Ravin Aircraft down in Camperdown - 15 March 2014

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Re: Ravin Aircraft down in Camperdown - 15 March 2014

Unread post by spokes » Sun Sep 01, 2019 7:59 pm

It sounds a lot like Galloping Ghost at Reno when the trim tab flew off.
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Re: Ravin Aircraft down in Camperdown - 15 March 2014

Unread post by StressMerchant » Mon Sep 02, 2019 11:00 am

I have mixed feelings about this report. I appreciate that light aircraft accidents give the accident investigators a really hard time. The aircraft usually have no flight data recording facility, and engineering data may be absent. The investigators have their work cut out for them. However, the lack of information sometimes leads to assumptions that cannot be validated. Looking through this report, I wonder if the cause was really established.

As I read the chain of events (Section 3.2.1):
1. The pilot flew above VNE (actually point 3.1.9)
2. He initiated a pull-up manoeuvre
3. The trim tab separated
4. The elevator separated
5. The wings separated

1. Pilot flew above VNE: The only evidence available is that the pilot did a high speed flypast in an aircraft known to be “slippery”. I see no actual evidence that VNE was exceeded. And given that the pilot held an ATP, and that another pilot was in the right hand seat, I’d have thought that a simple “did not monitor airspeed” would not necessarily hold true. Also interested to see that the report mentions the aircraft exceeding VNE in the synopsis and conclusion, but does not mention it as a cause.
2. Pilot initiated a pull-up manoeuvre: The report from the manufacturer suggests that the wing is good for 4.4G limit. Why would a pilot initiate a 4.4G pull-up? That’s more than needed for a loop. And why exceed it, go all the way to ultimate? Did something change in the aeroelastic or aerodynamic characteristics that prevented the pilot from unloading the controls?
3. Trim tab separated: Why? Static overload? Fatigue?
4. Elevator separated: Why? From what I can see the elevator is a conventional type, not a stabilator. Did it overload? And when did the horizontal stabilizer separate? Or did it remain with the aircraft?
5. The wings separated: The report quotes the ICAO manual on classic break-up sequences, which suggests that if the horizontal tail fails first then the wings will fail in downbending. This is usually the case if the horizontal fails first. Yet the information from the manufacturer says that the wings failed upwards. This implies that the wings broke earlier in the sequence than the report suggests. In fact, reading the manufacturer’s info, they suggest that the wing failed and hit the empennage. Why did the report bother to detail the “tail first” sequence, if the information from the manufacturer supports a “wing first” breakup?

A few things that I would like to have seen addressed:
  • Did the turn to the left have any bearing on the events? Was it a result of the commencement of the breakup, could it have triggered the breakup, or was it coincidental?
  • Did the wings fail up or down? The manufacturer says “up”, would be nice to see the investigator make a formal statement of concurrence if he or she agreed. If not in agreement, what was the basis for the difference?
  • The report says that the most critical structural case (highest bending moment) was found to be with 50% fuel in the tanks and with the aircraft loaded to its “maximum” take-off mass of 1620kg. At what speed was this? And why 50%, why not empty? Is there a MZFW restriction?
  • Ailerons were balanced down to zero, but what about the elevator? Where is the trim servo?
  • Flutter of the tab may not be an issue with the system intact (due to system rigidity), but what about the case of a disconnected element? Were there any single failure points in the system?
  • What is the condition of the wing root ribs and adjacent skins? Any sign of the root rib itself failing?
  • What was the failure mode of the attachment points aside from the main spar? How did the rear attachment fail?
In my opinion, after reading this report, I get the impression that the cause was not really determined with any real certainty. Blaming the pilots is the easiest path since they are not around to defend themselves. Given the various potential scenarios, the probabilities may favour a pilot error, but I think that the report is not conclusive evidence of this. I would like to have seen a statement to the effect that even though the most likely scenario was an error by the pilot, the cause could not be conclusively proven.
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Re: Ravin Aircraft down in Camperdown - 15 March 2014

Unread post by liaan » Mon Sep 02, 2019 12:38 pm

Quick question for the Aero engineer boffins.

If you flying above VNO (191 for this aircraft) and then do an aggressive aleron input (not saying what happened here) , could the wing twist "up" enough (up aleron, forcing trailing edge down, increasing angle of attack) to exceed G loading on the wing ? (again not saying this what happened, just like to learn something)

L:


This is part making me ask ...
" The aircraft then descended into the valley in a south easterly direction. As the aircraft turned sharply to the left, a piece of structure was observed falling off the aircraft. The aircraft pitched up during a left hand turn, then the left wing folded upwards and detached from the fuselage. The right wing then immediately detached with a thud that was clearly heard. "
bla bla fishpaste

I need spell Checker
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Re: Ravin Aircraft down in Camperdown - 15 March 2014

Unread post by StressMerchant » Mon Sep 02, 2019 1:43 pm

Depending on the design, aggressive rolling manoeuvres at higher speeds can cause structural damage. Basically a few things can go wrong:

- At that speed the control surfaces are not designed for full deflection. The basic design aim is to design the aileron for full deflection at Va, with the wing being designed to take the resulting roll rate. At Vno the requirement is that the aileron can be sufficiently deflected to reproduce the roll rate experienced by a full deflection at Va, which will result in a smaller deflection at Vno. So a full deflection at high speed can overstress the wing.
- Depending on the wing torsional strength, a full deflection can twist the wing itself to the extent that the local angle of attack changes. Look up the issues with roll control reversals experienced towards the end of WW2.
- "Checking" a roll, ie aggressive reversing the direction of control input to stop the roll, can also cause issues at high speeds. There is a detailed report from the UK CAA involving a Piper Arrow (?) about 20 years back that makes for some interesting reading.
- Then, of course, there's always the problem of doing any of the above when combined with a G loading. The design case is the required roll plus 2/3 of the G limit.

Whether any of these actually happened is questionable.
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Re: Ravin Aircraft down in Camperdown - 15 March 2014

Unread post by Wayne01 » Tue Sep 03, 2019 6:45 am

Would loosing elevator components not produce a sudden climb or dive. Would wing loading ability be reduced if aircraft flying above vne?, is it possible that a 2g pull up is equivalent to say a 4g if above a certain speed. I'm referring to the stresses on the wing.
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Re: Ravin Aircraft down in Camperdown - 15 March 2014

Unread post by StressMerchant » Wed Sep 04, 2019 10:32 am

The loss of an elevator, tab, or stabilizer certainly could cause a rapid change in pitch. In most stages of flight for a typical light aircraft the horizontal stabilizer produces a downforce, ie it presses down at the back of the aircraft to stop the aircraft from pitching nose-down [Edit: was Nose up. See Volo's post below]. If the stabilizer fails, the aircraft rapidly pitches nose down. The pitch change is often so sudden, and the negative G loads so large, that the wings immediately fail in downbending. If an investigator finds that the aircraft wings failed in downbending, the logical “next step” is to look at the horizontal stabilizer.

At certain stages of flight the stabilizer may be supplying a “tail-up” balancing force, and if it breaks the nose will pitch upwards. The sudden onset of the change can cause structural overload in the up direction..

The difficult issue is the effect of the loss of a portion of the tailplane, such as a trim tab. Trim tabs have a massive effect on control forces, particularly at high speeds. Loss of a trim tab could cause a sudden change in control forces, or a significant change in control position. As someone pointed out, this is considered to be the prime cause of the Galloping Ghost accident at Reno. If the pilot was using the trim tab to trim the nose down, and the trim tab departed, one can easily imagine a rapid nose-up pitch change taking place.

The problem is compounded by the effect of the applied torque on the wings at high speed. As an aircraft accelerates in level flight, the total amount of lift remains constant – refer to the standard “four forces” diagram. Although the amount of lift remains constant, the distribution changes. This is particularly true in the chordwise direction – you could think of it as the centre of pressure moving backwards or forwards as the airspeed changes, although my aerodynamics colleagues would probably cringe at the description. At high speeds (for light light aircraft) the wing torque can be a critical case. We had a test wing for a certification program fail its load tests in the whiffle tree rig from a high speed case, even though it was nowhere near the highest G loading. Failure mode was structural instability of the root rib, caused by the torque (twisting).

Which comes back to the current case and the root cause. Merely exceeding VNE, whilst not smart, will not instantly cause disaster by itself. It needs another factor, for example hitting a gust at high speed or a linkage having too much freeplay, to cause catastrophe.

Looking at this accident, I can envisage scenarios where it could be purely pilot error. I could also envisage scenarios where a pre-existing issue with the aircraft could cause the problem. To me, the report does not convincingly identify the root cause.
Last edited by StressMerchant on Wed Sep 04, 2019 11:30 am, edited 1 time in total.
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Re: Ravin Aircraft down in Camperdown - 15 March 2014

Unread post by Volo » Wed Sep 04, 2019 10:55 am

StressMerchant wrote:
Wed Sep 04, 2019 10:32 am
The loss of an elevator, tab, or stabilizer certainly could cause a rapid change in pitch. In most stages of flight for a typical light aircraft the horizontal stabilizer produces a downforce, ie it presses down at the back of the aircraft to stop the aircraft from pitching nose-up. If the stabilizer fails, the aircraft rapidly pitches nose down. The pitch change is often so sudden, and the negative G loads so large, that the wings immediately fail in downbending. If an investigator finds that the aircraft wings failed in downbending, the logical “next step” is to look at the horizontal stabilizer.

At certain stages of flight the stabilizer may be supplying a “tail-up” balancing force, and if it breaks the nose will pitch upwards. The sudden onset of the change can cause structural overload in the up direction..

The difficult issue is the effect of the loss of a portion of the tailplane, such as a trim tab. Trim tabs have a massive effect on control forces, particularly at high speeds. Loss of a trim tab could cause a sudden change in control forces, or a significant change in control position. As someone pointed out, this is considered to be the prime cause of the Galloping Ghost accident at Reno. If the pilot was using the trim tab to trim the nose down, and the trim tab departed, one can easily imagine a rapid nose-up pitch change taking place.

The problem is compounded by the effect of the applied torque on the wings at high speed. As an aircraft accelerates in level flight, the total amount of lift remains constant – refer to the standard “four forces” diagram. Although the amount of lift remains constant, the distribution changes. This is particularly true in the chordwise direction – you could think of it as the centre of pressure moving backwards or forwards as the airspeed changes, although my aerodynamics colleagues would probably cringe at the description. At high speeds (for light light aircraft) the wing torque can be a critical case. We had a test wing for a certification program fail its load tests in the whiffle tree rig from a high speed case, even though it was nowhere near the highest G loading. Failure mode was structural instability of the root rib, caused by the torque (twisting).

Which comes back to the current case and the root cause. Merely exceeding VNE, whilst not smart, will not instantly cause disaster by itself. It needs another factor, for example hitting a gust at high speed or a linkage having too much freeplay, to cause catastrophe.

Looking at this accident, I can envisage scenarios where it could be purely pilot error. I could also envisage scenarios where a pre-existing issue with the aircraft could cause the problem. To me, the report does not convincingly identify the root cause.
....................................

I presume in your opening remark you meant "Nose - down " and not "Nose - up" .
The rest of your description of the aerodynamics' at play all makes sense and using your logic I believe that the root cause was the trim tab departing due to trim tab flutter that led to the tailplane failure , however the tailplane may have been taken off by the departing wing after an aggressive pitch up for the reasons you mentioned .
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Re: Ravin Aircraft down in Camperdown - 15 March 2014

Unread post by Dragon » Wed Sep 04, 2019 11:25 am

Volo wrote:
Wed Sep 04, 2019 10:55 am
StressMerchant wrote:
Wed Sep 04, 2019 10:32 am
The loss of an elevator, tab, or stabilizer certainly could cause a rapid change in pitch. In most stages of flight for a typical light aircraft the horizontal stabilizer produces a downforce, ie it presses down at the back of the aircraft to stop the aircraft from pitching nose-up. If the stabilizer fails, the aircraft rapidly pitches nose down. The pitch change is often so sudden, and the negative G loads so large, that the wings immediately fail in downbending.
....................................

I presume in your opening remark you meant "Nose - down " and not "Nose - up" .
The rest of your description of the aerodynamics' at play all makes sense and using your logic I believe that the root cause was the trim tab departing due to trim tab flutter that led to the tailplane failure , however the tailplane may have been taken off by the departing wing after an aggressive pitch up for the reasons you mentioned .
Volo, you are correct about the “Nose-down” - I’m sure just a slip of the keyboard.
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Re: Ravin Aircraft down in Camperdown - 15 March 2014

Unread post by StressMerchant » Wed Sep 04, 2019 11:29 am

Slip of the keyboard, or too much red wine ;-)

Thanks, edit in progress
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Re: Ravin Aircraft down in Camperdown - 15 March 2014

Unread post by nrm » Wed Sep 04, 2019 3:22 pm

The rudder trim tab was found in the grounds of the school, a long way behind where the aircraft went down, and a long way off the centreline of the runway, to the left.
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Re: Ravin Aircraft down in Camperdown - 15 March 2014

Unread post by StressMerchant » Fri Sep 06, 2019 1:25 am

I spent a little time last night looking through the accident report for ZU-ENR at Potch together with this one, and I do find the two reports mildly disturbing. In both cases the sequence started with an experienced pilot doing a high speed pass. In both cases, at the end of the pass, there was a sudden pull-up. In one case the aircraft did a slight roll before breaking up, in the other case the aircraft did a barrel roll before impacting the ground.

Considering the second accident (ZU-FHX), the manufacturer assessed that the left wing failed, followed almost immediately by the right. The failure of the left wing could account for the change in bank angle observed by the witness – if the left wing had started to fail, the right wing (still rigidly attached at the stage) would tend to roll the aircraft to the left.

Now take this a step further. If the left wing had partially failed, but not separated, what would the outcome have been? With the left wing not providing full bending moment balancing the right wing, the aircraft would have rolled further left. And given it had already pitched up, the result would have been something like a barrel roll. Which does sound a bit like the description of the first accident.

If the aircraft had been flying at high speed, with nose down trim to relieve the stick forces, a failure of the trim tab would cause a sudden force on the control column pushing it backwards – climb. If the pilot did not have a really firm grip, would easily slam all the way to the aft stop – which at high speed would result in a pitch-up and a massive G-loading.

Although the pilots may have been exceeding the redline speeds, it may be worth taking a good look at the trim tabs of the fleet. I’m not familiar with the Ravin’s construction, but a debonding between a metal component and a composite one is not unheard of. And very difficult to detect on a cursory walk-around.
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Re: Ravin Aircraft down in Camperdown - 15 March 2014

Unread post by nrm » Fri Sep 06, 2019 2:04 pm

He did not do a sudden pull up. That is according to eye witnesses, experienced pilots.
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Re: Ravin Aircraft down in Camperdown - 15 March 2014

Unread post by jimdavis » Fri Sep 06, 2019 4:13 pm

nrm wrote:
Fri Sep 06, 2019 2:04 pm
He did not do a sudden pull up. That is according to eye witnesses, experienced pilots.
Nev, I suggest that the eye-witness not seeing it, doesn't mean it didn't happen. I imagine it could happen in the blink of an eye and that it wouldn't necessarily involve a noticeable pitch. I am sure Stressors will be able to shed light on how quickly breakup sequence could happen. Would one necessarily observe a noticeable pitch up before everything came apart?

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Re: Ravin Aircraft down in Camperdown - 15 March 2014

Unread post by Dragon » Fri Sep 06, 2019 4:25 pm

I speak in general terms now, as opposed to my opinions on the details of the specific accident: Keep in mind that a sudden pitch up where the wings fail instantaneously is not perceived by onlookers as a pitch up because the wings do not always have enough time in contact to visibly change the trajectory of the fuselage.

Here is a good example where normally the aircraft would have pitched up significantly.


https://m.youtube.com/watch?v=ybYeJVh1cew
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Re: Ravin Aircraft down in Camperdown - 15 March 2014

Unread post by jimdavis » Fri Sep 06, 2019 7:23 pm

Dragon wrote:
Fri Sep 06, 2019 4:25 pm
I speak in general terms now, as opposed to my opinions on the details of the specific accident: Keep in mind that a sudden pitch up where the wings fail instantaneously is not perceived by onlookers as a pitch up because the wings do not always have enough time in contact to visibly change the trajectory of the fuselage.

Here is a good example where normally the aircraft would have pitched up significantly.


https://m.youtube.com/watch?v=ybYeJVh1cew
Very interesting, Dragon. Thanks. That's exactly what I was thinking - the wings have take the additional load before the nose pitches up.

I also find it interesting that the commentator said the explosions blew the wings off. i suspect the wings detached spewing fuel and that subsequently ignited. The sound of the two explosions after the event would simply have been the time it takes for the sound to reach the eye (ear) witnesses.

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