Air Filters and Fires (and Volcanoes) – A Bad Combination

I heard about this one from a wanna-be pilot in the California area, which if you’ve been following the news, seems to be just about completely on fire these days, and I don’t mean in a good way!

FIRES TEND TO CROP UP FROM TIME TO TIME, AS DO OTHER NATURAL PHENOMENON SUCH AS THE OCCASIONAL VOLCANO. When these denizens turn up, our first thoughts go to the brave firemen and women who get out there and work to get the fires put out. Next we pray for the homeowners who happen to have their homes in the “line of fire,” so to speak. What we often don’t think about, but need to, is what effect fire has on the air we fly through, other than adding heat and smoke.

SMOKE IS MADE OF PARTICULATES. If you were to pick up a 100 micron air filter, and connect it to a proper sample pump — the pump will suck air through the filter, and turn it black with particulates in no time when exposed to a fire environment. The particulates coming from the fire are largely the soot or cinders that are left over from the burning process. These tiny particles are generally soft, but can at times be hard. In the case of a volcano, the dust and debris ejected into the air can be both toxic and abrasive. Regardless of which natural disaster you are flying near, you had better be thinking about what part of your airplane needs clean air to work — YOUR ENGINE.

No matter whether your engine is TURBINE, JET OR PISTON, particulates are a BAD THING. For jets and turbines, the particulates get drawn into the intake, where the spinning blades of the turbines get a chance to meet the particulates, if you’ll excuse the expression, head on. These tiny impacts don’t seem to be very important, but when heavy concentrations of volcanic debris or in some extreme cases, smoke particulates are in the air, they can cause jet engines to choke and fail.

DON’T BELIEVE IT? Tell it to the airline captain who was flying his commercial jet in the vicinity to Mount St. Helens following the big eruption. Both engines lost power, and the captain and his crew had to struggle to get the engines started back up. After that close encounter, more NOTAMS went up, urging pilots to stay clear of the plume from Mt. St. Helens. See this link: Volcanic Ash Avoidance

PISTON ENGINES USE A LOT OF AIR. All of that air goes through an AIR FILTER, which is typically on the front of the airplane. Air filters are rated for flight in clean air. This means that a typical air filter is good for between 50 and 100 hours, depending on the type of filter and the manufacturer, when the airplane is operated in clean air. When the airplane is operated around fires or volcanoes, all bets are off on how long the filter will last.

YOU MAY WONDER WHY THAT IS IMPORTANT. After all, most airplane engines have a bypass around the filter, so that the engine can get air even if the filter plugs up. The question you need to be asking is “How long will my engine continue to operate if that bypass is open when the air is full of particulates?

THINK OF IT THIS WAY: your airplane’s piston engine is turning at around 2300 rpm, give or take a couple of hundred. You are flying in the vicinity of an active volcano, or a very active fire. Your air filter quickly plugs, and the bypass ports open, allowing your engine to breathe in these fine, abrasive particulates, and run them around inside the cylinders for a while.

IF YOU ARE LUCKY, THE DAMAGE WILL TAKE A LONG TIME TO APPEAR. If you aren’t lucky (and when the worst thing can go wrong, we all know that our luck is sitting back at the airport bar, sipping on a cool one), your engine may start to sputter or cough. That means you have to be wondering how many more minutes you have as the pilot of an airplane … before you become the pilot of a very lousy glider!

VOLCANO DEBRIS IS DANGEROUS. It will bring down a piston or a jet plane with the same ease. FIRE DEBRIS IS LESS DANGEROUS, BUT IT IS STILL DANGEROUS. If you are flying a piston plane in and around fire areas — in the case of the recent California fires that could be a vast area — you need to keep your eyes on your air filter. If you see distress, or note smoky streaks around your air filter bypass ports, CHANGE THE FILTER. The price of the average air filter is insignificant when you compare it to the cost of an off-field landing both in terms of damage to the aircraft and your confidence. Do the right thing, and you’ll avoid getting in to trouble. Try to stay clear of fire and volcano areas whenever it is possible.