Twin Otter crash

[Concours77]

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...)

[Pilot DAR]

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....

[Liffy 1M]

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

N153QS photo

[bafanguy]

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 ?

[Pilot DAR]

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.

[jolihokistix]

Two killed in plane crash into Pacific Ocean near California (nydailynews.com) (https://www.nydailynews.com/news/national/ny-plane-crash-hawaii-pacific-ocean-california-20230521-wfnf663b4rdgtkrovczp6gupk4-story.html)

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

[WillFlyForCheese]

Two killed in plane crash into Pacific Ocean near California (nydailynews.com) (https://www.nydailynews.com/news/national/ny-plane-crash-hawaii-pacific-ocean-california-20230521-wfnf663b4rdgtkrovczp6gupk4-story.html)

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.

[BFSGrad]

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 (https://www.pressdemocrat.com/article/news/2-dead-after-plane-that-took-off-from-santa-rosa-crashes-40-miles-off-half/)

[Pilot DAR]

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....

[zerograv]

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.

[Liffy 1M]

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 (https://registry.faa.gov/aircraftinquiry/Search/NNumberInquiry)

[Pilot DAR]

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.

[sycamore]

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`...

[Compton3fox]

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

[Concours77]

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)

[Compton3fox]

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.

[Concours77]

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....

[cncpc]

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?

[Concours77]

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..."

[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.

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.

[Pilot DAR]

Assuming full rigid cabin ferry tanks: 6000 lbs (as reported by PilotDAR).

Just to be clear, I have not suggested a fuel quantity, nor tank type. I have only flown bladder tanks in the Twin Otter....

[Capt Fathom]

WideScreen, I’d hate to see a long analysis!

Was there a point to your short analysis?

[WideScreen]

Just to be clear, I have not suggested a fuel quantity, nor tank type. I have only flown bladder tanks in the Twin Otter....
Don't worry, bladder tanks will make the floating situation only worse. Not to say, without (nearly) empty rigid ferry tanks the Twin Otter would not be floating. The twin otter sinks to the bottom when only having (empty) main tanks. See, the calculation.

[WideScreen]

WideScreen, I’d hate to see a long analysis!

Was there a point to your short analysis?
Feel free to skip.

Without calculation, the analysis itself would just be an unfounded opinion, which we see so many of, at PP.

Giving calculations makes clear what the situation is, gives a proper base for discussion as well, and gives a proper explanation of what fundamental item has happened (IE somehow the ferry tanks emptied out).

[Concours77]

I'd rather post questions than opinions...I was tempted to ask if the main tanks were empty at launch, but it seemed too far fetched...the takeoff would
have been way too lively....what part could the wing tanks have played? Can they be replenished in flight? They are filled top of wing, yes? Do full wing tanks account for two hours of burn? If they launched with wing tanks, could there have been a switching issue when they got low, and the bladders/mains couldn't be accessed? Another direction going, could the lower tanks have been full of LL100?

If the wing tanks had only one load to offer, would the strategy be launch with wing fuel, but use only less than half, try switching to bladder/mains, then if inop, back to wing fuel and home???

conc

[First_Principal]

Feel free to skip.

Without calculation, the analysis itself would just be an unfounded opinion, which we see so many of, at PP.

Giving calculations makes clear what the situation is, gives a proper base for discussion as well, and gives a proper explanation of what fundamental item has happened (IE somehow the ferry tanks emptied out).

I logged in to 'like' this post, but see it's not possible on this particular forum. FWIW I appreciate you including some background/explanation to your opinion 👍

FP.

[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).

Wing tanks can be refuelled over the wing, but due to the height the 'standard' is to use the fuel pumps to transfer fuel from the mains to the wing tanks on the ground.

100LL burns a little more per hour than JET-A if I am not mistaken, but otherwise is a non-event unless you burn it for more than 150 hours.

The FAA registry shows Turtle Pacs installed in 2015. Nothing saying they used a different ferry tank system this time, though.

[cncpc]

I logged in to 'like' this post, but see it's not possible on this particular forum. FWIW I appreciate you including some background/explanation to your opinion 👍

FP.
Me too. I asked the question "Why is it still floating" and put the empty ferry tank scenario out there.
The very detailed response is much appreciated.

I too thought, if true, that the leak most likely came from between the bottom of the ferry tank and the top of the belly main.

[Concours77]

Me too. I asked the question "Why is it still floating" and put the empty ferry tank scenario out there.
The very detailed response is much appreciated.

I too thought, if true, that the leak most likely came from between the bottom of the ferry tank and the top of the belly main.

If the turtle tanks leaked out at the connection, that leaves the main tanks full, and might help explain inverted buoyancy, but how does that explain loss of all fuel, and fuel starvation ?? Main tanks supply engines from their bottom ?? Thx...

Having reread wide-screen, six hours of flight takes mains and wing tanks empty.

I should read for understanding .... my bad.

Except to query how the main tanks didn't show lowering fuel level for three hours post launch...shouldn't gauges show mains starting to empty within six hours of Honolulu??

[WideScreen]

I'd rather post questions than opinions........

Fortunately, there are also people who give answers. Answers in a different way than just populism (do I hear Trump, DeSantis, MTG ?), but with the substantial base knowledge, to provide real and realistic answers.

[Concours77]

Fortunately, there are also people who give answers. Answers in a different way than just populism (do I hear Trump, DeSantis, MTG ?), but with the substantial base knowledge, to provide real and realistic answers.

Thanks. Can you help with my last question?

If the ferry tanks were not available due leakage or not being filled, and the aircraft had only belly/wing tankage available for a total of six hours flight time, wouldn't the belly tank gauges start showing fuel level dropping just after wing tanks emptied? Or even sooner if the wing tanks were being held as a reserve?

Also, would a likely suspect for bladders leaking be the connection at main tank caps? Fuel coming out and offloading out the cap wells?

Thanks widescreen. That Weight/buoyancy post took me back to word problems in physics....ugh....

Come to think of it, the bladders may have been leaking until further out than return point. Then, Captain sees the gauges dropping, grasps the problem, and heads back to Half Moon Bay? Their flight time was about six hours total, so that doesn't work....belly tanks hold five hours fuel. I can't see a fuel exhaustion problem, given my lack of understanding of fuel system architecture. Only fuel unavailable....Exhaustion only works if ferry tanks were empty from the git...? It's intriguing the total time aloft is roughly equivalent to fuel avaliable from standard tankage...pump issues? How much weight could an external auxiliary fuel.pump add...?

Ferry fuel should gave gotten the aircraft to within five hours of Honolulu, at least?

[cncpc]

Are there photos of the aircraft floating at the site? Is a wing torn off? Is it reasonable to assume the pilots' bodies are not entirely underwater?

[WideScreen]

Thanks. Can you help with my last question?

If the ferry tanks were not available due leakage or not being filled, and the aircraft had only belly/wing tankage available for a total of six hours flight time, wouldn't the belly tank gauges start showing fuel level dropping just after wing tanks emptied? Or even sooner if the wing tanks were being held as a reserve?

Also, would a likely suspect for bladders leaking be the connection at main tank caps? Fuel coming out and offloading out the cap wells?

Thanks widescreen. That Weight/buoyancy post took me back to word problems in physics....ugh....

Come to think of it, the bladders may have been leaking until further out than return point. Then, Captain sees the gauges dropping, grasps the problem, and heads back to Half Moon Bay? Their flight time was about six hours total, so that doesn't work....belly tanks hold five hours fuel. I can't see a fuel exhaustion problem, given my lack of understanding of fuel system architecture. Only fuel unavailable....Exhaustion only works if ferry tanks were empty from the git...? It's intriguing the total time aloft is roughly equivalent to fuel avaliable from standard tankage...pump issues? How much weight could an external auxiliary fuel.pump add...?

Ferry fuel should gave gotten the aircraft to within five hours of Honolulu, at least?
Aha, now you want me to use the Crystal Ball?

I don't know the answers, though it certainly is possible, that the ferry fuel tanks emptied out through the main tanks (/vents).

When all of the ferry fuel is siphoned off in (for example) 30 minutes (yes, I know, it's quite a fuel flow), the main fuel tanks gauges will keep showing 100%, though this is not something that would be noticed, gauges tend to be inaccurate.

Or, maybe the crew did have a fuel setup, where the main tank was supposed to have a continuous feed from the ferry tanks, with the gauges showing 100% for quite some time.

Given the Twin Otter does have a pressurized cabin, I wonder, whether the ferry tanks venting was to the outside or in the cabin.

IF the ferry tank venting was into the cabin, a continuous feed to the main tanks would have created an overpressure on the main tanks. Have a sealing issue (or maybe just the main tank vent stop popping off due to the overpressure) and you have a nice "pump" to get rid of your ferry fuel.

One more item about the type of ferry tank. When this would be bladder tanks, a quick look over the shoulder by the pilots would have revealed the bladders would be emptying out very fast. With rigid ferry tanks, it would be more difficult to see this happening, needing a close look at the tank gauges, etc.

Regarding the ferry fuel being there, though not being available for use: I don't think so, since without (nearly) empty (rigid) ferry tanks, the Twin Otter would not stay afloat.

Fuel pump issues: I don't think so, the feed to the engines is through pumping means, being the standard, certified fuel handling mechanism for this airplane. And, the ferry tanks do have gravity feed to the main tanks, so, not really likely to fail, except for potential hose/tap issues.

[Pilot DAR]

Regarding the ferry fuel being there, though not being available for use: I don't think so,

This topic is probably at the point where some more facts are needed, and further thinking then based upon those facts. It is possible for an airplane, including a ferry configured Twin Otter, to have fuel aboard which is not available for use in flight - it happened to me this year in another type, and happened while I was right seat ferrying a Twin Otter in the mid '80's.

As to wheelplanes floating after ditching, many float for a long time, while wings slowly fill with water. Is there any factual information as to how long this Twin Otter floated?

Whether ferry fuel flow was a factor in this sad event is up for more factual information. In the mean time, the accident is a reminder about the importance of understanding the fuel system of your airplane as you're flying it, and knowing when to change plans - That is fresh in my mind!

[what next]

Given the Twin Otter does have a pressurized cabin,...

No, it is unpressurised!

[WideScreen]

No, it is unpressurised!
Strange, the wiki tells, it flies until 25000 - 30000 ft and is pressurized. What is your source ?

[what next]

Strange, the wiki tells, it flies until 25000 - 30000 ft and is pressurized. What is your source ?

My source is the manufacturer (Viking): https://resources.globalair.com/specs/aircraftbrochures/4249_Viking-Twin-Otter-Series-400-Technical-Specifications-R-01-2018.pdf

[Bksmithca]

My source is the manufacturer (Viking): https://resources.globalair.com/specs/aircraftbrochures/4249_Viking-Twin-Otter-Series-400-Technical-Specifications-R-01-2018.pdf
Having spent ten years working and flying on the CC138 Twin Otters operated by the RCAF they are not pressurized. Any flights over 12,000 ft the crew and pessangers were required to be on oxygen. The twins are fitted with onboard oxygen but the only time I was over 12,000 was when we were kicking the Skyhawks out for demonstrations.

[Concours77]

Having spent ten years working and flying on the CC138 Twin Otters operated by the RCAF they are not pressurized. Any flights over 12,000 ft the crew and pessangers were required to be on oxygen. The twins are fitted with onboard oxygen but the only time I was over 12,000 was when we were kicking the Skyhawks out for demonstrations.

According to FlightAware, these chaps were in cruise at 12,000 feet, and dropped down (on the way back) to 4,000 to scudrun a marine layer trying to get back to Half Moon Bay, keeping the Ocean visual. Anticipating a ditching... (that last from Juan Browne at Blancolirio)...

I'm guessing the aircraft did not have pressurization. Wouldn't a higher have been better on gas?

Add.. flown from Half Moon Bay many years.
He may have gotten a brief from locals, but it's likely the layer was offshore by 2 pm. From 12,000 at HMB, he could have almost glided to San Carlos...just sayin'....besides, the HMB runway is between two ridges, and frequently visual even if the layer is inland...

so very sorry for the friends and families...
Know that these two had the courage to fly this mission...I've been flying since 1972, I don't have what it takes to fly that route...

[Pilot DAR]

Posters, the Twin Otter is not a pressurized airplane!

[BFSGrad]

Posters, the Twin Otter is not a pressurized airplane!So you’re telling me I should scratch the DHC-6P off my bucket list?

Has there ever been a pressurized aircraft with a fixed gear?

[4forward8back]

Has there ever been a pressurized aircraft with a fixed gear?

The Mil Mi-26 (yes really).

[WideScreen]

Posters, the Twin Otter is not a pressurized airplane!
Thanks for correcting me. Makes the whole event even more of a mystery.

[henra]

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.

[megan]

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.

[Liffy 1M]

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. N153QS Comms

[Concours77]

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.

[WideScreen]

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).

[WideScreen]

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.

[Concours77]

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?

[Pilot DAR]

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).

[WideScreen]

..... 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?

[Concours77]

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

[Pilot DAR]

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...

https://cimg8.ibsrv.net/gimg/pprune.org-vbulletin/800x600/dhc6_on_water_877093b0fc0acc67cdb1204e35055336dd4e3a74.jpg

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.

[BFSGrad]

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 don’t think that is a typical airplane landing technique.

[netstruggler]

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?

[Concours77]

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...

[netstruggler]

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.

[Concours77]

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...

[WideScreen]

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.

[WideScreen]

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 don’t think that is a typical airplane landing technique.
For big commercial stuff, not advisable, though for small C172 alike, in principle yes (for the PPL, there is a zero -assumed malfunctioning- flaps training item to fly the C172 at a very shallow angle, without flare, against the deck, with suitable power). I'd say, a dead-stick twin otter should be treated like a (dead-stick) C172 (no power to support shallow-angle approaches. Feel free to correct me.

[Pilot DAR]

I'd say, a dead-stick twin otter should be treated like a (dead-stick) C172 (no power to support shallow-angle approaches. Feel free to correct me.

Yes, I'd like to correct you, there are a number of assumption errors with respect to the differences between how a wheelplane Twin Otter, and a wheelplane 172 fly, particularly in respect of flaps and power/propeller pitch. With some Twin Otter type training, these differences will become apparent, and result in change in your perspective on this type of flying.

One very common feature of both the Twin Otter and 172 will be that if you don't flare it, you're likely to bounce it off the nosewheel, and expensive damage will result.

[Concours77]

For big commercial stuff, not advisable, though for small C172 alike, in principle yes (for the PPL, there is a zero -assumed malfunctioning- flaps training item to fly the C172 at a very shallow angle, without flare, against the deck, with suitable power). I'd say, a dead-stick twin otter should be treated like a (dead-stick) C172 (no power to support shallow-angle approaches. Feel free to correct me.

That's the way I've been approaching the water entry.
It makes a lot of sense to ship the floats over in the belly of a big rig.
Less drag, and safer ditch if needed...

I think your summary makes sense, keep the nose up as long as possible, hoping when it hit, submergence is in the "shallow angle enough such that it pops back up, instead of pitching the whole business over" domain.

Also, hoping for a tail low aspect, so when the gear enters fully, the Nose coming down will plop, rather than sink and trip the a/c over its nose....(then again, if sufficiently entry vertical, the floats are sturdy enough to absorb alot of energy. Keeping them on for the journey for that reason however, implies alot of defeatism at the outset)

something like that?

[Pilot DAR]

The Viking/deHavilland Flight Manual for the Twin Otter describes the recommended ditching procedure for the wheelplane in section 3.9.7.

[Concours77]

Yes, I'd like to correct you, there are a number of assumption errors with respect to the differences between how a wheelplane Twin Otter, and a wheelplane 172 fly, particularly in respect of flaps and power/propeller pitch. With some Twin Otter type training, these differences will become apparent, and result in change in your perspective on this type of flying.

One very common feature of both the Twin Otter and 172 will be that if you don't flare it, you're likely to bounce it off the nosewheel, and expensive damage will result.
Done that, though in a 182. It didn't land first by much, but enough to cost me $483.00 when that was real money...

[Pilot DAR]

In a Twin Otter, it's called "Station 60 damage". Sadly common, and easy to do if careless in the flare. Always expensive, and usually done far from home....

[WideScreen]

Yes, I'd like to correct you, there are a number of assumption errors with respect to the differences between how a wheelplane Twin Otter, and a wheelplane 172 fly, particularly in respect of flaps and power/propeller pitch. With some Twin Otter type training, these differences will become apparent, and result in change in your perspective on this type of flying.
Please elaborate.

One very common feature of both the Twin Otter and 172 will be that if you don't flare it, you're likely to bounce it off the nosewheel, and expensive damage will result.
Yes, unless your approach angle is flat, IE, the no-flaps with power, fly the C172 against the deck situation.

[WideScreen]

That's the way I've been approaching the water entry.
It makes a lot of sense to ship the floats over in the belly of a big rig.
Less drag, and safer ditch if needed...

I think your summary makes sense, keep the nose up as long as possible, hoping when it hit, submergence is in the "shallow angle enough such that it pops back up, instead of pitching the whole business over" domain.

Also, hoping for a tail low aspect, so when the gear enters fully, the Nose coming down will plop, rather than sink and trip the a/c over its nose....(then again, if sufficiently entry vertical, the floats are sturdy enough to absorb alot of energy. Keeping them on for the journey for that reason however, implies alot of defeatism at the outset)

something like that?
Yep, something like that, a vertical splash, with minimal horizontal speed, though this twin otter is reported to be without floats, only regular wheel stuff.

With sufficient vertical speed, the time it takes for the gears to get submerged is less. Once the airplane's belly is on the water, there is a big floating surface at the front, that will take care that the flip-over tilt point will be much further forward, and as such, the energy/impulse required to flip the airplane (IE rotate over its nose) is much higher. As well, the time for the airplane to start the rotation before something else hits the water is much shorter. Add to that, the nose will dig in significantly more shallowly, which will also make the flip-over more difficult.

Still nasty to stop in such a way, though.

[Concours77]

In a Twin Otter, it's called "Station 60 damage". Sadly common, and easy to do if careless in the flare. Always expensive, and usually done far from home....

Knew what I'd done when the shimmy started... there goes the schedule... I just kept taxiing to maintenance past the tie down. Leo tried to make me feel better about it..."It's not uncommon..."

How Long? "Maybe a week?" There went two Revenue trips to the Bay...

A week later he called, "She's ready..." I walked in to the office past the coat rack, and the Naval flight jacket. "Good news, bad news, Batman. Good news first Leo. "The parts cost $8.50...."
I brightened up. "And labor is $474.50...."

Kidding aside, I am hoping to learn more soon. I believe our two friends would want there to be no mysteries.

[WideScreen]

In a Twin Otter, it's called "Station 60 damage". Sadly common, and easy to do if careless in the flare. Always expensive, and usually done far from home....
When ditching, the airplane would not be my worry. My worries would be how to survive the ditch (initially and later on), and I'd like to use every part of the airplane as a crumble zone ......

[WideScreen]

The Viking/deHavilland Flight Manual for the Twin Otter describes the recommended ditching procedure for the wheelplane in section 3.9.7.
What does it say?

[MechEngr]

Going out with a parachute, personal flotation vest, an emergency locator beacon, and a life raft still seems the best approach and should be included for long over-ocean ferry flights. Nothing good comes from riding it into the waves. I'd rather not be anywhere near a crumple zone if the way to avoid it was to open the door and leave.

[Concours77]

Going out with a parachute, personal flotation vest, an emergency locator beacon, and a life raft still seems the best approach and should be included for long over-ocean ferry flights. Nothing good comes from riding it into the waves. I'd rather not be anywhere near a crumple zone if the way to avoid it was to open the door and leave.

Sold... Knowing the C130 had your location to within ten feet, and Black Hawk enroute, the chute would be attractive...hoping I don't have to step over a ferry tank...
Spent hundreds of hours fishing the Pacific... once in awhile I would imagine a forced landing.

The ocean moves in mysterious waves....

[StudentPilot479]

Where does the '..about six hours airborne' figure come from?

This seems to be to most common (and verifiable via ADS-B) reported departure and ditching times. Five hours, fifty-four minutes.

Aviation-Safety.net (https://aviation-safety.net/database/record.php?id=20230520-0)
The aircraft had departed Santa Rosa-Sonoma County Airport, California (STS) at 15:21 UTC (08:21 local time) on a flight to Honolulu International Airport, Hawaii (HNL).
ADS-B data suggests that the aircraft at some point turned back, heading for HAF, until it crashed at 14:15 local time.

[Pilot DAR]

Yes, unless your approach angle is flat, IE, the no-flaps with power, fly the C172 against the deck situation.

Still... comparing the landing of a 172 and Twin Otter is not a good comparison. The affect of the use of flaps between the two types is very different, and specifically trained during Twin Otter type training.

But neither type suggests an "against the deck..." type landing for the wheelplane. In all cases, the airplane should be flared to a landing. The Twin Otter ditching procedure specifies the use of full flaps, land as normal, and not to stall the plane.

The use of full flaps in a Twin Otter results in very different handling than a full flap 172. Depending upon how either type is wrongly flown to a runway landing, contact of the nosewheel before the mains is possible, and really bad!

As I have said, type specific operation is appropriate, and we should credit the crew with doing their best to fly the airplane using prescribed techniques for the type. Second guessing how they ditched the plane while comparing what they had to do with flying a 172 is not really relevant, nor useful.

[Concours77]

Almost regardless of type, it would be very, very difficult not to Stall, trying to avoid the water til the last second.... Hopefully just as the tailcone kisses the surface, the wing pays off, the a/c flops horizontally onto the sea. And we're grabbing for survival gear, beacon first....

Without power, I don't think that happens. AOA isn't going to be nose high long enough to bleed off speed. Just trying to flare may immediately cause Stall. The airplane has two thirds of the planned fuel load still aboard, and how close is that to normal gross?

Thirty eight miles short, who wouldn't try to extend the flight path to land. My old house is next to that runway, on the Mavericks side... sad, just prayerful sad...rip

[ChrisVJ]

One thing not mentioned so far is the direction of ditching. Over the years there has been quite a bit of debate, especially about ditching smaller aircraft. One side had it that in any noticeable sea state one should ditch along the direction of the waves (swell) to avoid catching a wave top and going nose down into the next one. Certainly when I owned a seaplane in the early 2000s we were concerned that you could hit a wave top, get thrown back into the air at less than flying speed and stall into the sea nose first. Some thought that a smooth entry along a wave top might be better though I don't think the debate was ever settled.

[Pilot DAR]

one should ditch along the direction of the waves (swell)

Waves and swells are not always the same, there can be waves on swells, which are moving a different direction.

For the Lake LA-4 flight manual:

"Swells: (avoid if possible) Movement: Always parallel, never perpendicular - regardless of wind. "

So, never land/ditch across swells, always along them. If only waves, land/ditch into the wind (which is probably into the waves also).

[Concours77]

Waves and swells are not always the same, there can be waves on swells, which are moving a different direction.

For the Lake LA-4 flight manual:

"Swells: (avoid if possible) Movement: Always parallel, never perpendicular - regardless of wind. "

So, never land/ditch across swells, always along them. If only waves, land/ditch into the wind (which is probably into the waves also).

There is no "along them". Unless you can predict their velocity, and movement relative to the flight path. They are similar to the deck of the carrier, except not only do they move up and down, they travel sideways....so I'm told....

[Pilot DAR]

There is no "along them". Unless you can predict their velocity, and movement relative to the flight path

I have been able to predict the "along" of small swells sufficiently to land along the crest for practice, such that the landing was safe. Though that was only a foot or so swell, not the seemingly ten foot swells I have seen in the Pacific off Point Loma! It requires some judgement as to the small acute angle to be flown relative to the swell crest so the plane follows the crest as it moves. Those were short wavelength boat swells rather that longer wavelength open ocean swells I have seen, and fear! If I had to ditch in the open ocean, I sure would be trying to work out that acute angle to the swell crest, as slamming into the water (face of a swell at a near right angle) in an airplane is a much harder stop than it would seem! I have spent a lot of time looking down at ocean from the cockpit of a single, thinking how I would ditch it. In my flying boat, in mild conditions in the nearby lake, I can practice.

[Sea Plane Driver]

sure would be trying to work out that acute angle to the swell crest, as slamming into the water (face of a swell at a near right angle) in an airplane is a much harder stop than it would seem! I have spent a lot of time looking down at ocean from the cockpit of a single, thinking how I would ditch it. In my flying boat, in mild conditions in the nearby lake, I can practice

Agree 100% Mr. Pilot DAR:

I have also flown the Twin Otter on floats in salt water, even in a protected cove with good planning one can hit hard and shake the fillings out the teeth.
(No soft rubber tires, no soft shock absorbers on a rigid metal only float plane, like hitting concrete if you don't grease it on)

The Twin Otter on wheels is a pussy cat. On floats it can be a tiger.

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[Concours77]

Agree 100% Mr. Pilot DAR:

I have also flown the Twin Otter on floats in salt water, even in a protected cove with good planning one can hit hard and shake the fillings out the teeth.
(No soft rubber tires, no soft shock absorbers on a rigid metal only float plane, like hitting concrete if you don't grease it on)

The Twin Otter on wheels is a pussy cat. On floats it can be a tiger.


Thanks for the input. Just a nitpick, but the distance between swells, (period), can vary, the buggers are neither parallel nor concentric. Projecting a flight path based on a straight line (acute angle) can be deceptive. Since "they all look alike", trying to estimate height and rate is difficult, especially through a canopy, descending. As far as I can remember, only one successful ditching: PanAm Clipper...

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