I measured this on my TB20 a while ago… and rechecked it this morning at Brac before the expected long flight of around 900nm.
Avgas expands at 0.1% per degree C. So if you e.g. fill up with fuel at +15C (underground tank) and the wings reach +35C, that is 2% of expansion. A full 43 USG tank (TB20) will thus lose 0.86 USG (3.25 litres).
This morning, I found the fuel about 2.5cm below the rim, so taxied to the pumps to fill it right up. The fill was 10 litres (2.6 USG). I guess the max daytime temp there was +30C but clearly the wings got a bit warmer.
100% credit to the people at Brac airport for doing all this so fast. Everything including the top-off took 30 mins.
Although I (as most) fuel up after arrival, this is one argument to do it before departure when parked in a hot place.
I believe it is not only the expansion in a full tank (hinting dripping overboard as overflow when expanded with day’s heat) but also evaporation through vents.
This is not that strong on a full tank because evaporation is worse when more surface of the fuel is “exposed” inside the tank.
Sometimes I fly once a month and from the 28-28 Gallons mark where I always leave the TB20 tanks after each flight, I then find some 5~8 litters missing after a month of still parking.
The problem is that, at all but the most organised airports, fuelling before departure is asking for a disaster. I think all of us have re-learnt that “Flying 001” lesson a number of times 
Not just because at that point you are working against a deadline (the filed departure time) but the airport personnel doesn’t give a damn about you because you have already passed security (on arrival, they want you off airside ASAP so if you say you must have avgas they have a motive to organise it).
One solution might be to carry say 10 litres of avgas in a metal can.
Obviously this is relevant only on long flights.
Peter wrote:
The problem is that, at all but the most organised airports, fuelling before departure is asking for a disaster. I think all of us have re-learnt that “Flying 001” lesson a number of times
I agree, although this statement
Peter wrote:
but the airport personnel doesn’t give a damn about you
is perhaps true in the UK / Europe, certainly not in the US.
Another reason to fuel upon arrival is to make sure that you will indeed have fuel available for your planned trip. Two days later the fuel pump may have broken down or the tank emptied.
Should fuel not be available, it is also just as well to discover this upon arrival rather than at departure so you can plan accordingly.
In my 30 years of flying I seem not to have learnt the lesson 001 als I have never refuelled upon arrival but always before departure. I have never experienced any of the horror stories and should it once happen it will be a pure coincidence and not the norm. These OWT might have some relevance in Africa but not in Western Europe or the US. Not even in Sicily have I had any problems.
I think either way 9.9 times out of 10 you will be just fine.
Your profile doesn’t show your location, Placido
It helps if one’s profile (the avatar preferably) shows the location, because it helps to put a post in context. If maximum anonymity is required one could just show the country.
Our company SOPs are fuel on arrival, for all the previously mentioned good reasons. However, we would not pre-refuel with AVGAS if the OAT exceeded 35 Celsius and the AVGAS temp was more than 10 Celsius lower (yes, we test the temp of the fuel before issue). Under this circumstance, we would endeavour to refuel immediately prior to departure. Bearing in mind the aircraft carries 896l of AVGAS and burns about 160l/hr, the small numbers start to matter. Clearly we also consider the mass of the fuel (rule of thumb of 0.5% per 5 Celsius).
Turning to the oil burners (DA42/62), we have a similar issue although it is less important. An interesting spin-off from the diesel engines is that fuel temperature needs to be monitored throughout flight; generally speaking, coolant and oil temps are more stable than on an AVGAS engine.
placido wrote:
These OWT might
When relating to my own experience, it is hardly an OWT… Deauville frequently has NOTAMs concerning unavailability of AVGAS that pop up. Once at L2K it was the fuel bowser that became unserviceable between the time of arrival and departure the next day. When they refill the fuel tank at LFPT they shut down the whole fuel station for the whole time it takes to operate the transfer from the truck.
Therefore I would refuel when fuel is available because it might be too late later. That is particularly true of you arrive with too little fuel to reposition at another airport with fuel.
