Friday, December 10, 2010

I'm at the office as I write these words - taking a break from coding bills and updating some manuals. My plane is down for an engine hot section, which is what I'm gonna babble about today.

Our plane uses Pratt & Whitney JT15D-4 engines, which each produce 2,500 lbs of thrust. This model was the first turbofan engine that Pratt & Whitney ever made, and they got it right the first time.

The overhaul period on these engines is 3,500 hours, with a hot section interval of 1,750 hours. What that means is that after a new (or newly overhauled) engine has accumulated 1,750 hours of air time, they take it apart and check out the bits of it that spin around really fast and heat up - hence the name 'hot section'. After 3,500 hours, they take the whole engine apart and replace most of the moving parts. A typical hot section cost is about $60,000 and a typical overhaul cost is about $350,000. Aircraft ownership is not for the faint of heart :)

First, let's refresh ourselves with basic turbofan operation. This is a cross-section diagram of our engine. The arrows illustrate the airflow, and the colors illustrate the relative temperature.

Basically, air goes through the main (big) fan in the front, and is blown backwards. Some of the air gets ducted along the sides of the engine and never goes through the combustion process, and some of the air goes through another fan near the middle of the engine (called the boost fan because it's not really compressing the air, it's just speeding it up a little) and starts the journey toward the combustion chamber.

If the air goes through the engine, it first gets run through an axial compressor (the dark-shaded spinning disc where the air temperature turns from blue to yellow), then it does a 180 degree turn and goes into the combustion chamber. Fuel is sprayed into the combustion chamber and ignited (the air temperature goes from yellow to red), and the resulting expanded air does another 180, then starts to travel at a great rate of speed toward the back of the engine.

You'll see that the air spins another dark-shaded disk (called the high-pressure turbine because the air is traveling at its fastest when it goes through the turbine), which is directly connected to the same shaft the compressor at the front is - that's how the compressor is powered. Once the air passes through the high pressure turbine, it it blows through another couple of light-shaded disks (the low-pressure turbines, called that because a whole lot of the energy of the air has been depleted by the high-pressure turbine already and the air is moving slower), which are attached to (and power) the main fan and the boost fan at the front. Once it's done all that, the air blows out the back of the engine as straight jet thrust.

It's interesting to note that the air that's ducted along the sides of the engine (and only goes through the big fan at the front) is a much higher volume than the air the goes through the core (and the combustion process). The bypass ratio on our engine is 2.5 to 1, meaning that 2 1/2 times the volume of air goes along the sides of the engine than goes through the core.

It's also interesting to note that the airflow changes direction a couple of times as it goes through the combustion chamber - our engine is called a reverse-flow engine because of that, and it's a design that Pratt & Whitney have favored in many engine models over the years (like their PT-6 turboprop engine which is used in zillions of turboprop aircraft). One of the main advantages of the reverse-flow design is that it reduces the length of the engine; you can see in the drawing that if the red section was a straight line it would be considerably longer. Sure, it makes the engine cross-section wider, but I guess the engineering people have determined that a fatter, shorter engine is better than a long skinny one. Hmm, I'm thinking there's a joke there somewhere, but I'm gonna ignore it and move on...

These are relatively old engines, but they remain popular due to their low fuel consumption and great reliability - on this engine, there are only 6 moving parts (compared to hundreds in your car's engine) and out of the nearly 7,000 JT15D engines built (and more than 40 million flight hours), reliability is well over 99.9%

Okay, that's enough about how the engine works. Now let's talk about the hot section itself, and what they found. Fortunately for me, we got a nice shiny report on the condition of the engine, which I am passing on to you.

Phew, that's a relief. I'd be pretty concerned if the engine wasn't turning freely, or if there were metal shavings in the oil.

No cracks found in the first set of blades, which is welcome news.

The pins that hold the combustion chamber are starting to wear down a bit, but still well within limits.

Moving further inside the engine, still nothing bad to report.

A bunch of fuel nozzle sheaths need to be replaced, and so they shall be. They run about $300 each, but relative to the total cost of the hot section it's pretty minor.

This is definitely good news - the low pressure turbine is fine. Each of those blades is seven hundred bucks, so I'm glad to see they are in good shape.

This single nut needs to be replaced ($550) not because it's in bad shape, but because a newer betterer nut was created after this one was installed, and the maintenance bulletin that talks about this says the old nut has to be switched out during the next scheduled heavy maintenance (hot section or overhaul).

I obviously removed some identifying information, but in summary the engine is behaving how the engine should be behaving. That buys peace of mind, and to me that's well worth the $61.543.22 expense.

So basically that's it - the engine is halfway along the road to a complete overhaul, and it's holding together nice and tight. The overhaul will be in another 1,750 hours, during which they will pull the entire engine apart and replace damn near everything, (at about 6 times the cost of the hot section inspection). Yup, you gotta spend money to run an airplane, but as it's my hind end in the front seat on most of the trips, I am okay with spending it on stuff like this :) One additional note: at the rate we fly, we won't hit 1,750 hours more use for nearly another decade - I won't be trying to achieve that interval between inspections in my Honda Civic any time soon, but that just goes to illustrate how reliable these engines are.


SkinnyDennis said...

Very interesting!

What's the idea with the bypass air? Does it add to thrust?

Anonymous said...

Thanks for the post Sully! Very interesting!

Mario said...


At low altitude the fan efectively provides a good part of the thrust. The higher the bypass ration, the higher the thrust component.

But the bigger reason for the bypass air, is sound suppression: The bypass air of intermediate velocity isolates or shields the core high velocity and high temperature exhaust from the ambient air at low temperature. Just compare a Learjet 21 with a Boeing 747 at takeoff: The Learjet has an ear-splitting scream, while the 747 has a low rumble.

SkinnyDennis said...

Thanks. I like to understand how these things work.

Jonathan said...

Thanks for the post. I really enjoy your posts that dive into the technical "how it works" detail.

Merry Christmas and Happy New Year!


Jim said...

$350,000 after 3,500 hours, or $100 per hour per engine.

I'm part owner of a Cessna 172, and we budget $12.50 per hour for 2,000 hours, or $25,000, for our engine inspection/rebuild.

$100 vs $12.50. Not that big a difference, given the difference in the rides.

Jim said...

@Dennis: The engines in large commercial aircraft have a bypass ratio as high as 9:1, meaning for every pound of air going through the engine and being mixed with fuel, 9 pounds of air are being accelerated and pushed out the back bypassing the core.

Noise and fuel efficiency are the big reasons. Rather than getting thrust by mixing fuel and air and making it very hot and pushing it out the back at supersonic speeds (where every air molecule makes a sonic boom), one can get just as much thrust by using most of that energy to turn a very large fan and move massive amounts of cool (heavier) air backwards at not quite so high speed. And because most of the high-bypass engines energy is being used to move a massive amount of cooler air via the fan, rather than just raw thrust, the same amount of fueld moves much more air and thus gives better fuel efficiency.

And as mario said, not only is all that bypass air not only providing relatively silent thrust, it envelopes all the nosy air as well and mutes the exhaust somewhat. I remember a/c like the 707, DC-8, DC-9 and the original 737's with their narrow cigar-shaped engines.... and they were very very loud.

Anonymous said...

How much does one of these JT15D-4 engines cost?