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wazz'n'zoom
28th May 2010, 17:20
Just had the closest shave ever whilst conducting AP out hovering in a Bell 412EP with AHRS. On taking control from a Student pilot, who was slightly over-controlling the cyclic, the a/c started to bounce so severely in the vertical plane that vision was blurred- approaching G induced grey out, the helmet boom mike fell down out of position and the a/c information clip file in the Map Stowage slot burst open. As it was impossible to fly let alone land, I forced the cyclic forward (witnesses say 40deg nose down) and the bounce became even worse. The bounce was similar to a 'Computer Acceleration Control' or CAC runaway experienced on Lynx/S61/Sea Kings but a lot worse. Oddly none of the 4 crew saw any Tq swing or heard the engines hunting. The saviour was yelling to the right seat to engage the APs. Due to the bounce he really struggled to get his fingers anywhere near the AP Panel. On engagement the bounce dampened out almost immediately, a normal hover resumed and the a/c landed and shutdown 80m from the hover point. The whole incident took about 15 seconds and was witnessed by quite a few people who mentioned that they thought we were about to crash. Funny old thing-so did I! :eek:
Apparently the tailboom was flexing in all directions and to be honest, I'm amazed that it stayed attached given the severity of the bounce!!
The a/c was mid CofG, weighed 8900lbs and the wind was very light from the rear. Mast Tq was about 55% with no yaw. The pilot’s notes warn of an AP out condition where Tq and lever will de-synchronise at High Tq in an erratic yawing turn; none of these were met. The engineers could find no fault and the brave MTP took it for a spin with no fault found. This is the third 'odd' AP Out incident on our AHRS airframes. There have been no TARSYNS AP Out related incidents!
Anyone else had anything like this? I'm not convinced that we have got to the bottom of or heard the last of these potentially serious incidents!:suspect:

malabo
28th May 2010, 17:37
Never had a problem flying them with the AP's turned off, sometimes for weeks at a time.

Sounds like a control oscillation from a loose servo. Didn't this happen to a 412 in Australia a while ago?

wazz'n'zoom
28th May 2010, 17:48
Malabo
Is your 412 fitted with AHRS or TARSYNS? The Engineers said all the jacks, gear box feet mounts, control runs etc were within limits on inspection.
I've also flown many hours AP out, day and night, including some very untidy, nee scary student landings but never witnessed anything like this.

Encyclo
28th May 2010, 21:39
First of all, good job Wazz getting this on the ground safely :D.

As for possible causes, have your techs looked at collective minimum friction? My experience is that min. frinctions in all axis are often not kept up to spec and this leads to all sorts of strange issues.

Need to get the hydraulic cart out and a good spring scale. From the MM:

Attach a spring scale at center of upper throttle
grip within 0.50 inch (12.7 mm). Correct measurement
is 8 to 11 pounds (35.6 to 48.9 N) (helicopters without
4-axis Flight Director kit installed) or 4 to 4.5 pounds
(17.8 to 20.0 N) (helicopters with 4-axis AFCS/Flight
Director kit installed) up-collective stick force.


Hope this helps :ok:

wazz'n'zoom
28th May 2010, 22:25
Thanks Encyclo

Will pass the info on to our Gingerbeers. I did watch them set up the hyd rig and carry out several 'tests' but can't vouch for the type.

Via a different forum, I've le arnt that PIO 'Pilot Induced Oscillations' can affect the 3 axis autopilot fitted to the 412 especially when the collective is pumped up and down approx 1-2". This leads to a pulse in the collective jack that manifests itself into a full blown vertical bounce.
But why, with only 10% authority, would the AP engagement stop the oscillation within seconds.
Is there a semi redundant channel of the AP that is normally working in the collective channel for the 4 axis AP, that is still working in the 3 axis AFCS? But with no input to the collective lever is it still having an influence on the collective servo thus the bounce.
I now wish that I went up with the MTP to show him the situation that it happened in as this incident is making me doubt my integrity but moreover, I don't want a less experienced Instructor or Solo student getting himself into a situation that he/she can't recover from.
I've flown the same airframe twice since the incident but none of us have been stupid enough to disengage the AP in the air until more info is gathered.
Thoughts, theories anyone?

tottigol
29th May 2010, 01:22
In my experience flying the 412, the only time I had something similar happen, one of the droop restraints had become lodged and kept the relative blade out of path, boy I passed a stone the following day.
Symptoms were identical, but it had nothing to do with the TARSYNS or the HPs.
As a matter of fact, once you remove the TARSYNS and AFCS from the equation I cannot see a reason for the aircraft to react like that.

PO dust devil
30th May 2010, 03:34
I assume you put an incident report in so that there might be accountability for investigation and result?

It sounds like there have been a number of this type of excursion so it would be nice to build a profile on these events??

My 2 cents worth.

DD

Blackhawk9
30th May 2010, 06:28
Have seen all the mentioned faults (low collective friction, flogged out followup link on collective servo, worn out xmsn mounts, miss matched dash number xmsn mounts, pilot induced collective AND also Cyclic ossilations) make 412's buck all over the place you might want to ask your Bell Tech Rep what happened to the Jamaican Defence Force 412EP that crashed last year during training , eye witness reports said it bounced all over the sky then crashed, sounds like it could have been a collective/AP fault ( the jamaican machines are 4 axis machines).

wazz'n'zoom
30th May 2010, 11:20
PO
The incident signal was completed within 2 hrs of the incident but I haven't seen the engineers report yet.

wazz'n'zoom
30th May 2010, 12:33
Hi Blackhawk
We haven't heard of the JDF 412 crash last year. Will search the Post Crash Report out ASAP.
One of my colleagues, post my little incident, told me that he was flying the same cab, hovering AP out into wind when the a/c started to yaw left and right substantially, with no pedal inputs and both Tq's hunting between 20 to 70%. How odd is that? Obviously no fault was found again.
Cheers, Wazz

oldbeefer
30th May 2010, 13:59
As the 'ex MTP', has to be pilot error!

PO dust devil
30th May 2010, 15:47
:ok:

nice save and all

DD

Senior Pilot
30th May 2010, 21:32
We haven't heard of the JDF 412 crash last year. Will search the Post Crash Report out ASAP.


Discussed here: Jamaican Defense Force writes off Bell 412EP (http://www.pprune.org/rotorheads/380057-jamaican-defense-force-writes-off-bell-412ep.html)

gittijan
31st May 2010, 12:46
ATARS, a low time pilot under check/training with one engine in manual and the #1 hydraulic system switched off. No information as to whether they switched off AP1 or not.

wazz'n'zoom
31st May 2010, 16:30
Thanks for that info Gittajan. Sounds as if the compound emergency that they were practicing was rather complex.
We stopped putting the Eng Governor into Manual in flight, for practice, after one of our instructors blew the engine up by selecting manual with the throttle fully open!! It's now a simulator only emergency.
Re the JDF crash, I'd love to see the notes the Bell Team wrote. Wonder how you get hold of the JDFs Board of Inquiry outcome.

wazz'n'zoom
3rd Jun 2010, 19:00
Didn't this happen to a 412 in Australia a while ago?

Anyone have any more info on this 412 incident?

malabo
3rd Jun 2010, 20:02
Google is your friend.
AO-2008-039 (http://atsb.gov.au/publications/investigation_reports/2008/aair/ao-2008-039.aspx)

wazz'n'zoom
3rd Jun 2010, 20:46
Malabo
Good searching me hearty. I tried the same with the JDF crash but no main detail, just an outline official PR.
Wonder if he/her got a MAYDAY out! After all the years of training I didn't manage to bang one out; I was struggling to see let alone talk! Managed a PAN PAN once on the ground though.

Matthew Parsons
6th Jun 2010, 03:24
What autopilot is installed, and with AP off did you have Force Trim on or off?

Also, are there any modification to the collective head from a standard 412EP?

This sounds a bit like a control response to the aircraft movement causing the oscillation. With the AP turned on, the control in question no longer moves due to aircraft movement so the oscillation stops.

Although you said it was a vertical oscillation, I think it could have been either a heave oscillation or a pitch oscillation. Either may feel as a vertical from the cockpit. A modification that adds mass to the control head, or control frictions being too low could lead to this.

A little bit of theory, but mostly a wild guess.

Great job in bringing it down, keeping everyone safe!!

Buitenzorg
6th Jun 2010, 17:10
wazz'n'zoom,

where was your student's left hand during the (onset of the) bounce? If he was still holding on to the collective a small bounce may have moved his arm, there by moving the collective, creating another bounce etc., setting up a kind of resonance. Then, when you yelled at him to switch on the APs he had to take his hand off the collective to push the buttons, breaking open this "closed loop of resonance".

BTW I'm sure there is an official name for the kind of PIO I've described above but I can't remember it at the moment.

wazz'n'zoom
6th Jun 2010, 17:51
In answer to the various posts:
The Right Hand Seats left hand wasn't anywhere near the collective when the bounce started (as he had relaxed by then).
The Pedestal Master Force Trim switch was on but can't vouch if during the bounce I pressed the cyclic FTR button.
We don't fly the a/c in ATT mode except on long IFR transits.
Will find the exact Honeywell AFCS Model and Mod state and post it.

Other thoughts from a senior 412 operator were:
The Heads Pendulum Dampers could possibly set up a bounce if harmonised to the ?Hz oscillation. 24lb per blade root is enough mass to create a bounce.
My thoughts were- 'but why would engaging the AP damp it out?'

Still confused but 'good ideas club' inputs are heartily welcomed.:confused:

Shawn Coyle
7th Jun 2010, 12:01
wazz:
Why not fly it in ATT mode all the time? Learn to get used to the control forces and fly through it without using the force trim release all the time.
While it's not the greatest AFCS in the world, it's not bad, and when you get used to it, it actually does reduce workload, even in the hover.

wazz'n'zoom
7th Jun 2010, 21:10
Hi Shawn

Our 'customer' requires their student's to able to operate the aircraft VFR and IFR by Day, Night and NVG using SAS mode not via the luxury mode of ATT. Procedural transits can utilise the ATT mode but approaches are done in SAS mode.
With only the 3 axis system fitted however, it is only an area weapon. The 4 axis AP with full SAR mode is a lovely bit of kit though.:)

212man
7th Jun 2010, 21:14
Sounds like the customer is mixing coupled modes with basic AFCS modes! I agree with Shawn - press the FTR button to yank and bank, release it when you don't need to!

malabo
7th Jun 2010, 23:05
RFM Page 1-11, Limitations, AFCS shall be operated in ATT mode during IFR flight.

We experimented plenty with ATT and SAS. Most time we left it in ATT and used the FTR to move it around. Nothing "luxury" about it, just how the helicopter flies best. First I've heard of anyone wanting to keep it in SAS. I'd be interested to hear the reason.

wazz'n'zoom
8th Jun 2010, 09:36
Malabo
The 412s here are not operated 'fully' via the Bell RFM but by a 'MOD Military Release to Service'. This works in conjunction with the RFM but has several over-riding differences which, have been cleared by Bell.

The service does not want their student pilots using the ATT mode, as it is too automated and easy to fly but therefore prefer to use SAS mode for a more 'manual flight' mode therefore testing the student more. Thus, only the Honeywell SPZ-7600 3 axis AFCS was selected during the purchase of the aircraft in 1997.

When 2 extra 412EPs were purchased 10yrs ago, the a/c arrived with AHRS instead of TARSYNS. This is one of the a/c that had the 'Vertical Bounce' and some other 'odd' handling issues.

The service are requesting an inquiry into this area so that a student pilot does not suffer the same, potentially serious 'AP out' incident.

oldbeefer
9th Jun 2010, 08:36
Still just sounds like PIO from an inexperienced student - had it on Whirlwinds all the time!

212man
9th Jun 2010, 14:03
I tend to agree with Old Beefer and the other poster who correctly stated that a high frequency pitch oscillation can be misinterpreted as a vertical bounce. The Bell 212 with the basic SCAS sometimes suffered from a 'chattering' pitch gyro (loose wires) which would induce a severe vibration, as have other types (AS332 sometimes prone.) But, if the AP was 'out' in the first place then there should be no interaction at all with the controls from the AFCS. Reference to the collective channel is a red hering I think, because even where it to be functional, it would only be a trim actuator, and not a a SAS actuator, so would be unable to generate the frequency you describe.

You should not be surprised at how quickly the AP killed the problem. The disc is typically moved by the SAS far more rapidly than any pilot inputs and the fact it only has 10%, or so, authority is misleading too - try moving the cyclic back and to within 10% of it's range and see what the aircraft does!

I strongly suspect that there is something unrelated to the specific AFCS which has led to a PIO, and because it happens to be a unique AFCS fit within the fleet that is disracting your attenton.

I'm sure we will all be very interested to know the final outcome :ok:

PS. As FBH machines, do they have CVFDRs fited, or HUMS? I'm sure there will be some clues in their recordings.

wazz'n'zoom
10th Jun 2010, 12:18
212man
Interesting observations.
FBH did not send the CVR off to be read (it was impounded pending an inquiry but has since been re-used therefore overwriting the data:ugh:) and the a/c is not fitted with FDR (both deemed too expensive).

Quote: I strongly suspect that there is something unrelated to the specific AFCS which has led to a PIO, and because it happens to be a unique AFCS fit.
It's a shame the other pilots on the unit have not reported the 'other instances' relating to the 'odd handling issues' apertaining to only 2 of our 412EP's (both 3 axis AHRS). If i had a quid for every time someone has said "Oh i had something similar to that the other week/month" It'd pay for a few rounds at the local. As it is, most of us are unwilling to disengage the AP on these 2 AHRS a/c.

When my next delivery of 'brave pills' arrive, I'll go up with the test pilot and show him what led to the incident.:eek: Watch this space!

Shawn Coyle
10th Jun 2010, 12:46
Anytime something out of the ordinary happens to an AFCS, it should be reported!!!
Tell the folks who don't report things officially, but tell you after over a pint that how would they like it if a really serious problem didn't get report and was still present when they got the aircraft?
In fact, any time something out of the ordinary happens to a system it should be reported somewhere, somehow. A Bell 212 crashed with a total electrical failure (which is not supposed to ever be able to happen to it) and it was discovered that not only had that airframe had a total electrical failure a year before, but there was at least one other airframe that had a similar problem - neither got reported. 7 dead.

idle stop
11th Jun 2010, 17:13
Sorry to be pedantic, but I understand that the aircraft was check flown after the incident by 2 'real' (ie graduate) test pilots, who, like the engineers, were unable to find a fault with the aircraft.

It's quite easy to get 'in the loop' with the 412 in AP-out spot turns, but I had not heard of a vertical PIO. I wonder whether it started out as a bit of roll oscillation, which excited some yaw, and then coupled with a bit of heave, which got out of hand? Only 3-axis Helipilot, methinks, so can't blame the collective channel. And I gather that re-engaging the AP stopped it all.

Lesson: Don't taxy in AP out (especially with a tailwind..and NB critical azimuth..?) when you're about to RRF. There's a time and a place for practising what should be a 'degraded mode'.

Ah: the Force Trim! Another old chestnut. Personally, in high-gain situations, either 212/412: FT OFF. That means, basically, below about 250 ft. Above that, FT ON. Then ATT mode as soon as possible. Philosophy: use the FT and Helipilot system as it was designed to be used on the type in question, not because that's how another type's is used when you pop out of the flying training empire.

SASless
11th Jun 2010, 18:38
As I did not attend the vaunted CFS, and thus am not qualified to say anything about helicopter flying despite having done a life time of it....I will risk inciting riot and barbed responses by saying this.

I was taught to use "attitude" to hover and fly a helicopter. That being....select the attitude that provides the desired results....be it a steady hover or a rate of turn or airspeed or whatever. It was always about "attitude" and "sight picture". When I progressed to larger and more complex helicopters the basics did not change... just the way one went about it.

So....I ask you....if the helicopter has an "ATT" mode....and it functions with the FT "ON".....why not use it to "hold" the attitude you select by moving the cyclic stick? I might be awfully wrong but my memory of every single helicopter I ever flew....either manually moving the cyclic or selecting the FT "Off" by either switching it off or mashing the FT button on the cyclic allowed one to revert to "SAS" mode or override the autopilot.

Even in the venerable Huey one could use friction and FT to provide steadier hover and flight with a reduced work load on the pilot.

For training purposes....using reduced modes of autopilot/stability systems is good practice but should not be suggested as "The" way to fly the aircraft in normal operations.

The quickest way to get a Huey bouncing vertically (other than Engineering) is by moving the cyclic fore and aft in a fairly rapid cycle. Tranny mounts and Lift Link bearings are another way if not checked properly or replaced when worn. The 212/412 is no different in that regard.

212man
11th Jun 2010, 19:24
SAS beat me to it. In my limited exposure to the 212 (5 years and 2800 hours) I never found a need to turn off the trim. I flew plenty of 'high gain' manouevres - believe me, I did - and the use of the FTR was intuitive and seamless.

SASless
11th Jun 2010, 20:55
212man.....someone will come along with his pocket full of colored pencils and explain exactly how wrong we are on this. We seem to have embraced the KISS method of helicopter flying....that being "Keep It Simple Stupid!" which surely confuses heck out of CFS indoctrinated folks. The mere thought of using such a dangerous device as an "Attitude Hold" feature within 500 feet of the ground must strike fear into the Checklist Readers that lurk around this place.

Yet....the very same bunch will happily let a Sea King do so in a hover at night over a Sonar Pinger.

wazz'n'zoom
11th Jun 2010, 21:42
Guy's/Gal's

Let's not lose focus here and stop going down the 'force trim' rabbit hole.

Whether as some say 'You messed up' AP out, the fact remains that the a/c did something that no other helicopter I've flown has ever done....Taken control from me in a stable hover with virtually no wind!! Should that happen?...A big fat No!

It's a £2 million bit of kit that has some quirky handling and, via this forum, will stop someone else suffering the same incident. Nuff said??

p.s. Hopefully, if he is available, we should have the ex-ETPS in-flight test report/feed back next week. Wait out.

NewST
11th Jun 2010, 21:46
I am still amazed at the amount of people who will not engage the force trim on a 412 to this day. First thing on pre-takeoff check is "Force Trim Off' and the first thing on the after landing check is "Force Trim - On". Noticing the same thing now that I am getting into the 214ST as well.

SASless
12th Jun 2010, 00:36
in high-gain situations, either 212/412: FT OFF. That means, basically, below about 250 ft. Above that, FT ON. Then ATT mode as soon as possible.

The comment began with "Personally..." thus I shall be polite and say I firmly disagree "personally". Ya'll young and impressionable take note of the difference in personal opinion here.

One must note the difference from ATT Mode to SAS Mode is but a cyclic button click or click and hold away. Chinook pilots are well aware of this as real Chinook Pilots have a well formed callus on their right thumb....just as I did when flying 204/205/212 and 412's. I also used Cyclic friction on the non-SAS'd 204,205,212's I flew.

Now the Sixty-Four Dollara Question has been asked!

No Engineering fault found.

No Test Pilot, Test Flight reproduced the fault.

We have a Pilot who says he got quite a fright out of an unusually strong uncommanded reaction from an aircraft.

No one else has come out to say they have had anything remotely similar happen.

Pilots with thousands of hours in the same type aircraft have not reported any similar violent events in the particular aircraft or similar models.

No one has said the involved crew "cocked it up" but some have suggested PIO as a probable cause.

We all are interested in the answer for a multitude of reasons....some out of sheer boredom, some for technical reasons, and some for flying that particular aircraft or similar models.

One thing is sure....we are back asking the same question...."What Happened?"

Shawn Coyle
12th Jun 2010, 04:29
And they wonder why a FDR would be a good idea.
Not only are they very useful for incidents like this, and accidents, but they are also incredibly valuable if you're monitoring a fleet.
Sigh.

wazz'n'zoom
12th Jun 2010, 08:09
And the question still remains.. does anyone out there operate the 412EP with the Honeywell SPZ-7600 3 axis AFCS with AHRS or is it just our 2 FBH 412EPs? Have Bell sold us a lemon??

Ask about fellas/lasses.

UCLogic
12th Jun 2010, 09:13
We need to steer clear of tying AHRS to an AP out incident, if I remember correctly neither tarsyns or AHRs can influence the aircraft flight path if the AP is not engaged other than through the instrument-eye-hand-control route.
In SAS the ap will only be able to 'push' against the pilots stick position and therefore will always have a changing reference point, if there is control input being put in by the pilot the end effect on the flight path/attitude being unpredictable.

I believe that the mode of operation on this operation is not to use transparent piloting and either have force trim off or the button pressed. If so I seem to remember that pressing the button not only removes the 'fixed' reference point but also centres the AFCS actuators (check the API).

As ever awaiting the incoming!

forget
12th Jun 2010, 09:15
412EP factory Product Data below. AHRS was standard fit from at least October 2002. Note - nearly 300 pages.

http://www.bellhelicopter.com/en/aircraft/commercial/pdf/412EP_PDB_2002_r1_web.pdf

SASless
12th Jun 2010, 12:13
Perhaps the aircraft should be rendered the same standard of innocence any accused receives...."Innocent until proven guilty".

WASALOADIE
12th Jun 2010, 13:06
I was the crewman on Wazz& Zoom's aircraft. Just to back him up. the a/c was AP out so there should be no inteference from the AP system. We were AP out purely for a handling exercise for the student. The vertical bounce onset almost instantly and W&Z took appropraite action for us to clear a refuel area and in the process of landing/reselcting the AP in the problem solved itself. Witnesses from an adjacent building sadi they had never seen the underside or top of the a/c in such clear view in the space of a few seconds from about 50metres.

All I could do during the event was hold on ( was on harness in the cabin) and sit in a seat. The bounce was so severe that I could not focus within the cabin, only outside and I could not release my grip to strap into the seat.

Th incident has been reported to make sure people are aware of what happened.

A similar incident occured a while ago on the same a/c where instead of vertical bounce, whilst hover taxying downwind and AP out, there were oscillations from both engines with associated oscillations of RRPM and torque fluctuations and resultant yawing. This was reported to the engineers but no incident raised at the time. Again, no fault found!

forget
12th Jun 2010, 13:16
Probably too late but are there any local security cameras that may have this on record?

oldbeefer
12th Jun 2010, 14:42
It's very easy to confuse vertical bounce with rapid fore and aft movements of the cyclic - several Squirrels (AS 350) were snagged for vertical bounce when I was the MTP. They all turned out to have faulty vertical gyros which, at a particular pitch attitude, would produce this effect. Thus, I still feel the most likely reason for the situation was the stude simply overcontrolling in pitch - easy to do with both APs out on a 412 (and I still think it's a daft thing to do).

wazz'n'zoom
12th Jun 2010, 15:42
Forget
Probably too late but are there any local security cameras that may have this on record?
Been down that road and unfortunately not in the cameras view.
Wazz

SASless
12th Jun 2010, 15:47
Have ya'll called Bell to see what they have to say about it?

No doubting it happened or if it was an eye opener....just wondering what the actual cause of it is?

My hunch is something to do with Collective Friction or a bad collective bell crank or bearing.....everything all ticketyboo with the bellcranks, linkages, rod ends, collective friction setting? Any elongated bolt holes in the Bell Cranks?

If it were the friction setting....it should also show up in the AP mode or something similar.

Was the collective itself moving?

Did either pilot brace the collective with a knee so it could not move?

wazz'n'zoom
12th Jun 2010, 18:55
SASless
FBH have called Bell but they weren't interested; no surprise there then. If you don't pay up front then nothing will be done.
The gingerbeers checked the a/c as per the approved Bell post incident maintenance checks including the collective friction. Not sure about the bolt holes in the bell cranks though.
The collective was not moving of its own accord or being moved by me.

OldBeefer
The bounce happened after I took control.
22 Gp have now reduced the standard of AP out flying from Level 4 to 3, (For the unitiated: Level 1 crap, Level 5 Exceptional) therefore this is the only flying discipline in the Multi Engine Advanced course that is not a mandatory level 4 requirement to get the issue of RAF/RN wings! Haven't quite figured out why that is yet, and to be honest, I don't get paid enough to ask; you do as you're asked by the military commanders.

idle stop
12th Jun 2010, 19:45
FT: As I said, 'personally' a preference, and I know quite a few others who do the same. (SAS: I prefer the Huey this way too: sorry!)

What about critical azimuth? Any thoughts?

Encyclo
12th Jun 2010, 21:27
"FBH have called Bell but they weren't interested; no surprise there then. If you don't pay up front then nothing will be done.":eek:

A little surprised in that statement:

1) The Bell I know would jump on this and get to the root cause.
2) Last time I checked Bell doesn't charge for support.

spinwing
12th Jun 2010, 22:50
Mmmm ...

The company for whom I work (?) have 2 412EPs with AHRS and the 3 axis kit.

Nobody has reported anything like this on those machines in the past 8 years.

With the other 10+ (common or garden variety 412EPs) only one has reported anything like this occurrence and that was whilst carrying out an underslung operation (with both APs engaged) .... P1 still has the stains in his uniform pants, P2 reported (eyeballs uncaged rapidly) ...cause was put down to "collective bounce" don't know what engineering came up with ...


The noble quest (for information) continues ....

Good Luck :uhoh:

oldbeefer
13th Jun 2010, 08:33
OldBeefer
The bounce happened after I took control.


Well, there you are then! :}

212man
13th Jun 2010, 10:08
Well, there you are then!

seat-cyclic interface failure. Happens all the time....:E

wazz'n'zoom
13th Jun 2010, 11:03
212 & Oldbeefer.

Really constructive words and intelligently put.. not! :rolleyes: But if you put your head above the parapit then expect incoming....8-9-10 breathe.

Once again... the cyclic was not the root cause, IMHO it was the collective. I know what fore and aft oscillation feels like and this was not it. Quickly waggle the cyclic fore and aft a few inches and the 412 will shudder and buck about either AP in or out. We've all done this to prove a point to a student that small control movements will produce a stable hover.

Now if we had an FDR, we could analyse the data and work out if I f~+cked up or the cab is at fault.

Much like our 412 HAR2 that spun through 360 degs on initial take off a few months ago at Shawbury; everyone hear about that?? The FDR showed up exactly what caused the incident and we have all learnt from it.
I've learnt sod all from my incident except that I can't say MAYDAY when bouncing up and down like a fairground ride gone runaway and some other worldwide 412 drivers have had some scary incidents that they/engineers couldn't explain.

The quest continues.

Spanish Waltzer
13th Jun 2010, 11:26
Much like our 412 HAR2 that spun through 360 degs on initial take off a few months ago at Shawbury; everyone hear about that?? The FDR showed up exactly what caused the incident and we have all learnt from it.


What was the cause then?

wazz'n'zoom
13th Jun 2010, 11:51
What was the cause then?
Inadvertant selection of the wrong Mk 2 aircrew boot!
However it proved that the 412 can carry out a 360 deg spot turn in less than 3 seconds!!

oldbeefer
13th Jun 2010, 13:27
Not trying to be funny but, I'm still struggling to think of a scenario that you have described (after 8 yrs of trying to fix the beast) where putting the AP back in would cure the problem if it wasn't PIO.

SASless
13th Jun 2010, 13:53
Wazz...

Next time something like this happens....you might try taking your hand off the collective and very firmly brace the collective with your knee so it (the collective) cannot budge...not even a smidgen. Donuts to Dog droppings the wild ride will end almost immediately. That being said....there is something not quite right about the Collective friction set up on that aircraft I bet. Get the Gingerbeers to put the manuals aside....and do what they do best (well second best) and work their way through the collective control system link by link...bolt by bolt...and look for something subtle.

In the early days of flying the Chinook....we had a similar problem caused by a weak magnetic brake on the Thrust Lever (Collective Lever in Single Rotors), and every now and then took a wild rodeo kind of ride you mention. Trying to hold the Thrust Lever with one's hand only made it worse. The only way to effectively stop the Lever from bouncing was to do as I described....use a leg to act as a physical lock and firmly hold the Lever against it until the Bounce went away...usually only one or two bounces at most.

It would be worth trying anyway.

griffothefog
13th Jun 2010, 19:28
SaS,
with the utmost respect, I doubt if anyone has the reactions to do anything other than lower the lever After they have **** their pants with that sort of sudden occurrence!
Maybe it's my old age, but that's all I would probably manage :{
with over 3000 hrs on type, my only farts have been self inflicted :eek:

Encyclo
14th Jun 2010, 00:37
A Blast from the past :ok:

Here is General Operation Safety Notice 76-6:

OSN-GEN- 76-6
TO: ALL 206A/B/L, 204B, 205A1 AND 212 OPERATORS
SUBJECT: COLLECTIVE MINIMUM FRICTION
SOME OPERATORS ARE FAILING TO MAINTAIN THE REQUIRED COLLECTIVE MINIMUM FRICTION. ADJUSTMENT FOR COLLECTIVE MINIMUM FRICTION (NOT TO BE CONFUSED WITH COLLECTIVE FRICTION THAT IS AFFECTED BY ADJUSTMENT OF THE KNURLED NUT ON THE COLLECTIVE STICK) MUST BE ACCOMPLISHED ON THE GROUND BY QUALIFIED MAINTENANCE PERSONNEL AS SET FORTH IN EACH MODEL'S MAINTENANCE MANUAL*. MAINTENANCE OF COLLECTIVE MINIMUM FRICTION IS A CERTIFICATION REQUIREMENT BY THE FAA AND IS DESIGNED TO PRECLUDE "COLLECTIVE BOUNCE" BY:
1. REDUCING THE EFFECTS OF PILOT OVER-CONTROL.
2. REDUCING EFFECTS OF WIND GUSTS AND TURBULENCE THROUGH THE ROTOR INTO THE AIRFRANE.
3. PREVENTING FEEDBACK FROM EXTERNAL SLING LOADS INTO THE AIRFRAME.
IF "COLLECTIVE BOUNCE" SHOULD OCCUR WITH THE PROPER COLLECTIVE MINIMUM FRICTION, THE CAUSE MAY BE WORN LINKAGES, PILOT OVERCONTROL, EXTREME TURBULENCE OR HYDRAULIC CYLINDER IN NEED OF REWORK, WHICH SHOULD BE CORRECTED INMEDIATELY.
DO NOT FLY WITHOUT PROPER COLLECTIVE MINIMUM FRICTION.

The only reason the 412 is not listed at the top is because it wasn't invented yet (1976 versus 1980) but same logic applies.

Me agrees with SASless; collective bounce is the prime suspect :cool:

SASless
14th Jun 2010, 00:54
Griffo.....I thought perhaps I had given you the need for new undies a time or two when we flew together out of the sunny isle to the Ninian all those years ago! I know I was glad to be wearing Polish Depends....those Musk Ox rubber bags we rode around in.

As to my ease in suggesting the knee block is after those early years on Wokka's.....some of them had more road miles on them than air miles....having been towed back home more than a few times after a forced landing of some sort.....I got used to having the bejesus bounced out of me as the weak mag brake was a common problem and one got used to it after a while.

Being right under the forward rotor head made it all the more fun.....and unless one had the belts cinched right down....you could really make like a Mexican Jumping Bean.

We should all draw upon one another's experience and stow such little tricks in a handy place where we can call upon them when we first have the misfortune to redo what prompted the other fellow to pass on a word to the wise. I learned from other guys....as I felt I could not live long enough to make all their mistakes in addition to my own.

I make no claim to be a Wizard....just a Wizzer but I learn quick while leaned upon the Bar of Wisdom.....if you keep yer ears open while chugging down your Pint....the money spent for Beer is well spent as there can sometimes be a golden nugget picked up.

Matthew Parsons
14th Jun 2010, 17:49
I posted earlier, read all the added information and comments, and still think we're down to two possibilities:

1. Heave oscillation caused by airframe accelerations moving collective. This one seems to be eliminated by the fact that AP selection stopped the oscillation, however, AP selection required that at least one of the pilots put his hands elsewhere. PIO doesn't mean pilot's fault. Sometimes refered to as Pilot Involved Oscillation, it only requires that the pilot is somehow "in the loop". Hand placed on the control but applying no forces to move it is enough to change the control's reaction to aircraft acceleration.

Try placing both hands at the top of the steering wheel in your car, pull your body forward so the upper body is free to be moved by the car, then apply a small, quick input on the steering wheel. If your arms are locked, then every time the car swerves left, your body swerves right in the car frame of reference, causing the car to swerve right, etc.

2. Pitch oscillation (or multiple axis with cross coupling). I still think this is possible despite W&Z being a reliable eye witness, being fully aware of the pitch response of the 412, and insisting it was heave. The in-car experiment above reveals a completely different response to steering wheel inputs than if you just move the steering wheel back and forth a few times. By changing the interface of the pilot/driver with the control system, the (effective) mass properties of the controls change, and with that the frequency response and dynamic response will change. The pitch oscillation theory, I believe, is a stronger contender for the solution due to the fact that the APs selection appears to have eliminated the oscillation.

I am an ETPS graduate and have about 1500 hours on the 412, including extensive APs out flight and stability & control testing of standard and modified 412s. I have experienced divergent heave oscillations a number of times in an H46, so have learnt to quickly dampen them before it gets uncomfortable. There are many conditions on the 412 with APs out (or just about any aircraft) where an input may be able to incite a divergent control-input-due-to-aircraft-acceleration oscillation. However, most of the time our natural response is to dampen those vibrations. Changes to the control systems such as internal frictions, modifications to control heads, pilot involvement (active or not) and a short delay in initially damping the oscillations can drastically change the non-event into something similiar to what you've seen here.

wazz'n'zoom, good job in getting back safely and thanks for posting this event. The type of discussion it has generated is why I stay on this forum.

Caveat - I admit that this is all speculation and that one new piece of evidence could completely destroy these theories.

212man
18th Jun 2010, 05:01
I am an ETPS graduate and have about 1500 hours on the 412, including extensive APs out flight and stability & control testing of standard and modified 412s

At least if I'm wrong, I'll be wrong in good company:ok:

PS. The "seat-cyclic-interface" term was banter. My previous post was meant to be factual.

wazz'n'zoom
18th Jun 2010, 14:47
Matt P

Good theories from a sound POF and technical standpoint/grounding me old.

Will attempt to fly with one your ETPS colleagues next week (Night Sim permitting) to put the same a/c into the same configuration as the incident.

Wait out.

Spanish Waltzer
30th Jun 2010, 20:08
Wazz.........still waiting...........or did the re-fly not go too well:eek:

wazz'n'zoom
2nd May 2011, 21:30
At last a reply from Bell reference the "bounce incident". Names/ser numbers have been withdrawn for sensitivity reasons.

See what you lot reckon:

Firstly please accept my apologies for the long delay in answering your request regarding:
Flight Safety Occurrence Report fsor\Shawbury – RAF\60(R) Sqn\Griffin\##\#####2.
Bell Helicopter has reviewed the incident report of s/n #####7 regarding the abnormality your Griffin crew experienced and could not construe any plausible aircraft system fault that could duplicate the characteristic and duration of the vibration described.

Bell Helicopter considers the most likely catalyst for this short term vibration may have been from an external source.
It is possible that the rotor wash caused something to be picked up into the tail rotor which would create a momentary large span wise imbalance and generate a similar vibration to what was reported by your crew. Unfortunately without any physical evidence this cannot be proven.

Any debris caught up in the rotor would likely shed after a short duration eliminating the vibration which may have been in coincidence with the autopilots being selected ON.

During flight testing Bell Helicopter has observed that when the tail rotor is in a significant out of balanced span wise condition a vertical/fore/aft airframe (hop) vibration is felt at the pilot station.
It also should be noted that the AFCS was not designed to nor has any influence on dampening of vertical airframe vibrations.


Although I am confident that any of the following would have been mentioned in your very detailed report:
· By chance was there any video monitoring the helipad area at the time of this occurrence?
· After the incident was there any evidence of possible contact on either rotor system in particular the tail rotor blade and tail rotor hub?
· After the incident was there any evidence of FOD in the area where it occurred? (FOD can be thrown/ blown long distances).

If Bell Helicopter can provide any assistance with this or other items please do not hesitate to contact us again.
Regards,

It does not, for me, answer the questions posed to Bell in fact it leaves me none the wiser.