These are production specs. The end user have no idea what he gets. For instance:
The Comité Européen de Normalisation (CEN), also known as the European Committee
for Standardization, controls petrol volatility through EN 228 Automotive fuels – Unleaded
petrol – Requirements and test methods. The specification sets limits on vapor pressure,
distillation (percent evaporated at three temperatures), final boiling point (FBP), distillation
residue, and VLI. In addition, it employs six volatility classes (Table 1.2) that are assigned
by country for winter, summer, and transition period.
The vapor pressure varies from 45 kPa to 100 kPa, and you have no idea when or at what pump you will get the “best” blend. With UL91 all these uncertainties are taken out of the equation. Then you know exactly what you get, every time.
Yeah. Yet car engines still work, even under full load on the autobahn, for hours in a row, regardless of altitude despite “the end user not knowing what he gets”. With an aero engine the issue doesn’t even exist as you can’t run it at full power for any significant amount of time.
The avgas page for Air Shell (or BP) specifies the vapor pressure of avgas was standardised in 1918. Why, that’s reassuring…
On the other hand Transport Canada has removed their temperature and altitude restrictions for Mogas. I’m sure it was based on a back of a fag packet quick appraisal.
specifies the vapor pressure of avgas was standardised in 1918
Probably because no significant discoveries (other than stuff that’s relevant only right at the leading edge of materials science) have been made in thermodynamics since 1918.
Having replaced much of my fuel system after a “little incident” in May last year, I think quality control is really important in aero fuels. Not just mfg but also storage. All you need is an avgas bowser that’s been standing around for some months, unsealed, and you can be in for some “fun”.
Mogas is a very different scenario from avgas because
so the risk is severely limited, by pilot risk compensation and the “aircraft performance demographics”. This is the problem with most social science research… the population one is looking at has usually selected itself to demolish your conclusions before you even started 
You can smile at this one at your leisure
I recall reading some chat among microlight pilots. They said an engine failure every 200hrs is fairly normal. Do they get killed every 200hrs? Of course not. They end up in a field and either get the motor going or (if the “aircraft” is very small) get picked up.
even under full load on the autobahn, for hours in a row
For “hours”, not anywhere on the earth, unless it’s a 2CV
On the other hand Transport Canada has removed their temperature and altitude restrictions for Mogas. I’m sure it was based on a back of a fag packet quick appraisal.Quote
I flew about 500 hours of testing to gather data for TC for this testing and subsequent acceptance. Following the acceptance of four school C 150’s which were the focal point of initial C 150 testing, my C 150 was the first one to be “blanket TC approved” after which they all were by policy. We also flew and approved other types, including a carburetted 520 powered C 185.
The vapor pressure varies from 45 kPa to 100 kPa, and you have no idea when or at what pump you will get the “best” blend. With UL91 all these uncertainties are taken out of the equation. Then you know exactly what you get, every time.Quote
We demonstrated that for the approved 100 series Cessnas, this was not a factor of concern. The reality was that you could not buy fuel which was wrong for the season, and would not work safely. If you bought and stored it, the light ends boiled off and the VP went lower anyway. The worst case was filling with winter fuel in Canada, and flying that tank full directly south into warmer weather. But you could not get far enough in a single Cessna on a tank to matter. All of this turned out to be a tempest in a teapot.
It is true that you do not get an “airworthy release” with Mogas. Do you get it with Avgas? With engine oil? With oxygen? Who asks for it?
If you use Mogas in an engine airframe combination for which it is approved, in accordance with safe practice, it is safe to use.
nearly all of the pilots never fly to any significant altitude
That’s your claim without backing it up.
Half of our club fleet has been operating on MOGAS since before I joined (some ten years ago), and since the circuit altitude is almost 3000’ people have definitely been cruising way above that. I have personally flown an Archer at say ~FL130 in the alps on MOGAS during summer (read high temperatures) numerous times, with not a single “hiccup”.
It seems to be that people who use it don’t have a problem, while people not using it see problems. Notice the trend here? 
All of this turned out to be a tempest in a teapot.
That can be said about all issues related to safety. In 9999 out of 10000 cases nothing will happen. Mogas and avgas are different products for different purposes. Full up with UL91 and you can be 100% sure no fule related issues will occur. Fill up with mogas, and you are only 99.99% sure
@Peter – if England qualifies as anywhere on earth, you’ll want to know that Volvo raced a stock S60 T5 around a ring track at max speed for 24h only stopping to refuel and change drivers – precisely to demo durability in real world settings. Only mods from stock were steering angle to avoid constant steering input on the track and the roll cage. Oh and the steering wheel on the right.
Maybe 250km/h isn’t exactly max power as it is limited electronically but it is pretty close.
Things have moved on since the 2CV and the Cortina :-)
Full up with UL91 and you can be 100% sure no fule related issues will occur
You reckon?
You are probably right as far as the refinery and probably as far as the refinery-operated delivery vehicle.
After that, all bets are off and that is why I put IPA into avgas before any high altitude (=IFR) trip, despite avgas freezing at -58C.
Volvo raced a stock S60 T5 around a ring track at max speed for 24h
Indeed, 24hrs…
But that isn’t fuel related. I still think there is a need for a consistency in fuel if you are going to go beyond the “normal”. Mogas may well be fine to FL140. But a plane which can go to FL140 only without risking an engine failure, is not very useful for the sort of flying which many people do. And that brings us back to whether a hypothetical airport finds it economical to stock 2 or even 3 different fuels, with lower volumes of each one than before.
Full up with UL91 and you can be 100% sure no fule related issues will occur. Fill up with mogas, and you are only 99.99% sureQuote
Hmmm, I’ve never flown on UL91, but if we’re drawing a comparison to 100LL, I can assure you that it is not 100% dispatch reliable.
That said, if I can get 99.99% reliable on any fuel, I’ll be pretty happy with that. Nothing else in aviation comes close to those odds…
You reckon?
Partly, but it was more a general view of things, not necessarily 100.000% accurate
