View Full Version : L1011-500 oscillating question

9th Jan 2002, 10:03
As you all know I am but just a flight attendant and DO NOT claim to know "diddly" about the functions of jet engines. I have been on one of our Lockheed 500 series planes for two trips in a row. I know the sounds and feel of our Lockheed's inside and out and I know when something is a miss. There is a feeling right after take off ( only on the second leg of the trip.. strange I know) it feels like the aircraft is shuddering. I called it a vibration. I question the first front end crew and tho they said they felt it they did nothing. The next crew was more helpful and discussed it in length. The 3 crew members affirmed my suspicions that there was a irregular feel. They called it an oscillation tho. There were no indicators in the cockpit at all. They have since "logged" it.. yet another F/a said the feeling is still there. Anyone care to speculate?

Kermit 180
9th Jan 2002, 10:46
That the drinks trolley is not anchored, thus moving the center of gravity when the aircraft is pitched.


9th Jan 2002, 10:53
On take off Kermit?? EVERYTHING is secure

Tricky Woo
9th Jan 2002, 13:39
Probably that old bloke in first class with Parkinsons Disease.


gravity victim
9th Jan 2002, 14:57
Almost certainly it's the Guvnor, committing an act of furtive self-abuse under a blanket as he listens to the thrilling sound of a real L1011,and dreams his dreams of Celtic Wings... <img src="wink.gif" border="0">

Tcas climb
9th Jan 2002, 15:07
Could this be during slat and flap retraction?

Kalium Chloride
9th Jan 2002, 18:43
Not an ex-Eastern L-1011 is it? <img src="wink.gif" border="0">

Boss Raptor
9th Jan 2002, 18:58
used to be called 'pod nod' by Gulf Air crews I think...as I remember and it was a longtime ago...oscillation under certain atmospheric/flight conditions in the engine nacelle which causes side to side oscillation of the pods which translates to the nose going up and down...dont ask me why/what etc. that's all I remember!

9th Jan 2002, 20:22
That slight wobbly feeling is very probably all those wheels spinning right after liftoff. It stops when the landing gear is retracted.

9th Jan 2002, 21:01
interesting replies folks! <img src="wink.gif" border="0"> Tcasclimb.. simply.. NO.
Iknowthat these babies are dinosaurs, but this only happens on the one aircraft! we have several 500's and this is the only one doing this!

9th Jan 2002, 21:40
Keep reporting it !! If it is pod nod they might lube the fan blades. The engine actually oscillates in an orbit and usually vibrates the entire aircraft.

Send Clowns
9th Jan 2002, 22:12
Good to see the true principles of MCC (multi-crew co-operation) being applied. As we were told in our course, Locky, no-one is "just" an FA, they should all report anything they think might have safety implications!

10th Jan 2002, 00:15
no-one is "just" an FA

Yep, read the last chapter (the one before the epilogue) of Ernest K. Gann's "Fate is the Hunter."

10th Jan 2002, 00:21
awwwwwwwwwwwwwww ta SC and Brian! :) :) :) :)
feeling important now!!! :/
I am the deicing watchdog 2!

Willit Run
10th Jan 2002, 07:16
As much as I hate to agree with the Polzin man, he is right, although a big dork, that is probably what it is, POD wobble. its not that unusual.

Love ya Kenny!

10th Jan 2002, 08:12
Well locky, if you want to know, ask the L10 experts. Email is best.

10th Jan 2002, 09:14
Could you lead me in the right direction to these Lockheed experts?

10th Jan 2002, 14:52
If it is pod nod, try reducing thrust, back to idle if necessary, and after a minute, increase thrust again. Might stop the nod cycle. <img src="wink.gif" border="0">

Kermit 180
10th Jan 2002, 15:37
Whoops, sorry Locky! Because this topic was originally posted in JB I thought it was a joke of some sort hence my useless reply. Pardon me.

Kermie <img src="redface.gif" border="0">

Hot 'n' High
10th Jan 2002, 17:43
Hiya All,

This thread may have solved a little riddle for me. Back in '86 I was SLF on a Cathay 1011 out of BK and I noticed a vibration through the early part of the climbout (say to 5000 ft or so). Never could work out what it was as it appeared not to be wheel/flap related. Never came across it again so just pondered on the matter ever since then. So, years on, PPRuNers have come to the rescue and solved something that has been bugging me. At last I can sleep at night!

Cheers, H 'n' H <img src="smile.gif" border="0">

11th Jan 2002, 03:03
It is very probably 'Pod Nod' caused by fluctuation of the VIGV'S as the aircraft accel.up towards 250kts after t/o.
It should be reported to the engineers.

Pilot Pete
11th Jan 2002, 03:22

pulling the power back to idle in the initial climb out may stop more than just 'pod nod'..................! :) :) :)


11th Jan 2002, 08:38
Right on the money apfds, either the VIGV controller or....the master or slave pneumatic rams.
One of the few L10 experts I see....

FE Hoppy
11th Jan 2002, 09:41
Lot of talk of pod nod. Agree that its caused by VIGV's or fuel flow reg and should be reported but not sure I would call pod nod vibration! Rolls say pod nod has a frquency of 1-4 Hz and having had it a lot in the passed I wouldn't call it vib. I've had engine vibe that didn't show on the guages our boffins said it was too low a frequency but don't discount an airfame reason, gear door, cargodoor(aircraft doesn't have much diff on) or a wheel thats got a good flat spot vibrates like hell as its spinning down( you can feel the nosewheel realy well on the flightdeck or front gally).
Top marks for brining to the flightdecks attention. Free beer for you I think.

Kenny your buying but don't leave before she brings the tab.

16th Jan 2002, 03:42
Well thanks for all your replies. Still, nothing has been resolved. I'm in the process of writing the safety director for some answers. <img src="wink.gif" border="0">

16th Jan 2002, 07:48
Keep up the pressure as "news from the back" is always welcome.
Recall years ago at the old RUH airport I was taxiing out to runway 01 when the L4 F/A called on the intercom and mentioned that the floor was VERY hot. Back to the parking bay and had maintenance take a look...turned out to be a bleed air leak which burned thru a wire bundle.
Ah...not good.

FE Hoppy
16th Jan 2002, 09:06
I had a F/A point out a fuel leak on number 3 engine just after take off.She wouldn't take no for an answer and insisted I come and take a look right away. Would have taken a long time to show on the guages. But from the rear galley(-500) it was clear as day.
It's nice when you know your flying with pros in the cabin.

16th Jan 2002, 09:29
Thanks for the vote of confidence, I think I will need it come time to deliver this letter. I take safety VERY seriously and I agree that the cabin crew are helpful when alert and diligent.
As a side note though, there are the odd front end crew who humph and groan at some requests. We are only looking out for the safety of everyone.. and my motto is when in doubt "get your butt outta that seat and take a walk to the back!" Orrrr no coffee for you! :) <img src="wink.gif" border="0"> <img src="tongue.gif" border="0">

16th Jan 2002, 19:44
From my dim and distant past, POD NOD was a problem with the RB211-524 series, wing mounted engines, not -22B as Cathay Pacific flew. We would regularly replace the VIGV CONTROLLER for this problem.

[ 16 January 2002: Message edited by: All That Glitters ]</p>

20th Jan 2002, 22:30
I asked a couple of our pilots if they had ever heard of "pod nod".. they all looked like I had spoken in tounges!
gesshhhhhhh. CAN A REAL PILOT PLEASE STAND UP? At my company anyways! Hahahaha <img src="wink.gif" border="0">

23rd Jan 2002, 20:42
could it be the wheels precessing during retraction , with a failure of auto retract brakes ....??

23rd Jan 2002, 21:07
FE Hopey............Had some ice go thru number 2 once at rotation. It was so noisy in the tail that people had to yell at each other . No indication on the vib meter. Pulled the throttle back a little , noise went away , flew the trip and replaced 5 fan blades. I was amazed that it didnt show on the vibs. Was told that it senses sideways movement but not forward and back. There was no vibration in the flight deck and we would not known we had a problem if a flight attendant had not called us.

John Farley
24th Jan 2002, 00:13

Nobody has mentioned your original comment that it only happens on second sectors. Sounds like a clue. If this IS invariably the case, then the trick will be to decide what component might be working under different circumstances from the first sector - temperature most likely, but not necessarily. Are you always operating from the same few airfields? Could it be related to a specially long or short taxy distance or is it not airfield related. Even negative answers help to eliminate possibilities.

Anyhow, you stick at it. Aeroplanes are only machines and there will always be a logical answer to a persistent characteristic. Aeroplanes fail only because somebody has not done something right. In this case you are not the weakest link.

26th Jan 2002, 01:46

I have in front of me an Operating Information Memo issued by Lockheed in May 20th 1980 called "Wing Engine Pod Motion" and a Notice To Operators issued by Rolls Royce on the 22nd of May 1981 called "Engine Thrust Oscillations". Looks to me like it will answer your questions.

A fairly long one to type, unfortunately no time doing it today, but I could have it posted in next couple of days. Interested or not???. .Cheers!!!

28th Jan 2002, 21:50
May 20, 1981. .L1011 Tristar - Operating Information Memo. .Subject: Wing Engine Pod Motion

Recently there have been a few pilot reports of small lateral and/or vertical airframe motion felt in the cockpit and/or cabin of 524 powered aircraft. Observation of the wing engine pods appeared to be moving in a somewhat elliptical path with the major axis in a lateral direction and at a frequency of 2 hz. The amplitude of the motion was reported at about 2 inches at the cowl inlet with 80% of the motion in the lateral direction and 20% vertically.

Testing and analysis has determined that this phenomenon is the result of thrust oscillations attributable primarily to the variable inlet guide vane system and possibly the engine fuel system (FFR, PAC). The oscillations, although felt through the airframe may not be detected by reference to engine instruments because of intrument damping characteristics.

The pod movement does not affect structural integrity or flight safety as it is a non-divergent limit cycle oscillation and the loads are very low - less than those normally associated with very light turbulence. Thrust oscillations of one engine usually results in observed motion of both wing pods.

If this phenomenon occurs in flight, and it has only been encountered in extremely smooth air, the offending engine can easily be found by individually retarding each power lever to flight idle and then restoring power. The oscillations will disappear when the power lever of the affected engine is moved to flight idle - and when restoring power, if the power is reset either slightly higher or lower than the earlier setting, the oscillation will not recur. The wing/pylon response to thrust oscillations is dependent somewhat on airspeed and fuel loading, thus wing pod motion has been reported for a range of power settings ranging from hold conditions to lightweight climbs usually around 250 knots indicated airspeed.

A Lockheed Information letter and a Rolls-Royce Notice to Operators Number 198, are being issued to advise Engineering and Maintenance personnel or recommended corrective actions in the event the phenomenon be encountered.. .End.

Rolls-Royce Notice to Operators. .Engine Thrust Oscillation:

......Rolls-Royce are informed by LCC (???) that the natural frequency of the L1011 wing/pylon suspension is in the order of 2.0 to 2.5 Hz, varying with wing tank loading. The coincidence of the frequency of engine control systems instability with the natural frequency of suspension, results in the amplitude of movement that has been observed. The resultant pod movement has on occasions also been referred to as a pod nod, rocking, circular, or figure of eight movement.

LCC confirms that such movement is not a safety item.

Reports to date show that the problem:. .a) Occurs in the power range 1.2 to 1.4 EPR and can be eliminated by moving the throttle to a setting above or below the power range.. .b) May, but not always, be accompanied by noticeable cyclic fluctuations of engine flight deck parameters.. .c) Is not always repeatable on ground run.. .d) Has occured with engines incorporating the P4/PF and P4/PS1 (not clearly readable) variable inlet guide vane (VIGV) control system.. .e) Has not been encountered with the B747 aircraft installation.

Rolls-Royce has recently taken full advantage of the opportunity to investigate this problem in flight with a L1011 aircraft at Palmdale and also with a service engine on a Derby test bed. Testing involved comprehensive instrumentation, particularly so on the test bed, to monitor the stability of the engine control system. In each investigation, instability of the VIGV control system was detected and was thereupon identified as originating from the VIGV controller.

From the initial experience gained to date in investigating this problem, Rolls-Royce consider the following order of approach to be appropriate when trouble shooting:. .a) Check air signal lines for leaks (VIGV and FF? loops).. .b) Change VIGV controller.. .c) Change separate bleed valve control unit (SBVCU).. .d) Change fuel flow regulator (FFR).. .END.

That's it folks. I skipped a few notes to make it shorter a bit, in the hope that it still makes sense.. .Cheers!!!

30th Jan 2002, 07:24
READY;. .thank you thank you thank you!. .John.. thank you as well. .I think I will forward this to our company.I haven't flown on this peticular jet in about a month.. .As for second sector flights, they are usually from down south so I assume the runways are much shorter.. .Thanks for the input fellas <img src="smile.gif" border="0"> it is very much appreciated

[ 30 January 2002: Message edited by: locky ]</p>

1st Feb 2002, 22:11
I don't have to tell you Locky, but report it, report it, report it, and then keep reporting it, be a real pain in the butt over it, and something will get done. Recurring vibration or oscilation, in a part which really shouldn't be doing it, needs to be looked at.

I once grounded a BA 737 as SLF, when I noticed, on a climb out from Hamburg, a strangely coloured (QED essential) fluid, which appeared on the port wing leading edge. I told the FA, who got the Captain to come look. His response "Oh that's nothing to worry about" My response "I am a Pilot" His response "Ah" lol

Net result aircraft grounded at Birmingham, and my reward? A very waspish male flight attendant, on my depature from the aircraft hissed at me "Thanks to you, I won't get home tonight"

My response..... "Bite me" :)

So the moral is, maybe ask one of the SLF's if they notice something too, get the Captain, and the SLF will more than happily break the good news, something might get done quickly then

Tony. .(Happily grounding BA's fleet)

3rd Feb 2002, 00:11
If what you heard/felt was a vibration it was not the common "pod nod", which is more like the a subdued version of the lurches you get on the underground.. .In hot climates, probably your "second sector",. .the F/E has a tendency to send the ram air doors a little too wide open, and depending on how open the ram air doors are, you can get a rumble, rattle, or chugging. At some openings it does feel like vibration, and is usually reported by F/As at L1 door.

FE Hoppy
7th Feb 2002, 03:05
Agree that pod nod is not vib but don't like the F/E dig.. .The ram air "LOUVERS" are controlled automatically unles there is a fault in the pack.

7th Feb 2002, 20:54
The "Louvers" can be manipulated when the respective Packs are in "MAN" position and when "Cool" (Turbine Bypass) or "Warm" (Ram Air) buttons are pushed. <img src="rolleyes.gif" border="0">

8th Feb 2002, 05:20
Hoppy. .No aspersions intended, when it is 35C or more outside, and humidity is 95%, any attempt by F/E to cool things down is much appreciated. Manual is often the best way.