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R44 Crash Reenactment (Video)


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Just ran across this channel on vimeo about reenactments of various plane crashes (some with CVR's). It's a great learning tool to the risks that many of us face flying and to always be on guard.

 

This particular video is of a R44 that crashed and how the fuel take ruptured killing all 4 occupants on board. The helicopter was over gross and departing down wind in a high density altitude environment. Eyewitness reports said the helicopter was buffeting back and forth and subsequently hit the ground fracturing the transmission (which ruptured the fuel tank).

 

The video doesn't explain it at all, but I'm curious as if anyone has a more detailed explanation as to why the helicopter would buffet back and forth so bad. The details of being overweight, high density altitude, and flying downwind (16kts) all contribute, but I'd be interested on how they all came together.

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I find the re enactment interesting. The first up close shot makes me think a damper bushing on the tail rotor drive shaft may have been out, out of alignment, or even maybe a bent drive shaft?

 

Then after reading some details, a high DA makes for some serious power usage in an R44. Then departing downwind. Sometimes I have really had to dance on the pedals while hovering if turned downwind. It makes the tail rotor seem like its failing, but in reality its just having to work harder.

 

Dancing on the pedals if the pilot were, may have caused the buffetting.

 

The disk in the hover may have been able to produce lift with a tail wind of 16kts IGE hover, giving a false sence of available power, then as speed increased over the ground in a downwind direction, the extra lift that had been provided by the wind over the disk was removed as the helicopter matched the wind speed.

 

This resulted in a reflection of the gross over weight over coming available power to remain airborn.

 

Loss of control from possible pilot induced occilation of the tail rotor, loss of lift resulting from high DA, over weight, lack of lift available and lack of power available for that condition.

 

If in doubt, as many here have said dont depart down wind. Pilot could have hovered without passengers across the terrain downwind nice and slowly to position the helicopter for a better option to depart. Then loaded them, used the wind as an advantage and possibly made the departure....

 

Depends on what else may have been a factor in the vibrations, and mechanical condition of the ship in question.

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The pilot broke the most basic rudimentary fundamentals of operating a helicopter that any beginner pilot should know about and respect…… When this happens, bad things will happen every time. -Know this. -Understand this. No one is immune. While flying helicopters, ignorance is not bliss. Its death……

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I thought then and I still believe the shake was pilot induced. I think she was able to pull it into a 40' AGL OGE hover because essentially had translational lift in a hover and lost it as she stated to accelerate downwind. The ship danced and she lost altitude.

This one was over the second she decided to takeoff downwind near goss weight at altitude.

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Since the release of SB regarding the bladder fuel tanks, RHC added an entire lecture to their course about how the tanks are more likely to rupture in an R44 (without bladders) than in an R22. I'm really surprised that they didn't show this video as a case study - I think it explains the situation very well.

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The NTSB determines the probable cause(s) of this accident to be:

 

The pilot's improper planning/decision in attempting a downwind takeoff under high-density altitude conditions that resulted in a loss of control and impact with terrain. Contributing to the accident were the helicopter's gross weight in excess of the maximum hover out of ground effect limit, a high density altitude, and the gusty tailwind.

 

Ref link: Full narrative available

 

This accident is more characteristic of “settling with power” or power settling” and illustrates that those two terms should not be used interchangeably with Vortex Ring State (VRS), in most cases they have nothing to do with it.

 

In the case of this accident, with the wind blowing down hill toward the valley, not only was there a downwind, but also most likely there may have been down drafts coming off the hill. The combined effects of high-density altitude, gusty tailwinds, possible down drafts, being over gross weight, and OGE with too little engine power, were beyond the pilots capability to recognize or handle.

 

As the helicopter started to settle, the pilot may have tried to recover with collective; however, the power required far exceeds the power available. The result was decay in rotor rpm due the engine inability to produce enough torque to support the increase in blade pitch, ending in the torque rising and rpm falling.

 

The rotor rpm decay feeds on itself and continues to fall through 90% - 85%. At 85% of normal RPM the main rotors and the tail rotor are only producing 70% of their normal thrust. This is due to the square law that states the thrust a rotor can produce is proportional to the rotors angle of attack times the square of its rpm, (T~ α x (NR)2). The loss of thrust therefore, is proportional to the square of the loss in rpm.

 

The pilot under these conditions is now trying to control a helicopter that’s not responding normally due to the decaying rpm; moreover, the tail rotor’s ineffectiveness, which may explain the so-called wobble/sway as the pilot tries to recover.

 

As the rotor rpm decay continues, the tail rotor thrust is no longer able to counter the main rotors torque reaction and the helicopter yaws right, exposing the left side of the helicopter. The main rotor, due to the decaying rpm is unable to arrest the descent and the helicopter settles to the ground left side first, hits the ground and rolls over on its left side. If not for the fuel cell issue causing the post fire, this may have just been a non-fatal rollover.

 

The diagram below shows, if they were hovering OGE with a 15-knot tailwind at point B on the power curve, with a small power margin between available power and required power, any takeoff downwind from that point would require going through zero airspeed, were the power required would be greater than where they started from at point B.

 

Once you start downwind under these conductions, each knot of ground speed subtracts from (reduces) net relative airflow across the main rotor and once you cross the power available line the helicopter will start settling. The true meaning of power settling, nothing to do with VRS.

 

Moreover, they may have thought, no problem with power, since they were hovering OGE at 40 feet. They didn’t realize the initial power required is considerably greater when you fly downwind in gusty 15-knot conditions.

 

Had they made the transition to forward flight into a 15-knot headwind (point A), each additional knot of ground speed (speed relative to the ground) would have increased the net relative airflow across the main rotor; thereby, decreasing the power required throughout the takeoff phase.

 

DraftCopy_zpsf7a2ede1.jpg

Edited by iChris
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