What caused the tragic helicopter crash in Sao Paulo?

[Ivan Seeking]

Some pretty dramatic footage. Unfortunately, the pilot did die in this crash; I think it was reported that two or three others were onboard and survived. The pilot is being called a hero for avoiding the homes just short of the field.

https://www.youtube.com/watch?v=k55yMo8q6Us

 

[FredGarvin]

Tail rotor loss is a b**ch! It looked like he completely lost the main transmission shortly there after. Ugh. What a way to go. The only good thing is that he was probably working so hard at keeping the aircraft righted that he never knew just how fast the ground was coming up.

 

[Astronuc]

Ouch! I guess the main rotor froze up with the main transmission gone.

 

[Andre]

Sad indeed, tail rotor failure is end of story. Basic mechanics, the torque interaction between rotor and main frame causes the uncontrollable spin, simple Newton mechanics. We had an accident like that some decades ago, with the centrifugal forces so strong that the pilot was thrown out of the seat and ejected through the canopy despite the seatbelts. The fabric just failed, which was caused by aging and fatique.

In those circumstances there is little controlling to do, no houses to avoid. Also, if a gear box failure would have stopped the main rotor, then the main frame would not have spun so violently.

Edit: About how fast things go, there are many reports about apparent http://www.informaworld.com/smpp/content~content=a713697891&db=all, for instance from jet pilots, wondering why it took so long between activating the ejection seat and the actual ejection (normally around 0.5 sec). So while waiting they did a complete cockpit check, noting the indications of all the dials. It could that in those conditions the ground approached in perceived slow motion.

 

[Ivan Seeking]

Why did it lose altitude so fast due to a rotor failure?

 

[FredGarvin]

It looks like it was doing OK until that big poof of white smoke. That appears to be the main transmission. At that point the rotors are still turning, but the transmission is toast and is probably eating itself in the process and thus killing rotor speed. Low rotor speed = drop like a rock.

 

[Cyrus]

Why did it lose altitude so fast due to a rotor failure?

When the rotor stalls (RPM gets too low), you can't unstall it like an airplane by pitching the nose back down. So, you basically become an 10k lb brick. He was probably coming down (easily) at 2000 feet/min.

They way they landed was the worst possible. They landed belly down, so all the impact went straight through the fuselage. Had they hit the side/rotors first, they would have absorbed a lot of the energy.

 

[Ivan Seeking]

Okay, I can understand falling when the main rotor goes, but they looked to be falling pretty quickly due to a tail rotor failure. That is that part that I don't understand. Maybe he was trying to lose altitude quickly at that point, but when the transmission went, he was hosed?

 

[Cyrus]

It's hard to say. You're right that a tail rotor failure alone shouldn't make you come down like that. If he was in forward flight, a tail rotor failure could have been easily handled by keeping his airspeed up and making a running landing. The helicopter is designed to off load the need of a tail by its fins during high speed flight.

My guess is that he was in a hover and more than one thing went wrong.

 

[Ivan Seeking]

Could he get into a spin and a drop like that due to a general power loss? Could that have been a precursor to the catastrophic failure of the transmission?

 

[Cyrus]

Well, a power loss would just decrease rotor RPM to all rotors. I don't think you would start spinning like that. That was absolutely a tail rotor failure, at the very least.

 

[Andre]

Absolutely, this is really a clear cut tail rotor failure. Most helicopters can auto-gyro land with an engine failure.

If something happens with a tail rotor you may have only one very slim chance, 'collective' pitch full down to reduce the torque as much as possible and cyclic/ nose down to pick up airspeed and hope that the tail will wind vane enough.

But collective down also means no more lift and become a brick aerodynamically.

http://en.wikipedia.org/wiki/Helicopter_flight_controls

 

[Ivan Seeking]

Would it make sense that he was trying to lose altitude, tried to throttle-up before hitting, and that's when we see the smoke?

What bugs me is that based on what you are all saying, he had more than one failure, which seems unlikely. Why would more than one system fail within a few seconds of each other?

 

[Andre]

No, the tail rotor failure was long before the smoke, you can see the helicopter spinning long before that. This causes extreme stress on the airframe, which can lead to all kinds of other failures. So, the smoke can be caused by a number of possible secondary failures. For instance the engine may just as well have blown up, due to excessive precession forces on the main shaft. The board of investigation will find out.

 

[Ivan Seeking]

No, the tail rotor failure was long before the smoke,

Yes, that is clear.

you can see the helicopter spinning long before that. This causes extreme stress on the airframe, which can lead to all kinds of other failures. So, the smoke can be caused by a number of possible secondary failures. For instance the engine may just as well have blown up, due to excessive precession forces on the main shaft. The board of investigation will find out.

Okay, so once you get into a spin all bets are off. That makes sense.

 

[FredGarvin]

My guess is he was doing some major control surface excursions and overstressed something in the mast assembly. He could have possibly had a droop stop failure as well and had a sudden stoppage. I'm not sure if the A-Star (I think that's what he was flying) has them though.

At the very beginning of the video, it is tough to see, but he is almost at a hover, but has lost his tail rotor and is spinning.