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RR three spool engine

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RR three spool engine

Old 16th Jan 2015, 23:38
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RR three spool engine

Currently reading up on some info about the CF-6 engine which has a variable bleed valve to prevent compressor stalling on start. The author hints that the RR engine does not need this because of its three spool engine. The statement is that they never stall. Is this correct?
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Old 17th Jan 2015, 00:26
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In theory, 3 spool engines don't need variable geometry since the individual spools could better speed match relative to the 2 spool engines. The original RB211 3 spool engine was "intended" to not need variable vanes or bleeds.
It didn't exactly work out that way - and by the time it was certified it used pneumatically controlled vane and bleed systems (although the only the first stage of the mid spool had movable vanes - compared to the multiple stages of variable vanes on the same generation 2-spool Pratt JT9D/PW4000 and GE CF6 engines. Theory doesn't always work out in the real world

The current Trent engines have nearly as complex of variable geometry systems as the modern day GE and Pratt engines.
As for 3 spool Rolls engines not experiencing compressor stalls during start, someone forgot to tell the engine . The RB211 engines on the 747-400, 757, and 767 were far and away the worst starting engines relative to the 2-spool engines from Pratt and GE, both on the ground and in-flight.
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Old 17th Jan 2015, 01:34
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@ tdracer,

I think that is very true regarding the ease of engine starting. Years ago a Boeing 767 departing LAX for CVG had started it's climb out over the Pacific Ocean, when the flight crew inadvertently shut off the fuel flow to the CF-6-80A engines. Apparently the shut off switches were in the same position for other switch features on the Boeing 727 that the crew had recently transitioned from. They were able to restart the first engine in time to keep from landing in the ocean. An article in Aviation Weekly gave air start times for the GE, P&W and RR engines at that time and for sure if you were on that flight, the GE engines were the ones to have!
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Old 17th Jan 2015, 07:29
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Yep and after that Boeing fitted silly looking guards beside the Fuel Control Switches....
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Old 17th Jan 2015, 07:38
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You can start the Rollers in pairs though, which is rather nice.
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Old 17th Jan 2015, 08:58
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GE as well.
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Old 18th Jan 2015, 01:56
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There are a lot of message threads on PPRuNe about 3 spools vs 2 spools, although the information doesn't always agree

http://www.pprune.org/tech-log/41714...ble-spool.html

http://www.pprune.org/tech-log/44499...l-engines.html
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Old 18th Jan 2015, 02:11
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Turbine D, it was actually a little worse than that. The early 767s with JT9D-7R4 and CF6-80A engines had "supervisory" electronic engine controls - a fully capable hydromechanical control with a digital electronic EEC that would trim the fuel flow from the hydro control (basically a mid step between pure hydro and FADEC controls). If there was a malfunction or problem with the supervisory control, the procedure was to simply switch the EECs off and use the purely hydro control. At the time, the EEC switches were located on the aisle stand - right above the fuel RUN/CUTOFF switches.
There had been a previous event on a JT9D powered 767 where a pilot had, shortly after takeoff, reached down for the EEC ON/OFF switches, but instead set both the RUN/CUTOFF switches to CUTOFF - he quickly realized his mistake and turned the fuel back on - one engine restarted, the other went into a non-recoverable stall. They basically circled back around and landed.
When we (Boeing) investigated, we figured the pilots were idiots and it would never happen again . Then it happened again (I'm thinking it was ~12 months later) with the CF6-80A event you referenced. After that it was quickly determined we had a human factors issue, and the EEC switches were moved to the overhead (with the 'half a broomstick' switch guards ACMS referenced as an interim fix until the EEC switches could be moved).
BTW Al Murdoch - dual engine start on the 747-400 was a function of autostart, (which was an extra cost option - most Rolls and nearly all GE customers but only ~ half of Pratt purchased autostart). The APU could handle dual engine start for all three engine options on the 747-400, at least near sea level.
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Old 18th Jan 2015, 06:32
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Any pics available of the EEC fuel control switch set up?
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Old 18th Jan 2015, 09:15
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Originally Posted by tdracer View Post
There had been a previous event on a JT9D powered 767 where a pilot had, shortly after takeoff, reached down for the EEC ON/OFF switches, but instead set both the RUN/CUTOFF switches to CUTOFF - he quickly realized his mistake and turned the fuel back on - one engine restarted, the other went into a non-recoverable stall. They basically circled back around and landed.
When we (Boeing) investigated, we figured the pilots were idiots and it would never happen again . Then it happened again
Anybody know why he was intending to select both the EEC's off in the first place at such a low altitude?
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Old 18th Jan 2015, 11:13
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Jammedstab:
Anybody know why he was intending to select both the EEC's off in the first place at such a low altitude?
tdracer:
the 'half a broomstick' switch guards ACMS referenced as an interim fix until the EEC switches could be moved
Apologies for the thread drift, but it's an interesting diversion. The 'interim fix' was nothing short of brilliant because for a few cents it stopped clever dick co-pilots like me wrapping 4 fingers around the two cut off switches and shutting both off at once - very neat at the right time, but potentially fatal if for whatever reason the 'shut down' action got triggered wrongly. Jammedstab's pertinent question is not quite the right one: the prompt to go to the EEC switchlights was the EEC warning, the question should be 'why did he go for the cut-off switches instead?'. Probably no one knows, but if you've ever put salt in coffee, right action, wrong time, you can sort of understand. Once the EEC switches were moved to the overhead panel (took about a year to get through our fleet), then, in a really dumb move, the simple switch guards were removed, presumably on the questionable assumption that only the EEC lights would trigger the wrong action.... hmmm
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Old 18th Jan 2015, 13:51
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Just a bit more on CF6 starting -

DC-10's had an APU that didn't like running at its full rating, and so (at least for a time??) had to be de-rated. It resulted in substandard pneumatics for engine start.

There was much consternation in engineering offices, but to the best of my knowledge, no real problem with CF6-6 or CF6-50 starting in airline operations.
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