I've had very good success with Patagonia Performance jeans, now made in dark gray as well as classic blue denim (note to self: dark blue denim hides coffee and food spills better than dark gray!). Patagonia has two fits (at least in their regular line of jeans, I think in Performance too) - slim and slightly looser - which helps. The weight - thickness - is lighter than Levi 501's while not super thin like Prana etc. So they do look like jeans but you don't feel like you are hauling around a ton of weight. The 20% Coolmax and thinner weight makes sink washing possible, they dry overnight in an aircon room (removes ambient humidity) if I twist them in a dry shower towel (or stomp-to-dry). And oh yeah the 20% Coolmax helps them deal with sweat better, the Coolmax evaporates sweat better than cotton which fiercely retains water.

I'm talking about jeans even though the original poster definitely wanted to stay away from them due to sweating issues. BUT for most of us, jeans - and cargo shorts - are Universal Travel Wear. If you watch YouTube videos of travel destinations you see a lot of jeans and a lot of shorts.

For inflight travel, I share the earlier commenter's concerns about fires.

The real risk is not nearby flames from fuel spills, its usually fireballs from fuel vapors.

Fireballs have a momentary "flash heat" effect. They are generally survivable, but it turns out the worst damage isn't from the intense momentary heat on your skin (even on bare skin), it's from tight stretch fabrics (Under Armour Heat Shield compression tees are the worst) melting while they sink into your skin, fusing fabric to skin and requiring painful debridement. Loose synthetics will melt ONto your skin; compression synthetics (Heat Shield, yoga tights, nylons) melt INto your skin.

That having been said, personally I go with the Patagonia Performance jeans - 80% cotton, 18% Coolmax synthetic, 2% elastane/Spandex "slight stretch". I feel, though I have no evidence for it, that 80% cotton, and lack of compression levels of Spandex, means I won't have the melt INto risk.

Unlike the thread-starter, I NEED the extra warmth of thicker pants on the flight. And, I WANT the Universal Travel Wear look - for both the flight and then again on the ground (I look around my flights apart from a few business travelers, it's almost all jeans and shorts - with increasing numbers of yoga pants). Jeans are definitely upscale compared to shorts.

Finally, if I want jeans on the ground, GOTTA wear them in flight, they are thick compared to travel pants and shorts, and just take up too much space in the checked 22" wheelie I use as luggage. Luggage gets thin, flat-folding (or tight rolling) specialized travel pants instead - REI, Royal Robbins, even the very versatile -in some designs - Kuhl, which aren't travel-specific but are still thinner than chinos. Heck I can pack 3 Kuhls in the space of 1 Levi 501's.

BTW your mileage may vary on jeans - the fits from different brands are all over the place. Thanks to the poster who recommended stretch Lucky, I'll check them out. I can't fit "young, trim, athletic" jeans anymore. I need granddad jeans.