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