So they haven't found all the bodies yet and we have a probable cause. Don't know whay there is an NTSB then, just call pprune.


To add a little data. The PT-6 was originally designed as a stationary power unit for water pumping in very hot dry foreign places. In it's aviation use it is a reverse flow turbing, that is, it takes air in toward the lower rear of the engine and then the air flows foward to the first stage compressor. There is a huge amound of centrifical separation that occurs and tends to shed rocks, bolts, and other FOD before it gets into the engine. How anything but massive fire hosing of water would get through this I don't know. Please don't compare PT-6 to T-56, Orpheus, and other stuff like that. The TF-34 on the S-3 aircraft has a phenomenol reliability record and is maintained as has been previously suggested with frest water washdowns etc.

In many installations (Beech King Air etc) of the PT-6 the engine oil provides the power for the prop pitch control so if the engine fails in a mode that results in oil pressure or quantity loss the prop feathers.

Sympathy to the Chalk organization, pax and families. It is bad enough to get it like that, but to have lots of holiday pics on CNN of it...must be horible for the families.

I recall that the PT-6 even with flights over the sea, from one island to another required a regular compressor wash.

http://roger.ecn.purdue.edu/.../ jets/tprops/pt6.html

hopefully this pic works. The PT6 is equipped with inertial separators, on most a/c these are selectable either by Bowden Cable or electric actuators. these are used in icing conditions and most ground situations to avoid FOD/Ice ingestion. The actual engine inlet is annular, but is normally fed by a bottom pitot or ramp inlent. Further to the CS prop control, engine oil pressure is boosted from around 100 psi to about 750 psi and enters the prop hub thru an gland system into a single acting hub. With loss of oil pressure a spring will drive the blades to a feather position. The normal failure mode for the PT6 is due to a leak in the P3 line of of the prop governor to the FCU, the engine then falls back to min flow around 55%. good for electrics and bleed air but essentially produces no power. Thankfully the AFX system will normally feather the affected side. Several days back we had an low speed reject, due to a lose fuel line. the Captain shutdown after clearing the runway.

Agreed. FAR's require containment of a turbine blade failure, and the resulting imbalance should be within the ultimate load capability of the mounts.

A DISK failure, however, is something else entirely. I've seen it cut a fuselage in two. (However, the fuselage appears intact in the CNN video)

Do we know what brand props are used on the G-73T? If there is a fine-pitch failure mode (i.e. runaway), that might be a major issue.

I've seen those Chalks flights takeoff from the sealane right from the Macarthur Causeway leading to Miami Beach many times. Almost went aboard once - ended up on a Metro gambling junket instead (30.00 US round trip to Nassau).

They are a good outfit - never lost a passenger til now, although 2 crew were lost near a cayos.

Link to the beauty of flying Chalks from Miami to the Carribean:

http://www.worldairroutes.com/Chalks.html

This is a repost- my original was deleted .

http://h1.ripway.com/eisler99/519_picture.gif

PT6 shelling
Overtalk,
I can see your point of view. I have witnessed Compressor blade failures and seizures and and Turbine blade failures. But I think everytime the Autofeather did the trick. Do these Turbine Mallards come with autofeather. Also I never heard a turbine failure resulting in uneven running and moving the engine off its mounts. They usually go "pop and stop". More than likely the reason for this would be some sort of prop failure, tip loss or blade loss. And lastly of all the PT6 failures Ive heard none of the Compressor blade, Turbine blades failures, have resulted in the engine shelling its components through the casing. I maybe wrong but never heard of it. The Casing that contains these whirling parts is extremely strong and tested.

Sheep

P.S. A few questions to be answered. Probably only from A&P Mechanics or LAMES that have worked on or have done the PT6 conversion.
1. Does the PT6 Mallard rebuild come with Autofeathering Props and also does it come with Fire Bottles? I am assuming for 135 ops it would be mandatory in such an aircraft?

PT6A Engine
 

Saw the video on CNN last night, I actually found it hard to watch. Poor souls never had a chance going in at that angle. May they rest in peace.

Mere curious speculation:
I was thinking maybe an engine fire might have heated the wing spar to the point of failure? Didnt such a failure down a turboprop in Canada a few years back, after a landing gear fire in the gear well started from a brake dragging on T/O?

The CNN amateur video shows the a/c going down trailed by what looks like a wing section in flames.

This could be a structural failure that ignited fuel by arcing of breaking electrical wires.

That was a Metro: http://www.tsb.gc.ca/en/reports/air/...7/a98q0087.asp and (correct me if I'm wrong) there are no hydraulics in a Mallard's wing, the wheels retracting into the fuselage which appears undamaged in the video.

PaperTiger-
Thank you for the clarification-and for posting the link to the Metro incident.
My suggestion though, was not that a hydraulic or landing gear issue caused this, but simply that heat from a fire could cause a structural failure of the wing. From the TSB report on the Metro incident-"The initial event in the break-up sequence was the failure of the front wing spar, caused by intense heat." Of course a fire may ignite due to an unkown number of circumstances, but an engine nacelle is one of the more susceptible locations for obvious reasons. I really need to stop speculating though....I think I'll bow out till the NTSB has had a chance to do their job.

Regarding PT6 operations in salt water, several companies in Vancouver and Seattle have used turbo beavers, otters, and caravans for years. The standard (I believe) is a compressor wash after every flight day. There is a freshwater garden hose hookup on the PT6 for this purpose. I'm sure any seaplane operation worth its, er, salt would do same.

unctuous thanks that was the pic I was trying to load the pt6 has 3 governors. one for the FCU, two for the prop, and one for prop runaway ie: the hydraulic overspeed governor. The N1 or FCU controls fuel flow governing N1 speed set on a rear mounted accesory pad. prop governor sets speed of prop with the FTG set at normally 106% of selected prop releases p3 bleed air which reduce fuel flow on the FCU mounted on the top of the reduction gear box primary prop gov/FTG are combined, the HOG releases prop hub oil to govern as a last ditch in Prop overspeed situations. Don't know if the Turbo Mallard has the centripetal start lock system, this allows props to start in the ground idle position.

Anyway guys we hijacked this thread long enough lets move this to a separate thread. My Heart goes out to the families. My Condolensces

That's undoubtedly true, but I don't see how it fits in this case. The Mallard crashed immediately after takeoff so even had there been an engine fire it surely would not have destroyed the wing in what couldn't have been more than a minute or two. To me the video is sadly reminiscent of that of the P4Y breakup a couple of years back. Pure speculation (so spare me the finger-wagging !), but it looks to me like a structural failure precipitated the explosion/fire and not the other way round.

Just wondering...

Are those planes fitted with CVR / FDR ?

--alexT

I believe that it has a CVR only. Unrecovered as yet.

(Non-reporting journo here.) In these older aircraft, is it possible within reason to retrofit them with FDR's? In my radio station, we can take any paramaters we wish from the meters into whatever state-of-art devices come on the market, with little difficulty. Are there any basic FDR's which can be hooked up this way to at least give some clues in the event of an incident?

Just saw the video on the news here tonight and I am extremely saddened.

I have a couple thousand hours Twin Otter on floats and several thousand more on wheels and all I can say is that seeing that video on the news here tonight has been one of the most distressing I have seen and reminds me of one of the main reasons I got out of the float game.

Structural failure was the biggest worry I have ever had having seen how some of the aircraft we flew were cleaned. Little wonder DHC mandated stainless control cables. I later flew the –6 Island hopping and even their we washed the bird at the end of every day.

Where ever or for whatever reason….. once that wing let go it was game over.

I love old planes and I love the sea but unfortunately they don’t mix, nothing will replace the Mallard, or even the Twin Otter on the few remaining niche jobs that exist and require these specialised machines.

Condolences to all involved

PaperTiger
I see your point. Now that you mention it, the C 130 wing failure in 02 comes to mind as well-although there were other issues invloved there anyway.
http://ntsb.gov/ntsb/brief.asp?ev_id...21X00954&key=1

One no-go item on the Twin Otter is the PT6 Beta Back-up system which dumps the oil if the prop. spring busts so the prop. doesn't 'go into reverse'.