hanski wrote:
That “unuseable fuel” seems to confuse pretty many. The definition is something like “not useable in ALL normal angles of attack” or such. Usually you can use all fuel in level flight. Like someone mentioned, the fuel line sucks from the bottom of the tank. In level flight all fuel is “useable”.
Neither “useable fuel” means usable in go-around & takeoff & landing (or aerobatics !)
In my POH, in the fuel tank you need min 3USG to cruise, min 12USG to takeoff and min 8USG to land
One can call these fuel quantities as they wish 
Indeed, you can’t use the last 3USG in Mooney in cruise now matter what you do (unless you fly some aerobatics), but you still can’t use the last 8USG to “land safely” and the last 12USG to “takeoff safely”, so one has to be careful with “usable fuel” in POH, anything between 3USG-12USG is not very usable for flying low near the ground !
Peter wrote:
May be type dependent
It surely is type dependent and may be read in the POH. It depends on the constructional details of the specific fuel system layout, where the “sump” is situated, whether the fuel is drawn from below, or e.g. from back or front side. There are fuel systems which cannot run dry completely, or can only run dry when climbing and so on.
Have seen a pilot report on youtube (one of those from the series of accident case studies of the AOPA, “trapped on top” it was called) where this literally saved a Mooney in the end, as on short final upon evading to collide with a wall (and being short of the runway for some meters only) the engine came back to life, as the rest of the fuel was said to be drawn from the rearside of the fuel tank.
To give another input, the outer wing tanks in the Comanche shall only be used in level flight. Therefore, the unusable fuel is indicated in the POH as zero for these tanks – which is consistent in itself.
I should add, that it might be a last line of help if the pilot was aware of where the fuel is being drawn in his plane. I think in typical Cessnas (172) and in Mooneys it’s drawn at the rearside, but have no wide insight myself of the differences in between types. This means, that when fuel feed is starving, that it might be possible to regain fuel flow upon climbing (and 3 Gallons might save the day). But this however must be done quickly, as long as airspeed may still be converted to the right pitch attitude.
In my Comanche I would possibly fly as leveled as possible, if I were facing such troubles – but haven’t thought this to an end so far.
Peter wrote:
Hence I think the recommendation to run the pump when switching tanks, regardless of whether you expect a gap in the inlet fluid flow, is based on pretty flimsy evidence.IMHO most of the purpose of the electric pump is a backup for the engine driven one.
I think you’re right. On long trips I do tend to run tanks dry. (On the Comanche, I have Mains, AUX and TIPS bot L&R). I do this as I want to know that whatever my fuel totaliser is indicating I have onboard, I actually have on board (and it’s useable – more on that in a bit). As soon as the engine starts to sputter I switch. So, usually take-off is on the mains, in cruise switch to tips first which I will run dry. that way I know I have gotten my full 2.5 hours from the tips (which I then check). I use no pump for this, and it will take about 2 mins for the fuel totaliser to settle again at the desired fuel flow (when leaned at say FL100 it is around 48L). It runs richer for a little bit and I know it’s settle when the fuel burn is back to where it was before. I then keep a very close eye on the burn between left and right, often (4 out of 5 times I’d say) when I then switch to the AUX tanks there is an unequal fuel burn, due to air bubbles in the line, so the fuel is drawn from one side not both, I then close the tank that is giving fuel (position off) and leave the tank that is still full on for about 15 mins so things equalise. Again no pump.
In the Baron, on restarting the engine (there are 2 so it’s a non-event ;-)) the checklist is this.
fuel selector on
throttle 1/4 forward
Mixture full rich
magnetos both
propeller pitch, move to fine pitch (move slowly which should give about 2000RPM
If the propeller does not unfeather, move to full coarse pitch, engage starter then 2000RPM 12Inches
Oil pressure checked
Alternator on (it’s turned off in a voluntary shut down, there are 2 so no issue)
Cylinder head temp >100 deg
No pump, all nav systems stay on…
