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Pen Air Saab Overrun Unilaska with Injuries

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Pen Air Saab Overrun Unilaska with Injuries

Old 20th Oct 2019, 21:29
  #41 (permalink)  
 
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As is often the case in the code-share era, it appears that the branding of the flight is somewhat convoluted. Isn't Peninsula Aviation Services currently doing business as PenAir as a successor to the original company using a Part 121 certificate?

From the Anchorage Daily News:

The flight, PenAir Flight 3296, was sold by Alaska Airlines, which markets flights to Dutch Harbor. But the actual operator is more complicated.

The carrier is owned by Ravn Air Group and not PenAir, an Alaskan flying institution started in 1955 that filed for bankruptcy in 2017. A company called Peninsula Aviation Services Inc., which is affiliated with Ravn, bought PenAir’s assets last year in Chapter 11 proceedings.

The name on the federal certificate as the operator of the plane involved in the crash is Peninsula Aviation, a Ravn spokeswoman said. But information distributed after the crash by Ravn and Alaska Airlines repeatedly names PenAir as the operator of the flight.

Orin Seybert, who founded PenAir, said his company had nothing to do with the crash except for providing the leased Swedish Saab 2000 twin-engine turboprop that’s in the process of getting transferred over to the new owner.
PenAir did not provide or train the pilots, Seybert said.

"I really hate that it happened,” he said. “But this accident was not PenAir’s fault even though it was one of our airplanes. That was a total Ravn operation.”

Ravn operates several Alaska-based air services under its brand: Corvus Airlines (the former Era Aviation), Hageland Aviation Services and Frontier Flying Service. The company
overhauled its management team in 2017 following scrutiny for a 2016 crash in Southwest Alaska that killed two pilots and their passenger.


https://www.adn.com/alaska-news/2019...red-10-others/

Last edited by Airbubba; 21st Oct 2019 at 06:00.
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Old 21st Oct 2019, 16:13
  #42 (permalink)  
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So the last two US passenger fatalities were both due to penetration of the hull by engine/prop shrapnel.
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Old 21st Oct 2019, 16:45
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Originally Posted by DaveReidUK View Post
i don't think that's necessarily the case. The same principle applies to ice being shed from a blade as does to the release of the blade itself (apart from the vastly different amounts of energy involved, obviously).

In the former case, airframe manufacturers reinforce a relatively small length of the fuselage in the plane of the prop, because that's the part of the cabin that's at risk from ice being shed. Exactly the same applies to the trajectory of a departing blade, except of course that's there's no hope of stopping it if it's heading for the fuselage (which is why it's not supposed to happen).
You make an excellent point; however, I'm not sure that the loads on ice accretions are the same as those on the propeller blades. While the blades themselves have significant fore and aft loads (depending on propeller pitch), ice accretions might not. Ice accretions usually occur initially at the propeller blade root and often extend spanwise (blade span) along the leading edge of the blade toward the tip. My theory (guess?) is that ice accretions, being on the leading edge of the blade, do not experience pressure differentials created by the blade pitch and are primarily affected by centrifugal force. If that is the case, then the ice could be expected to shed within the plane of propeller rotation. Due to the aerodynamic loads on the propeller blade, I would expect that it (they) would shed out of the plane of propeller rotation.

Below is a link to a study titled, "Propeller Icing Tunnel Test on a Full-Scale Turboprop Engine" conducted under the auspices of the FAA (U.S.). I must admit that nowhere in the study was I able to find any specific reference to aircraft propeller ice shedding patterns; consequently, my theory is no more than a somewhat insufficiently educated guess (speculation).

The link:

http://www.tc.faa.gov/its/worldpac/techrpt/ar0660.pdf
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Old 21st Oct 2019, 16:46
  #44 (permalink)  
 
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Originally Posted by Airbubba View Post
That's what I'm thinking is the only other Part 121 passenger fatality in the past decade. FedEx, UPS and Atlas have had fatal freighter crashes since the Colgan mishap but is this only the second passenger fatality in this time frame?
For the record, I noticed that on aviation-safety.net database the 2018 Southwest Airlines flight 1380 incident is classified as Forced landing on runway, instead of Uncontained engine failure. See: https://aviation-safety.net/database...?id=20180417-0
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Old 21st Oct 2019, 20:57
  #45 (permalink)  
 
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Originally Posted by GordonR_Cape View Post
For the record, I noticed that on aviation-safety.net database the 2018 Southwest Airlines flight 1380 incident is classified as Forced landing on runway, instead of Uncontained engine failure. See: https://aviation-safety.net/database...?id=20180417-0
Right, but the narrative for SWA 1380 begins:

Narrative:
Southwest Airlines flight 1380 diverted to Philadelphia Airport, Pennsylvania, USA, after suffering an in-flight uncontained engine failure.
We know it was an uncontained failure, because we've all seen the separated parts and the results of the impact on the fuselage (although the parts that hit the fuselage appear to have been pieces of cowling, not fan blades).
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Old 21st Oct 2019, 21:15
  #46 (permalink)  
 
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Originally Posted by GordonR_Cape View Post
For the record, I noticed that on aviation-safety.net database the 2018 Southwest Airlines flight 1380 incident is classified as Forced landing on runway, instead of Uncontained engine failure. See: https://aviation-safety.net/database...?id=20180417-0
I think you're reading too much into the ASN classifications, which aren't intended to be definitive.

In ICAO-speak, the SWA occurrence would be classed as SCF-PP (System/component failure or malfunction - powerplant).

See Aviation Occurrence Categories - Definitions and Usage Notes
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Old 22nd Oct 2019, 01:39
  #47 (permalink)  
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I think that the FAA's perception of propeller blade shedding can be inferred from the design requirement:

Sec. 25.771
Pilot compartment.
............
(b) The primary controls listed in Sec. 25.779(a), excluding cables and control rods, must be located with respect to the propellers so that no member of the minimum flight crew (established under Sec. 25.1523), or part of the controls, lies in the region between the plane of rotation of any inboard propeller and the surface generated by a line passing through the center of the propeller hub making an angle of five degrees forward or aft of the plane of rotation of the propeller. .............
This is a design requirement for the pilots, but not the passengers.

Ice seems to come off a propeller in line with the disc. I have broken a propeller ice panel on the baggage door of a light twin with the prop shedding large chunks of ice in flight. The contact point was directly in the prop disc. A blade developing thrust would seem to have a range of 5 degrees if the FAA design requirement is an indicator of experience. It's worth noting that the instant that a blade might release from the hub, it stops developing thrust, so would be acted upon by centrifugal force primarily. That said, when the propeller damage is a result of a ground strike, I imaging the physics change.
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Old 22nd Oct 2019, 06:21
  #48 (permalink)  
 
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The separated blade should retain its angular momentum and instantaneously spin like a boomerang about its CG at the same rate as it was rotating when attached. The path of the CG of the blade will instantaneously be tangential to the rotation as the force causing the blade path to curve around the axis of the powerplant will no longer exist.

Over long distances lift and drag will cause the blade to undergo a complex trajectory, but over a very short distance and time, the blade should maintain its orientation and convert some of that angular momentum into lift, much like a maple seed does, though the advance relative to the airframe may not be much in that interval. I'd say that 5 degrees forward is easily a possibility and that armoring a plane against a blade is unrealistic.
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Old 22nd Oct 2019, 06:34
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I notice that about 90% of pictures showing actual results of props shedding and hitting a fuselage show that the blades hit within a fraction of a degree of the prop plane. (Image-google "propeller blade separation").

And there is a good explanation for that - whatever the aerodynamic forces acting on the blade after separation, they likely have on the close order of 0.01 seconds to produce any relative motion fore or aft. Once released, the blade will be moving laterally at about 94m/s (339kph), and only has to travel about 1m to reach the fuselage (assuming it will hit it at all) on most transport turboprops.

(Math is: 2-meter blade, 1-meter half radius. At 1800 rpm the half-radius is travelling at pi x R x 1800 = 5654m/minute or 94m/sec average speed. The motion of a thrown blade is a pirouette around the blade's center of mass, tip over root, like a thrown boomerang).

Interestingly, in cases where the entire prop departs (gearbox failure and such) the prop almost always moves backwards - drag on a now-unpowered prop vastly outweighing any residual thrust. E.G. Marine C-130 crash, Mississippi, 2017 - #3 prop knocked loose by shock of #2 blade failure, went backwards over the top of the fuselage like the iceberg ripping the Titanic - dit-dit-dit.

As to "prop-proofing" the fuselage: at 94m/s, a prop blade will be travelling at only ~1/8th the velocity of an average firearm muzzle velocity (800m/s) - but it will mass about 333 times as much as the average bullet and have around 40 times the momentum (p, where p=mv). Kevlar won't do much - would require steel armor. Assumes bullet masses 30g and prop blade masses 10kg.
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Old 22nd Oct 2019, 07:23
  #50 (permalink)  
 
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Originally Posted by Pilot DAR View Post
I think that the FAA's perception of propeller blade shedding can be inferred from the design requirement:



.. It's worth noting that the instant that a blade might release from the hub, it stops developing thrust, so would be acted upon by centrifugal force primarily. ..
At the instant the blade is released from the hub there is no more centripetal force. It is now obeying Newton's first law w.r.t. the hub and will go in whatever direction it was already going. OTOH, it is subject to drag and lift from the shape of the blade and the airflow around it,so there are other forces acting on it that might cause it to accelerate in one direction or another.
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Old 22nd Oct 2019, 07:55
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"Centrifugal force", while non-existent, is a useful concept.

OTOH, it is subject to drag and lift from the shape of the blade and the airflow around it,so there are other forces acting on it that might cause it to accelerate in one direction or another.
Correct. But, as pointed out above, if the blade is going to hit the fuselage it will do so almost instantaneously, so the time during which those aerodynamic forces can alter its trajectory is very short indeed.
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Old 22nd Oct 2019, 16:29
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Even if the area could be reduced - the energy of a released prop blade is so high that is is nearly impossible to protect the fuselage and passengers.

There have been cases in the past were a released prop blade nearly cut the fuselage in half.
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Old 22nd Oct 2019, 16:42
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Several Lockheed Electras have lost props, sometimes with considerable damage.
I know of four incidents, none of which caused injury to the occupants.
I never sit in line with the props.
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Old 22nd Oct 2019, 17:05
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Yes, something like that can ruin your entire day.



Note both the entry and (on top of the fuselage) exit holes made by the errant blade. Ouch.

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Old 22nd Oct 2019, 17:27
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Correct. But, as pointed out above, if the blade is going to hit the fuselage it will do so almost instantaneously, so the time during which those aerodynamic forces can alter its trajectory is very short indeed.
It's all relative. In order to hit the fuselage above the window line a clockwise spinning prop has to come from the outboard side of the nacele and up and over the wing (not sure which way the subject event went) Take the center of mass velocity vector and degrade it against drag and then add in the axial drag against the direction of the fuselage flight and you get a curvilinear result. As others have said the blade is also tumbling about it's center so if it is a long slender shape two impact points may be realized. followed by gash intersections between them.

I used to compute this kind of stuff and then to document the results in a Photo sketch by overlaying wet spaghetti strands (along the trajectories) on my many aircraft models that I kept in my office.

In the 90's we reviewed all the documented instances of prop sheds in an effort to update the containment regulations. After consideration it was decided the the tried and true method of designing the aircraft flight controls to best possible redundancy and separation (keel beam to crown) was the only practical way forward. (of course we always kept the pilot out of harms way)
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Old 22nd Oct 2019, 18:00
  #56 (permalink)  
 
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Originally Posted by lomapaseo View Post
It's all relative. In order to hit the fuselage above the window line a clockwise spinning prop has to come from the outboard side of the nacele and up and over the wing (not sure which way the subject event went)
The Saab's props do indeed rotate clockwise when viewed, as is the convention, from the rear.

Take the center of mass velocity vector and degrade it against drag and then add in the axial drag against the direction of the fuselage flight and you get a curvilinear result. As others have said the blade is also tumbling about it's center so if it is a long slender shape two impact points may be realized. followed by gash intersections between them.
I don't think any posters are disputing that the trajectory will be curvilinear. Just not very.
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Old 22nd Oct 2019, 20:41
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Well I wouldn't want to sit one row behind it
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Old 22nd Oct 2019, 22:19
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Originally Posted by EDML View Post
Even if the area could be reduced - the energy of a released prop blade is so high that is is nearly impossible to protect the fuselage and passengers.

There have been cases in the past were a released prop blade nearly cut the fuselage in half.
Since the area could not be reduced and the energy is so high of a released prop blade, I do agree that is nearly impossible to protect the fuselage, however there is a way to protect the passenger, do not have seating on the projected trajectory of the released prop blade
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Old 22nd Oct 2019, 22:51
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Nice pictures showing the energy and also width of the damage. Interesting how wide the damage on the exit side of the blade is:

AVSIG: Slight C-130 Problem...

This one crashed because the fuselage failed after a prop blade separation (CV580):

https://aviation-safety.net/database...?id=19670305-0
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Old 22nd Oct 2019, 23:04
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Used to have the toilets in the plane of the props.
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