Despite there being GA material aplenty in Croatia nowadays, for this next piece I decided once again to go for one of my “periodic airliner exceptions”. The reason this time was the opportunity to photograph an entire CFM International CFM56 engine in the nude, something so rare even for us airliner drivers that I simply could not leave it at “just another set of photos” somewhere on my hard drive.
Admittedly, the mass of bits and bobs that is a jet engine is somewhat underwhelming to look at on a computer screen, especially with no sense of scale; but even then, the sight is more than enough to send the geek-o-meter right off the scale. Working under the assumption that mine wouldn’t be the only one, I rolled up my sleeves and dialed up my carrier’s PR department for an amen to go to town in true Achtung, Skyhawk! style…
The Power Of Flight
Being such an ubiquitous engine, much has already been written about it – so, if no one objects, we’ll cut straight to the chase. The first bit we need to address is the name: C, F, M, 5 and 6. Even a brief search online will quickly reveal that this name is actually a portmanteau of General Electric’s CF6 (Commercial Fan 6) and Snecma’s M56 (Moteur 56). The “problem” here is that this is a bit misleading, since it implies that the CFM56 is a mashup of bits from these two engines. This doesn’t quite work since a) the CF6 is a big brute of an engine that develops 240 kN of thrust (twice that of the most common CFM56)… and b) the M56 never really existed in the real world. Intended to be Snecma’s stab at the 90 kN thrust range, the M56 was still a paper project by the time the CFM International joint venture was announced, with barely the essential tech and concepts worked out.
To make it slightly more complicated, GE’s contribution to the CFM56 is actually based on the F101, a reheated high-performance military engine developed for the original supersonic B-1A Lancer. Use of the CF6 designation came in handy purely out of convenience, since GE and Snecma had already collaborated previously on European production of parts the CF6-50 powering the A300 and A310.
The 50-50 division of work established within CFMI thus sees GE contribute a modified version of the F101’s high pressure section (the high pressure compressor, high pressure turbine and the entire combustion chamber), while Safran (today’s Snecma) supplies the fan, low pressure compressor, low pressure turbine, accessory gearbox and exhaust section, all of which were developed for the stillborn M56.
The specific engine I had the opportunity to photograph is called the CFM56-5B4 SAC, a typical engineering sausage that immediately tells you pretty much all you need to know:
5: developed for use on the Airbus A320 Family*
B: an upgrade of the “first gen” 5A with a modified fan and low pressure compressor
4: developing 120.1 kN for take off and 108.4 continuously
SAC: variant featuring the Single Annular Combustor**
* while it’s easy to assume that “a CFM56 is a CFM56”, quite a lot of engineering has to go into making the engine suitable for use on other airframes. Perhaps the best examples are the -3 and -7 variants developed for the 737-300/400/500 and 600/700/800 respectively, whose gearboxes had to be repositioned and fans made shorted to make them fit under the type’s low wing with reasonable ground clearance (other changes on the -7 include just 24 fan blades versus the 36 on the -5, and a redesigned core to compensate for the fan’s reduced efficiency)
** normally not part of its formal designation, the combustor type does have some bearing on the engine’s performance. In the modern turbofan, the combustion chamber is a doughnut-shaped cavity in the middle of the engine in which fuel is mixed with air and turned into noise. Fitted at the front of this chamber is the combustor, in essence a ring mount for 20 fuel injectors that spray atomized fuel into the chamber. In SAC engines, there is only one set of injectors; in 1995 however, CFMI started offering the Dual Annular Combustor (DAC) option, which featured – obviously – two sets of injectors. The first – called the Pilot Dome – is optimized for lower power settings, while the second one – called the Main Dome – is optimized for high power settings. The Pilot Dome operates in all stages of flight; however, when extra oomph is required (such as during take off or climbout), the Main Dome comes online to increase efficiency and better use the energy in the fuel. While there are some fuel consumption benefits to this setup, the main party piece is better throttle response – and a significant reduction in NOx emissions, reported to be 40-45% over SAC models
Other details? Well, despite being a sizable lump of machinery – 2.6 m long, 1.9 m wide and 2.1 m tall – it tips the scales at just 2,382 kg dry (without oil & hydraulics loaded), and 2,420 kgwet, i.e. fully loaded and ready for installation into the nacelle.
Since I believe that continuing to solely talk numbers would defeat the (hopefully) educational point of this article, it’s high time to get down to the best bits: the photos! 😀 Alas and unfortunately, some of them will not be up to my usual standard; the aircraft that I got to inspect – a pretty stock A319-112 – was undergoing deep maintenance at the time, parked in a hangar with a scorching sunlit day outside. Thus, the situation was one of extreme backlight and unfavorable contrast, which took quite a deal of both physical and electronic work to compensate for (not to mention a ton of sweat). Hopefully though, the sight of all those purposeful do-dads and thingamajigs will make up for it!
Another interesting “feature” of shrouds is the noise they make when the engine is windmilling – captured (rather loudly!) in the video below. In essence, the blades of the fan, compressors and turbines are not rigidly fixed to their associated shaft, but are free to move about longitudinally in their mounts. In normal operation and at normal rotation speeds, they set themselves into one position and stay there; but when windmilling (or turning at lower speeds), they jiggle up and down making a racket. Among several benefits, the main point of this freedom of motion is to absorb and dampen the shaft’s vibration, and prevent some of that energy from being transferred to the blades themselves – energy that can cause internal cracks and various fatigue damage, stuff that is very problematic and very hard to detect (and had brought down more than one airliner in the past).
Shrouds add their bit by turning the volume up to 11, since during windmilling they tend to clap loudly against the next blade in line with every rotation. One needs to look no further than a comparison with the -7 series on the 737NG (below): its smaller fan does not need shrouds at all, since its shorter and lighter blades can be dampened just as effectively already at their root (just like compressor and turbine blades). Positive peace & quiet compared to the -5!
Airbus – A319/320 Flight Crew Operating Manual (FCOM)