Polish HEMS EC135 almost losing it on landing
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"Failure to anticipate" I do agree that is a factor, however, once full right pedal is applied too quickly and that aircraft is spinning, maintaining full right pedal may not stop the yaw. Under normal circumstances, air only flows in one direction through the fenestron, I suspect that once full pitch is applied quickly, we are asking too much of the fenestron and it can no longer cope (stalled), possibly even allowing air to flow in the wrong direction. One possible solution would be to unstall it with left pedal and re-apply gently......try doing that when spinning to the left at a very disorientating rate and no yaw control.
As always, from the comfort of an armchair, it is very easy to spout "mishandling", "it doesn't exist", but when out of control and applying full pedal to correct with no response, it does exist, something has gone wrong down the back and if it isn't mechanical then it must be aerodynamic (fenestron stall/LTE...call it whatever we like).
As always, from the comfort of an armchair, it is very easy to spout "mishandling", "it doesn't exist", but when out of control and applying full pedal to correct with no response, it does exist, something has gone wrong down the back and if it isn't mechanical then it must be aerodynamic (fenestron stall/LTE...call it whatever we like).
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Leprechaun
Accidents occurred on US Army OH58 helicopters. The phenomenon was initially called "Tail rotor stall" and the pilot was asked to add pedal in the turn direction to "unstall" the tail rotor. I am convinced that it contributed to increase the number of accident untill tests demonstrated that there was no tail rotor stall.
Accidents occurred on UK Army Gazelle helicopters (and almost only there), attibuted to a "Fenestron stall". The pilot was asked to add first pedal in the turn direction to "unstall" the fenestron. I am convinced that this is the reason why so many accidents occurred on the UK Gazelles. I never heard of any accident after this strange procedure was removed.
Wing stall is a physical phenomenon. It is demonstrated daily to student pilots. Whenever you reproduce the stall conditions, it occurs. Fenestron stall is only a mind's creation, not confirmed by any experiment.
This is what happens when you apply full right pedal. The upper curve gives the pedal position and the lower the heading. You may see that the Gazelle was rotating 360° in about 3 seconds. In 2 seconds it was stopped.
In that test, only 75% pedal was applied, quite far from the stop. The rotation was eventually stopped, even if it took more than 720°.
A fully operative helicopter rotating left with full right pedal is a nonsense. Exceeding 360° rotation means that no pedal reaction was done or that opposite pedal reaction was abandonned after an initial attempt that seemed to have no effect.
An improper wording of a phenomenon may have unexpected consequences. I believe that the "Loss of Tail rotor Effectiveness" wording that followed the "Tail rotor stall" is a reason why such accidents still happen. The tail rotor does not lose effectiveness and saying that it does cannot give the pilot confidence in his tail rotor, which is however the best chance he has to exit unanticipated yaw. So please do not use "Fenestron stall" . It does not exist and recommending to "unstall" a fenestron or a tail rotor has already been tested. I can guarantee the result.
Please consider using leprechaun to designate the phenomenon, it will be less harmfull.
Accidents occurred on UK Army Gazelle helicopters (and almost only there), attibuted to a "Fenestron stall". The pilot was asked to add first pedal in the turn direction to "unstall" the fenestron. I am convinced that this is the reason why so many accidents occurred on the UK Gazelles. I never heard of any accident after this strange procedure was removed.
Wing stall is a physical phenomenon. It is demonstrated daily to student pilots. Whenever you reproduce the stall conditions, it occurs. Fenestron stall is only a mind's creation, not confirmed by any experiment.
This is what happens when you apply full right pedal. The upper curve gives the pedal position and the lower the heading. You may see that the Gazelle was rotating 360° in about 3 seconds. In 2 seconds it was stopped.
In that test, only 75% pedal was applied, quite far from the stop. The rotation was eventually stopped, even if it took more than 720°.
A fully operative helicopter rotating left with full right pedal is a nonsense. Exceeding 360° rotation means that no pedal reaction was done or that opposite pedal reaction was abandonned after an initial attempt that seemed to have no effect.
An improper wording of a phenomenon may have unexpected consequences. I believe that the "Loss of Tail rotor Effectiveness" wording that followed the "Tail rotor stall" is a reason why such accidents still happen. The tail rotor does not lose effectiveness and saying that it does cannot give the pilot confidence in his tail rotor, which is however the best chance he has to exit unanticipated yaw. So please do not use "Fenestron stall" . It does not exist and recommending to "unstall" a fenestron or a tail rotor has already been tested. I can guarantee the result.
Please consider using leprechaun to designate the phenomenon, it will be less harmfull.
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Checking my log book, first flew a fenestron tail in 2009, last flew a non-fenestron tail in 2016 (B206 L4), since then only fenestron.
Have experienced the "loss of angle of attack" effect mentioned by Craab, have learned the non-linear pedal input and have seen the torque spike it creates. Also experienced the "tail the size of a barn" (not just the door) effect when yawing through the wind.
Never once experienced an inability to control the tail, which I put down to good training and being ahead of the aircraft (I did throw-away one approach and go-around due to (upon reflection) being behind the aircraft in yaw control as I bled speed, but I am of the view that this is what I was trained to do). Coming back the second time, no issues.
Have experienced the "loss of angle of attack" effect mentioned by Craab, have learned the non-linear pedal input and have seen the torque spike it creates. Also experienced the "tail the size of a barn" (not just the door) effect when yawing through the wind.
Never once experienced an inability to control the tail, which I put down to good training and being ahead of the aircraft (I did throw-away one approach and go-around due to (upon reflection) being behind the aircraft in yaw control as I bled speed, but I am of the view that this is what I was trained to do). Coming back the second time, no issues.
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AMDEC
"A fully operative helicopter rotating left with full right pedal is a nonsense. Exceeding 360° rotation means that no pedal reaction was done or that opposite pedal reaction was abandonned after an initial attempt that seemed to have no effect."
I can assure you my friend that you are quite wrong. I was sitting in the left rear seat of an RAF Gazelle and the right seat pilot was unable to stop the yaw, I looked specifically at this feet and he definitely had full right pedal applied. He made no attempt to unstall the tail, we didn't know what was going on other than he could not stop the yaw. After several times 360 deg, he took the hard landing option. A strip-down of the tail rotor control showed no fault existed. Aerospatiale test pilots were unable to re-create the same condition, consequently, Aerospatiale's assessment was that it was not a phenomenon. I disagree. Not that it matters.
I have over 5000 hours in the Gazelle, I loved it, and I never experienced any control issues, but he certainly did that day.
"A fully operative helicopter rotating left with full right pedal is a nonsense. Exceeding 360° rotation means that no pedal reaction was done or that opposite pedal reaction was abandonned after an initial attempt that seemed to have no effect."
I can assure you my friend that you are quite wrong. I was sitting in the left rear seat of an RAF Gazelle and the right seat pilot was unable to stop the yaw, I looked specifically at this feet and he definitely had full right pedal applied. He made no attempt to unstall the tail, we didn't know what was going on other than he could not stop the yaw. After several times 360 deg, he took the hard landing option. A strip-down of the tail rotor control showed no fault existed. Aerospatiale test pilots were unable to re-create the same condition, consequently, Aerospatiale's assessment was that it was not a phenomenon. I disagree. Not that it matters.
I have over 5000 hours in the Gazelle, I loved it, and I never experienced any control issues, but he certainly did that day.
If Fenestron stall was real and was a function of too much or too little airflow through the Fenestron, it would be simple to reproduce it by hovering crosswind in a strong wind - it doesn't happen.
Not sure about air only flowing one way through the Fenestron either - max sideways demo on 365 provided a noticeable kick and change in pedal position as you accelerated and decelerated moving right as the airflow switched direction.
Fenestrons do use a lot of power but I really don't believe they stall - I've mishandled enough of them and never had a problem correcting yaw.
Not sure about air only flowing one way through the Fenestron either - max sideways demo on 365 provided a noticeable kick and change in pedal position as you accelerated and decelerated moving right as the airflow switched direction.
Fenestrons do use a lot of power but I really don't believe they stall - I've mishandled enough of them and never had a problem correcting yaw.
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Crab,
What can I say mate? I am open to any and all further explanations as to what happened that day. It wasn't mechanical and his right pedal was fully forward, I have to conclude therefore, that it was aerodynamic. Nonetheless, as I say, I am open to further explanation. The name "Fenestron Stall" may grate with purists, I don't really care what we call it.
What can I say mate? I am open to any and all further explanations as to what happened that day. It wasn't mechanical and his right pedal was fully forward, I have to conclude therefore, that it was aerodynamic. Nonetheless, as I say, I am open to further explanation. The name "Fenestron Stall" may grate with purists, I don't really care what we call it.
I know and we have discussed this before but I just can't see any hidden aerodynamic anomaly that could account for it - maybe we should just stick to the 139
