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Twin Otter crash

Old 29th May 2023, 02:36
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Originally Posted by Pilot DAR
Posters, the Twin Otter is not a pressurized airplane!
So youíre telling me I should scratch the DHC-6P off my bucket list?

Has there ever been a pressurized aircraft with a fixed gear?
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Old 29th May 2023, 04:23
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Originally Posted by BFSGrad
Has there ever been a pressurized aircraft with a fixed gear?
The Mil Mi-26 (yes really).
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Old 29th May 2023, 07:12
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Originally Posted by Pilot DAR
Posters, the Twin Otter is not a pressurized airplane!
Thanks for correcting me. Makes the whole event even more of a mystery.
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Old 29th May 2023, 08:55
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Originally Posted by WideScreen
Some calculations, all figures rounded, and short analysis.

Empty weight twin otter: 7000 lbs
Fuel in main tanks: 1500L, equals 3300 lbs buoyancy from empty fuel tanks.
Assuming full rigid cabin ferry tanks: 6000 lbs (as reported by PilotDAR).
Specific weight aluminum: 3000 kg/m3 = 6500 lbs/m3
Specific weight iron: 8000 kg/m3 = 17500 lbs/m3
Specific weight jet fuel: 780-840 kg/m3, assume 800/m3

Assuming a 50:50 weight ratio in iron and aluminum, the "weight"
of the submerged airplane would be:
Aluminum: 3500 - (3500/6500 * 2200) = 2300 lbs
Iron: 3500 - (3500/17500) * 2200) = 3060 lbs
Total submerged weight: 5400 lbs
Subtract the buoyancy of the empty fuel main tanks, giving the remaining weight: 2100 lbs

The buoyancy of 6000 lbs Jetfuel:
6000/0.8 - 6000 = 1500 lbs

When the ferry tanks are full, the submerged weight becomes: 2100 - 1500 = 600 lbs

The positive buoyancy contribution of 2 human bodies is neglectable.
The crew probably would have some additional stuff with them, assuming 200 lbs.
The cabin ferry tank structure would add some weight, assuming 200 lbs.

Gives a submerged weight of 1000 lbs.
In general I really appreciate a calculation based approach. And the figures appear reasonable. So nothing wrong with your Maths.
But there are unknowns:
Is the aircraft still afloat after an extended period of time or was it just still floating when the rescuers arrived, roughly 15 minutes after the crash? How much was it still floating? Solidly or just barely?
In the first case (aircraft just barely floating 15 minutes after the dirching) this could be because there might be an air bubble trapped in the wing structure (outside the tanks). Compensating the ~1000lbs doesn't take much captured air bubble compared to the volume of a twin otter. The fuselage has a volume of ~20m^3 alone. 5% remaining would cater for this 1000lbs. Even more likely is the wing not completely flooding (at least not so quickly).
So, unless we get pictures of the ditched aircraft or more detailed information I would not draw too much conclusions from the little information we have. The flight time until exhaustion is also interesting. It matches surprisingly well the fuel out of the regular fuel system. And yet even from this fact it is difficult to draw meaningfull conclusions. What can be concluded is the fact that it looks like they identified the problem too late. Either due to accidentally not noticing a slowly deteriorating (fuel-) situation (due to a possible leakage) or by not having checked the general functioning of the auxiliary fuel system in time for a safe return. Unfortunately, at the moment, I guess that is all which can be concluded with a reasonable probability. Maybe the preliminary report will give some more usefull information.
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Old 29th May 2023, 14:04
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The Mil Mi-26 (yes really)
For NBC reasons apparently, nothing to do with crew/pax comfort, other than getting gassed by NBC of course.
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Old 29th May 2023, 17:07
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Some of the comms relating to this crash are contained in this video. One of the last communications from the Twin Otter refers to them being "dead-stick", which was presumably after they lost power.
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Old 29th May 2023, 18:15
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Wondering if a bit of fuel for flare might have affected the outcome. Better a flop than a pitch pole?
Maybe launching from HMB instead of Schulz? Maybe stay at 12000 and on AutoPilot? How long at 4000? Maybe flameouts at 12, with loss of AP? Forty miles, jeez.

Last edited by Concours77; 29th May 2023 at 18:54.
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Old 29th May 2023, 19:12
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Originally Posted by henra
In general I really appreciate a calculation based approach. And the figures appear reasonable. So nothing wrong with your Maths.
But there are unknowns:
Is the aircraft still afloat after an extended period of time or was it just still floating when the rescuers arrived, roughly 15 minutes after the crash? How much was it still floating? Solidly or just barely?
In the first case (aircraft just barely floating 15 minutes after the dirching) this could be because there might be an air bubble trapped in the wing structure (outside the tanks). Compensating the ~1000lbs doesn't take much captured air bubble compared to the volume of a twin otter. The fuselage has a volume of ~20m^3 alone. 5% remaining would cater for this 1000lbs. Even more likely is the wing not completely flooding (at least not so quickly).
So, unless we get pictures of the ditched aircraft or more detailed information I would not draw too much conclusions from the little information we have. The flight time until exhaustion is also interesting. It matches surprisingly well the fuel out of the regular fuel system. And yet even from this fact it is difficult to draw meaningfull conclusions. What can be concluded is the fact that it looks like they identified the problem too late. Either due to accidentally not noticing a slowly deteriorating (fuel-) situation (due to a possible leakage) or by not having checked the general functioning of the auxiliary fuel system in time for a safe return. Unfortunately, at the moment, I guess that is all which can be concluded with a reasonable probability. Maybe the preliminary report will give some more usefull information.
Yep, the unknowns, as such, writing extensively about the factors I included, so, others can feedback with better info, etc.

I don't know about the weather situation, though in this thread, there was a mention of potential swells of 3+ meters. Not to say, when the sea is smooth, a ditch would not have to be catastrophic for the crew, still in the seatbelts when found, etc.

With significant swells, I have my doubts, the cabin would stay sufficiently intact on impact to keep air bubbles inside, let alone, a torn cabin in significant swells will rapidly release any air bubbles.

Regarding the wings containing remaining air pockets: Maybe, though even when a perfect carbon-fiber/polyester Cirrus SR22 sinks in no time, when ditched in the open ocean, I have my doubts, about whether a 30+-year-old design based on plated frames will hold any longer.

Though, as you mention, quite a lot of unknown aspects. My calculation was just a first approach to "under what conditions would this twin otter be able to stay afloat".

The answer to that was: When the ferry tanks are rigid and (nearly) empty. That gives sufficient buoyancy. All other items are highly dependent on what happened with the ditching. I can personally hardly imagine that a fixed undercarriage on a twin otter will not tear open the cabin floor when ditching. Not to say, when the deceleration on ditching is so high that the crew doesn't survive, I have my doubts about the leak-free status of both cabin and wings.

We'll see with a preliminary report (or maybe earlier in the press when the wreckage is retrieved, either floating or from the bottom).
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Old 29th May 2023, 19:19
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Originally Posted by Concours77
Wondering if a bit of fuel for flare might have affected the outcome. ....
Not sure about that, every flight ends with a stall, just above the landing surface, with or without fuel. Once you get used to it, a dead stick landing isn't that difficult, especially not, when the whole landing surface is your runway, IE, no specific touch-down point needs to be your aiming point, only Height and Speed are relevant.
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Old 29th May 2023, 19:48
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Originally Posted by WideScreen
Not sure about that, every flight ends with a stall, just above the landing surface, with or without fuel. Once you get used to it, a dead stick landing isn't that difficult, especially not, when the whole landing surface is your runway, IE, no specific touch-down point needs to be your aiming point, only Height and Speed are relevant.
Well, power extends, and lowers Vs.

Wondering if, after three hours of ferry fuel, what remained was not enough to "pull through" a partially blocked vent. Then ferry tanks equalized and got them back to exhaustion...the "second" block preventing main tanks from providing any further fuel.
Do the bladders "spread" when emptying, perhaps squishing a vent?

Last edited by Concours77; 29th May 2023 at 19:58.
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Old 30th May 2023, 01:47
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Once you get used to it, a dead stick landing isn't that difficult, especially not, when the whole landing surface is your runway, IE, no specific touch-down point needs to be your aiming point, only Height and Speed are relevant.
On a hard surface runway of suitable dimension, perhaps so, I have done power off landings in the Twin Otter to a runway. Open ocean is entirely different, particularly with swells. I have landed in open ocean in a Twin Otter, though with no swells. It is not a power off maneuver.

Do the bladders "spread" when emptying, perhaps squishing a vent?
Probably not. But for certain, when the fuel venting is changed for the ferry tank installation, careless filling of the cabin tanks can result in a blocked vent, I have seen it, and guarded against it first hand (by filling the cabin tanks myself).
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Old 30th May 2023, 04:05
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Originally Posted by Pilot DAR
..... I have landed in open ocean in a Twin Otter, though with no swells. It is not a power off maneuver.......
Interesting, can you elaborate?
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Old 30th May 2023, 05:33
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For henra...

I too was interested in six hours flight time from standard tanks, and the duration of the flight. The problem with that as I see it, is using fuel from the mains is really draining from the ferry tanks. Pumping from the bottom of the mains, and replenishing with gravity feed from ferry tanks. Turning around makes it clear that starvation was imminent. Venting would have ceased at the cabin supplies. So, where did the next 3 hours flight time come from? Joining the main tanks and ferry tanks into one very large inseparable system? No valves, bleeds, or gsuges? The Otter has a forward belly and an aft belly tank. Two separate pumping systems. Each belly tank would hold 2.5 hours of fuel. Approximately 40 gallons per hour per engine. So around 480 gallons of fuel consumed in six hours. The first three hours would have used up roughly 240 gallons, equal to the contents of one belly tank. If the plan was to use Standard tanks for the first 5-6 hours, then transfer from ferry tanks, then repeat, it would simplify fuel.management, and not have to rely on one massive, lashup system: Depending on ferry fuel being available..Now that does not involve the wing tanks, was the aircraft so equipped? Because if fuel burn was higher, and no wing tankage, that would put them right around where the Coast Guard found them. The good news is the support folks in Santa Rosa probably know already what misfortune befell Quebec Sierra...

Regards
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Old 30th May 2023, 13:43
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We do power off floatplane landings for training, though they are not always safely possible (so we choose the water/wind conditions for this training). Sometimes, landing with power is the only safe way to get a floatplane on the water. A power on landing affords a stretched out flare, so the flare altitude can be judged better, and adjusted (with power) if a mis-judgement is recognized. Power off, you get one chance. On a hard runway, a hard landing, or bounce is a nuisance, or possibly damaging. On water, a hard landing, or bounce, particularly power off, will much more likely result in the plane being upside down in the water. I have been right seat for this, it happens fast, and is very bad.

Waves pound the floatplane, and may result in the plane being thrown back into the air a little, perhaps slower than flying speed. The resulting second landing will be bad. Swells greater than a meter (for a Twin Otter) will certainly result in the airplane being thrown back into the air if you hit one wrong. I'm uncertain whether the subject Twin Otter ditched as a wheelplane or amphibian, I have heard it both ways. In the case of an amphibian, an open ocean water landing which resulted in the airplane remaining upright would be a happily very unusual event.

A floatplane Twin Otter in relatively calm (2 - 3 foot waves / no swells) ocean...



As for ferry tanks, if the ferry fuel could not be accessed in flight (as nearly happened to us once ferrying a Twin Otter) ditching a very heavy powerless Twin Otter in the open ocean would be an intense maneuver with little chance of a good outcome. Once in the water, full cabin tanks are hardly bouyant. An inverted floatplane with undamaged floats will float indefinitely. A Twin Otter wheelpane will sink when flooded. If the pilots opened the cockpit doors before the ditching, flooding the cabin could happen fairly quickly. A crash into the water can result in unexpectedly high crash forces (we exceeded 14G). And underwater egress is a skill (I retrained for it two weeks ago). So, if things did not go well, fatalities in the pilot's seats would be possible.



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Old 30th May 2023, 17:53
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Originally Posted by WideScreen
Not sure about that, every flight ends with a stall, just above the landing surface, with or without fuel. .
That would mean an aircraft would cross the threshold at 1.3Vso, bleed off 30% of the airspeed in the flare, and touch down at Vso with the wing at the critical angle of attack?

I donít think that is a typical airplane landing technique.
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Old 31st May 2023, 09:30
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Originally Posted by StudentPilot479
Twin Otter main fuel tanks are in the belly. Wing tanks are an option and only hold 88 gallons - about an hours worth of fuel. Mains plus wing tanks gives a bit under six hours fuel, about the same time as they were airborne. That is the only reason I would assume they had wing tanks. At this point I would be quite surprised if there was an issue with the standard or optional fuel systems (ie it appears to be an issue with the ferry tank installation).
Where does the '..about six hours airborne' figure come from?
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Old 31st May 2023, 11:19
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Tanks...

Originally Posted by netstruggler
Where does the '..about six hours airborne' figure come from?
Maybe from "time aloft" and the assumption that standard (plus wing tanks) was the only fuel available; the ferry fuel somehow "not accessible...." ?

Investigatively, arse about?

There is some tank data available from Juan Browne's excellent video, "Blancolirio" snd a pro ferry pilot, Tom Lopes...
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Old 31st May 2023, 12:00
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Originally Posted by Concours77
Maybe from "time aloft" and the assumption that standard (plus wing tanks) was the only fuel available; the ferry fuel somehow "not accessible...." ?

Investigatively, arse about?

There is some tank data available from Juan Browne's excellent video, "Blancolirio" snd a pro ferry pilot, Tom Lopes...
Thank you - I was confused by a couple of references to the turn back point and/or the crash being at 10:30, a couple of hours after take off. That was obviously incorrect information.
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Old 31st May 2023, 12:57
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Originally Posted by netstruggler
Thank you - I was confused by a couple of references to the turn back point and/or the crash being at 10:30, a couple of hours after take off. That was obviously incorrect information.
Same happened for me...
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Old 31st May 2023, 15:14
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Originally Posted by Pilot DAR
We do power off floatplane landings for training,.....
While a very interesting explanation, there are reports, the floats were removed, so this twin otter was no longer a float plane, and nothing more than a big C172, for which the instruction is to ditch at the lowest possible speed.

What I might consider for such a twin otter ditch, would be:
- Full flaps for the last 300-400 feet in height.
- With suitable speed, descent until roughly 1-2 m height.
- Props in full fine pitch, bleeding off the airspeed very fast, and
- When the stall warning goes off,
- Raise the flaps,
- And let the airplane "drop" on the water, with the lowest horizontal speed possible and a bearable vertical speed.

Because there is an as low as possible horizontal speed and a significant vertical speed, the chances for a flip-over (when the wheels catch the water) are, as I judge it, the minimal possible. It'll "dive in" with the nose, bleed off the energy in maybe 1-2 track meters or so, using the airplane structure to absorb the energy, and subsequently resurface in a few seconds. Feel free to correct me.
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