Puzzle for RR Viper experts.
 

JP3, with Viper 101 engine.

On initial spool up for takeoff the engine, rather than overswinging to 102%, and then settling at 100%, as normal, slows, and stops on 100% exactly, without overswing.

During acceleration on the ground, maintains 100%.

Once airborne, and accelerating, slowly rolls back to 98%.

Increase in speed causes further roll back to 97%.

Reduction in speed causes RPM to increase.

The HP fuel pump, and BCU have both been changed. Fuel filter has been checked and clean. LP 'FUEL' caption is out.

The JP3 has not much thrust to start with, and so 2% makes a big difference to performance, esp in warm weather.

Anyone any ideas what might be the problem?

I'd be very grateful for any help with this.

Thanks

MJ:ok:

Can't say I know the JP/Viper in depth, or at all (although we did use it for training during my time at RAF Halton sootie school 30 years ago....ahem!) so not sure if Viper has this kind of arrangement.....but generically, my first thought was intake pressue sensing (P1/P0) either leaking or semi-blocked giving inaccurate pressure signal to a fuel control unit, or jet pipe pressure (P7 in RR Conway terms) limiting fuel flow. HP Fuel Pump and Fuel Control Unit governors adjusted correctly? EGT normal so not temp limiting? Anti-ice/bleed valve not fully closed?

Long time since I worked on non-FADEC engine also!

This is from the dim and distant past....
Accel times OK? Acceleration is set on the AFRCU, and from memory it does the intake sensing.
I would try setting the accel to the lower, shortest limit, see if your overswing comes back.
If it doesn't then I would try the AFRCU. We used to set them at lowest accel time, set the Governor to 100.5, that would overswing to 102.5, which I think is the limit.
Usual, I have no manuals handy, so see what your maintenance thinks.
I did set up a few, though...

Check with another gauge(CRPM),if you can..Does the JPT follow CRPM...?Tried a compressor wash..?
Checked the throttle control runs are not`backing off``where there are those `rubber`gaiters get stretched,and `pull-back`on a control run??.Rate-reset valve possibly..?
Otherwise ,try the guys at N Weald...

Thanks for your suggestions, guys. I'll pass them on to the engineers.

MJ:ok: