but it’s unnecessary risk to pull those useless last 2USG full of dirt & sediment on cross-country,
@lbra agree, and with older bladder tanks you may be picking up water/slush/ice
Done that and had no problems.
You know that you would actually SEE 2 Gallons (regardless whether U.S. or imperial) of mud in a fuel tank?
Ibra wrote:
Not referring to unusable fuel but I am under the impression fuel quality is not the same between the first USG and the last USG
But wouldn’t the “last USG” be in the unuseable fuel?
I mean the last 1USG of the useable fuel, not the last 1USG of total capacity 
I think the tanks will quit when you burn all useable fuel in cruise
In my aircraft it’s 3USG is the unusable in cruise, it has to do with not being able to access it during level flight in cruise and tank geometry? surely not because it’s the dirty remains?
Also one has to be careful with useable fuel and total capacity, it’s only applicable in cruise, in my aircraft takeoff/go-around you need > 14USG on the tank, for landing on sideslip you need > 8USG on the tank
UdoR wrote:
(I can now foresee a soon engine stoppage on my fuel flow indicator, which will start to fluctuate a bit before the engine starts to reduce RPM)
This recent video of mine shows this behaviour. The fuel flow indicator starts to shoot up shortly before the RPM reduces. The instrument panel is rather dark unfortunately, but if you play it back in full screen mode you can see the white FF needle creeping up:
Ibra wrote:
I mean the last 1USG of the useable fuel, not the last 1USG of total capacity
I agree with Airborne_Again: surely all useable fuel is of roughly the same quality?
Ibra wrote:
I mean the last 1USG of the useable fuel, not the last 1USG of total capacity
I think the tanks will quit when you burn all useable fuel in cruise
I understand that’s what you mean. What I’m saying is that any dirt at the bottom of the tank should be in the unusable fuel as that fuel is at the bottom of the tank. If there’s actually enough dirt that usable fuel is also contaminated, then why just the last USG? Why not the last 2, 3, 4 etc. USG?
Gotcha, will make that 4USG !
Ibra wrote:
it’s unnecessary risk to pull those useless last 2USG full of dirt & sediment on cross-country,
This is such a common misconception.
As a rule, all the fuel you use is drawn from the bottom of the tank (save for the unuseable which still puzzles me as to why anyone would build such a fuel tank). If it were not so your unuseable would be as much as is left in the tank after the outlet port is exposed to air due to (eg) running down to half tank…
So there is zero quality risk running the last few USG of usable fuel?
Unusable fuel as it’s name indicates, can’t be used/accessed in flight
Just like fuel in Total station or fuel in dodgy calculations
To clarify my earlier point about bladder tanks, as they age they can develop wrinkles where sediment and water build up. Trying to get the last drop, especially above the freezing level, might lead to engine stoppage as ice or slush clogs the fuel filter.
Conversely bladder tanks may carry a few gallons more than stated capacity. This sometimes becomes apparent when the aircraft reweighed.
