Twin Otter crash
 

Twin Otter flying to Honolulu reported crashed 40 miles West of Half Moon Bay, California. Aircraft took off from Santa Rosa at 8:20 AM and crashed in the ocean at approx. 2:15 local after turning back toward Half Moon Bay. NTSB investigating, two on board presumed lost. That's all I have, from a newspaper, the San Francisco Chronicle. RIP

)Corrected for time of ditching...)

When I have done very long ferry flights in the Twin Otter, we carried cabin fuel, and were rather heavy. It's an additional system which requires careful management, though not usually early in the flight....

Registration N153QS: https://aviation-safety.net/database...?id=20230520-0

I know almost nothing about float planes. But, from the pic above it had floats. I assume no CVR or FDR ?

So if they ran into trouble keeping the thing airborne, couldn't they land on water rather than crashing ? At this point nothing is known.

I think I've heard that float planes aren't supposed land in open ocean but under the circumstances would such a thing have been possible ?

"Supposed to...." is simply the water conditions. I have landed a number of floatplanes in the ocean - in very calm water conditions. Open ocean conditions would most likely be too much for even a Twin Otter amphibian, during most common ocean conditions, particularly if it's a very heavy weight landing, and unusually urgent. And if (though I expect not, with good cockpit discipline and checklist use) an amphibian's wheels were down for a water landing, it's a certain overturn in the water. I'm not saying at all that that was a factor in this case, just an important consideration for every amphibian.

Two killed in plane crash into Pacific Ocean near California (nydailynews.com)

They've located the submerged plane, according to the above.

I suspect they’ve found floating debris. 40nm offshore of Half Moon Bay the water is at least 2000’ deep.

Key points from linked article:

Aircraft departed KSTS at 0820L. Aircraft turned around at 1040L, reporting a fuel transfer system malfunction.

Pilots issued a distress call about 70 nm west of land that aircraft “was nearly out of fuel” indicating intention to ditch.

USCG spotted aircraft in water, upside down. Rescue swimmer noted both pilots still strapped into cockpit seats and unresponsive.

2 pilots killed on Hawaii-bound plane

For my experience ferrying Twin Otters with cabin fuel, it is vital to assure that the cabin fuel venting is working as required while still within return to safe landing range on the belly tanks. On one of our flights, my captain, very aware of this, spent 45 minutes in the cabin sorting out a venting problem so we could continue, reminding me that if he could not get the cabin fuel venting, we were headed for an overweight landing (that leg was not over water). He got it venting, and our further legs were trouble free with caution. Careless filling of the tank can splash fuel out the vent, and the "U tube" of fuel in the vent line prevents correct venting of the tanks. I know of a Twin Otter which ditched many years ago because the pilots flew beyond the return point, and could not use cabin fuel.

An overweight ditching (even on floats) in open ocean would be very worrisome!

I have no special knowledge of this event, but reported details thus far bring to mind experiences of the past....

Hello Pilot DAR,

As someone that has no knowledge whatsoever of the Twin Otter, from what you described, there was one detail that I found a bit uncommon when you mentioned 'belly tanks'.
The aircraft has its Fuel Tanks not in the wing itself, but instead this are located in the 'belly', or it is a combination of both locations ? or I misunderstood something ?
Tanks in the belly would allow a more simple transfer of fuel of the Cabin Fuel. I'm guessing it will be done by gravity IF, as you mention, venting is working properly.
Transfer of fuel from Cabin to a Wing tank would imply some sort of pumping to get it done.
As I mentioned earlier no knowledge of the system arrangement, but the Cabin Fuel should be designed in a way that wouldn't require that long to make sure that it is working properly.
From what you describe, it seems to be a very tricky system, that can end up in a very bad outcome.

Details of the ferry fuel system are contained in the FAA Registry page for the aircraft. Enter "153QS" in the search box and scroll to the bottom of page. FAA Register Look-up

Interesting reading the ferry fuel system on the register. Yes, that generally describes the system I have flown twice in the '80's. Yes, the cabin ferry fuel will drain into the airframe belly tanks by gravity. I don't remember the exact details of the venting, but it is very memorable my captain being in the cabin for 45 minutes cursing at it while he tried to clear it. He succeeded, and we continued as planned. That leg of the flight was 13 hours, Rotterdam to Rhodos, Greece.

I would have thought an airtest to check the system would have been prudent before the planned departure .
Surely ,if the tanks are rigid,then opening the filler cap should vent the system; or carry a hammer and `spike`...

The floats were removed. Too much drag on the ferry flight.

WAG. Wouldn't the main tanks filler necks and caps have to be sealed if connected to turtle tanks above, on the cabin floor? Were they planning to fly through wing tanks? That requires pumping fuel up. If they launched on wing tanks, and made it out an hour before realizing they couldn't pump fuel up and through the wing tanks to the engines, then were beyond return point? Well, running out of gas and still over gross with fuel, jeez.

Dual flameout that heavy? How to flare into what were probably at least 6 foot swells? (That's a flat Pacific in my experience)

From what i read, the ferry tanks fed the belly tank, so no external pumping required. Gravity fed.

Yes that's understood. If the bladders weren't vented, but collapsed as emptied there would be no venting for the belly tanks.
Belly tanks would have been sealed at caps, to prevent leaking from above (bladders). But that would only have prevented the pumping of fuel after the bladders emptied. They ran out of gas after what I presume was burning off wing tanks to empty. Or, more precisely, a little more than half of the fuel available from the wing tanks. No more was then available due to failure of the great remainder to pump to the engines. Crimped supply line, crimped vent, something like that....

Just a thought, but one possibility is that the ferry tank was empty.

Perhaps supporting that theory is that aircraft was not on floats, but floated after impact. I saw PilotDar comment that there would be 6000 lbs of fuel in that tank. At 6.8 lbs a gallon, I take that to be empty, sealed, space capable of holding 882 US gallons. Surrounded by sea water, that tank empty would provide buoyancy of some 7540 lbs. If the aircraft was out of fuel in the other tanks, and empty wing and tailfeather spaces were not opened, the total buoyant space may have exceeded the empty weight of the airplane. It does seem odd that the airplane would float with 6000 lbs of fuel in the ferry tanks.
On the other hand, a ditching with 6000 lbs of fuel behind you is going to be an awful smack into the water with no power to control the flare and touchdown.

I saw a report where a Navy swimmer put into the water observed the two crew strapped into their seats, and pulled the leg of one of them, without response. Let's say there is a 30 knot wind out there, and that the forced can be made into wind. Flown perfectly to a touchdown at stall, that would mean a waterspeed of 28 knots at touchdown. What we have is a touchdown, or a loss of control, that was sufficient to inflict fatal trauma on the crew, strapped into their seats.

It's not realistic to think this crew, or someone they relied on, didn't fill the tank with every drop it would take. The empty tank scenario, which I'm certainly not wedded to, would have to result in all of the fuel porting overboard. I can't imagine the crew wouldn't have noticed that. Do those ferry tanks have fuel guages built in, or is it time and power setting that is the only check on what is left in it?

If it's down to "out of fuel", or "fuel system malfunction", then it's semantics, ATC, or some Otter jock with a better opinion than mine... was there a Mayday? I would want to consider carefully the wording of the nature of my emergency... especially if ATC asked me the "fuel left on board..."

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.

Or so to say, the twin otter with empty main fuel tanks and full rigid cabin ferry tanks will clearly sink.
And, the twin otter with empty main fuel tanks and full or empty flexible material cabin ferry tanks will clearly sink.

Given the twin otter does float, the cabin ferry fuel tanks are rigid and
aren't full, IE the tanks are at least partially empty.

When the ferry tanks aren't rigid, the buoyancy contribution situation will
be worse (when emptying out) and will give no extra buoyancy due to the
ferry tanks inflating on emptying out.

Given, the twin otter did run out of (usable) fuel, it's unlikely they did
manage to use part of the ferry tank's fuel (either all fuel is available
or nothing, some way or another). So, given the wreckage is found floating,
it looks like the cabin ferry tanks are (nearly) empty.

The fact, the twin otter seems to be floating upside down, is an indication of
a strong buoyancy due to empty main fuel tanks and empty rigid cabin ferry tanks.


Now the departure situation: Assuming experienced twin otter pilots, I can not imagine,
they would not notice the anomaly of "empty" cabin ferry tanks, since that would
represent a pretty low-weight twin otter and certainly noticeable due to its STOL
take-off capabilities (vs. an expected overweight departure).

Or, so to say, the twin otter quite probably lost the cabin ferry fuel when in the air, on its way to its destination.

How to accomplish that:
- Hose detaching, somewhere down the line to the main fuel tanks.
- Given the gravity fuel flow to the main fuel tanks, maybe a lost sealing of the main fuel tank caps, fuel tank venting being open(-ing up), or something like that.

Feel free to correct the figures, etc.