> Gordon's point is that the best commercial-grade service ... didn't move much in the seventy-eight years* from 1936--2014. It's been nearly another decade and that story's largely the same.
The fact that total passenger miles has been largely flat since 2000 (is that even accurate? [1]) has no bearing on that claim. Furthermore, the number of passengers has basically tripled in the last 20 years alone [2], at the cost of some comfort certainly, but this also must count as efficiency gains. I mean, if we ignore an order of magnitude reduction in cost and increase in number of passengers, yeah, I guess the industry is the same since the Wright Brothers.
This shows flattenings 1979--1984 (Iranian oil embargo, Reagan recession), again 1990--1992 (First Gulf War / recession), a strong climg through 2000, then a large post-9/11 drop and relatively flat trend through late 2004. Omitted is the 2007-8 Global Financial Crisis.
There are two separate questions under consideration here.
One is the capabilities of commercial general aviation air travel in terms of total elapsed travel time.
The second is the general commercial utilisation of air travel.
In the case of the first, advances in aircraft speed have been all but entirely offset by increased frictions in other parts of the air transport system. 2014 travel between NYC and Chicago is only barely faster than it was over 80 years ago. Moreover, aircraft speed is flat or declining since the late 1950s and the introduction of jet-powered aircraft.
The cost side is based on economics and other factors. In part, it is a question of both supply and demand. Costs have fallen through technological efficiencies; jet engines of today are roughly as efficient as propellor engines of the 1950s, not merely faster as was initially the case. They've also fallen through nonaeronotical advances: more efficient booking, higher seating utilisation, tighter seating spacing, and the like.
Statista's data extend only to 2007. Gordon gives data to the 1940s.
Among the big surprises to me a few years back was that peak avaiation fuel (at least through the mid-2010s) had occurred by 2000. Total passenger miles increased, but only through much higher load factors.
Gordon's data show inflation-adjusted passenger price per mile as essentially flat since 1990. Given miles travelled is a function of price per mile ... a similar relationship holds. Rise and Fall, p. 401.
> 2014 travel between NYC and Chicago is only barely faster than it was over 80 years ago.
Your issue is twofold: you are focusing solely on one metric to measure efficiency, arguably the worst one (speed), and you then further narrow the scope by saying efficiency gains must come from one dimension of the business ("aeronautical" advances).
There are obvious reasons for speed to not increase further, namely the speed of sound as you already mentioned, so this is a non-starter. We are simply not trying to make commercial planes faster anymore because that would be useless.
Regarding the second point, I think it is remarkable that you present data that directly contradicts your hypothesis and fail to realize that. If we are starting more flights, carrying more people and using the same fuel, the precise definition of it is an efficiency increase. To achieve that we had improvements in booking, avionics, crew training, business models (Ryanair and other low-costs), etc. The fact that we could make a few flights at incredible cost 80 years ago does not mean that we routinely achieving far safer, cheaper and frequent flights at the same speed today is stagnation, rather the opposite in my view.
I'm given a measure, one specifically of technological capability, and one that was specifically highlighted by a significant author within the field (and whose work was cited within TFA), to make a point.
I get the strong impression you don't like that point, and would prefer to use different measures. That's fine.
But it doesn't invalidate the measure or point made.
There are obvious reasons for speed to not increase further, namely the speed of sound as you already mentioned, so this is a non-starter.
You see my point. You admit it. You state it freely yourself. And yet you reject it.
SST would be useful, and is used in specific domains (satellite launches, military). On a costs/benefit commercial basis, given externalities and legal frameworks, however, technology has hit if not a dead end, then a remarkably robust barrier to further progress.
The areas in which air travel has expanded, and the mechanisms by which it's done so, are notable because they are largely not related to aeronautical engineering: flight controls or power plants.
There's some exception to the latter, jet engines have increase in efficiency both in terms of fuel consumption and power-to-weight ratios. But that's limited incremental improvement. (I'm pretty sure Vaclav Smil addresses this in Energy and Civilisation, as one of his power-plant efficiency curves. These tend to approach asymptotic limits with time.)
Most of the efficiencies though come from smaller aircraft, economies of scale, increased load factors, some materials advances leading to lightweighting, and removal of both price and amenities lower-bounds which have made available lower-cost, but also far-less comfortable, air travel. There's also been a significant, though largely incremental over time increase in overall safety. No major breakthroughs, but chipping away at major risks with time.
Some of those risks are reversing. The number of commercial aircraft downed by missile attack seems to be increasing in recent decades, at least on an absolute basis. It's still likely falling relative to total passenger miles. Still sucks to be you if you're on the wrong end of that ratio.
But aeronautical engineering has hit a hard stop.
Seeing and understanding that point viscerally is key to grasping the premise of technological limits. Many people seem exceedingly averse to doing so.
