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View Full Version : 747-400 Fan Blade Vibration (RB 211 G rated)


cunningplanmylord
16th Feb 2011, 13:33
Dear All,

I have been on the 747 for about 3 years now and only over the last 6-7 months have been accutely aware of what must be a low wave level of cycling vibration from the N1 fans.

It is audible at a relatively low and discreet level, but I think by far far the biggest issue is the level of resonant vibration that u feel cycling through the seat at a constant interval. This is particularly irritating and when spending nights out of bed tired on long haul night flights ; it is like chinese water torture !!

I have tried to sync the n1's as close as i can but does not really eliminate the problem, numerous pillows and cushions on the seat make no effect whatsoever. We always as pilots feel and expect vibration levels in the flt dck but it is the cyclical nature that causes the stress. Is there any other experience of this problem out there and a way/method to troubleshoot this or even cognitive ways of helping cope overcome a persistent and irritating problem, as im sure it affects only a minority of us ?

Many thanks in advance

CPML

DERG
16th Feb 2011, 18:40
Years ago there was a seat cover made up of stringed beads..they sort of took up the vibrations. Must be 30/40 years since I saw them for sale. I hope this helps. I cannot tell you if they work or not but I would try with them.

Regards

barit1
17th Feb 2011, 13:20
Tell the mgmt the aeroplane needs a syncrophaser a la prop aircraft of the 50s.

I once rode a Falcon bizjet with mismatched engines (lo vs hi time) and to keep the fans synced, the throttles were staggered such that core speeds were 3-4% apart. On a twin it's not too hard to manage, but harnessing four donks can't be much fun.

lomapaseo
17th Feb 2011, 14:20
It aint the engine RPM it's the aircraft.

Try working that side of the equation. Many sources of harmonic inputs. pumps, gears, airflow, etc.

cunningplanmylord
17th Feb 2011, 15:41
I did think about the issues of air conditioning packs, duct pressures etc, but I am convinced its linked to eng rpm as it goes away with high eprs in clb thrust settings and at idle in the descent. It is improved by tweaking thrust settings or worsened eg the cycles of vibration either speed up or slow down

lomapaseo
17th Feb 2011, 17:44
Time to wake up seems to be the solution

Vibrating cockpit seat proposed for pilot alerts (http://www.flightglobal.com/articles/2011/02/17/353283/vibrating-cockpit-seat-proposed-for-pilot-alerts.html)

bearfoil
17th Feb 2011, 17:59
I had a girlfriend who liked the hard mounted twin in my Bonneville.

Ask the women if a vibrating seat would be a nice option.

bear

TopBunk
17th Feb 2011, 18:08
I tend to agree with Lomapeso - it is likely to be not engine related.

Having flown about 8000 hours on RB211 B747-400's, the autothrust iirc balances EPR normally, even if you displace one lever away from the others.

I would ask how many airframes you have and whether this happens on all or only one airframe?

cunningplanmylord
18th Feb 2011, 12:52
Thanks for the replies.

Top Bunk,
I know the a/t balances epr, however the 747's (49 of them ) we have are aging badly and whilst the epr is balanced the n1's are never in sync and its not every airframe that suffers from this cycling vibration.
I did wonder whether disengaging the a/t and having a play might be more beneficial than the large thrust lever movement required to move out of the FADEC gate.
The solution the more i think of it rests in a seat cover than has a material to heavily dampen vibration, any ideas here would be great, neoprene seems a popular choice on the net !?