That Sucking Sound — Engine Induction Woes

Did you ever take a good look at your aircraft’s induction system? Constructed of hoses, clamps, and even tubing, the induction system conveys air — or an air fuel mixture — to each cylinder. The cylinder draws in the charge, compresses it and, through the ignition system, turns it into an explosion, which drives your propeller through the air.

THE PREFLIGHT WAS BORING AND UNEVENTFUL for our Cessna 170 owner. This particular 170 had been his for years, and knew it inside and out. He carefully went over each item on the checklist, and obtained a satisfactory result. With the plane fueled, and the preflight completed, he was ready to go.

After obtaining his clearance, he taxied out to the active runway for departure. The runup was uneventful, and the mags checked out fine. Full power was applied, and the engine responded, pulling the plane down the runway until it became airborne and started to climb…

WHICH IS EXACTLY WHEN THE ENGINE STARTED TO SPUTTER AND SHAKE. Our pilot was a couple hundred feet in the air, and around a thousand feet away from the runway when the engine decided to pitch a fit. Well, a fit might not describe the situation well enough — the engine wasn’t making enough power to keep the plane climbing.

Our pilot, who had experience in everything from trainers through jets at the time, remained calm and referenced his emergency checklist. He tried the fuel selector with no effect, he tried the primer, he tried full throttle, he tried the carb heat.

NOTHING HE TRIED WORKED, and the corn was getting closer. Our pilot continued to follow his emergency procedures, and started to set up for an emergency landing, even as he struggled to nurse the plane back to the airport. It became clear he wasn’t going to make it, so he started to reach down to pull back the mixture to stop the engine and when he did, the engine power increased!

WHEN YOU PULL THE MIXTURE, YOU EXPECT THE ENGINE TO DIE, RIGHT? In this case, our pilot was slowly pulling the mixture out. He tweaked it back and forth, and watched his engine power rise. With this extra power, he was able to nurse the plane back to the airport, and make a safe — but shaken — landing.

THE POST-MORTEM INSPECTION WAS INTERESTING. Our pilot suspected a problem with the air filter, but inspection of the filter showed that it was clean and free of debris. The pilot then had the filter removed and found the problem — the induction hose between the engine and the air filter had started to fail. The inner lining of the hose had actually started to get sucked into the engine, somewhat like pulling a sock through backwards.

Since it was being pulled in backwards, the normal size of the hose was reduced by more than a factor of two — the hole that remained was roughly an inch wide. This means that the engine couldn’t get enough air to run smoothly — it had excessive fuel in the mixture — exactly the opposite of what normally happens!

OUR PILOT FOLLOWED THE CHECKLIST, AND WON. Had he gone in with the power he was making, he likely would have caused significant damage to the aircraft, engine and prop. But his decision to shut down the engine with the mixture was to preserve the engine and the prop, so he got lucky.

WHEN ALL ELSE FAILS, TRY SOMETHING ELSE. Just because it isn’t in the emergency checklist doesn’t mean that it can’t happen. If you have exhausted all the options, there is no harm in trying whatever else you may have at your fingertips. This includes making adjustments to the mixture in an effort to find a workable air/fuel mixture to keep your plane flying.

BOTTOM LINE: Watch those induction system hoses. If a hose shows signs of fraying, take action to have it replaced immediately. Don’t get sucked in by a bad hose — the effects of such an event on your engine — and your heart — aren’t recommended!