Bell 412EP AP Out Instability
Griffo.....I thought perhaps I had given you the need for new undies a time or two when we flew together out of the sunny isle to the Ninian all those years ago! I know I was glad to be wearing Polish Depends....those Musk Ox rubber bags we rode around in.
As to my ease in suggesting the knee block is after those early years on Wokka's.....some of them had more road miles on them than air miles....having been towed back home more than a few times after a forced landing of some sort.....I got used to having the bejesus bounced out of me as the weak mag brake was a common problem and one got used to it after a while.
Being right under the forward rotor head made it all the more fun.....and unless one had the belts cinched right down....you could really make like a Mexican Jumping Bean.
We should all draw upon one another's experience and stow such little tricks in a handy place where we can call upon them when we first have the misfortune to redo what prompted the other fellow to pass on a word to the wise. I learned from other guys....as I felt I could not live long enough to make all their mistakes in addition to my own.
I make no claim to be a Wizard....just a Wizzer but I learn quick while leaned upon the Bar of Wisdom.....if you keep yer ears open while chugging down your Pint....the money spent for Beer is well spent as there can sometimes be a golden nugget picked up.
As to my ease in suggesting the knee block is after those early years on Wokka's.....some of them had more road miles on them than air miles....having been towed back home more than a few times after a forced landing of some sort.....I got used to having the bejesus bounced out of me as the weak mag brake was a common problem and one got used to it after a while.
Being right under the forward rotor head made it all the more fun.....and unless one had the belts cinched right down....you could really make like a Mexican Jumping Bean.
We should all draw upon one another's experience and stow such little tricks in a handy place where we can call upon them when we first have the misfortune to redo what prompted the other fellow to pass on a word to the wise. I learned from other guys....as I felt I could not live long enough to make all their mistakes in addition to my own.
I make no claim to be a Wizard....just a Wizzer but I learn quick while leaned upon the Bar of Wisdom.....if you keep yer ears open while chugging down your Pint....the money spent for Beer is well spent as there can sometimes be a golden nugget picked up.
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I posted earlier, read all the added information and comments, and still think we're down to two possibilities:
1. Heave oscillation caused by airframe accelerations moving collective. This one seems to be eliminated by the fact that AP selection stopped the oscillation, however, AP selection required that at least one of the pilots put his hands elsewhere. PIO doesn't mean pilot's fault. Sometimes refered to as Pilot Involved Oscillation, it only requires that the pilot is somehow "in the loop". Hand placed on the control but applying no forces to move it is enough to change the control's reaction to aircraft acceleration.
Try placing both hands at the top of the steering wheel in your car, pull your body forward so the upper body is free to be moved by the car, then apply a small, quick input on the steering wheel. If your arms are locked, then every time the car swerves left, your body swerves right in the car frame of reference, causing the car to swerve right, etc.
2. Pitch oscillation (or multiple axis with cross coupling). I still think this is possible despite W&Z being a reliable eye witness, being fully aware of the pitch response of the 412, and insisting it was heave. The in-car experiment above reveals a completely different response to steering wheel inputs than if you just move the steering wheel back and forth a few times. By changing the interface of the pilot/driver with the control system, the (effective) mass properties of the controls change, and with that the frequency response and dynamic response will change. The pitch oscillation theory, I believe, is a stronger contender for the solution due to the fact that the APs selection appears to have eliminated the oscillation.
I am an ETPS graduate and have about 1500 hours on the 412, including extensive APs out flight and stability & control testing of standard and modified 412s. I have experienced divergent heave oscillations a number of times in an H46, so have learnt to quickly dampen them before it gets uncomfortable. There are many conditions on the 412 with APs out (or just about any aircraft) where an input may be able to incite a divergent control-input-due-to-aircraft-acceleration oscillation. However, most of the time our natural response is to dampen those vibrations. Changes to the control systems such as internal frictions, modifications to control heads, pilot involvement (active or not) and a short delay in initially damping the oscillations can drastically change the non-event into something similiar to what you've seen here.
wazz'n'zoom, good job in getting back safely and thanks for posting this event. The type of discussion it has generated is why I stay on this forum.
Caveat - I admit that this is all speculation and that one new piece of evidence could completely destroy these theories.
1. Heave oscillation caused by airframe accelerations moving collective. This one seems to be eliminated by the fact that AP selection stopped the oscillation, however, AP selection required that at least one of the pilots put his hands elsewhere. PIO doesn't mean pilot's fault. Sometimes refered to as Pilot Involved Oscillation, it only requires that the pilot is somehow "in the loop". Hand placed on the control but applying no forces to move it is enough to change the control's reaction to aircraft acceleration.
Try placing both hands at the top of the steering wheel in your car, pull your body forward so the upper body is free to be moved by the car, then apply a small, quick input on the steering wheel. If your arms are locked, then every time the car swerves left, your body swerves right in the car frame of reference, causing the car to swerve right, etc.
2. Pitch oscillation (or multiple axis with cross coupling). I still think this is possible despite W&Z being a reliable eye witness, being fully aware of the pitch response of the 412, and insisting it was heave. The in-car experiment above reveals a completely different response to steering wheel inputs than if you just move the steering wheel back and forth a few times. By changing the interface of the pilot/driver with the control system, the (effective) mass properties of the controls change, and with that the frequency response and dynamic response will change. The pitch oscillation theory, I believe, is a stronger contender for the solution due to the fact that the APs selection appears to have eliminated the oscillation.
I am an ETPS graduate and have about 1500 hours on the 412, including extensive APs out flight and stability & control testing of standard and modified 412s. I have experienced divergent heave oscillations a number of times in an H46, so have learnt to quickly dampen them before it gets uncomfortable. There are many conditions on the 412 with APs out (or just about any aircraft) where an input may be able to incite a divergent control-input-due-to-aircraft-acceleration oscillation. However, most of the time our natural response is to dampen those vibrations. Changes to the control systems such as internal frictions, modifications to control heads, pilot involvement (active or not) and a short delay in initially damping the oscillations can drastically change the non-event into something similiar to what you've seen here.
wazz'n'zoom, good job in getting back safely and thanks for posting this event. The type of discussion it has generated is why I stay on this forum.
Caveat - I admit that this is all speculation and that one new piece of evidence could completely destroy these theories.
I am an ETPS graduate and have about 1500 hours on the 412, including extensive APs out flight and stability & control testing of standard and modified 412s
PS. The "seat-cyclic-interface" term was banter. My previous post was meant to be factual.
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Matt P
Good theories from a sound POF and technical standpoint/grounding me old.
Will attempt to fly with one your ETPS colleagues next week (Night Sim permitting) to put the same a/c into the same configuration as the incident.
Wait out.
Good theories from a sound POF and technical standpoint/grounding me old.
Will attempt to fly with one your ETPS colleagues next week (Night Sim permitting) to put the same a/c into the same configuration as the incident.
Wait out.
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At last a reply from Bell reference the "bounce incident". Names/ser numbers have been withdrawn for sensitivity reasons.
See what you lot reckon:
Firstly please accept my apologies for the long delay in answering your request regarding:
Flight Safety Occurrence Report fsor\Shawbury – RAF\60(R) Sqn\Griffin\##\#####2.
Bell Helicopter has reviewed the incident report of s/n #####7 regarding the abnormality your Griffin crew experienced and could not construe any plausible aircraft system fault that could duplicate the characteristic and duration of the vibration described.
Bell Helicopter considers the most likely catalyst for this short term vibration may have been from an external source.
It is possible that the rotor wash caused something to be picked up into the tail rotor which would create a momentary large span wise imbalance and generate a similar vibration to what was reported by your crew. Unfortunately without any physical evidence this cannot be proven.
Any debris caught up in the rotor would likely shed after a short duration eliminating the vibration which may have been in coincidence with the autopilots being selected ON.
During flight testing Bell Helicopter has observed that when the tail rotor is in a significant out of balanced span wise condition a vertical/fore/aft airframe (hop) vibration is felt at the pilot station.
It also should be noted that the AFCS was not designed to nor has any influence on dampening of vertical airframe vibrations.
Although I am confident that any of the following would have been mentioned in your very detailed report:
· By chance was there any video monitoring the helipad area at the time of this occurrence?
· After the incident was there any evidence of possible contact on either rotor system in particular the tail rotor blade and tail rotor hub?
· After the incident was there any evidence of FOD in the area where it occurred? (FOD can be thrown/ blown long distances).
If Bell Helicopter can provide any assistance with this or other items please do not hesitate to contact us again.
Regards,
It does not, for me, answer the questions posed to Bell in fact it leaves me none the wiser.
See what you lot reckon:
Firstly please accept my apologies for the long delay in answering your request regarding:
Flight Safety Occurrence Report fsor\Shawbury – RAF\60(R) Sqn\Griffin\##\#####2.
Bell Helicopter has reviewed the incident report of s/n #####7 regarding the abnormality your Griffin crew experienced and could not construe any plausible aircraft system fault that could duplicate the characteristic and duration of the vibration described.
Bell Helicopter considers the most likely catalyst for this short term vibration may have been from an external source.
It is possible that the rotor wash caused something to be picked up into the tail rotor which would create a momentary large span wise imbalance and generate a similar vibration to what was reported by your crew. Unfortunately without any physical evidence this cannot be proven.
Any debris caught up in the rotor would likely shed after a short duration eliminating the vibration which may have been in coincidence with the autopilots being selected ON.
During flight testing Bell Helicopter has observed that when the tail rotor is in a significant out of balanced span wise condition a vertical/fore/aft airframe (hop) vibration is felt at the pilot station.
It also should be noted that the AFCS was not designed to nor has any influence on dampening of vertical airframe vibrations.
Although I am confident that any of the following would have been mentioned in your very detailed report:
· By chance was there any video monitoring the helipad area at the time of this occurrence?
· After the incident was there any evidence of possible contact on either rotor system in particular the tail rotor blade and tail rotor hub?
· After the incident was there any evidence of FOD in the area where it occurred? (FOD can be thrown/ blown long distances).
If Bell Helicopter can provide any assistance with this or other items please do not hesitate to contact us again.
Regards,
It does not, for me, answer the questions posed to Bell in fact it leaves me none the wiser.
