AS 350 SHORT SHAFT FAILURE
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AS 350 SHORT SHAFT FAILURE
Has anyone ever had a short shaft failure on a squirrell?
The way I understand it, if the shaft breaks your RRPM will decrease (giving a low RRPM horn) and the engine RPM will increase giving one hell of a yaw due to the RPM increase through to the tail rotor...
What do you do then........?
Enter an auto, and try correct the yaw
But then what do you do with the engine...?
I haven't found any Eurocopter papers on it and what the published emergency would be.
The way I understand it, if the shaft breaks your RRPM will decrease (giving a low RRPM horn) and the engine RPM will increase giving one hell of a yaw due to the RPM increase through to the tail rotor...
What do you do then........?
Enter an auto, and try correct the yaw
But then what do you do with the engine...?
- If you cut it you're going to most probably end up in a sideways flare at the bottom of the flare and a possible roll over on the landing, or do you keep the speed up at the end of the flare so the tail fin is effective for a slide in landing..?
- Do you take it to flight idle position and normal auto...?
- Do you keep the engine running at flight position and control the yaw with pedals...?
I haven't found any Eurocopter papers on it and what the published emergency would be.
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I merged your original thread into our AS350 thread >>> AS350 Astar / Squirrel
I merge discussions relating to specific types to save people having to search through lots of threads in the future.
We now have an enormous database of type-specific information.
Examples - not the full list:
AS365
Agusta 109
Bell 206
Bell 407
Bell UH-1 Huey
Boeing AH-64 Apache
Boeing CH-47 Chinook
EC135
EH101 Merlin
MD Explorer
Sikosrky S-76
Sikorksy S-92
V-22 Osprey
Heliport
I merge discussions relating to specific types to save people having to search through lots of threads in the future.
We now have an enormous database of type-specific information.
Examples - not the full list:
AS365
Agusta 109
Bell 206
Bell 407
Bell UH-1 Huey
Boeing AH-64 Apache
Boeing CH-47 Chinook
EC135
EH101 Merlin
MD Explorer
Sikosrky S-76
Sikorksy S-92
V-22 Osprey
Heliport
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AS350 Short Shaft Failure
Good question Whirlybirda@yahoo, I read an article in "Rotor & Wing" magazine some years ago (I still have a copy of the article) of what to do if your experience an AS350 short shaft failure.
If the short-shaft between the engine and MGB fails you will indeed experience 2 things;
1. Increase in engine RPM
2. Decrease in M/R RPM.
As in any machine, the collective follows the MR RPM, that is DOWN in this scenario and enter autorotation.
Your engine will be running away like a rocket, so to control that the technique is to REDUCE THE FCL (throttle) TO IDLE.
IDLE is slected because if you shut down the engine completely, the tail-rotor will also slow down (or stop eventually) as nothing is driving it!!
If you leave the FCL in the flight position, when you flare at the bottom and raise the collective to cushion the touch-down the aniticipator will kick in and produce a big yaw, especially if you are not heading directly into wind. This will cause you to get very untidy and crash at the bottom most likely.
If your would like a copy of this article, email me your contact details at [email protected]
Safe flying
Nigel
If the short-shaft between the engine and MGB fails you will indeed experience 2 things;
1. Increase in engine RPM
2. Decrease in M/R RPM.
As in any machine, the collective follows the MR RPM, that is DOWN in this scenario and enter autorotation.
Your engine will be running away like a rocket, so to control that the technique is to REDUCE THE FCL (throttle) TO IDLE.
IDLE is slected because if you shut down the engine completely, the tail-rotor will also slow down (or stop eventually) as nothing is driving it!!
If you leave the FCL in the flight position, when you flare at the bottom and raise the collective to cushion the touch-down the aniticipator will kick in and produce a big yaw, especially if you are not heading directly into wind. This will cause you to get very untidy and crash at the bottom most likely.
If your would like a copy of this article, email me your contact details at [email protected]
Safe flying
Nigel
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AS350 SHORT SHAFT FAILURE - Ray Prouty article
Reasonable Pilot and Design Decisions Produce Tragic Result
Author: Ray Prouty / Rotor & Wing Sept 1994
There are times when aircraft designers make reasonable decisions and pilots make reasonable decisions, but a tragic result can still occur. In this example, a pilot was killed and a helicopter destroyed due to the accumulative effect of a series of otherwise reasonable decisions.
Designers of the Eurcopter AS350D chose to depart from conventional method of driving the tail rotor directly from the main-rotor transmission. Instead, they elected to drive it from the speed-reduction gearbox mounted on the engine.
The Eurocopter design saves the weight and cost of the tailrotor drive shaft part (B206 also). Under normal circumstances, there is no differance between the methods in the way the tailrotor operates. It is driven by the engine in powered flight and by the main rotor in autorotation.
It is only upon failure of the short shaft between the engine and the main transmission that things change. With the alternate system, the tail rotor is not directly connected to the main rotor, but is driven by the engine.
Short Shaft Failure
In this accident, the helicopter had the "alternate" system and while it was in cruise flight about 500ft AGL a coupling on the short shaft failed and disconnected the transmission from the engine.
At this point, another design decision came into play. Like many AS350s flying today, this one had separate speed tachometer for the rotor and for the gas producer part of the engine (Ng). The pilot would have seen the loss of mainrotor speed, but on switching his gaze to the engine instruments, been confused to find that the engine was still running. It is estimated that the pilot had less than 30 sec to decide how to react.
Pilots who investigated the accident felt a "dual" tachometer, which has pointers for both the rotor and the engine's power turbine on tha same dial, would have given the pilot a better chance of figuring out what had happened. The needles would have immediately indicated an abnormal situation.
The pilot then elected to leave the engine lever in the "fly" position while otherwise doing everything right for an autorotative landing. The AS350D flight manual tells the pilot to completely close the throttle - or the fuel flow control (FFC) valve - in the event of an engine failure. In the case of a short-shaft failure, doing this would result in stopping the tailrotor. Perhaps seeing that the engine was still running, the pilot left the FFC fully forward and followed the axiom "use power if you have it."
With luck, the landing might have been successful. There was a narrow field bordered by trees and houses within gliding range, but the wind was at right angles to it. Thus, on his final approach, the pilot was holding some tailrotor pitch in crabbing flight to touch-down with no side drift. He made a successful cyclic flair and at the proper moment, pulled the collective stick up to soften his landing.
The anticipator system
Now another Eurocopter design decision came into play. To lesson "rotor droop" during normal collective inputs, an "anticipator system" was installed. Under normal circumstances, when collective pitch is increased, the system warns the engine that a power demand will soon follow and increases fuel flow immediately. The system only works when the FFC is fully forward - in the "fly" mode.
Without the main rotor's inertia and drag, the engine oversped. Subsequent tests showed that the overspeed could have been as high as 15% almost instantaneously. In this situation, the tailrotor speed also increased by 15%.
Not much would have happened if the pilot could have made his approach into wind, and the tailrotor pitch was near zero. But because of the crabbing flight, the tailrotor pitch was not zero and the increase in speed resulted in a sudden and unexpected thrust increase of about 30%. This caused the helicopter to swing around as it landed, and it crashed into a large tree trunk.
There were no obviously bad decisions, but some tentative recommendations can be made:
1. The first is that the flight manual should address the possibility of a short shaft failure by describing its symptoms and calling out proper pilot action - such as partially retarding the FFC to the flight-idle position. This would keep the tailrotor going, but disengage the anticipator system.
2. The second is that a dual rotor/power-turbine tachometer should be used to give a quicker indication of this unusual situation.
Author: Ray Prouty / Rotor & Wing Sept 1994
There are times when aircraft designers make reasonable decisions and pilots make reasonable decisions, but a tragic result can still occur. In this example, a pilot was killed and a helicopter destroyed due to the accumulative effect of a series of otherwise reasonable decisions.
Designers of the Eurcopter AS350D chose to depart from conventional method of driving the tail rotor directly from the main-rotor transmission. Instead, they elected to drive it from the speed-reduction gearbox mounted on the engine.
The Eurocopter design saves the weight and cost of the tailrotor drive shaft part (B206 also). Under normal circumstances, there is no differance between the methods in the way the tailrotor operates. It is driven by the engine in powered flight and by the main rotor in autorotation.
It is only upon failure of the short shaft between the engine and the main transmission that things change. With the alternate system, the tail rotor is not directly connected to the main rotor, but is driven by the engine.
Short Shaft Failure
In this accident, the helicopter had the "alternate" system and while it was in cruise flight about 500ft AGL a coupling on the short shaft failed and disconnected the transmission from the engine.
At this point, another design decision came into play. Like many AS350s flying today, this one had separate speed tachometer for the rotor and for the gas producer part of the engine (Ng). The pilot would have seen the loss of mainrotor speed, but on switching his gaze to the engine instruments, been confused to find that the engine was still running. It is estimated that the pilot had less than 30 sec to decide how to react.
Pilots who investigated the accident felt a "dual" tachometer, which has pointers for both the rotor and the engine's power turbine on tha same dial, would have given the pilot a better chance of figuring out what had happened. The needles would have immediately indicated an abnormal situation.
The pilot then elected to leave the engine lever in the "fly" position while otherwise doing everything right for an autorotative landing. The AS350D flight manual tells the pilot to completely close the throttle - or the fuel flow control (FFC) valve - in the event of an engine failure. In the case of a short-shaft failure, doing this would result in stopping the tailrotor. Perhaps seeing that the engine was still running, the pilot left the FFC fully forward and followed the axiom "use power if you have it."
With luck, the landing might have been successful. There was a narrow field bordered by trees and houses within gliding range, but the wind was at right angles to it. Thus, on his final approach, the pilot was holding some tailrotor pitch in crabbing flight to touch-down with no side drift. He made a successful cyclic flair and at the proper moment, pulled the collective stick up to soften his landing.
The anticipator system
Now another Eurocopter design decision came into play. To lesson "rotor droop" during normal collective inputs, an "anticipator system" was installed. Under normal circumstances, when collective pitch is increased, the system warns the engine that a power demand will soon follow and increases fuel flow immediately. The system only works when the FFC is fully forward - in the "fly" mode.
Without the main rotor's inertia and drag, the engine oversped. Subsequent tests showed that the overspeed could have been as high as 15% almost instantaneously. In this situation, the tailrotor speed also increased by 15%.
Not much would have happened if the pilot could have made his approach into wind, and the tailrotor pitch was near zero. But because of the crabbing flight, the tailrotor pitch was not zero and the increase in speed resulted in a sudden and unexpected thrust increase of about 30%. This caused the helicopter to swing around as it landed, and it crashed into a large tree trunk.
There were no obviously bad decisions, but some tentative recommendations can be made:
1. The first is that the flight manual should address the possibility of a short shaft failure by describing its symptoms and calling out proper pilot action - such as partially retarding the FFC to the flight-idle position. This would keep the tailrotor going, but disengage the anticipator system.
2. The second is that a dual rotor/power-turbine tachometer should be used to give a quicker indication of this unusual situation.
and how about:
3. Always design the throttle to be on the collective and have sufficient arc of movement to allow accurate pilot manipulation. Like the 205/212/412. Not like the 206/L.
3. Always design the throttle to be on the collective and have sufficient arc of movement to allow accurate pilot manipulation. Like the 205/212/412. Not like the 206/L.
