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-   -   CRJ down in Sweden (https://www.pprune.org/rumours-news/572882-crj-down-sweden.html)

FIRESYSOK 13th Jan 2016 16:26

There is no manual trim wheel on this type either.

It's happened to me that both pitch trim channels failed at cruise speed and we were left with an uncomfortably out-of-trim airplane as we slowed for approach.

I can only imagine if a HS runaway were not disconnected early via disconnect switches or CB, what the elevator forces would be with no manual trim reversion to correct the problem.

Machinbird 13th Jan 2016 17:51


I can only imagine if a HS runaway were not disconnected early via disconnect switches or CB, what the elevator forces would be with no manual trim reversion to correct the problem.
I'm surprised that the aircraft would be certified without some means to recover to a good trim setting. If all you have is "Jack be nimble, Jack be quick, as a defense before it gets away from you, then that isn't too good. If you are flying on autopilot and the first you know of a problem is that the autopilot has been overpowered, and the aircraft is pitching down, then you are already too late. Even the relatively simple Douglas designed, THS equipped, A-4 Skyhawk had a pitch trim "crowbar" lever that let you manually overpower whatever signals were trying to move the pitch trim actuator screwjack.

Limited information I've seen on the CRJ indicates that there is probably a pitch trim force warning light before the actual A/P disconnect. Is that correct?

Chronus 13th Jan 2016 18:37


Originally Posted by _Phoenix (Post 9235920)
It think the crash site is about at right distance for descent initiation.
Is it possible cargo shifted forward once aircraft pitched down?

Phoenix`s question is worthy of further consideration.

Flight was airborne at 23:10 and crashed at 00:18. I estimate distance to destination was approx 1000 miles, possibly duration of some 2hrs or so.
The crash site near Akkajaure Lake and Tipel (FIR Bdry) would be some 400 miles to run to destination.
I would have thought it early for TOD.

Am not familiar with route. Anyone familiar with the route may perhaps enlighten us further.

172driver 13th Jan 2016 18:59


I estimate distance to destination was approx 1000 miles
604 nm according to Great Circle Mapper

AliTee 13th Jan 2016 19:23

I have experienced it in the sim. It is violent and uncontrollable unless immediate action is taken. At low altitude it is fatal. The change in attitude is so sudden that if you don't have your wits about you, you forget to push the master disconnect and just pull on the yoke for dear life. I suspect that they may have had a runaway, were taken completely by surprise and didn't know what hit them. If I remember correctly, Stab Trim Runaway did not even have memory items until late 2000's. Can any old CRJ guys remember when this was changed in the QRH?

AliTee 13th Jan 2016 19:44

The initial report leads one to believe that they were still at cruise altitude. No report of a request or clearance to descend. Descending normally from FL330 would only require about 120 NM (including decel). Additionally there is high terrain south of Tromso that keeps you at or above 5000ft until inside about 11 miles if landing Rwy 01 (~80% or more), so no, I don't think they were descending.

khorton 13th Jan 2016 22:27


Originally Posted by Machinbird
I'm surprised that the aircraft would be certified without some means to recover to a good trim setting. If all you have is "Jack be nimble, Jack be quick, as a defense before it gets away from you, then that isn't too good.

The CRJ trim system, by design, should detect un-commanded motion and shutdown. The problem referred to by some earlier posts is that there was a manufacturing defect at a supplier, and the circuit boards for the horizontal stab trim control unit (HSTCU) did not have the required conformal coating applied to the boards during manufacturing, which left the traces on the circuit boards exposed to the atmosphere. The result was that after many years in service, the traces on the boards could corrode, and the corrosion could bridge adjacent traces, creating circuits that weren't supposed to be there. This resulted effectively in a circuit diagram that differed from the design, creating paths for electricity to flow where it shouldn't, resulting in uncommanded trim motion.

It is well and good to have a perfect design, but you need to ensure the delivered product actually conforms to the design, or all the bright work of the designers is for naught.

This issue was covered by Transport Canada AD CF-2006-20. This AD, among other things, required the installation of new HSTCU, with conformal coating on the circuit boards. The accident aircraft should have had the new HSTCU installed many years ago, so accident should not be directly related to this old issue.

Machinbird 14th Jan 2016 00:32


The CRJ trim system, by design, should detect un-commanded motion and shutdown.
Thanks khorton,
The AD indicates that there are disconnect switches on each control wheel and I have seen pictures indicating a red 'pitch disconnect' switch on the left side of the center console.

When the autopilot is engaged, it has authority to command pitch trim motion, doesn't it? This type trim in a mechanical control system is usually activated by force sensing systems in the control linkage. Is that the case on the CRJ?
If so, then jams in the pitch control system can probably cause trim runaways.

Important caveat. I know next to nothing about the specific design characteristics of the CRJ. It is just that this specific situation has resulted in trim runaways in a number of different types of aircraft. Is there a common link? Design engineers are clever people and they learn from past problems and come up with novel solutions. They may well have circumvented this type problem with their HSTCU design.

_Phoenix 14th Jan 2016 04:21


Descending normally from FL330 would only require about 120 NM
... the crash site is about 125 NM out of Tromso
Great Circle Mapper

khorton 14th Jan 2016 09:53

There are stabilizer trim disconnect buttons on each control wheel. The red "PITCH DISC" T handle on the left, front of the centre console is for jams in the control linkages in the pitch axis - it disconnects the linkage that connects the left and right control wheel in the pitch axis. If you pull that T handle, the left control wheel is connected only to the left elevator, and the right control wheel is connected only to the right elevator, allowing for continued control by one of the pilots after a jam in the pitch control system. A similar ROLL DISC handle on the right side of the centre console is used for jams in the roll axis control linkages.

The autopilot will command pitch trim to zero out any required elevator servo commands, after a short time delay (to prevent it from commanding trim in response to short duration elevator commands). The autopilot receives elevator position information, and I believe it would disconnect if the elevator position did not match the result expected from the amount of servo current, but I'm not completely certain of this.

Machinbird 14th Jan 2016 15:19


The autopilot will command pitch trim to zero out any required elevator servo commands, after a short time delay (to prevent it from commanding trim in response to short duration elevator commands). The autopilot receives elevator position information, and I believe it would disconnect if the elevator position did not match the result expected from the amount of servo current, but I'm not completely certain of this.
khorton
Thank you again for your helpful descriptions.
This implies that there must be both right and left elevator servo valves and that the linkages between the control columns and the elevator servo valves are duplicated, probably all the way back from the cockpit. Since each elevator side is actuated by 3 cylinders on 3 different hydraulic systems, the servo valves are either a single valve body with 3 sections or actually 3 separate valves on each elevator side. Followup signals would probably be by means of a linkage attached to the individual elevators which is then summed with the associated input linkage. Autopilot trim inputs would then be taken directly off the summing linkage per your description. Then the question arises, are autopilot trim inputs taken from both sides and compared or from just one side? Or have I run well off the track?:}

Those design engineers are clever fellows and often do the unexpected to achieve their goals. I tried to download system information from Scribd, however didn't find much of great use.

khorton 14th Jan 2016 15:46

There are three separate hydraulic actuators for each elevator. The servo valve controls have jam-tolerant mechanisms, so a jam in one servo valve control will not cause the whole system to be jammed. I'm not sure how the servo valves receive their position feedback from the elevators.

There are pitch control runs from each control wheel back to the elevator servos - left control wheel connected to left elevator servos, and right control wheel connected to right elevator servos. The two control wheels are connected via the disconnect mechanism (actuated by the PITCH DISC T-handle).

The autopilot commands to the HS trim have their source in the autopilot servo torque that is commanded. If the servo torque required exceeds some threshold for a minimum time, then the autopilot sends a command to move the HS.

Studying the maintenance manual a bit more deeply, it looks like my earlier statement that the autopilot receives elevator position feedback was probably incorrect. I don't see any such input on the spaghetti diagram for the autopilot.

Chronus 14th Jan 2016 18:20

[QUOTE=_Phoenix;9237934]... the crash site is about 125 NM out of Tromso
[url=http://www.gcmap.com/mapui?

TIPEL is 158.4 nm R-196 TRO. I would therefore agree with Phoenix that the likelihood of loss of control initiated with the descent from cruise level is probable.

captplaystation 14th Jan 2016 20:02

It may be if the ATC transcript included a request for & clearance to . . descend. If not, well, I am guessing, as professional pilots, they didn't just start on down.

158 out is a bit early for a descent from F330.

espenjoh 15th Jan 2016 16:04

mil radar data show that plane make a right turn to the east when desending.
LINK

balaton 15th Jan 2016 21:34

Hi All,

Cl-604 incident (similar a/c systemwise).
https://www.gov.uk/aaib-reports/aar-...-november-2005

Cheers

Tamas

khorton 16th Jan 2016 22:42


Originally Posted by balaton
Cl-604 incident (similar a/c systemwise).
https://www.gov.uk/aaib-reports/aar-...-november-2005

That incident was caused by corrosion on the horizontal stab trim control unit (HSTCU). There were several similar incidents, which triggered the AD mentioned earlier in this thread. That AD required replacement of the HSTCU with ones that had a conformal coating on the circuit board, to prevent such corrosion. Thus this accident should not have been caused by the same problem.

despegue 17th Jan 2016 09:14

Is there any info on the loadplan?

A loadshift forward when starting descend due to no or broken forward uplocks will make a Freighter going in an unrecoverable vertical dive.

Livesinafield 17th Jan 2016 10:55

The laodshift theory doesn't work, the aircraft is loaded with mail trolleys, each bay is packed tight with trolleys that are strapped to the floor, then each compartment has a spider net close the compartment off

I do this kind of loading all the time and feel its much safer than carrying a bulk load of loose items or even containers, each trolley is a fairly light weight 80-110KG on average, if the load was not secure correctly im pretty sure some catastrophic would have happened on take off or in the climb

Just toadd that this loading is done with the FO as a supervisor instructing where to put each trolley so I would be very very surprised if it wasn't secured

YOUNGBUCK 19th Jan 2016 16:37

I'm not type rated on the CRJ so don't jump at my vague information.

I've heard that looking thru the tech log pages there was a problem with the stab motor which had been replaced, perhaps that wasn't the fix. Descent and path down suggest problems with the tail.
Condolances to the families friends and colleagues.


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