Rotorheads: why did this helicopter crash yesterday in Hawaii?

Caught on video by some lucky bystanders; the heli almost came down on top of them.

So what happened? The news yesterday said that “you can tell from the video” that the tail rotor had stopped, but I suspect that’s a wagon-wheel video artifact; there was only a little bit of yaw happening as the heli came down.

At the start of the descent, the RPMs appeared to climb as if the engine had become unloaded (either a break in the transmission, or a break in the collective pitch link). A frame-by-frame shows the main rotor blades appear to be bent upwards as if they’re trying to generate a lot of lift, but this too could be a rolling-scan video artifact.

So what happened? Settling with power?

I’m going to guess the transmission broke between engine and main rotor, so the engine races but the main rotor just freewheels.

As a result, no lift (obvious from the video). It’s not rotor power and no tail rotor, or the body would start spinning like a top.

I have heard the dropping aircraft can be used to drive (windmill) the rotor to mitigate the drop speed, but it appears this was not done effectively.

Looks like a problem with the collective (rotor blade pitch control) to me.

I’d go with md2000’s assessment too: main transmission failure.

As soon as the trans fails the rotor RPM starts to fade. Which immediately leads to the rotor blades coning up. Hearing the engine spin up not down is the thing which clues us to transmission failure rather than engine failure or a pilot error.

They were probably outside the realm of safe autorotation. Which may or may not have been a pilot mistake to operate in that regime. You’d like to avoid it always, but sometimes you simply can’t while still getting into and out of the intended landing area.

The real takeaway here is how darn fast they went from utterly normal to utterly screwed. Then it was just a matter of waiting the next couple seconds for the water to arrive.

I’ve known a couple of chopper pilots who survived crashes via auto-rotation (which is what you’re talking about). Even for a skilled pilot it’s not easy, and typically all you do is take a fatal crash and convert it into a survivable crash where the occupants suffer significant injuries but are alive enough to survive it.

I gather auto-rotation landings aren’t intrinsically a problem; just like a deadstick landing in a fixed-wing plane, everything’s fine if you’re in a good location with plenty of altitude. The problem is that often times you’re not in a good location and/or don’t have a lot of altitude to work with. A powered landing right here and right now into a thick forest isn’t going to be much better than an autorotation landing.

here’s a trooper repeatedly practicing autorotation landings at an airfield.

Yes, that’s the problem - you need sufficient altitude in which to react. I also gather that auto-rotation is easier if you also have forward motion (I’m sure if that’s incorrect our rotorheads will correct me).

But what are helicopters frequently used for? Yep - low-altitude hovering. Which gives you little time and space in which to react.

And you are correct about fixed-wing deadstick landings - the big problems are do you have enough time/altitude to deal with the situation, and do you have a place to land safely?

Dead man’s curve.

For a successful auto-rotation you initially need height or speed. The more height you have, the less speed you need. As you can see in the attached diagram, if you are very low you don’t need speed anymore. So there is a profile you can fly on both landing and take-off that will always allow for an auto-rotation, assuming you have adequate terrain below you.

Ref the info in Richard’s excellent post. …

It’s a lot easier to stay out of the dead area on a typical takeoff than landing. And it can be impossible if you’re approaching a small obstructed landing area.

The real simple version is keep the speed above something around 30 or 40 knots until close enough to the ground that you can stand to fall that far and live. Then slow down in nearly level flight to a touchdown on target.

We can see they can approach over the water at low altitude easily enough (ignoring boats & local hazard regs), but we don’t get a look at the pad that’s presumably off to the camera’s right. It may be surrounded by trees or a fence or something. Or be on a raised platform.

Which leaves you stuck during the last parts of the approach being too slow to autorotate with the ground too close to gain enough diving speed before impact, yet too far away to fall to survivably.

Do you have a ref to a news article? How did the occupants fare? If they’re all alive that would be considered a “successful” outcome from the spot they were in, regardless of how hard (read $$) it’ll be to reuse any of that particular helicopter.

Lots of news articles. One (out of five onboard) critically injured, no fatalities.

Is that helicopter salvageable at all? They crashed into Pearl Harbor, so it was in salt water for who-knows-how-long before they hauled it out. Besides, even without the salt, I’m guessing a sudden influx of nice, cool water isn’t good for smoking-hot turbine parts. Also, that was a pretty hard impact, so I’m guessing the whole airframe got wracked.

Which is why autogyros can fly. (A powered propeller keeps the craft moving forward, causing the rotor to spin and generate lift. Basically it’s in auto-ration all the time.)

I think the guy did a good job. Considering.

Got it close to the land without putting it down on top a bunch of tourists.

I was there back at Christmas. There is no nearby pad. It’s all tourist area, with monuments and trees, and lots of people. They went down about here.

Good to hear about the people, relatively speaking.

That helo is just parts now. Some stuff will be salvageable, but not a lot. As you say, between impact damage, thermal shock, & salt water intrusion it won’t be lots.

That was amazingly lucky timing. Had they failed any time more than a second or two later they would have impacted land with no survivors. And perhaps landed in the crowd.

No survivors ? … In the past there was a lot of that due to unsafe fuel tanks… a minor crash and the fuel is burst everywhere and it erupts in a fireball…

Hopefully the helicopters now have safer fuel tanks.

I’d say that it may have resulted in more injuries, but its unclear if it would have been fatal to hit ground… depending on the ground…

For those with an interest in “fall down, go boom” in aircraft, here is the link to the
NTSB’s Aviation Accident Inquiry Search Page.

It’s been recovered. 1 pm local, Friday. http://www.staradvertiser.com/breaking-news/crashed-helicopter-remains-submerged-arizona-memorial-partially-closed/

As a helicopter pilot, I was trying to guess what happened.

First: Autorotation landings can be/should be as gentle as any other landing – given a suitable landing area. Autorotations are actually rather fun. :slight_smile:

Since it was a jet helicopter, I think what we’re hearing is the rotors spinning up. I didn’t notice the sound of a turbine running away. As to coning, coning happens whenever the helicopter is flying.

I couldn’t tell from the video I watched exactly when there was an emergency. Was the helicopter in an emergency situation before it fell out of the sky? It didn’t look like it, but if the approach was not to a normal landing place, the pilot may have had a chip detector light or some other indication of something bad about to happen. As for the sudden drop, I think we can rule out the vortex ring state (settling with power). Speed would need to be under about 15 knots (which it appeared to be), and the rate of descent would have to be greater than about 300 feet per minute (which it didn’t appear to be).

I’m guessing – and this is only a guess – that the pilot was making a normal or precautionary landing when Something Bad happened and he attempted to enter autorotation. The first thing you do to enter autorotation is to lower the collective. This does a couple of things: it allows the rotor RPM to be maintained or increased, and the helicopter drops rapidly. Richard Pearse linked to the Dead Man’s Curve (Height-Velocity Diagram). The video shows the helicopter coming in slow, and close to the surface. It looks to me like Something Bad happened and there just wasn’t enough height or airspeed to deal with it. I hope someone will link to the NTSB report or a statement by the pilot to let us know.

What is Something Bad? Could be any of a lot of things. Transmission failures are rare, but they do happen. As someone already said, the rotor freewheel and you enter autorotation. Tail rotor failures seem to happen more often, but they are also fairly rare unless you hit something. Fuel exhaustion will make you autorotate. Helicopters have chip detectors in their transmissions (main and tail rotors). If your gears start falling apart, there will be metal fragments that will be picked up by magnetic plugs. If one of your chip lights comes on, you land immediately and prepare for the loss of one of the transmissions on the way down. Maybe the pilot had a chip light and was making a precautionary landing, and the main rotor transmission failed just before landing? It’s possible. There are other Bad Things that can happen to induce a pilot to get onto the ground ASAP, but from what I can see in the video they probably don’t apply.

Anyway, that’s my 2¢. Opinions and educated guesses only, subject to change as new facts emerge.

Radio traffic from helicopter pilot from my cite above indicates a sudden emergency. At that location, the helicopter would be under direct ATC guidance from the airbase/airport (it’s a joint base Pearl Harbor-Hickham with Honolulu International).

Radio communication between the Honolulu air traffic control tower and the pilot of the helicopter suggest that whatever went wrong happened quickly.

At 10:22 a.m., according to a recording of the air traffic control conversations provided by liveATC.net, the pilot tells the controller he is near the south ramp at Ford Island.

Seconds later, the pilot issues a distress call: “Tower. Chopper 8. I think I’m going down.”

“Chopper 8, Roger. You’re east of Ford Island right,” the controller replies.

The pilot’s response can’t be understood.

The tower tries to reach the helicopter again, but there is no response.

The air traffic controller then asks another aircraft to fly toward Ford Island to see if they can spot the helicopter.

A preliminary report on the crash is expected in 10 days. Another report on the cause of the crash should be released within 12 to 18 months.