747-400 engine differences
 

Hello,
As an aviation enthusiast and a 747-400 fan, I would like to know what are the differences between the GE, Pratt and the Rolls in terms of fuel burn, operating range, reliability, weight, mainenance costs... And also, as pilots, your preference, if any !
Thank you !
Alex

Never flew Rolls powered 74s and don’t know much of that geeky stuff but operating GE engines was easier. We could start 2 at a time versus one Pratt at a time. And GE just uses N1 and not EPR so less to look at.

The GE had the better fuel burn and reliability - especially early on - and the market share reflects that GE getting the lions share (IIRC, the CF6-80C2 powered more 747-400s than Pratt and Rolls combined).
Pratt upgraded the PW4000/94" with the ring case compressor configuration (mandated by AD due to takeoff surge issues with the original segmented case compressor configuration) which matched up very favorably with the CF6-80C2 installation. But by the time the RCC came on-line in the mid-naughts, the 747-400 was past its prime and was only being sold as a freighter.
The Rolls wasn't bad - fuel burn on a par with the pre-RCC Pratt - but it had issues with reliability, especially on the engine control side. Unlike the Pratt and GE, the RB211-524G/H wasn't a true FADEC - it was a "FAFC" - Full Authority Fuel Control. The vanes and bleeds retained the fluidic control from the earlier marks of the engine - and both the FAFC and fluid vane and bleed controls proved to be very problematic.

Thank you for your answers ! Very interesting and very much appreciated.

Were there any meaningful differences in the CF6 that "made it" more reliable than the Pratt or Rolls offerings? Interesting to see the note GE out-sold both PW& Rolls put together!

GE's big advantage with the CF6 is that it was a major derivative - not an all-new engine. Normally, a clean sheet new engine gives better fuel burn - but generally at the price of reliability early on until the bugs get ironed out. However in this case, GE pretty much matched the fuel burn of the PW4000/94" and had outstanding reliability at EIS. Worse, Pratt had issues performance retention - a brand new PW4000 had marginally better fuel burn than a CF6-80C2, but the Pratt rapidly deteriorated such that after 1,000 hours the CF6 had the fuel burn advantage. Pratt finally addressed that with the Ring Case Compressor upgrade, but that didn't come for another 15 years - too little, too late.
The RB211-524G/H was a good, reliable engine (aside from the previously mentioned engine control issues) and had very good performance retention (big advantage of the 3 spool design - less bending moments that rub seals) but was heavier than the other two and was a bit worse on fuel burn.
In short, GE really nailed the design of the CF6-80C2 - even though it was a derivative it gave nothing away to the 'state of the art' PW4000 and was more reliable to boot. Ironically, Boeing thought the PW4000 would have the dominate market share and the CF6 wasn't going to be a big seller. As a result, Boeing elected not to invest in developing the engine nacelle for the CF6-80C2, instead buying the nacelle from GE (at a much higher price than an in-house design would have cost). An expensive mistake.

td, you could write a most interesting book giving an insight into your career.

I'd read it!

An awesome & comprehensive reply, muchly appreciated, tdracer. :ok:

It would never sell - at least not enough to make it worth the effort. But thanks for the thought.

Some sample numbers from the book to illustrate:

For a 390T aircraft to climb to optimum CRZ alt:
CF6-80C2B1F: 18mins & 7500kg
PW4056-3: 20mins & 8100kg
RB211-524H2: 19mins & 8100kg

Long Range Cruise at optimum alt for 390T (The PW and RR fall behind slightly further for lighter weights):
CF6-80C2B1F: 13144kg/h
PW4056-3: 13184kg/h
RB211-524H2: 13328kg/h

The GE and PW are pretty close overall in the cruise with the Rolls lagging behind by about 1.5%.
Interestingly in climb performance the CF6 really shines for some reason.

Having flown all three engine types, I have to say the CF6 is a joy to start, very quick to spool up while the PW and RR take an age to get going (God help you if its summer and you're in Nairobi or Addis).

PW very reliable though, once had an ENG OIL FILT contamination message which would have meant retarding the thrust and possible shutdown in a CF6 but PW decided having a secondary filter was worth it, no action required.

The Rolls does have one thing going for it however, it by far sounds the best of the three!

Telekon

Have to 100% agree with you on that one. The RB211 sounds amazing, no matter what it’s attached to.

Except, very little residual thrust on the RR… needed to add quite a handful of thrust to get going on taxy if heavy, and also woe betide the pilot used to PW/CF6s who retarded the thrust levers on an RB211 too soon during the flare :}

Fortunate to have flown all 3
GEnX is my pick, it just delivers.
Eng Ice Crystal Icing events a concern but suspect much the same for all power plants.
Interested to hear your thoughts on this tdtracer.

I assume you mean the CF6-80C2 (unless you've also flown the 747-8)?
Ice Crystal Icing (ICI) is an interesting phenomenon. Our understanding of ICI is way better than it was 30+ years ago when it first raised it's ugly head on the CF6-80C2, but I still wouldn't say that it's well understood. For the uninitiated, 'conventional' icing involves super-cooled liquid water droplets - liquid water that is well below freezing so it's just looking for something to freeze to like a nice cold wing or inlet leading edge. It's readily addressed by adding heat to the susceptible surfaces. ICI is fundamentally different - ICI is literally very small ice crystals - and they simply bounce off cold surfaces. BUT, warm surfaces can present an issue - the IC hit the warm surface and melt. If the ice crystals are present in sufficient quantities, additional crystals hit the liquid water and cool it sufficiently that it re-freezes - resulting in ice accumulation on surfaces that would normally be considered way too warm to ice. In testing, we saw ice accumulation in areas of the compressor where the air temp was as high as 85 deg F/30 deg C!:eek:
There seems to be something about the GE compressor design practices that make it more prone to ICI than Pratt or RR. For a long time, the CF6-80C2 was the only engine that had issues with ICI, although eventually the PW2000 suffered a few ICI events (but nowhere near as bad as the CF6. The GE90 had minor issues with ICI (no shutdowns, but some minor compressor damage events) but ICI hit the GEnx hard. We had a 747-8/GEnx-2B event fairly early on that - although the engines re-lit and ran fine for the balance of the flight, there was severe compressor damage on 3 engines. GE came up with a fairly ingenious method of detecting the presence of ICI and opening the compressor bleeds to send most of the ice overboard and that effectively solved the ICI issue with the GEnx.

NASA and the FAA did ice crystal icing research with a Falcon 20 and DC-8.

https://www.nasa.gov/aero/nasa-compl...ign-in-florida

The RB211-524H2 was only ever meant to be a stepping stone to the H3 engine. The H3 was to use the latest technology FADEC from the then new Trent series just coming online. However the H3 was a bridge too far in terms of meeting performance targets, and it ran too hot and fast. The final straw was an almighty bang on a H3 dev engine one nightshift in the Derby test beds, debris all over the test bed floor, and not long afterwards the H3 was canned, (along with similar developments of the 535 as well the E5/F5) So the 747-400 was stuck with the slightly older technology -524H2.
Very interesting times at RR, with the birth of the trents, V2500 and RB211 developments. There was a final development, with fitting the Trent 700 HP module, which actually gave a marked improvement of fuel burn and TGT.

Thanks a lot guys, very interesting to read! And special thanks to you tdracer for your very informative posts! :)

One question just popped. At the very beginning Cargolux had some 400s with GE and later switched to RR. Besides financial reasons what were the other considerations, if any, that could have justified this choice ?

Just my tuppence worth.
From a line maintenance point of view the RR has always been easier to service. The gearbox is on the fan case with all the accessories. Starter motor, IDG, Fuel and oil pump packs etc. Access is simple, just open the fan cowl and away you go. The GE and PW philosophy was to cram as much as possible around the hot core section which means you need a pump to open the C Ducts/ Thrust Reverser halves. More time consuming and in my opinion more difficult access.
The GE CF6 is a brilliant bit of kit though. Incredibly reliable.

Thanks HOVIS for your response. Very informative.