This will (hopefully) be the first of a series of diaries about how a modern airliner functions. These diaries are meant to be informative and easy to understand for those with little or no technical knowledge.

First off - I'm not an aeronautical engineer. My degree was in electrical and computer engineering. I'm also not an aircraft mechanic. I'm just a guy who drives airplanes for a living and has a workingman's knowledge of them.

Pilots and aviation enthusiasts may find this a bit on the simplistic side. I'm writing these for those with an interest in how things work but little or no aviation background. That way if I get something wrong I can just say "I knew that. I was just trying to simplify it."

So if you've ever been sitting back there in seat 15C and wondered "what was that noise?" or "what's that thing moving out there on the wing?" then you might find these interesting.

I'm starting with the engines - because we're not going anywhere without them.

An airliner won't fly without engines, at least not for very long. In physics, you don't get something for nothing.

In simplest terms, in order to get something to move energy must be expended.

That energy is stored in our fuel tanks, and the engines convert the chemical energy locked up in the fuel into thrust (kinetic energy) to make the plane go forwards. Once we get the thing going forwards fast enough, the wings do their aerodynamic magic and we're flying.

At one time all aircraft were powered by piston engines, similar to the one in your car. Today piston engines or "recips" are found mostly in light aircraft.

A recip engine has pistons which travel up and down and go through 4 cycles. Intake, Compression, Combustion, Exhaust. Or as I like to call it "Suck, Squeeze, Bang and Blow".

We're going to suck in some air, mix something flammable with it, squeeze it tight, explode it, and push out what's left over. The force of the explosion drives the piston, which turns the engine, which turns (in an aircraft) a propeller. Simple enough.

Airliners switched to turbine engines starting in the 1950s. There are three main types of these: turbojets, turbofans and turboprops. I'll explain the differences later.

A turbine engine actually does the same 4 things, it just does them continuously. It's constantly sucking, squeezing, banging and blowing. In a sense, we have a continuous controlled explosion that's driving us forwards.

So what makes it better? A couple things. Speed of course. You can only go so fast with a propeller. There are aerodynamic limits that prevent you from going faster.

Then there's reliability. By the late 1950s we'd pushed piston engine technology about as far as it could go. The last piston engined airliners like the DC-7 had monster 18-cylinder supercharged engines cranking out over 3,000 horsepower. These were wonderfully complex machines that tended to beat themselves to death over time and required a lot of maintenance. A DC-7 or Super Constellation had 72 pistons, 72 connecting rods, plus 144 valves all thrashing back and forth 2000 or so times per minute. The Flight Engineer actually had an oscilloscope to check on each of the 144 spark plugs in flight.

On a side note: piston engines are a lot more complicated for the pilot, as I learned when getting my Airline Transport Rating in a Beech Baron. "Throttles! Props! Mixtures! Carb heat! Cowl flaps! Boost pumps! Help meeeeeeeee!"

In contrast, a turbojet has very few moving parts. We have a compressor, which is basically just a type of fan that sucks air in the front. That air gets squeezed into the combustion chamber, mixed with fuel, and ignited. The hot gases go out the back, past a turbine (basically a reverse fan) which is connected by a shaft back to the compressor. Once you get the the process going it's self sustaining. We don't even need spark plugs except for starting it. Once you light the fire it keeps going as long as you add fuel.

Typical Turbojet Engine
So why didn't we have these sooner? Mainly because we're dealing with some extreme temperatures and pressures in there and it took a while for metallurgy to catch up. Plus we're going to spin it at 20,000 rpm or more. The first jets had a life of about 25 hours before they had to be rebuilt.

"Pure" turbojets didn't stick around for very long. They were noisy, burned a lot of fuel and polluted badly. They only ran efficiently at relatively high speeds and altitudes. We needed something that was a bit better.

We came up with the turbofan, sometimes called a "fan jet". Take a turbojet and stick an extra turbine at the back. Connect that second turbine to a second, larger fan at the front of the engine. Now "bypass" most of the air coming out of that large fan around the engine. It's almost like having a small, covered propeller. What makes this better? Well it turns out that moving a lot of air slower is a better way to make thrust than moving a little air really fast.

Turbofan Engine
In later generations they made the fan much larger, calling it a "high bypass" engine. That's why newer engines are so much fatter than the engines on 1960s airliners. The vast majority of the thrust is generated by the fan with a little bit coming from the jet exhaust.

These are most efficient at medium-high altitudes and air speeds. They're in their happy zone up around 35,000 feet and around .80 mach.

For shorter flights that don't go up as high, a turboprop is actually more efficient. They're similar, except instead of turning a fan that second turbine drives a propeller via a gearbox like the transmission in your car.

Now we just need a way to start the engine and a way to control it.

Only the very smallest of jet engines use an electric starter like your car does. A starter capable of cranking an airliner engine would weigh a lot more than we'd want to carry around. We start them with compressed air, which can come from a ground cart or our auxiliary power unit (APU).

The APU is just a baby jet engine that is small enough to start electrically. It doesn't provide any thrust but it can produce enough air to start the engines plus electricity to power the plane on the ground.

The starter on a jet engine is a small turbine that is geared to the engine. When we send air to it the engine will spin up enough where we can start it. This is usually around 25% rpm. We measure jet engine rpm in % of maximum. Once the engine is running the starter has a clutch that disengages it (or it would come apart in a short time).

At the same time, the igniters will start to fire continuously. These are very similar to the spark plugs in your car, except we only need them when we start the engine. We do sometimes turn them on in flight, usually during takeoff and landing, "just in case".

Once we get the engine spinning to the magic number we move a switch or lever that sends fuel to the engine. It should "light off" in a matter of seconds. Then it's just a matter of watching the gauges to make sure it starts properly. The whole process might take a minute or so.

Once we get one engine running we can even siphon some of the excess air, called "bleed air", from it to start the other engine(s).

To control a jet engine we just have one lever for each engine, called a "thrust lever". Technically only piston engines have throttles, but we still sometimes call them "throttles", old habits die hard.

The thrust lever is connected to the engine's fuel control, which is similar to the fuel injection on your car. In the old days this was a purely mechanical device and you had to manually keep the engines from spinning too fast or producing more than their rated power "over boost".

In the old days you had to be careful not to move the thrust levers too quickly or you might cause the engine to compressor stall (popping and chugging) or to torch (shoot flames out the back). The old J57s on my B-52 would do that occasionally.

Today it's all done electronically. The thrust lever is connected to a computerized engine control - called an EEC (Electronic Engine Control) or a FADEC (Fully Automated Digital Engine Control). In simple terms it keeps the engine happy all the time. No matter what I do with the thrust lever it won't let me hurt the engine.

We also have a few extra gauges that you probably don't have in your car (unless you happen to be Batman).

N1 - This is like the tachometer in your car. It measures how fast the fan is turning in % of maximum. Some number between 0 and 100.

For some engines (General Electric ) we use the N1 gauge to set our power.

N2 - Same thing except this is the speed of the compressor and turbine.

EGT - Exhaust Gas Temperature, how hot the engine is running. Usually in degrees Centigrade. Some really big number around 400 degrees Centigrade.

Oil Pressure - Same as your car.

Fuel Flow - how much fuel the engine in using in pounds per hour.

EPR - Exhaust Pressure Ratio (pronounced "eeper"). This is basically the difference in pressure between the air going in the front and the air going out the back. It's a measure of how much power the engine is producing.  Some number between 1.00 and 2.00 usually.

On Rolls Royce and Pratt & Whitney engines this is how we set the power.

That's about it. They're very simple in concept but difficult to build because of the stresses that they operate under. Until someone perfects the hypersonic scramjet or the warp drive this is what we're stuck with.

Fortunately modern jet engines are very reliable. That's why most airliners have only two engines these days. The odds of one failing are quite low and an airliner will fly just fine with an engine out. In 29 years of flying I have never had to shut down an engine in flight and I hope to keep that streak going.

I hope this has been informative. I'd like to do a whole series of them if people like them.

Originally posted to Major Kong on Mon Apr 01, 2013 at 11:35 PM PDT.

Also republished by Kossack Air Force, SciTech, Aviation & Pilots, and Central Ohio Kossacks.


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