Bristow S76 down in Lagos discussion (Not condolences)
Hello Copterline,
Is there an evidence that the control rod linking the pilot's and copilot's cyclics have actually been broken by the crew's only physical strength rather than by the force of impact ?
Couldn't this be detected on the FDR, as a discrepancy between the actions on the cyclic and the rotor (swachplate) response ? (actually, I can't check here, where the cyclic positions are recorded, maybe at the transducers ? )
Is there an evidence that the control rod linking the pilot's and copilot's cyclics have actually been broken by the crew's only physical strength rather than by the force of impact ?
Couldn't this be detected on the FDR, as a discrepancy between the actions on the cyclic and the rotor (swachplate) response ? (actually, I can't check here, where the cyclic positions are recorded, maybe at the transducers ? )
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cpt ,
CVR was explaining what was on going in our accident. Flight Controls were stuck and the crew was fighting and making all the efforts to get these stuck controls to move.
The crew broke the top of collective when the captain was trying to got fully (max) up position standing collective down. Also the crew was using all available hand power to move cyclic stick. Pilot’s side cyclic stick was bended forward. The connecting Rod (connecting pilot and co-pilots controls was separated (the tube and the Rod end).
The design of the flight controls tubes and rod ends are designed to be used in normal condition. In normal conditions there is no major load at all. These tubes and control Rod End are not designed to be fail safe with a power what could be used by two dying men. The basic assumption is that (FAR29) redundant Servo Actuator systems should not be stuck.
I can sent an email to you and attach NTSB’s ADAMS HYDRAULIC SIMULATION. There are scenarios where one C3 return port is blocked and the second simulation is that the both of these C3 return ports are blocked.
The scenario of one C3 is block will make Forward and Lateral Servo Actuator full extension within 3,5 seconds. If the both C3 ports are blocked then the Forward and Lateral Servo Actuators full extension will happened within 1,5 seconds.
Simulation affect (air speed 0,8 VNE and G force 1,0 cruise flight phase) is that when Servo actuator is fully extended then the helicopter will pitch up 60 degrees, banking to the left, at the beginning turning to the left which after (the collective is max up position then mixing unit will mechanically put the right pedal in fully right down position. After this helicopter will start to spiral to the right.
NTSB Servo simulation found out that Lateral Servo Actuator is under very heavy stress (load). The NTSB test report shows that, the stress force will variate between -1400 LBF to -400 LBF. This variation is very quick to fluctuate. It will fluctuate from high -1400 LBF (equal -6227 Newton) to -400 LBF (equal -1779 Newton) average 8 times every second.
If you will study an accident in Sutton, USA in 1986 the root case for accident is unknown. The S76B helicopter was cruising at 6000 ft. The crew was two Sikorsky’s test pilot and two Sikorsky’s technicians. Suddenly helicopter came unflyable. The radar recording is proving that helicopter speed, heading start to variate quickly. The helicopter was losing the speed and turning to the left. Very soon after this helicopter was starting to spiral to the right. The rate of decent was about 3000 ft/min. At time when this S76 hit to the ground the impact attitude was 12 degrees bank to the right and nose was at zero level. The Baltic Sea accident rate of decent was 3000 ft/ min and when the helicopter hit to the water it had 12 degrees right bank.
We all know the law of aerodynamics. If the conditions are same the result is always the same.
It is very interested in to see all the data from Lagos. Flight data recorder and CVR recording will tell within five minutes what this case was. ICAO ANNEX 13 procedures should bring Interim Accident Report to be published within 30 days. What is the reason for delay?
FAA has issued an EAD. My opinion is that the Jam nut is not the issue at all. It idea of Jam nut in this Control Rod tube / Rod End is only for to keep this Rod assemblies length stuck at time when the Rod assembly in not connected to the helicopter flight controls, mixing unit or the Servo Actuators. When Rod Assembly is connected by locking bolts (by each end) the Rod end is not able to rotate. If the Jam nut is totally missing or if the Jam nut is in its place with safety wiring there is no change of strength of the Rod assembly.
The other end of the Rod tube and Rod end is riveted (fix installation). Also the other Rod end is a fix installation due to the bolt connection (Servo Actuator). This makes impossible that the Rod End can be rotating (loosens or tightens).
I don’t understand how this Jam nut could be a case at all? And what is the point of this EAD?
The crew had 37 seconds time to try everything to have helicopter be a flyable helicopter. During this 37 seconds time the helicopter was spiraling to the right 13 times.
There is a procedure to get an unflyable helicopter back to flyable if this stuck controls case happen.
CVR was explaining what was on going in our accident. Flight Controls were stuck and the crew was fighting and making all the efforts to get these stuck controls to move.
The crew broke the top of collective when the captain was trying to got fully (max) up position standing collective down. Also the crew was using all available hand power to move cyclic stick. Pilot’s side cyclic stick was bended forward. The connecting Rod (connecting pilot and co-pilots controls was separated (the tube and the Rod end).
The design of the flight controls tubes and rod ends are designed to be used in normal condition. In normal conditions there is no major load at all. These tubes and control Rod End are not designed to be fail safe with a power what could be used by two dying men. The basic assumption is that (FAR29) redundant Servo Actuator systems should not be stuck.
I can sent an email to you and attach NTSB’s ADAMS HYDRAULIC SIMULATION. There are scenarios where one C3 return port is blocked and the second simulation is that the both of these C3 return ports are blocked.
The scenario of one C3 is block will make Forward and Lateral Servo Actuator full extension within 3,5 seconds. If the both C3 ports are blocked then the Forward and Lateral Servo Actuators full extension will happened within 1,5 seconds.
Simulation affect (air speed 0,8 VNE and G force 1,0 cruise flight phase) is that when Servo actuator is fully extended then the helicopter will pitch up 60 degrees, banking to the left, at the beginning turning to the left which after (the collective is max up position then mixing unit will mechanically put the right pedal in fully right down position. After this helicopter will start to spiral to the right.
NTSB Servo simulation found out that Lateral Servo Actuator is under very heavy stress (load). The NTSB test report shows that, the stress force will variate between -1400 LBF to -400 LBF. This variation is very quick to fluctuate. It will fluctuate from high -1400 LBF (equal -6227 Newton) to -400 LBF (equal -1779 Newton) average 8 times every second.
If you will study an accident in Sutton, USA in 1986 the root case for accident is unknown. The S76B helicopter was cruising at 6000 ft. The crew was two Sikorsky’s test pilot and two Sikorsky’s technicians. Suddenly helicopter came unflyable. The radar recording is proving that helicopter speed, heading start to variate quickly. The helicopter was losing the speed and turning to the left. Very soon after this helicopter was starting to spiral to the right. The rate of decent was about 3000 ft/min. At time when this S76 hit to the ground the impact attitude was 12 degrees bank to the right and nose was at zero level. The Baltic Sea accident rate of decent was 3000 ft/ min and when the helicopter hit to the water it had 12 degrees right bank.
We all know the law of aerodynamics. If the conditions are same the result is always the same.
It is very interested in to see all the data from Lagos. Flight data recorder and CVR recording will tell within five minutes what this case was. ICAO ANNEX 13 procedures should bring Interim Accident Report to be published within 30 days. What is the reason for delay?
FAA has issued an EAD. My opinion is that the Jam nut is not the issue at all. It idea of Jam nut in this Control Rod tube / Rod End is only for to keep this Rod assemblies length stuck at time when the Rod assembly in not connected to the helicopter flight controls, mixing unit or the Servo Actuators. When Rod Assembly is connected by locking bolts (by each end) the Rod end is not able to rotate. If the Jam nut is totally missing or if the Jam nut is in its place with safety wiring there is no change of strength of the Rod assembly.
The other end of the Rod tube and Rod end is riveted (fix installation). Also the other Rod end is a fix installation due to the bolt connection (Servo Actuator). This makes impossible that the Rod End can be rotating (loosens or tightens).
I don’t understand how this Jam nut could be a case at all? And what is the point of this EAD?
The crew had 37 seconds time to try everything to have helicopter be a flyable helicopter. During this 37 seconds time the helicopter was spiraling to the right 13 times.
There is a procedure to get an unflyable helicopter back to flyable if this stuck controls case happen.
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Copterline, I think the issue is that without the jam nut being tight, the threads of the rod and body can start to chatter, and thus wear, and thus eventually pull out - no rotation of the parts needed. This is just my guess, hopefully a technician will confirm.
.... there's also this #2 engine uncontained failure wich is a concern. The damage seems to be aft of the armouring shield but there's obviously no apparent link with the disconnected rod wich is located much more forward. A power turbine shedding its blades here, would rather damage the tail rotor shaft.
But the point is that the similarities, with the "Baltic Sea case" are strikingly evident, at least in the first stage of the flight upset.
But the point is that the similarities, with the "Baltic Sea case" are strikingly evident, at least in the first stage of the flight upset.
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I am very rusty now, having been retired for some years, but I was on the 76 from day one, as a senior engineer and I find this accident very strange. If the rod end was engage to past the witness hole, I cannot see why it should pull out just because the lock nut was not tight. From memory there is very little stress on these rods, hydraulics on or off, they only operate the pilot valves.
If the rod had been fitted with just a few threads engaged, then I would think it unlikely that the correct rigging figures could have been obtained. The only way I can see that the servo could have been correctly rigged like that, is for the whole system to have been miss rigged all the way back to the stick, an unlikely scenario.
The rod ends cannot rotate, so are they saying that if the nut is not tight, then chattering is enough to pull the rod apart.
If the rod had been fitted with just a few threads engaged, then I would think it unlikely that the correct rigging figures could have been obtained. The only way I can see that the servo could have been correctly rigged like that, is for the whole system to have been miss rigged all the way back to the stick, an unlikely scenario.
The rod ends cannot rotate, so are they saying that if the nut is not tight, then chattering is enough to pull the rod apart.
I took my Subaru in to a dealer for a 4-wheel alignment. 3 months and about 2000 miles later it was due an MOT, I gave it a quick check-over including jacking it up. I noticed detectable play in one of the front wheels. Transpired the jam nut on the track rod was backed off a couple of turns. Clearly it had never been tightened. The threads had worn sufficiently for detectable play caused mostly by a slight buckling effect even though there was lots of mating thread length.
So with that in mind I can believe that the accident helicopter's rod end could have worn and pulled out. Perhaps less force than a car's steering, but a much more rapid cycling of that force.
G'd evening Dave,
Do you think it could have been possible that this loose lock nut with only a few threads engaged had no consequences on the rod integrity in a normal servo valves operation, but may have allowed a separation of the rod with its body under the constrains of a struggling pilot's inputs against a frozen (jammed) servo ?
Do you think it could have been possible that this loose lock nut with only a few threads engaged had no consequences on the rod integrity in a normal servo valves operation, but may have allowed a separation of the rod with its body under the constrains of a struggling pilot's inputs against a frozen (jammed) servo ?
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Perhaps less force than a car's steering, but a much more rapid cycling of that force.
The strength of the Control tubes and Rod Ends are much higher (at least 100 times higher) than this 5 lbf load.
However that is not taking into account the very destructive effects of vibration on the masses of the parts. I'll confess to knowing nothing about the control topology of the S76 flight control system, but if you know more and can suggest an alternative explanation (other than a jack runaway, which doesn't seem to be supported by the evidence) then let's hear it please.
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I am not going to argue with the AIB, they are very clever people, and I have worked with some. I will however offer an alternative for discussion.
If the uncontained engine took out some hydraulic pipes, then you could lose both systems pressure, the S76 cannot fly in manual control, but you are obviously going to try to save the aircraft. Could the control rod have pulled out under severe loading.
As I said already I am rusty now, but I seem to remember that both 1&2 systems run pipes through the tail boom to the tail rotor, if I am getting confused in my dotage, then someone please correct me.
If the uncontained engine took out some hydraulic pipes, then you could lose both systems pressure, the S76 cannot fly in manual control, but you are obviously going to try to save the aircraft. Could the control rod have pulled out under severe loading.
As I said already I am rusty now, but I seem to remember that both 1&2 systems run pipes through the tail boom to the tail rotor, if I am getting confused in my dotage, then someone please correct me.
Yes Dave, you are right, hydraulic lines run all along the tail boom to supply the tail rotor servos, but if these lines are severed the "tail rotor shut off valve" will automatically isolate the main rotor stage 1 servos from the leak, as the hydraulic level drops. Thus, in case of a major dual system hydraulic leak aft of the the main rotor servos, control of the main rotor is normally still possible via hydraulic stage1.
....I'm not "speculating" but as a S76 flyer, it seems normal to share ideas and experiences on such a case.
For sure, investigators will soon find out what happened.
....I'm not "speculating" but as a S76 flyer, it seems normal to share ideas and experiences on such a case.
For sure, investigators will soon find out what happened.
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S76 Wizard wrote "My speculation is that when they rigged the input they misread the tape measure and made this an inch longer than it should be and then compensated by adjusting the pitch links to bring it in rig. The problem is that too much maintenance gets done to helicopters by engineers that don't have enough experience on type."
The input rods are not measured. With the controls set in a fixed position and rigging blocks installed in the servos, the inputs are adjusted to fit. Later in the process they maybe adjusted again for control clearance.
The S92 TR servo follow through linkage had a steel rod screwed into an aluminum block on early servos, with no jam nut. Vibration would cause the movement between the rod and block, steel won. No clue what that follow through linkage does, the servo will work when the steel rod falls out of the aluminum block. That steel rod augered that thing out like a drill bit, we didn't notice until it was in two pieces, we- in fact I didn't, a coworker spotted it while inspecting some unrelated work I had done. The design has been improved.
If the jam nut was loose for what ever reason, I can see the same type of wear occurring.
The input rods are not measured. With the controls set in a fixed position and rigging blocks installed in the servos, the inputs are adjusted to fit. Later in the process they maybe adjusted again for control clearance.
The S92 TR servo follow through linkage had a steel rod screwed into an aluminum block on early servos, with no jam nut. Vibration would cause the movement between the rod and block, steel won. No clue what that follow through linkage does, the servo will work when the steel rod falls out of the aluminum block. That steel rod augered that thing out like a drill bit, we didn't notice until it was in two pieces, we- in fact I didn't, a coworker spotted it while inspecting some unrelated work I had done. The design has been improved.
If the jam nut was loose for what ever reason, I can see the same type of wear occurring.
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Hi helicopterkeys
The report has yet to say if the rodend came out of the input or if it was hanging on by a thread. My guess is the input was hanging by a thread or we would be doing more inspections on these input threads.
The report has yet to say if the rodend came out of the input or if it was hanging on by a thread. My guess is the input was hanging by a thread or we would be doing more inspections on these input threads.
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Lynnx
What is your point with the waiving finger?
Those of us in the business have all read these documents, carried out the inspections as required by the EAD and ASB and released the aircraft back to service.
What is your point with the waiving finger?
Those of us in the business have all read these documents, carried out the inspections as required by the EAD and ASB and released the aircraft back to service.
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Brother,
My waving finger was mainly - ( though not clearly enough - obviously) -pointed at 76 Wizard and/or Copterline.
The accident report is a marvel of brevity.
It simply states that the separation was a "pre impact condition"
Nothing more.
EVERYthing on this thread other than that simple comment is therefore pure speculation as of now, and until, a further report is released.
Perhaps that is the purpose of the "Proffessional Pilots Rumour Network" ?
My waving finger was mainly - ( though not clearly enough - obviously) -pointed at 76 Wizard and/or Copterline.
The accident report is a marvel of brevity.
It simply states that the separation was a "pre impact condition"
Nothing more.
EVERYthing on this thread other than that simple comment is therefore pure speculation as of now, and until, a further report is released.
Perhaps that is the purpose of the "Proffessional Pilots Rumour Network" ?
OK....we have a pre-impact failure of a rod end....and an un-contained engine failure in the same crashed aircraft. Two completely different components in two different areas of the aircraft.
Just what do we take out of that situation?
Are we overlooking something in all of the wild assed guessing that is going on about what caused the Crash?
Just what do we take out of that situation?
Are we overlooking something in all of the wild assed guessing that is going on about what caused the Crash?