A CTLS comes in two weight categories: 472,5 and 600kg.
The airplane weights empty 330kg. Takes 130lt of fuel for huge endurance. Let’s take “only” 100lt of fuel onboard. That leaves us with 70kg and 200kg to go respectively.
Where’s the catch? Even with no fuel two passengers make the 472,5 overweight!
This issue presents itself in virtually all ultralights.
Obviously you cannot operate an ultralight of 472,5kg overweight. Are there any differences between a CTLS of 472,5 and one of 600kg?
That same CT will have a MTOW of 450 kg as a Belgian ultralight, unless equipped with the ballistic chute (it is mandatory in Germany, but not here, and few microlights have one). I agree that an empty weight of 330 kg is unacceptable, 270-280 is possible for a 2-place though not easy). On the other hand 100 litres of fuel is quite liberal, who wants to sit in a little noisy cabin for six hours?
That said, there is a consensus that NO microlight takes off with two people on board weighing less than the legal MTOW, be it 450 kg or 472,5. And indeed, the limit is purely theoretical, the exact same planes are flown as LSA’s with 600 kg MTOW. The low MTOW is one of the restrictions we must accept in exchange for the low requirements on training and medical fitness: if things go wrong, the damage caused by our slow and lightweight machines will not be excessive. But of course, there will alway be some people pushing the limits.
To be on the safe side, I always fly solo in a two-seater, except for a short bimble around the village spire. Makes for very modest runway requirements, too.
Since the mass limit was introduced prior to the development of most ultralight aircraft, I don’t think the limit itself is the problem, but bad design and engineering. It is certainly possible to develop a microlight two seat airplane with the capability to carry two persons for about an hour or two with the necessary VFR reserves, but then it won’t cruise at 130kts with autopilot, retractable gear, aero tow hook, multi-screen glass cockpit systems, leather seats, seat heating, full interior lining, fridge and what else is found on those heavy microlights these days.
There is a myth that many of these ULM would be built for a 600kg MTOM within the LSA class, but this is often just based on outward appearance. Many ULM do save in structure mass and therefore the 472.5 kg are not just a bureaucratic limit, but an engineering limit. Those ULM that are built the same as LSA and as ULM are far to heavy for a ULM. Bad engineering, one might say.
The amount of overweight is striking, at least in the Germany accident reports. You can easily find accidents with ULM being easy 80kg over gross, even without fuel. This is 17% overweight. I’d have to fill our 172b with 170kg over gross to archive the same ratio. I have done some calculations for some of the ULM, nearly all of them will leave the rear stability margin with this load. With the 172, I am not so sure, but I haven’t calculated it, yet.
Surely 100 ltrs of fuel on board gives a very acceptable range given the low fuel burn of most engines fitted to microlight aircraft. One of the microlights that I fly has a 50ltr fuel capacity and a fuel burn the averages out at 10ltrs per hour, but as it has been stated a full fuel load is very unlikely with 2 persons aboard. It does however carry a placard giving maximum fuel for given seat loadings so getting legal is very straightforward.
In the UK all microlights types have to provably comply with Section S before they can be registered, Part of this compliance is the ability to carry a stated cockpit load and fuel for 1 hour at max continuous revs. So the aircraft is certainly able to fly legally, not to do so is the choice of the pilot with the very high probability of being unable to have a successful insurance claim should he have an accident. All UK registered microlights are weighed every 5 years to ensure that they still comply, while this cannot regulate the removable items that can be carried at least the basic aircraft can be flown meeting requirements. The main thing is to keep it simple and as light as you can every added toy is fuel that cannot be carried.
It is of course very possible to have an aircraft designed such that it can carry both a practical cockpit load and fuel for a flight much longer than most would be willing to sit somewhat cramped and noisy cockpit, so as in all things you chose the compromise that suits your own requirements best.