Diagnosing an engine failure by sound

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Dad began his aviation career in the US Air Force, where he was trained as an aircraft mechanic and was assigned to be the mechanic responsible for a particular C-47 and a particular C-45. In other words, he was a crew chief.

Dad eventually went to Eastern Air Lines as a professional flight engineer, trained to operate the engines and aircraft systems of all of Eastern Air Lines’ four-engine airliners. They included the DC-6, DC-7B, L-749 Constellation, L-1049 Constellation, and L-1049 Super C and Super G Constellations. Dad was also flight engineer on the Lockheed L-188 Electra with its four turboprop Allison 501-D13 engines. The DC-6 used the Pratt & Whitney R-2800 radial engine. The DC-7B and later Constellation variants like the Super C and Super G used the Wright R-3350 engines but added power recovery turbines, or PRTs. These added about 550 maximum takeoff horsepower at sea level on a standard temperature and pressure day to the crankshaft through hydromatic transmission technology, or hydraulic pressure torque directly added to the crankshaft. Basic Constellations used the plain Wright 3350 without PRTs.

Large radial engines like the Wright 3350 use all kinds of tricks to boost power output.

We lived close to a metropolitan airport. It was 1977, and I had just started as an aircraft parts specialist on Labor Day of 1977. Where I worked at that nearby airport for exactly three years, I learned all I could about airplanes, aircraft maintenance, and added pilot ratings and certificates at my company’s flight school.

I learned how to overhaul a VW engine by watching and helping Dad on several occasions. I recall asking him how I could tell if an airplane engine had a supercharger or turbocharger one day. A Piper PA-31 Navajo was flying overhead, and we were outside. I knew it had a turbocharged type of engine, spinning a turbine on a shaft that also spun a compressor to increase air pressure coming into an engine.

Dad asked me, “Do you hear a high-pitched whine?”

I answered, ”I can hear it!”

“That’s a supercharger or turbocharger,” Dad replied.

In the summer of 1987, I watched a DC-3 take off from a paved runway south of Atlanta, where I was working to add fuel and oil to airplanes. I heard an inrush of air as a “waa – waa” sound. This was recurring at a noticeable but slowing rate. I interpreted this slowing repetition as an engine inlet manifold splitting open behind a supercharger. A supercharger or turbocharger spins a compressor to pack dense air into an intake air chamber called an intake manifold. An intake manifold distributes air into every cylinder evenly, airflow and pressure being controlled by a supercharger or turbocharger and waste gate, and a throttle. The slowing repeat of inrushing compressed air indicated a widening split of the intake manifold.

As the intake manifold split open wider and wider, its rate of repeating the sound would slow down somewhat until the intake manifold completely split open.

The splitting intake manifold, I felt, would decrease intake manifold pressure. A non-flying pilot would be increasing throttle position with reference to a desired constant intake pressure, and try to stabilize the decreasing intake manifold pressure. But intake manifold pressure would continue to decrease as the intake air box split wider and wider.

The non-flying pilot would continue to advance the throttle and maintain intake pressures of that engine as the intake manifold continued to split open. This happens because a splitting intake manifold or splitting intake air box will release air pressures increased by supercharging or turbocharging as it continues to split open. The intake manifold pressure would continuously decrease until the manifold completely split open, so the second and non-flying pilot would keep increasing throttle to make up for a splitting intake air box behind the supercharger compressor.

In the process, I felt it was reasonable to assume that perhaps one or two cylinders would become “overboosted” to the point of blowing a jug, or pushing a cylinder head up and off of its cylinder base. When this happens, a piston can be seen moving up and down with no cylinder walls around it.

What causes a cylinder head to blow off of a supercharged radial engine?

Each cylinder head of a radial engine is actually heated up to expand it and the cylinder head or “jug” is hydraulically pushed onto its cylinder’s base with what I understand is thousands of pounds of force, perhaps even well over ten thousand pounds of force. After being heated to expand it, the cylinder head of a radial engine is hydraulically pressed onto its cylinder base. Then it is cooled to contract onto its cylinder base. If I understand correctly, a few low-compression radial engines do allow a mechanic to screw cylinder heads onto their respective cylinder bases.

But if the incoming air pressure inside the cylinder is increased too much, pressures will literally blow the top off of a cylinder base. This was often called “blowing a jug” when radial engines were in wide use during World War II and during the heyday of piston-engine airliners.

But let’s remember that I was simply listening to a DC-3 take off with its two Pratt & Whitney R-1830 engines (R= Radial Engine or cylinders arranged in a circle on a radius, while 1830 = total approximate displacement of cylinders).

I am saying you can hear many sounds of a piston engine, including inrushing compressed air. I could also hear an intake manifold split open.

Well, I ran and told George (the aircraft mechanic) what I had heard. I predicted the aircraft would return very soon with one engine stopped and its propeller feathered. There would be no tendency for the feathered propeller to spin the engine, causing great drag and the need for more power. I mentioned the splitting intake manifold and predicted that one or two jugs would likely be blown.

DC-3 engine

Managing the engines on a DC-3 means understanding what all those parts do.

The aircraft immediately appeared over the trees, headed for the runway. One engine and its propeller were obviously not turning. George smiled: “I’ll come get you in the morning!”

The next morning George came and got me. He had apparently placed engine parts on a table.

“What’s that?” I asked him.

“You don’t know? Look at it!” George replied.

A long rectangular box lay on the table.

“It’s completely split!” I said. “Is that an intake manifold?”

George smiled and nodded.

I asked, “How many cylinders did it get?”

George smiled again as he held up one finger.

One jug had been blown!

I WAS RIGHT. And I was stunned that I was right!

Back during the 1970s and 1980s, we called Knowledge Tests “Written Tests.” First, I studied for the FEX written, a combination of Flight Engineer Basic and Boeing 727 Jet Flight Engineer. I scored 100% on the first try.

Then one day I went to Atlanta International Airport and asked a Zantop International (Cargo) Airlines pilot what I needed to go to work for Zantop. He said I needed the Commercial Pilot Certificate with an Instrument Rating and Multi Engine Rating, plus a Flight Engineer Basic Written Test and a Turboprop Written Test. I had everything but the required Flight Engineer Basic Written Test and Flight Engineer Turbopropeller Written Test.

I would soon pass the Flight Engineer Basic Knowledge Test and the Flight Engineer L-188 Turbopropeller Knowledge Test. I initially scored 96% on the L-188 Written, and then I mentally reviewed my test and realized that I had studied for a cargo version but tested for a passenger version. This meant that AUX VENT “ZERO POSITION” causes smoke to clear air in the cabin for passengers, but traps smoke in the cabin in order to extinguish a fire on board a cargo version. The FAA awarded me a 97% after I asked Dad if he felt I should ask for reconsideration. He said I should, so I did.

I would be hired as a Zantop flight engineer on the Lockheed L-188, qualifying as a pilot and not as an aircraft mechanic as Dad had done. Also, like my father, I would fly several of Eastern Air Lines’ former L-188 Electra turboprops a generation after my father had.

I would constantly ask aircraft mechanics questions in order to understand my aircraft better, and eventually move up to copilot and later captain. The knowledge I gained from asking questions was later very helpful.

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