Bell 412EP AP Out Severe Vertical Bounce
 

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:

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?

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.

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:

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?

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.

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

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).

PO
The incident signal was completed within 2 hrs of the incident but I haven't seen the engineers report yet.

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

As the 'ex MTP', has to be pilot error!

:ok:

nice save and all

DD

Discussed here: Jamaican Defense Force writes off Bell 412EP

JDF 412
 

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.

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.

Anyone have any more info on this 412 incident?

Google is your friend.
AO-2008-039

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.

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!!

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.