Twin Otter crash
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Assuming full rigid cabin ferry tanks: 6000 lbs (as reported by PilotDAR).
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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).
Thread Starter
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
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
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).
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).
FP.
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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.
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.
Last edited by StudentPilot479; 27th May 2023 at 01:15.
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.
Thread Starter
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.
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.
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??
Last edited by Concours77; 27th May 2023 at 07:49.
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Thread Starter
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?
Last edited by Concours77; 27th May 2023 at 17:24.
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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?
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?
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.
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Regarding the ferry fuel being there, though not being available for use: I don't think so,
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!
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My source is the manufacturer (Viking): https://resources.globalair.com/spec...-R-01-2018.pdf
Thread Starter
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.
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...
Last edited by Concours77; 28th May 2023 at 21:02.