Helicopter down this evening in Norway: July 2011 [Archive]

GenuineHoverBug

2nd Nov 2012, 14:39

The report was published today and the english summary goes like this:

As the helicopter started the descent for an approach in mountainous terrain, witnesses observed it turning tightly to the right. During the turn, control of the helicopter appeared to be lost, with a high bank angle and a steep descent. At the end, it seemed as if control was about to be regained, but the helicopter hit the ground hard about 500 metres short of the planned landing site and immediately caught fire. All five persons on board were fatally injured. The helicopter was a total loss.

The investigation has not revealed any technical defects or irregularities that could have influenced the course of events. The Accident Investigation Board Norway considers it likely that abrupt manoeuvring initiated a sequence where control of the helicopter was partly lost for a period, and that the height was insufficient for the commander to recover in time. The AIBN believes that the hydraulic system may have reached its limitation during the manoeuvring, resulting in the phenomenon servo transparency, (also called jack stall) occurring.

Three safety recommendations are issued in the report, dealing with limitations during manoeuvring with passengers on board, warning of servo transparency hazards and the need for flight recorders.

An english translation of the full report is availble here: AIBN 2012/13 eng (http://www.aibn.no/Aviation/Reports/2012-13-eng)

Anthony Supplebottom

2nd Nov 2012, 17:07

Is someone willing to provide a description of servo transparency, how it happens, what is feels like and how to avoid it in the Squirrel?

John R81

2nd Nov 2012, 17:56

Try here http://www.aaib.gov.uk/cms_resources.cfm?file=/Eurocopter%20AS350B2%20Squirrel,%20G-CBHL%2002-09.pdf from page 90.

jymil

2nd Nov 2012, 19:02

In short, aggressive maneuvers and/or excessive G load can result in control forces which overpower the hydraulic system, leading to stiff controls and an uncommanded roll and pitch up.

You can easily avoid it by not doing any crazy stunts.

GenuineHoverBug

2nd Nov 2012, 19:27

Chapter 1.18 (page 30-45) in the AIBN report also has a quite detailed description of what it is and what it might feel like. It looks in particular at ST in right turns, similar to the AAIB report on the Scottish accident referenced above. The AIBN description and calculations appears to have been developed with/accepted by Eurocopter.

Flyting

2nd Nov 2012, 20:11

It's a crap your pants feeling... I've had it in a BA while doing a hard right hand turn when all of a sudden the controls locked up. Luckily I had space to fight through the recovery. Also had it in a B3 with a faulty servo while coming in to land...with only a 20-30° bank at about 40 kts... Luckily it was only a partial effect but being that close to the ground wasn't comfortable :sad:

Like Jymil says..... avoid the excessively hard menouvres and it will avoid you...

Anthony Supplebottom

2nd Nov 2012, 20:14

Thanks for the feedback.

leading to stiff controls and an uncommanded roll and pitch up.

The report citing Eurocopter states:

the phenomenon will induce an uncommanded right cyclic force and an associated down collective reaction.

Will the a/c always pitch up with down collective?

Is the Squirrel more sensitive to jack stall/servo transparency than other types? Does it manifest the same way in other types?

ShyTorque

2nd Nov 2012, 20:21

Rather puzzling that it occurred in the descent though. Collective pitch would normally be reduced to descend, taking the aircraft further from the limit of the hydraulic system.

Also, from the photos posted here, the tail rotor blades appear to be almost undamaged.

Aesir

2nd Nov 2012, 21:24

Seems to me they have no clue what happened.

Actually after reading the full report that´s what they say. it´s an educated guess. Excellent report i must say.

Servo transparency is a good a guess as any other but to get into it to the point of crashing into the ground takes some pretty aggressive high speed manuvering.

The AIBN believes excessive maneuvering may have led to servo transparency, and that this
may have contributed to the accident. It is important that helicopter pilots flying helicopter
types that are equipped with a single hydraulic system are familiar with the limitations and are
aware that the margins in some cases are reduced faster than expected. The AIBN has noticed
that more knowledge and understanding of the phenomenon is needed.

GenuineHoverBug

2nd Nov 2012, 21:36

I guess that is one of the possibilities that are discussed. If the collective was not reduced before the 60-90 degrees bank turn, that could have made all the difference, and caused the forces to exceed the threshold for ST onset. Ther are some calculations and graphs in there, with Eurocopter's blessing, which they use to justify their speculations.

Although most would see it as natural to reduce the collective before a descending turn, some might wish to keep the speed up if they e.g. should fall for, as discussed in the report, "the temptation of giving the passengers an extraordinary experience at the end of the ride". (Which may be an AIB way of saying "showing off"?)

As to what happens in the controls, that is explained by the EC training department slide on the very last page of the report. It will move the cyclic to roll right, pitch up and lower the collective IF the pilot takes no action to counter the ST. But as demonstrated in FAA testing, it can be held in ST for a prolonged time.

500guy

2nd Nov 2012, 22:15

Thank god. No hydraulics!
But it soulds like this one could also be an in-flight loss of t/r drive.

Jack Carson

2nd Nov 2012, 23:42

It is my understanding that to encounter servo transparency one has to have a pretty significant pitch rate. This can be encountered in either a high bank angle (high “g”) turn or during a sharp pitch attitude pull up. In any case, relaxing the severity of the maneuver unloads the servo and returns the system to normal operation. The relative violence the mishap aircrafts’ maneuvering show similarity to the ENG AS-350 that crashed in New York City some years back. In that case, I believe that the cause was a hydraulic failure at a very critical point in the flight envelope.

Anthony Supplebottom

3rd Nov 2012, 07:06

Are there many cases of this happening in Bells?

GenuineHoverBug

3rd Nov 2012, 08:12

If this report is anything to go by, you do not need to speculate anymore where servo transparency may be encountered. The table on page 35 shows the g-force required at different TAS for the accident flight conditions:

90 kts -2.09 g
100 kts -1.97 g
110 kts -1.86 g
120 kts -1.74 g
130 kts -1.63 g
140 kts -1.51 g
150 kts -1.40 g

At 130 kts 1.63 g is reqired. If I remember correctly that equals a level, constant speed turn at just over 50 degrees of bank. Which is perhaps not that agressive a manouvere?

The graphs on the next couple of pages shows the ST onset threshold for other masses, altitudes and temperatures.

[email protected]

3rd Nov 2012, 11:06

Somewhat worse than the Gazelle which would tolerate 2g @ 120kts without much fuss but would jackstall at VL (168 kts) without too harsh an application of aft cyclic).

1.4g is about where the g becomes perceptible (a 45 degree level turn)

GenuineHoverBug

3rd Nov 2012, 11:30

I should perhaps have specified the parameters:
Mass 2 148 kg
Pressure alt 3 150 ft
OAT + 18 degrees C

And 130 TAS equals about 120 IAS at that DA

One of the other graphs shows that at sea level on a standard day, the threshold would increase to about 1,85 g at that mass.

jymil

3rd Nov 2012, 11:31

Load factor formula is: n = 1 / cos bank angle. 1.74 G would be 55 degrees.

Keep in mind this formula is only valid for balanced turns. Otherwise it would be impossible to fly a 90 degrees turn with a helicopter, because the G force would be infinite.

Aesir

3rd Nov 2012, 11:56

Are there many cases of this happening in Bells?

Servo transparency (ST) can not be reproduced in Bell helicopters. At least not in any flyable extreme manuvering. Parts will be breaking off the aircraft before the jacks will stall.

However on AS350 this is really no problem as long as pilots know and understand it´s limitations. True for any helicopter type.

I teach and show pilots actual jack stall in 350´s during training and LPC/OPC´s so people will know where the limits are and avoid the area at lower altitudes. The new AS350e should be less susceptible to ST but at the moment has other problems which will be fixed.

GenuineHoverBug

3rd Nov 2012, 12:03

It apparently does not have to be very aggressive. The quote from the EASA OEB report (http://easa.europa.eu/certification/experts/docs/oeb-reports/eurocopter/EASA-OEB-Final-Report-Eurocopter_AS350_Family_(B3e)-04-06082012.pdf) seems to describe a rather benign excercise:

“8.9.2 Demonstration methodology for Flight Instructors and Type Rating Instructors:
Servo-transparency (called also servo-reversibility):
Except for EC 130 B4 and AS 350 B3 Arriel 2B1 & AS 350 B3e when fitted with dual Hydraulic system.
The servo-transparency training could be performed in the following way:
- Complete procedure should be performed above 1000 ft (AGL),
- Achieve airspeed between 130 and VNE (with a rate of descend),
- Perform a 30° left turn,
- Slowly increase the load factor by a backwards cyclic action,
- When the servo-transparency is achieved, the tendency of the aircraft is to pitch up and turn to the right,
- As soon as the load decreases, servo-transparency disappears
Pay attention to the following:
- Due to control loads linked to servo-transparency, the collective pitch tendency is to decrease. The collective pitch decrease and the pitch up may lead to rpm increase.
- The procedure should not be done too aggressively
- The exercise is easier when high All Up Weight and/or high density altitude.”

Is this what everybody is doing when demonstrating ST in the aircraft?

Aesir

3rd Nov 2012, 12:09

Is this what everybody is doing when demonstrating ST in the aircraft?

It´s what I do yes.

But please anyone! As always don´t try this without instructor and approval of the company/owner. It´s hard on the aircraft and can be quite violent if not done right. However it is invaluable to show the limits of the flight envelope for the AS350. As it´s a powerful and manuverable helicopter it is also a lot of fun to fly and many do like to fly it agressively. It´s is neccessary for those people to know where the aircraft will stop flying!

GenuineHoverBug

3rd Nov 2012, 12:20

Another quote from the report:

They have explained that for a long time, it was difficult to get a clear statement from Eurocopter on whether you could train for servo transparency or not. In October 2011, Airlift at last received clarification from Eurocopter that servo transparency could be practiced regularly, on one and the same helicopter without risk of damage, providing it was demonstrated by a qualified instructor and in accordance with the procedures.

Anthony Supplebottom

3rd Nov 2012, 12:24

Except for EC 130 B4 and AS 350 B3 Arriel 2B1 & AS 350 B3e when fitted with dual Hydraulic system.

So when fitted with dual hydraulic systems ST doesn't happen?

GenuineHoverBug

3rd Nov 2012, 13:11

Another couple of quotes:

Eurocopter has developed a Technical Improvement Proposal TIP-350-29-10-01, where the original hydraulic system in the helicopter type Eurocopter AS 350 is replaced by a duplicated hydraulic system. This duplicated system equals the system installed on the twin-engine version, Eurocopter AS 355. The modification increases the helicopters load capacity by 100 kg. The modification is comprehensive, time-consuming and costly (estimated at about EUR 380 000). It was introduced to meet a need of operators who would use the helicopter type for instrument flying.

The duplicated hydraulic system has double servos in the sense that the system pressure is supplied from two hydraulic pumps. If hydraulic pressure in one system is lost, the helicopter's servos can continue to function with pressure from the remaining hydraulic system. Simply stated, the servo has two hydraulic pistons installed on a common piston rod. Each of these pistons must separately be able to overcome the aerodynamic forces imposed on the main rotor blades. When both the systems work together, the servos can, however, transmit more force than the main rotor can tolerate. To reduce the risk of such overloads, the servos have been equipped with an integrated load sensor. When the load reaches a set limit, a LIMIT warning light is activated on the instrument panel.

and:

... A dual system also eliminates the servo transparency problem, but does not prevent structural overload.

I believe the duplex servos are standard on the B4, and can be retrofitted on the older ones (if you can afford it).

Anthony Supplebottom

3rd Nov 2012, 13:19

Thanks.

I don't know if the greatest risk of encountering ST comes from flying without due regard for the aircraft's limitations/showboating (being a plonker) or from inadvertently entering IMC and ending-up in an attitude which may invoke high g.

My guess is that the latter is a more likely risk.

GenuineHoverBug

3rd Nov 2012, 13:49

Agree that the latter is more obviously dangerous (and for various reasons).

What they seem to be grinding on about in this report, is that ST can be even more dangerous in the not so obviously risky situations. And especially that it may catch pilots by surprise and cause confusion and a delayed reaction, which in this case may have proven fatal due to the low height. If you diagnose it correctly, I understand it is possible to pull through the forces and recover the aircraft even with fully developed ST.

One problem is that the aircraft's limitations are perhaps not described in the Flight Manual in a way that pilots easily can relate to, and I read from the report (and the FM) that encountering ST is not necessarily outside the envelope, however continued operations are.

Shawn Coyle

3rd Nov 2012, 14:04

HeliboyDreamer

3rd Nov 2012, 14:07

The new AS350e should be less susceptible to ST but at the moment has other problems which will be fixed.

Can someone extrapolate on this, I am interested to know what other issues the 350e is facing?

Jack Carson

3rd Nov 2012, 14:58

To answer the question concerning if the phenomenon of servo transparency has even been experienced in Bell helicopters, the answer is yes. During our testing of the AH-1T, the predecessor to the AH-1W, at Patuxent River we experienced very significant pitch cyclic control feed back while performing symmetric pull ups at indicated airspeeds greater than 120KIAS and greater than 2.5 gs. In those situations, control of the helicopter was never compromised. Recovery required only a small reduction in g at the point where feedback was felt.

[email protected]

3rd Nov 2012, 16:25

If you diagnose it correctly, I understand it is possible to pull through the forces and recover the aircraft even with fully developed ST. I don't think so - the only way to recover is to reduce the severity of the manoeuvre.

Just hovering a squirrel with the hyds out for a short while is tiring enough but it would take a lot of brute force to handle the aircraft at 120 kts.

The type of relatively benign manoeuvre that could catch out the unaware is a descending right hand turn which is tightened up using lever and/or aft cyclic - the tendency of the aircraft to pitch up and roll right would roll the aircraft towards the inverted which could be interesting close to the ground.

Strange that Eurocopter now say it is OK to repeatedly stress an aircraft teaching ST because that is the reason they stopped us teaching jackstall on the Gazelle - repeated stress fatiguing the rotor system.

Gordy

3rd Nov 2012, 16:42

performing symmetric pull ups at indicated airspeeds greater than 120 KIAS and greater than 2.5 gs

Kinda like this:

sH56Kx70wxw

Anthony Supplebottom

3rd Nov 2012, 16:59

During our testing of the AH-1T,

Not an AH1T and not at high speed but maybe this brought the pilot close to the extremity of the a/c's cyclic authority.

ye5dRqS7Pm4

Gordy

3rd Nov 2012, 17:15

Anthony

Your video shows an aircraft almost getting dynamic rollover, not the same as servo transparency. Good video though.

Anthony Supplebottom

3rd Nov 2012, 17:51

Yes am aware its a different situation.

Your video "nails it" as you would say over there and it makes me a little weary of the Eurocopter. I mean you should be able to perform a zoom like that without the thing losing aerodynamic stability. The way the tail hangs down (I'm guessing that's the loss of pitch) is concerning.

GenuineHoverBug

3rd Nov 2012, 18:29

I haven't read the report but I'm trying to figure out how a helicopter supposedly coming into the hover suffers servo transparency. The gyrations described afterwards don't sound like servo transparency.

The possible servo transparency happened in the turn onto a long final, banking steeply to the right (60-90 degrees according to eye witnesses) with a relatively high speed. Ground speed from GPS is given as 120-125 kts.

The forward speed at ground impact has been calculated to about 105 kt based on tracks at the accident site. The helicopter hit the ground with a nearly flat pitch angle and about 45° of bank to the right. It came to rest about 25 meters past the first point of impact.

I have not seen any gyrations mentioned, it entered a steep descend while still banking steeply, but rolling out somewhat before impact.

Were the actuators at all recoverable after the fire?

From the report:
Extensive damage to the main gearbox and main rotor system indicated high torque when the helicopter collided with the ground. The position of each of the three servo control actuators could be determined, and these corresponded to a control position that would give a significant bank to the right.
But also:
The positions of the servos were probably affected by the impact, as was the cyclic stick, and they do not necessarily correlate with the positions of the flight controls prior to the crash.

I don't think so - the only way to recover is to reduce the severity of the manoeuvre.

Just hovering a squirrel with the hyds out for a short while is tiring enough but it would take a lot of brute force to handle the aircraft at 120 kts.

Reducing the severity is definitely the FM recommended way of recovering. But in this case it is possible the pilot ran out of altitude in an awkward attitude, and in that case the last thing on ones mind may be to reduce the severity. Especially to lower the collective.

I have not seen anywhere how the control forces experienced in servo transparency and those during a total loss of hyd power compare. I would have thought that with a total loss of hyd, the pilot has to do all the work, while in servo transparency he only needs to put in "a little extra". The FAA test flights from 2003 mentioned in 1.18.2 (page 33-34) in the report shows a diagram of the lateral control forces required.

The data that form the basis for the diagram show that the forces on the collective were between 3.5 and 10 daN (not shown in the figure), while the cyclic was pushed to the left with a force of between 2.5 and 12 daN.

Aesir

3rd Nov 2012, 19:01

Can someone extrapolate on this, I am interested to know what other issues the 350e is facing?

They have the really annoying tendency to loose tailrotor in flight. Therefore emergency AD, limitations to Vne, manuvering and inspections every 3 hours.

Today is not a good day to own shares in Eurocopter. I´m sure they will fix these problems but it may take a few months.

EASA Airworthiness Directives Publishing Tool (http://ad.easa.europa.eu/ad/2012-0217-E)

jymil

3rd Nov 2012, 19:06

From the AS350 AFM chapter 3:

Actuate the switch on the collective pitch control lever, to cut off hydraulic pressure.
Load feedback will be felt immediately ; load feedback may be heavy if the helicopter is flying at high speed :

. collective pitch : 20 daN (44 lbs) approx. pitch increase load
. cyclic : 7 to 12 daN (15 to 26 lbs) approx. left-hand cyclic load
. cyclic : 2 to 4 daN (4 to 9 lbs) approx. forward cyclic load
. yaw pedals : practically no load in cruising flight.

So it seems at least on the cyclic, it can get as bad as a full hydraulic loss (12daN).

Jack Carson

3rd Nov 2012, 20:28

I am really surprised that the AS-350 series was successfully certified with published control forces so high. Even after a single system failure (Hyd or Servo transparency) the resulting control forces must not exceed those outlined as Arm strength N (lb) Design criteria levels in Chapter 14 of the US Federal Aviation Administration Human Factors Guide. Table 14.5.2.1 of this guide clearly states the maximum force for a right hand, lateral control motion to the left should be no more than 6 to 8 pounds. This is far in excess of what is published in the AS-350 RFM. US designed and certified aircraft are required to meet these requirements. These standards were applied when Sikorsky included viscous cyclic control dampers in the H-53 Series to insure that in the event of a control damper bind it could be easily over ridden within the force limitations of the aforementioned table.

Jack Carson

3rd Nov 2012, 20:30

My bad the 6-8 lb limit was for a right hand control motion to the RIGHT, the back hand direction of motion.:rolleyes:

Nubian

4th Nov 2012, 09:14

Reducing the severity is definitely the FM recommended way of recovering. But in this case it is possible the pilot ran out of altitude in an awkward attitude, and in that case the last thing on ones mind may be to reduce the severity. Especially to lower the collective.

Spot on!

I think this report is almost identical to the accident of Colin McRae a few years back.

A result from showboating around without leaving sufficient room to recover if things not go as planned.

John R81

4th Nov 2012, 20:43

My thoughts. Hence the cross link when asked to explain JS

GenuineHoverBug

4th Nov 2012, 20:46

After the accident the involved operator introduced manoeuvre limitations on flights with passengers:

... Roll and pitch angle during passenger flights
Maximum roll and pitch angle during operations with passengers on board below 500’ AGL is set to 30 degrees roll and 15 degrees pitch.

... Initiating a right descending turn at low level
Initiation of a right descending turn at altitudes below 500’ AGL with passengers on board without first lowering the collective is prohibited.

That will probably make it safer for the passengers (of that operator), but what about the rest of us?

If I interpret it correctly, the EASA OEB recommended method of training servo transparency should be included in (all?) AS 350 training in Europe. The report seems to indicate that this has not been too well known. It would be interesting if anyone knew if:

It has made its way into the syllabi of the flight schools?
It is tested during Skill Tests / Proficiency Checks? (should it?)
Operators include this training in their OPC's?

And finally, my view is that it could be asked if the training method is adequate. (The accident pilot had been through ST training) Training is important mainly to teach pilots where the limit is and how to avoid it, but also to recognise it and intervene early if it should still happen.

It could be said that the approved method with a demonstration of ST in a diving left turn is confidence building, and reinforces the feeling you get from reading the statement in the FM that the phenomenon is self-correcting. (Rolls back towards level and the nose pitches up. The collective goes down - and you are out of it.) Could that give the impression that this is really no big deal?

If the demonstration was done in a diving right turn, it could, if AAIB and AIBN are correct, be a more violent thing. As I understand it, the turn would then tighten both by an increase in bank and a pitch up. As the collective drops at the same time, rate of descend increases. If you try to roll out to the left, you would load the servos even more. Experiencing this during training would perhaps get the pilots' attention and indicate that it not always self-correcting, at least not if you are in a right turn in a tight spot.