Helicopter down this evening in Norway: July 2011
Seems to me they have no clue what happened.
Actually after reading the full report thatīs what they say. itīs 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.
Actually after reading the full report thatīs what they say. itīs 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.
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.
Last edited by Aesir; 4th Nov 2012 at 13:06. Reason: added comment after reading full report
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.
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.
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.
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.
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.
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)
1.4g is about where the g becomes perceptible (a 45 degree level turn)
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.
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.
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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.
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.
Are there many cases of this happening in Bells?
However on AS350 this is really no problem as long as pilots know and understand itīs limitations. True for any helicopter type.
I teach and show pilots actual jack stall in 350īs during training and LPC/OPCīs 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.
Last edited by Aesir; 3rd Nov 2012 at 12:04. Reason: spelling
It apparently does not have to be very aggressive. The quote from the EASA OEB report 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?
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?
Last edited by GenuineHoverBug; 3rd Nov 2012 at 12:16. Reason: Added link, as the link in the report appears not to work
Is this what everybody is doing when demonstrating ST in the aircraft?
But please anyone! As always donīt try this without instructor and approval of the company/owner. Itīs 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 itīs a powerful and manuverable helicopter it is also a lot of fun to fly and many do like to fly it agressively. Itīs is neccessary for those people to know where the aircraft will stop flying!
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.
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.
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Except for EC 130 B4 and AS 350 B3 Arriel 2B1 & AS 350 B3e when fitted with dual Hydraulic system.
Last edited by Anthony Supplebottom; 3rd Nov 2012 at 12:24.
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).
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).
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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.
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.
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.
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.
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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.
Were the actuators at all recoverable after the fire?
Were the actuators at all recoverable after the fire?
The new AS350e should be less susceptible to ST but at the moment has other problems which will be fixed.
Servo Transparency in Bells
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.