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AS332L2 Ditching off Shetland: 23rd August 2013

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AS332L2 Ditching off Shetland: 23rd August 2013

Old 12th Sep 2020, 14:18
  #2481 (permalink)  
 
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I've been using that accident regularly as an example of how not to use a 3-axis AP.
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Old 14th Sep 2020, 17:25
  #2482 (permalink)  
 
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Originally Posted by nomorehelosforme View Post

He said in the UK, the pool is reasonably warm, and you are given a thorough briefing before.

UK News
Things have changed since his day - the pool at Yeovilton is somewhat chilly these days!
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Old 15th Sep 2020, 10:17
  #2483 (permalink)  
 
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Originally Posted by [email protected] View Post
I've been using that accident regularly as an example of how not to use a 3-axis AP.
or how not to use a 4-axis AP!
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Old 15th Sep 2020, 10:41
  #2484 (permalink)  
 
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or how not to use a 4-axis AP!
Yes, absolutely
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Old 15th Sep 2020, 20:21
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Originally Posted by [email protected] View Post
Yes, absolutely
So. Why was this approach not flown fully coupled?

The AAIB reviewed the last 1000 ft of over 3000 onshore approaches where upper modes had been used. They found that less than 20% of these were flown with 4-axes engaged.


Last edited by marcr; 15th Sep 2020 at 22:31. Reason: Better description of data used and punctuation.
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Old 16th Sep 2020, 18:40
  #2486 (permalink)  
 
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It was common practice to fly the L2 in 3 axis as the IAS hold was quite poor. To fly 4 axis, you had to be very gentle with any speed changes or the aircraft would exchange altitude for speed. During a 200’ ARA this could be quite exciting. There’s no issue flying 3 axis as long as you monitor and apply power as the aircraft levels. Have I ever had to remind the PF to raise the lever in 3 axis...yes.
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Old 17th Sep 2020, 02:57
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An enlightening insight, cyclic.
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Old 17th Sep 2020, 13:59
  #2488 (permalink)  
 
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Originally Posted by cyclic View Post
It was common practice to fly the L2 in 3 axis as the IAS hold was quite poor. To fly 4 axis, you had to be very gentle with any speed changes or the aircraft would exchange altitude for speed. During a 200’ ARA this could be quite exciting. There’s no issue flying 3 axis as long as you monitor and apply power as the aircraft levels. Have I ever had to remind the PF to raise the lever in 3 axis...yes.
Not wanting to dispute the above but the AS332L2 deserves a bit more credit than this. ALL helicopters have a the same Power-Required for Level Flight Curve. This curve reverses as we pass through Vy!
If flying in 3 axis, with ALT hold engaged and the power setting is insufficient for level flight at Vy, the venerable AP will continue to raise the nose, speed decaying, to maintain the commanded altitude ([ALT]
If the power has been set low enough (paradoxically), the speed will decay through Vy. Now things get really exciting as the power demand for level flight increase exponentially as each knot of IAS is lost.
The AP keeps raising the nose in a departed effort to maintain the commanded ALT. Eventually a ROD will set in, now at low IAS, and if the crew do not intervene quickly enough and/or the surface is not far below the helicopter, a powered recovery may simply not be possible in time to prevent impact with the surface.

NOTE, the IAS and the ALT hold on the AS332 is more than competent. (as actually demonstrated by this accident). However, in 3 axis mode, power set below that required for level flight at Vy in any part of the envelope will result in a descent, required or otherwise.

This problem should be the No1 lesson for all 3 axis systems and 4 axis systems when operated in 3 axis.

As an aside, AIRBUS APM 2000 (EC225/175/145/135) incorporates flight envelope protections to "save" the crew/helicopter from these potentially disastrous mistakes.

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Old 17th Sep 2020, 15:35
  #2489 (permalink)  
 
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This was not with ALT set, it was with VS engaged but the same concept applies.

You would expect a channel to disengage along with a warning with reducing IAS below a certain level (60 Kts for example) rather than stay engaged all the way to VRS.
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Old 17th Sep 2020, 16:01
  #2490 (permalink)  
 
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Hi Crab, in the AS332L2, an engaged mode that cannot hold the selected parameter +minus a small value, will flash amber. No aural warning.

My understanding of this approach was the V/S was active to a preset ALT.A. So ALT would deploy at the set altitude. The previous stabilised approach during the descent then destabilised as there was no automatic or manual modulation of the power (collective).

As an aside, we spent many years teaching ourselves to reduce the IAS for an approach to minimums. Odd really when our FW brethren cant do that and have higher speeds and lower minimums than us. In 3 axis AP and modes its seems more prudent to mandate an approach speed at a minimum IAS of 100 KIAS. This provides some reaction time to modulate power etc. Flying a descent at Vy leaves little or no margin for correction if the IAS is allowed to tumble backwards with the PV curve working against the crew.

For 3 axis approach it is better to fly with IAS engaged for the descent as in order to maintain the glidepath the PF must routinely modify the collective setting which in turn keeps the control in his conscious mind at any level out for an MDH/A. This combined with a sensible IAS for the descent provides some protection albeit human rather than automatic.
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Old 17th Sep 2020, 16:49
  #2491 (permalink)  
 
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Agreed DB - on N3 and 412EP I have either flown with IAS coupled and manual collective or ILS/GS coupled just helping it with a small collective reduction at top of drop when the GS captures - all flown between 90 and 120 kts. Or just done it the old fashioned manual way

If I remember the accident report, they were already behind the drag curve of reducing IAS (not enough power applied) before they reached MDA (which is what they would presumably have had the ALTA set to)
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Old 17th Sep 2020, 18:01
  #2492 (permalink)  
 
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As an aside, we spent many years teaching ourselves to reduce the IAS for an approach to minimums. Odd really when our FW brethren cant do that and have higher speeds and lower minimums than us.
Every ILS I have flown all around the World have had thousands of feet of concrete behind them to allow you to stop. Why make things difficult by slowing down on the approach.

The faster you come down the slope the less time you have to cock it up; smaller groundspeed differences and less drift.

Last edited by Fareastdriver; 17th Sep 2020 at 19:48.
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Old 17th Sep 2020, 22:07
  #2493 (permalink)  
 
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Whether or not you can, or indeed how you should, carry out a 3-axes approach is not the issue here.

The question remains - why would anybody CHOOSE, especially in marginal weather, to perform a 3-axes approach in a 4-axes capable aircraft? Or are we now advocating mixed mode flying?

If a similar coupled approach data set was analysed for the 225 we are unlikely to see even 2% of approaches flown 3-axes, let alone 82%.

So, is it the case, as Cyclic contends, that the L2 "IAS hold was quite poor" or is it reasonable to argue that, as DB puts it, "the IAS and the ALT hold on the AS332 is more than competent"?

The factual information gathered by the AAIB, and presented in their report, seems to support Cyclic's statement. Whether or not they were correct in doing so, for some reason, pilots were voting with their feet and electing to fly an approach in the L2 with only 3-axes coupled, the majority of the time.

This data was drawn from the operator's FDM records and that raises some further questions. If the clearly identifiable, mixed mode, 3-axes approaches were not neccessary, as the autopilot "was more than competent", then you might be forgiven for assuming that these events would be flagged to the training department for consideration and correction. On the other hand, if the evidence suggested that the "IAS hold was quite poor" you would perhaps expect to see some communication with the OEM to highlight the issue, and the associated feedback regarding the fault investigation and intended resolution.

However, if nothing was being done to address the perceived, or otherwise, problem with engaging a 4-axes coupled approach on the AS332L2 Super Puma, and pilots were being left to establish their own "work around" - then some might say, this was already "an accident waiting to happen".

Taking the quoted news headline,

"Helicopter would not have crashed if pilot had......"

I'll let you fill in the many blanks, which I'm sure will also be explained in detail to the inquiry.


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Old 18th Sep 2020, 05:57
  #2494 (permalink)  
 
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Marcr, I cannot ever remember flying an IAP in 3 axis mode in the L2 except during training for degraded AFCS approaches. Prior to the EC225 it was the bestAP out there., L2 AP had no real disabilities when operated within its published AFM limitations. Certainly the IAS hold was competent as were all other modes.

Not wanting to go over old ground but the I remember that the general consensus at the time was had the helicopter been operated in 4 axis this accident would most likely not have happened. It was unusual approach. LOC ONLY. 09 to LSI. Maybe the mindset of the lack of G/S continued into the AFCS approach plan.

After this accident further tightening of industry policies for use of upper modes was widespread as it had started to be after the EC225 went into the sea on a night approach several years earlier. Hopefully, in this respect, crews are better informed and controlled by mandatory policies. Cue the old nutmeg “What about manual flying skills” well, we have to train hard to fight easy. All skills must be kept fresh......but just don’t do the degraded ones in IMC unless you have a real degrade.

Culture and common sense have to coincide.
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Old 18th Sep 2020, 06:39
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From the summary of the accident report
The commander was the Pilot Flying (PF) on the accident sector. The weather conditions were such that the final approach to Runway 09 at Sumburgh Airport was flown in cloud, requiring the approach to be made by sole reference to the helicopter’s instruments, in accordance with the Standard Operating Procedure (SOP) set out in the operator’s Operating Manual (OM). The approach was flown with the autopilot in 3-axes with Vertical Speed (V/S) mode, which required the commander to operate the collective pitch control manually to control the helicopter’s airspeed. The co-pilot was responsible for monitoring the helicopter’s vertical flightpath against the published approach vertical profile and for seeking the external visual references necessary to continue with the approach and landing. The procedures permitted the helicopter to descend to a height of 300 ft, the Minimum Descent Altitude (MDA) for the approach, at which point a level-off was required if visual references had not yet been acquired.

Although the approach vertical profile was maintained initially, insufficient collective pitch control input was applied by the commander to maintain the approach profile and the target approach airspeed of 80 kt. This resulted in insufficient engine power being provided and the helicopter’s airspeed reduced continuously during the final approach. Control of the flightpath was lost and the helicopter continued to descend below the MDA. During the latter stages of the approach the helicopter’s airspeed had decreased below 35 kt and a high rate of descent had developed.
The investigation identified the following causal factors in the accident:
  • The helicopter’s flight instruments were not monitored effectively during the latter stages of the non-precision instrument approach. This allowed the helicopter to enter a critically low energy state, from which recovery was not possible.
  • Visual references had not been acquired by the Minimum Descent Altitude (MDA) and no effective action was taken to level the helicopter, as required by the operator’s procedure for an instrument approach.
The following contributory factors were identified:
  • The operator’s SOP for this type of approach was not clearly defined and the pilots had not developed a shared, unambiguous understanding of how the approach was to be flown.
  • The operator’s SOPs at the time did not optimise the use of the helicopter’s automated systems during a Non-Precision Approach.
  • The decision to fly a 3-axes with V/S mode, decelerating approach in marginal weather conditions did not make optimum use of the helicopter’s automated systems and required closer monitoring of the instruments by the crew.
  • Despite the poorer than forecast weather conditions at Sumburgh Airport, the commander had not altered his expectation of being able to land from a Non-Precision Approach.
Interestingly (just re-reading the report) They had identified an intermittent problem with the collective friction on climb out - it appeared to throw off a small amount of pitch after the lever was set at the desired Tq.

Second interesting point for DB - they did not arm the ALTA for the approach.

Last edited by [email protected]; 18th Sep 2020 at 06:50.
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Old 18th Sep 2020, 06:51
  #2496 (permalink)  
 
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Hi Crab, with the V/S installed in 3 axis mode it is active on the Cyclic "Pitch" channel. I was fairly certain they had set an ALT.A at MDA.
The incipient reduction in IAS during the descent would have been small compared to the effect when the ALT engaged automatically at the MDA.

During our L2 Type Training at the same Operator, the Training Staff went to great lengths to demonstrate this effect to us to dissuade us from engaging in this potentially dangerous practice,. We called it "The L2 tail slide" as that is what it felt like at the extreme.

As an aside, the PM took a hit for not monitoring the IAS. However, at the point of level off at MDA, the PF should be primarily monitoring the Instruments and the PM looking out for the Required Visual References. It raises an interesting argument and further supports the practice of CDFA where there is no level off and therefore no prolonged attempt to gain visual references and thus less opportunity to lose the IAS.

This single accident raised a lot of issues. All inter related and all significant.
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Old 18th Sep 2020, 08:44
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This single accident raised a lot of issues. All inter related and all significant.
Agreed - if you had a known or suspected collective issue, wouldn't you be extra vigilant knowing you had selected a cyclic mode (VS) where power controls speed? he seems to have made a power adjustment at 80 Kts but not looked to see if it had the desired effect of stablising IAS.

Personally I dislike CDFA profiles - may be my age - but the ability to level off at MDA and run in until the MAPt is ideally suited to helicopters in marginal conditions.

The fact that they managed the 'L2 tail slide' without using ALT.A speaks volumes for the lack of CRM.
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Old 19th Sep 2020, 02:29
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You have to remember, the L2 was a very early first generation glass cockpit helicopter and coupler.

Compared to anything else I’d flown before, it was without doubt the best thing since sliced bread.

Treated with respect the 3 axis AP function was more than adequate

Like any other it did have vices which could jump up and bite.

The vertical bar indication for IAS was difficult and non intuitive

Thankfully with time progress has been made with AP flight envelope protection
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Old 19th Sep 2020, 12:02
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Despite DBs assertions that the L2 AP was great, that wasn't a view shared by the majority of line pilots. The collective coupling was very slow to react to required changes and then had a habit of overreacting in an attempt to play "catch-up" and could cause momentary overtorques. As the data in the AAIB report indicates, most crews would chose not to use the collective channel i.e. only use 3 axis coupling.

The choice of 3 axis rather than 4 axis coupling wasn't itself a fundamental mistake, it was the choice to couple altitude to pitch rather than speed. The lack of speed monitoring at 80 KIAS and below was the final hole in the Emmental.
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Old 19th Sep 2020, 12:38
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it was the choice to couple altitude to pitch rather than speed.
or rather to ask the AP to use the cyclic channel to control RoD (VS) leaving the pilot in the counter-intuitive position of controlling speed with collective.
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