Airplane engines suck. Pistons move up and down (or in horizontally opposed engines, in and out) and create tremendous suction that draws air in through the induction system. Although some engines benefit somewhat from “ram air” induction, and others have turbochargers to boost the airflow, all depend primarily on this internal suction to draw air in for combustion.
In a perfect world air would flow unimpeded into the engine for maximum manifold pressure and therefore the potential for maximum power. In reality we prefer to keep bugs, dust and other contaminants out of the engine’s innards to protect it from friction and damage. Hence aircraft engines have some sort of inlet air filter, often soaked in lubricants to collect dust and further prevent internal engine contamination.
But what happens if the filter collects too much dust? What if you fly through a swarm of locusts that substantially or completely block the filter? What occurs if you encounter airframe ice and the filter becomes obstructed?
Visualize pistons moving in and out in their cylinders. Slow down the image in your mind so you can see each cylinder moving slowly in and out, each intake and exhaust valve opening and closing individually to permit the passage of air. This slowly-turning “air pump” draws air in, through and out in a fairly smooth motion, with some internal turbulence and air exchange between individual cylinders’ intakes, but without interruption when considered as a whole.
Imagine now the inlet air filter begins to get clogged. The engine’s speed is not instantaneously affected, but as the engine tries to draw in the same amount of air through an effectively smaller inlet the suction in the intake manifold increases. With this increase in flow the intake pressure begins to drop; the engine starts to loose power. If the inlet filter is significantly blocked the engine will starve for air, drawing the last from the intake manifold with rapidly decreasing induction air pressure.
Fuel injected engines have an “alternate air door” to permit continued engine operation with a blocked inlet filter. A rectangular cut in the induction manifold upstream of the throttle plate, the alternate air door is held closed by a spring. The force of this spring, coupled with air pressure through the “normal” induction inlet, keeps the alternate air door closed in routine operation. But if the inlet filter becomes obstructed, the almost instantaneous drop in induction air pressure (spin your slowed-down mental image of the engine up to 2300 or more revolutions per minute) overpowers the alternate air door spring; the door is sucked open to admit air from the engine compartment. The engine may “burp,” but it continues to run.
OPERATION ON ALTERNATE AIR
Flying a normally aspirated airplane you may not even notice if you go to alternate air. Air pressure in the engine compartment is lower than ambient, but in most cases the resulting manifold pressure drops by only about half an inch at full throttle. If you look closely you’ll see a drop in cylinder head and exhaust gas temperatures when cruising at rich of peak settings, hotter if starting from lean of peak. Manifold pressure will rise and fall normally, albeit with this slightly lower maximum value, when you change altitude.
Turbocharged engines suffer the most from operation on the alternate air system. Manifold pressure can drop as much as six inches when flying above about 15,000 feet, and critical altitude drops to 12,000 to 13,000 feet. Inside Information: Cowling airflow creates a lower air pressure in this area and the turbocharger has to work very hard to make up the loss.
ALTERNATE AIR HANDLE
Many fuel-injected airplanes have a T-handle connected by a cable to a metal “finger” mounted alongside the alternate air door, outside of the induction manifold. Pull the cockpit control and this “finger” pivots to push inward on the door from the outside. Release the handle and the “finger” springs back outward; if the inlet air filter is unobstructed the spring and intake air flow will blow the door closed again, but if the air filter is blocked suction created by the engine will hold the alternate air door open.
Important: Some pilots new to these airplanes confuse this with the alternate static air source on the left sidewall near the pilot’s knee. That, of course, is an entirely different system, used to provide an air reference to flight instruments.
The function of the manual alternate air control is to force open a door that is frozen closed, or stuck by moisture or dirt.
The hazard of operating on alternate air, of course, is that induction air is not filtered and contaminants are free to be sucked into the engine. If you have reason to believe you’re running with the alternate air door open and the reason doesn’t clear up by itself (for instance, ice that melts off), clean or replace your air filter as soon as possible and certainly before flying in dusty or bug-infested areas. Conversely, moist environments can cause the alternate air door to stick closed as dirt or grime forms a “glue” in the induction manifold.
Maintenance Tip: It’s a good idea once a month or so to check that the alternate air door opens.
How To: With the engine shut down, push the door open with a metal pointer or some other object that will not break off if it gets closed in the air door. Airplane owners might watch while someone in the cockpit pulls the alternate air control to verify it really pushes the door open, and that the “finger” pulls out of the way when the cockpit handle is released.
LOSS OF POWER/ENGINE FAILURE IN FLIGHT
The real purpose of the alternate air door is to prevent total power loss. In most fuel-injected airplanes the alternate air door should open automatically when needed. That’s why activating the manual control is just about the last item on “Engine Failure” checklists — it’s a “last ditch effort” to get an engine restarted if all other troubleshooting steps fail. The alternate air system is simple, reliable and easily forgotten; just remember to keep your air filter clean, check that your alternate air door doesn’t stick closed, and understand the indications and implications of running on alternate induction air.
WHAT ABOUT CARBURETED ENGINES?
Carbureted engines do not have an alternate air door. Carburetion adds the possibility of “carb ice” even in clear skies in humid air. These airplanes have a carburetor heat knob in the cockpit; pull the knob and exhaust-heated air is ducted through the carburetor, hopefully melting the ice. Heated air’s lower density results in a loss of power. Carb heat takes the place of an alternate air system as a backup to the filter in carbureted aircraft engines, but in most installations won’t keep the engine running if the inlet air filter is blocked with ice, dirt or insects.
The Bottom Line: Read the POH for the airplane you’re flying, and understand how its systems are alike and dissimilar from those in other airplanes. Apply your knowledge to normal and emergency checklists.