Helicopter Stall

[Ibra]

The flight was to get there as soon as possible and highly likely even faster to catch up some clerance delays (VIP pressure), so they cruised at 120kts, the physics at that speed vs weather/terrain constraints would be same as fixed wings (high likely, don't see it coming or spatial disorientation)

I was told helis are hard to slowdown or hover in IMC without increasing bank angle too much for instrument or going sideways, "safe imc speed" is toward 40kts, with the guy experience (he probably flew low SVFR in that area more than anyone else) slowing down to 50kts in that flight would have been 999/1000 doable but his pax would be 1h late...

 

[chrisk]

I doubt the pilot "stalled the helicopter", with anything similar to what a fixed wing pilot would consider a stall.  

Things like a retreating blade stall happen with relatively high  speed.   The advancing blade has a high speed relative wind.   The retreating blade has low relative wind speed, and can stall if the relative wind is to slow.   Given the helicopter was flying slowly at the time of the crash, this is not likely.  

Blades can stall if the rotor RPM is to low.  I can't imagine that happening.   

I can picture disorientation and accompanying  low or negative "Gs" with the helicopter.   Stability is essentially lost when operating in low/negative Gs.

If "hovering to a descent" I could picture a vortex ring.  That might be tough to recognize and exit in IMC.

[0TreeLemur]

Because of the need for sufficient centrifugal force to hold them outward, rotors will fold up before getting to a high enough angle of attack at slow speed to stall like an airplane wing.  Rotor speed is the most important variable in keeping a helicopter aloft.

[jetdriven]

What happens when you point the nose of The helicopter up 30°? The airspeed drops,, and then you lose a lot of the transitional lift. I would guess the helicopter would behave like an airplane it would roll off to the left or right, then nose down and increase airspeed, gain transitional lift, then pull up and fly level again. 

[Hank]

8 minutes ago, jetdriven said:

What happens when you point the nose of The helicopter up 30°? The airspeed drops,, and then you lose a lot of the transitional lift. I would guess the helicopter would behave like an airplane it would roll off to the left or right, then nose down and increase airspeed, gain transitional lift, then pull up and fly level again. 

Unless this happens as you go inadvertent IMC 500' agl over rising terrain, surrounded by higher hills . . . .

[PT20J]

1 hour ago, jetdriven said:

What happens when you point the nose of The helicopter up 30°? The airspeed drops,, and then you lose a lot of the transitional lift. I would guess the helicopter would behave like an airplane it would roll off to the left or right, then nose down and increase airspeed, gain transitional lift, then pull up and fly level again. 

Interesting question. It takes more collective to hover than to fly level because a forward component of airflow over the rotor has the effect of increasing the angle of attack for a given blade angle.This is the translational or transitional lift effect. So, if you were to pitch up, the airspeed would drop and the lift would decrease and there would also be a tendency to roll because of asymmetric blade effects as you surmised. It is interesting to contemplate what would happen when it ran out of forward speed and momentum. It's not like an airplane stall where the inherent stability would necessarily cause the nose to pitch down. I'm not sure what would happen, but it probably isn't good. If you were to reduce collective, it would probably go into autorotation. Helicopter pilots don't fly like that though, and it's hard to imagine an experienced pilot doing such a thing. The cyclic is used to get the helicopter moving left, right, forward or back and the collective, coordinated with power, is used to control climbs and descents.

Skip

[carusoam]

7 minutes ago, PT20J said:

 

Helicopter pilots don't fly like that though, and it's hard to imagine an experienced pilot doing such a thing.

Skip

This is what I was thinking while watching the video of the helicopter doing tight circles over a neighborhood...

It was clear that the pilot preferred flying in circles, over hovering or landing....

Then seeing the tracks on google maps showing the circles, left and right...

If i we’re doing that in the Mooney with all the seats full...

The complaint meter and barf meter would be pointing towards the pegs... 

The internal communications must have been interesting...

Best regards,

-a-

[gsxrpilot]

• We all know how perishable a skill, IMC flying is. Professional fixed wing pilots typically fly a lot of IFR. But this professional pilot worked for a Part 135 outfit that was VFR only. I imagine he didn't get much, if any IFR/IMC practice or experience. 
• It's unlike that he had his own helicopter for practice IMC/IFR flight time. So was likely limited to a sim ride once a year or so.
• Scud running under a low layer, while following the highway, meant it's unlikely he was on the autopilot. So at a relatively high rate of speed, climbing into IMC, the autopilot would have taken a few extra steps to engage. So unlikely the autopilot was engaged.
• Facing questions about going IMC/declaring an emergency would have taken a few extra seconds to process. He also wasn't authorized to ask for an IFR clearance as the Part 135 certificate was restricted to VFR. 

It seems likely to me that he ran out of options, tried to hand fly it out, and just didn't have the skill/experience necessary to fly the maneuver in those conditions... tragic.

[Ibra]

5 hours ago, jetdriven said:

What happens when you point the nose of The helicopter up 30°? The airspeed drops,, and then you lose a lot of the transitional lift. I would guess the helicopter would behave like an airplane it would roll off to the left or right, then nose down and increase airspeed, gain transitional lift, then pull up and fly level again. 

My guess blades stall in high nose up is probably like flying a fixed wing nose up on crossed controls and zero forward speed?
The outcome and flight path after 30 degrees nose up to stall on fully crossed controls config would be impossible to predict

Maybe something Chinook CH47 can do? it has lot more symmetry but that comes at huge cost of zero lift anytime forward speed is low

Fixed Wing Pilot only (flying R22 is not my thing)

[ChrisV]

As a licensed (not practicing) Helicopter CFI, I would highly doubt retreating blade stall, settling with power or a nose up attitude caused this.

Retreating blade stall - This one is possible if he was at a high airspeed and attempted to duck under one of the lower clouds, but that would be REALLY poor judgement.  Not something I would expect of a high time commercial pilot, that close to the ground.

Settling with power (ring vortex state) - basically the opposite of retreating blade stall would need to happen.  He would have had to get under 30ish (probably lower) knots of forward airspeed and be descending greater than 400 FPM.  Not likely when he was so low already.  

Raising the nose (not collective) - this won't stall a helicopter unless you flipped over.  It will slow it down.  I've never came close to raising the nose of a helicopter 30 degrees, not even when performing "Quick Stops" (an emergency slow down maneuver).  The only time I could imagine a helicopter doing this would be landing under hostile fire.

My guess is he went inadvertent IMC, panicked due to the close mountains that he knew were but couldn't see, and instead of switching to instruments he looked for an opening in the clouds.  Of course it could have been a control surface failure too.

[0TreeLemur]

10 hours ago, jetdriven said:

What happens when you point the nose of The helicopter up 30°? The airspeed drops,, and then you lose a lot of the transitional lift. I would guess the helicopter would behave like an airplane it would roll off to the left or right, then nose down and increase airspeed, gain transitional lift, then pull up and fly level again. 

Efficient climbing is done with nose down.   Raising the nose causes helicopters to move backwards at low speeds, and is unsustainable at high speeds unless you have enough momentum to do a loop, because you are going to lose forward speed. 

[Garryowen]

The accident model helicopter has a normal cruise speed of 140-155kias at the altitude it was flying at. 
 

With 150kias in level forward flight I can pitch up 30° and achieve a near 4000 ft./min. rate of climb. Airspeed will bleed off but the resulting altitude change is dramatic. 
 

Pitching the nose up 30° in ‘wings-level’ flight in any helicopter at any forward indicated airspeed will result in a positive rate of climb until the indicated airspeed is zero. 

Upon reaching zero indicated airspeed and with no change in the nose up pitch the helicopter will then begin to accelerate in the opposite direction now trading altitude for airspeed - in a negative direction of course. 
 

All things being equal (same power setting and same 30° nose up ) the helicopter will return to the speed and altitude as when the nose up maneuver began - albeit with a vector 180° opposite. 
 

Think of it as if you’re rolling a marble up a giant skateboard ramp at 50 kts. Once the marble runs out of kinetic energy it will roll right back down. Same thing happens in a helicopter - all things being equal. Just like everything aerodynamic helicopters like to fly pointy end forwards. But I can fly this helicopter backwards and sideways at 35kts relative airspeed all day long (normal operating limitation).

Pardon the crude drawing.

 

 

[PT20J]

2 hours ago, Garryowen said:

The accident model helicopter has a normal cruise speed of 140-155kias at the altitude it was flying at. 
 

With 150kias in level forward flight I can pitch up 30° and achieve a near 4000 ft./min. rate of climb. Airspeed will bleed off but the resulting altitude change is dramatic. 
 

Pitching the nose up 30° in ‘wings-level’ flight in any helicopter at any forward indicated airspeed will result in a positive rate of climb until the indicated airspeed is zero. 

Upon reaching zero indicated airspeed and with no change in the nose up pitch the helicopter will then begin to accelerate in the opposite direction now trading altitude for airspeed - in a negative direction of course. 
 

All things being equal (same power setting and same 30° nose up ) the helicopter will return to the speed and altitude as when the nose up maneuver began - albeit with a vector 180° opposite. 
 

Think of it as if you’re rolling a marble up a giant skateboard ramp at 50 kts. Once the marble runs out of kinetic energy it will roll right back down. Same thing happens in a helicopter - all things being equal. Just like everything aerodynamic helicopters like to fly pointy end forwards. But I can fly this helicopter backwards and sideways at 35kts relative airspeed all day long (normal operating limitation).

Pardon the crude drawing.

 

 

Now that you jogged my memory, I recall an air show performance in a Schweitzer 300 (articulated rotor) where a steep climb ended with a zero airspeed 180 pedal turn with a descent as you described.

So, it’s clearly possible to do this maneuver in a fully articulated rotor system. 

Is it possible to do it safely with a semi-rigid rotor like a Robinson or a Jetranger?

Skip

[Garryowen]

11 minutes ago, PT20J said:

Is it possible to do it safely with a semi-rigid rotor like a Robinson or a Jetranger?

Absolutely possible to do it with semi rigid designs. The only limitation is you have to maintain positive G. Unloading a rotor of that design has flapping and feathering issues that may cause a portion of the system to contact the mast (aka mast bumping). 
 

Fully articulated rotor heads on the other hand are fully aerobatic and will even take negative G. That is the system used on all S-76 series of helicopters. 

[PT20J]

@Garryowen how good is the SAS on the S-76?

Skip

[RobertGary1]

On 1/28/2020 at 9:39 AM, ChrisV said:

I've not flown a turbine helo, do they have the turbine power lag like turbine planes?

I've also not flown a turbine helicopter but in the piston helicopter if you want to change RPMs you do it on the ground. The RPM's never change once you are in flight. At least in the Bell. 

 

-Robert

[Garryowen]

2 hours ago, PT20J said:

@Garryowen how good is the SAS on the S-76?

Skip

SAS on the S-76 is one component of a complex Automatic Flight Control System that easily rivals advanced systems on high flying jets. The AFCS is a combination of SAS, autopilot/yaw damper and flight director functions that completely couple to all 4-axis (pitch, roll, yaw and collective).

Hand-flying in IMC is a non event. The SAS smooths the ride by automatically providing up to 10% control authority with out any input from the pilot. It basically dampens out any external inputs. Hand flying I set the collective and forget it. Feet on the floor because yaw is controlled by turn coordination and once the cyclic is trimmed out all that is required is pressure/counter-pressure to maintain any desired attitude.

With the flight director coupled, 100% control authority is given to the AFCS via the autopilot, flight control computers and trim motors attached to the various flight controls. The pilot tells the flight director what vertical and lateral path to take and with the fourth axis (collective/power) also directs the speed to fly. As an example, pressing one button on the collective will automatically roll wings level with pitch and climb set to a pre-computed attitude and power setting which guarantees recovery and climb out....”go around mode.” 
 

Long way around to say that the S76 is one of the safest and pilot friendly IFR machines around. 
 

Nothing unsafe about helicopter IFR/IMC with the right training. Having a nice helicopter to do it in is even better. Just like everything in aviation, helicopter IFR is just a tool to use when it’s the safer or faster way to do the trip. Holding outside controlled airspace waiting to pick up a SVFR is a waste of everyone’s time and money considering I’m in a machine that is capable of almost every single approach available at any particular airport. And I have lower IFR landing minimums because I’m in a helicopter!!! (CAT 1 lower mins only)  

Here’s a cockpit shot of a fully coupled ILS into JFK’s 4L. Appch told me to keep the speed up because I was ahead of an a380. Then had to tell me to slow down because I had 25 kts on an a320 in front of me.

 

I broke out at minimums and used a decel function on the flight director that slows me to 70kts and an automatic level off at 50ft right down the centerline. On this flight I ended up air taxiing over to parking and was inside eating a sheltair cookie before the a320 was even to the gate. 

Edited February 13, 2020 by Garryowen

[carusoam]

Thanks for the awesome helicopter insights, gents!

Best regards,

-a-

[MooneyMitch]

Interesting AvWeb Paul Bertorelli nsight regarding Kobe Bryant crash. 

https://www.avweb.com/multimedia/votw/kobe-bryant-crash-risk-by-the-numbers/

[Garryowen]

Another great video by Paul.  Helicopters are truly a niche within aviation and he breaks it down very well. 

[ilovecornfields]

Lawsuits have predictably started:

https://www.tmz.com/2020/02/24/kobe-bryant-wrongful-death-lawsuit-island-express-helicopters-vanessa-gigi/
 

Not where I usually get my news , but I was sent the article by a friend. I read a few comments before I couldn’t stand it anymore. Seems complete ignorance of a situation is not sufficient to keep most people from posting their opinions. I especially love the “FAA shouldn’t have let them fly that day” comments. I’ll be sure to call FSDO next time I’m planning on departing VFR just to make sure it’s ok with them...

[carusoam]

TMZ and USAToday...

Very important news gathering agencies...

 

-a-

[ilovecornfields]

1 hour ago, carusoam said:

TMZ and USAToday...

Very important news gathering agencies...

 

-a-

Touché! Not my usual source of news but in this case I believe the story is accurate. Soon the public will be calling to ban all those irresponsible pilots from flying without a flight plan.

[Ross Taylor]

Not related to helicopter stalls... but related to this crash and the media coverage of it.  Last week there were several "the pilot previously violated FAA safety rules" articles about this pilot... because he busted a class Bravo a few years ago.  Regardless of the pilot's actions during this crashed flight, those articles sort of implied a pattern of unsafe flying.  I consider myself a pretty safe and attentive pilot, but I came within a 1/2 mile of busting a Bravo last year when I didn't catch the SFC start on an approach to Santa Monica.  It was only thanks to the Santa Monica tower's warning that I didn't end up "copying a number" myself. 

[201er]

Just learned the probable cause from the NTSB about the Kobe Bryant helicopter crash: the basketball player’s persistence to continue a flight into deteriorating weather conditions was the main cause of the accident. Contributing factors include having too much fame, money, and ambition. No flight plan was filed. And the pilot had spatial disorientation. Not that we didn’t know all this a year ago.