cf6 Question
 

What is the maximum time for a CF6-80c2a5 to accelerate from ground idle to take off power?I have observed some engines that are very slow up to 70%N1 but normal ablve this.

like all other big fan from the days of the JT3d to today no fixed time as long as they all come up together from 70 % thow even this can have a bit of spread, unlike the old conway 508 where there was a 1 second spread between the slowest and fastest.

10 -12 seconds from flight idle is the limit that springs to mind on the 767 and 744 applications..:ok:

"clipped" is on the money 6 - 8 sec sounds correct

For a non-Fadec CF6-80C2, acceleration from Approach Idle N2 to 95% of Take Off N1 target is 6 to 8 secs.

Hope that helps.

Thank you for all of your replies.It is usually engines that are slow in the low to 70%N1 I was interested in.

I think "cornwallis" this is the only figure that the engineers can work on, i.e. in the manuals.:confused:

mmm, I've known this number, but it's a long time ago I had to use this. I tend to remember it is more than 6-8 seconds, like 12-15 seconds from ground idle to max power. I've had several occasions where we had pilots reporting slow acceleration, most of the time due too a bad MEC (main engine control). This controls the variable bleed valves and variable stator vanes. Maybe the high stage bleed valve is sticky/worn, so after 70% all is going well.

CF6 has no high stage bleed valve - it only has bypass valves at the booster (LPC) exit.

25+ years ago the CF6-50 could "hangup" out of ground idle - actually a low-speed compressor stall, no accel, slow EGT increase. It only occurred on "bleed off" takeoff - you could clear it by turning on A/C pack bleed. I doubt this is what happens on the CF6-80C2 though.

On the 767 CF6 powered aircraft I have experienced tech crews snagging an engine for slow accel only to find out after a HMU/MEC change that the defect remains.
After consultation with GE and time it was discovered that the slow engine was in limits and fast accelerating engine was spooling up to fast which could result in unstable compressor airflow etc etc.
I think a clever upgrade on the ACMS software solved the troubleshooting problems with this defect. (years ago)

Its much easier to pick an engine with different parameters when you have more than one engine to compare with...:ok:

Yes it does it is called the HPSOV (High Pressure Shut Off Valve).

Sorry - I was distinguishing between the bleed valve(s) required for engine functionality (ATA 72-00-00) vs the one(s) providing airframe-required bleed air (ATA 70-00-00). The CF6 VBV's I was alluding to are of the former type; obviously the HPSOV airframe bleed (a pneumatic PTO, if you like) is the latter. :8

Quite true but is shut after 75% N2 (approx) not much use for airflow control.
:ok: