nearest garbage can."
The next common mistake involves regapping spark plugs: There are so many ways to do it wrong! For instance, regapping aviation spark plugs with their parallel ground electrodes can't be done properly with a cheap automotive gapping device.
"Those over-the-counter gapping tools don't apply pressure to the electrodes in the right way," Staudt says. "In fact, that can result in the electrodes being set at slight angles to each other. What this means is that the spark will naturally leap the gap at the closest points, causing extensive metal erosion at that location, and thus shortening the plug's life."
"Some do-it-yourselfers don't even use the right tools," says Sontag. "Instead, they try to close the gap using needle-nose pliers! Needless to say, this can be very bad for the spark plug. The amount of pressure applied by needle-noses can easily crack the ceramic insulator. Meanwhile, you are very unlikely to get an even, precise gap."
The third common error is improper cleaning. "Using glass bead cleaners is fine for spark plugs, but you have to make sure that the beads are the right size," Staudt says. "Too large, and they won't do the job. Too small, and they can get lodged into the seams of the spark plug." In the same vein, lead deposits need to be removed using the right tool, as opposed to an ice pick or some other available pointed object.
A fourth common error is not replacing the gasket, notes Hansen. "When the gasket is new it is soft and conforms to any irregularities in the spark plug and cylinder. After they are run they cannot conform as well and they can have small leaks, and may not transfer heat to the cylinder as well.
"We also see plugs that have had the harness attach nut overtorqued and deformed, and the spark plug wire kinked by not using two wrenches to tighten them," he adds. "One wrench should be used to tighten the nut and another wrench should be used to hold the wire."
And, all of the experts caution against using too much lubricant when reseating spark plugs. "You need to use this lubricant very sparingly," says Alex Fiels, Champion Aerospace's product manager for airframe and general aviation. "Use too much, and it will get onto the electrodes and foul them."
Spark plugs can be the margin between life and death in an aircraft. Proper maintenance is vital, and anyone handling them needs to treat them with the utmost care and attentiveness possible.