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Helicopter down outside Leicester City Football Club

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Helicopter down outside Leicester City Football Club

Old 7th Dec 2018, 20:14
  #981 (permalink)  

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Originally Posted by Pozidrive
Interesting, but your hub nut still has a split pin?
No, it doesn't have a split pin. The outer edge or lip of the nut is turned to a thinner profile and once the nut is torqued up the lip is knocked with a hammer and punch into a longitudinal groove on the end of the stub axle. From personal experience, this isn't quite as secure a method as a castellated nut and split pin. However, a car can also be driven in reverse so it is important to secure it in both directions. Unlike a tail rotor, which only runs in one direction.
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Old 7th Dec 2018, 21:25
  #982 (permalink)  

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Originally Posted by Ahernar
Ofc , i was meaning the rpm of the cabin !!!
@ Uplinker
You lose TR , you spin faster and faster until at certain rpm the drag from the spinning body of the heli counterracts the rotor drag . Unfortunately no human can control a helicopter spinning like that , but a computer surely can ,go watch that rocket gently land after spinning from several km height . I'm not expecting the control system to be able to keep it right afterward it lands so the machine will be lost but the landing would be gentle and very survivable ( except some bad cases of fire / bad terrain ) .
If the cabin rotates at 130 rpm, this will result in a net loss of 130 rotor rpm. This would almost certainly result in the same outcome - it is a loss of almost half the rotor rpm needed to sustain lift and the integrity of the disc would probably be lost, especially as the aircraft would be descending at a high rate.
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Old 7th Dec 2018, 22:01
  #983 (permalink)  
 
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Originally Posted by ShyTorque
If the cabin rotates at 130 rpm, this will result in a net loss of 130 rotor rpm. This would almost certainly result in the same outcome - it is a loss of almost half the rotor rpm needed to sustain lift and the integrity of the disc would probably be lost, especially as the aircraft would be descending at a high rate.
Out of interest where does the 130rpm come from?
The only other time I've seen data from an event like this it was about 120 deg/sec.
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Old 8th Dec 2018, 04:41
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Originally Posted by dClbydalpha
Out of interest where does the 130rpm come from?
The only other time I've seen data from an event like this it was about 120 deg/sec.
An exageration from my belly ( sort of worst case scenario for the control system ) . I would have to calculate real rpm values . The airframe coud be further designed to lower this value , with an enlarged fin aft , maybe even a little pirotechnically launched drogue chute (that's my new ideea , lol ).
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Old 8th Dec 2018, 07:05
  #985 (permalink)  

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(that's my new ideea , lol ).
Actually, neither of those are new ideas. The late Ray Prouty discussed both in his books, years ago.
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Old 8th Dec 2018, 07:32
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There will be a reason for the bearing failure that probably is the main cause of the subsequent accident. Questions about the overall design of the input assembly might come into it. However, there will be great effort made to identify why the bearing failed and, the mitigation of that process. It might be anything from: a major redesign, an improved bearing, improved QC of the bearing, revised servicing or an improved inspection.

OAP
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Old 8th Dec 2018, 07:43
  #987 (permalink)  

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Let's hope so!

I'm sure that all the RW pilots reading this are very concerned that the failure of a relatively small but highly critical item such as this bearing could bring down an aircraft and that the design wasn't in any way "fail safe". I certainly am.
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Old 8th Dec 2018, 08:34
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Originally Posted by ShyTorque
Let's hope so!

I'm sure that all the RW pilots reading this are very concerned that the failure of a relatively small but highly critical item such as this bearing could bring down an aircraft and that the design wasn't in any way "fail safe". I certainly am.
'Twas always thus. Years ago, I was fortunate to notice the clutch light flicker a couple of times during run-up on an R22 and shut down. It turned out to be the upper sheave bearing breaking up. Not a good failure to have.
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Old 8th Dec 2018, 08:59
  #989 (permalink)  
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Can anyone explain why the "big nut” holding the damaged bearings onto the shaft tightens up in this scenario? Looks from the photo to be a right handed thread and I would have thought it would tend to be undone a little if the bearings failed, assuming the appropriate direction to undo the small nut at the other end....?
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Old 8th Dec 2018, 09:52
  #990 (permalink)  

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As someone who has been flying helicopters for a living for almost four decades, the prospect of this type of tail rotor control failure has always greatly concerned me.

I've been writing about it on this forum and elsewhere (initially when I flew RAF helicopters) since the mid 1980s. Some would say I "bang on" about it, but imho with good reason. This situation is a helicopter pilot's worst nightmare. Tail rotor control is completely lost and the tail rotor pitch then goes to either full power at "push" or "pull". I don't know of any present helicopter where this situation is recoverable by the pilot, whatever his level of skill and experience.

The answer is to mandate that by design that all helicopter tail rotor control systems are equipped with a centreing device for the servo valve mechanism, to prevent a servo runaway in the event of the pilot's flight yaw controls becoming disconnected, such as now appears to have occurred here. With such a device fitted, the servo is automatically biased to move to a fixed, central position enabling controlled flight to still be available and a safe landing of some sort to be carried out.

Some aircraft manufacturers, such as Sikorsky, having been fitting such devices to their aircraft for many years. Other manufacturers, as appears to be the case here, obviously still do not.

These devices are not difficult to design. Most consist of a simple spring mechanism. The simplest one I've seen (Super Puma) consists of something that looks very much like a bicycle tyre pump, with a spring either side of a piston, sitting inside a cylindrical body. The body of the device is attached to the aircraft structure and the "piston rod" is attached to the servo control valve arm. The piston rod moves in and out of the body under the influence of the normal movement of the servo valve mechanism, such as when the pilot moves the yaw pedals. As it moves it compresses one spring and relaxes the other and is "invisible". However, in the event of a pilot's control input disconnection (e.g. a cable break), the springs equalise to bring the piston to a central / neutral position, so that the servo valve cannot not run away. The Sikorsky devices I'm familiar with are slightly more sophisticated and consist of quadrant arms controlled by springs, but the devices all work in a similar way.

Had such a device been fitted to this 169, the outcome might have been very different.
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Old 8th Dec 2018, 10:36
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As ShyTorque says, it is probably one of two things we, as helicopter pilots, would worry about failing the most, save for a rotor blade departing in flight. Ok so three. But a main gearbox failure (we don't carry a spare) or a tail rotor failure (for which we can only practice in the sim, and then its what the computer modellers/engineers and designers 'think' will actually happen).

My personal worry is a tail rotor failure, everything else we are pretty much well trained for, but in this day and age of modern technology, the 169 design does appear very simple. Too simple?
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Old 8th Dec 2018, 10:37
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Originally Posted by ShyTorque
...The answer is to mandate that by design that all helicopter tail rotor control systems are equipped with a centreing device for the servo valve mechanism, to prevent a servo runaway in the event of the pilot's flight yaw controls becoming disconnected, such as now appears to have occurred here. With such a device fitted, the servo is automatically biased to move to a fixed, central position enabling controlled flight to still be available and a safe landing of some sort to be carried out...
This wouldn't help if the failure was beyond the servo, i.e. the shaft to the tail rotor. There is also the down side of adding weight and complexity.
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Old 8th Dec 2018, 10:39
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Perhaps a way of removing hydraulic power to the actuator in an emergency as well?

The TR pitch control linkage of this aircraft looks insanely complicated and convoluted to my untrained eye - so many linkages, pivots, rose joints, cranks and other components: they must be a nightmare to maintain and keep working smoothly and check during the pre-flight inspection?

Seems bizarre to have only a single yaw control system, which if it fails will guarantee a crash.





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Old 8th Dec 2018, 10:48
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NOTAR technology was meant to solve this failure mode, wasn't it? Having flown an MD-600 and a Jet Ranger I couldn't 'feel' any difference.

Is there a reason why the design hasn't found wider adoption?
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Old 8th Dec 2018, 11:57
  #995 (permalink)  
 
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Originally Posted by ShyTorque
Actually, neither of those are new ideas. The late Ray Prouty discussed both in his books, years ago.
Probably yes but not in conjunction with computerised control .
Just for fun - kind of amazing - 2.20
Anyway i just read that fly by wire for helicopters is being researched in the military so if it's capable of coping with TR loss we will find someday .
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Old 8th Dec 2018, 12:58
  #996 (permalink)  

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Originally Posted by Pozidrive
This wouldn't help if the failure was beyond the servo, i.e. the shaft to the tail rotor. There is also the down side of adding weight and complexity.
A) Maybe not but what is the point of that statement?

B) The size and weight of such a mechanism (they do exist!) are very small, possibly adding not much more than couple of kilos, if as much as that. All we're talking about here is a device to spring bias the hydraulic servo control valve to a central position, i.e. to replicate the pilot's normal pedal input.

Can I suggest you read the following publication? Section 3 is of relevance to this accident:

https://publicapps.caa.co.uk/docs/33/CAPAP2003_01.PDF
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Old 8th Dec 2018, 13:12
  #997 (permalink)  
 
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The answer is to mandate that by design that all helicopter tail rotor control systems are equipped with a centreing device for the servo valve mechanism, to prevent a servo runaway in the event of the pilot's flight yaw controls becoming disconnected, such as now appears to have occurred here.
Really? Go back and read the report.
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Old 8th Dec 2018, 13:35
  #998 (permalink)  

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RVDT, Your point isn't clear. I've read the report, thanks, more than once. It would help if you would explain what your disagreement is based on, rather than asking a very vague question and giving an order..
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Old 8th Dec 2018, 13:55
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I think what RVDT is pointing out is that it wasn't 'a servo runaway'. That would be when a servo malfunctions and moves uncommanded. The hydraulic systems and servo were operating correctly, as far as has been revealed, and the servo was just doing what it had been commanded to do before the feedback link stopped working.
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Old 8th Dec 2018, 14:10
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Shy take a look at how the thing works. It was not the input that failed but lets say the "output" to cancel the input via the feedback mechanism.

It's pretty simple - when all 3 points attached to the lever line up with the servo valve in the neutral position nothing happens as the control shaft moves and cancels the input request.

As the feed back end became detached - nothing to cancel the input - servo motors to the stop and in this case full right pedal?

BTW - Sikorsky has been fitting centreing as the result of accidents or incidents not unlike this one - not pro-actively.

The design and acceptance of the AW models may be under review shortly. If not I will be very surprised.
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