Jul 25 2017

The Speed of Commercial Jets

nasa_supersonic_conceptWhen I was about 10 my family flew to California to visit my uncle. The flight from New York to LA took about 6 hours. Forty years later a flight from New York to LA still takes about 6 hours (exact flight times vary by airline, but 6 hours is typical).

The pace of technological advance in the last century has been so fast that we generally expect everything to get faster, better, cheaper as a matter of course. So the flat commercial travel times seems to be a weird exception.

Airline travel has improved over this time. Jets have become safer and more fuel efficient. Average costs of flights have decreased by about 50%, although apparently this had more to do with deregulation than technology. It may also have something to do with the shrinking legroom in economy class. But still, why does it take me 6 hours to fly across the country?

There are a number of reasons for the flat commercial flight speeds, which make it seem that there is unlikely to be a change anytime soon.

Supersonic Flight

The first factor that most people likely consider is that we are pushing up against the sound barrier. This is partly true, but not the whole story. The Concorde, which was the only commercial supersonic jet, was retired in 2003. This was mainly because flights were too expensive and it wasn’t commercially viable. There have been no plans to bring it back or replace it.

There are frequent news reports about this or that company working on a supersonic design, but these never seem to get past the concept stage. NASA has been working on developing commercially viable supersonic flight. They gave a contract to Lockheed Martin to develop low sonic-boom technology (QueSST – Quiet Supersonic Travel).

These types of projects lead to countless news reports of “Flying from New York to LA in 2 hours.” Despite 20 years of such headlines, it doesn’t seem as if we are any closer.

The idea is solid, however. The sonic boom is a huge limiting factor. It is, in fact, currently illegal for commercial jets to fly faster than Mach 1.o (the speed of sound in air) over land in the US. The Concorde only flew intercontinental flights for that reason. This is because the noise is too disruptive and even potentially damaging.

So, developing technology to limit the sonic boom is probably a necessary step if we are to ever have significant supersonic commercial flights. It may still not be enough, however.

Further, even approaching Mach 1.0 is problematic because of something called the critical mach number. As planes approach Mach 1.0 the airspeed across parts of the frame may exceed 1.0, even if the plane itself isn’t. So planes have to stay below the critical mach number.

Boeing tried to address this issue with their Sonic Cruiser, which was designed to achieve Mach 0.98. They cancelled the development of this jet in 2002, however, and shifted instead to the Dreamliner, which has a cruising speed of Mach 0.85.

But there is yet another issue that may be the ultimate limiting factor – cost efficiency. The dropping price of airline travel has been great for consumers, but has significantly reduced margins for the industry. This has resulted in some annoyances, like paying for luggage, less leg room, and getting fewer amenities in flight. But hey, I would rather have a cheaper flight than pay more to get a non-crappy meal on the plane.

Another result, however, is that airline are highly motivated to make each flight as cost effective as possible. Airspeed is part of this calculation. As jets go faster, air resistance increases fuel use. Drag is about proportional to square of velocity. This means that a 10% increase in speed results in a 21% increase in fuel use.

Airlines, therefore, calculate optimal speed for cost efficiency. The sweet spot is about Mach 0.85, so that is where most airlines operate. A six hour flight at Mach 0.85 would take 5:22 at Mach 0.95. The shorter flight would be nice, but not worth the extra fuel cost. Most people would likely notice the higher ticket cost than the slightly faster travel time.

The time saving would be more noticeable on longer flights, but there already is higher fuel costs on longer flights because the jets have to carry more fuel for the longer range. For this reason some of the longest routes were canceled. In any case, on longer flights the airlines have even more of an incentive to keep at the optimal speed for fuel use, rather than flight time.

While jet design has certainly improved and will continue to improve, the result of these advances has been in reduced ticket cost, not reduced flight time. It seems like this is going to continue to be the cast for the foreseeable future – we are stuck cruising at Mach 0.85.

This will only change when commercial supersonic flight becomes viable. Reducing the sonic boom will help, but by itself won’t be enough. The overall cost of such flights would have to come down to the point that enough flyers would be willing to pay the premium for the convenience.

At the end of the day flight times have more to do with economics than technology. While I am hopeful that NASA’s project will bear fruit, if I had to predict, I doubt I will ever take a commercial supersonic flight.

24 responses so far

24 thoughts on “The Speed of Commercial Jets”

  1. kvh says:

    Additionally I think once we started hitting limits on the speed of moving atoms we switched to using electrons to move information instead. Even with supersonic flight you can’t beat the few seconds it takes to route info around the world a few times.

    And surface cargo transport started increasing their bandwidth by moving from increasing speed to increasing volume. So now we have container ships larger than aircraft carriers.

    The need to move things through the air faster & faster doesn’t have the economic push as other means are available.

  2. Belgarath says:

    Actually most of the US commercial flights cruise around Mach 0.78, the typical airplane you fly across the US (737, A320 etc) all cruise right around .74 to .78. Over water flights will cruise a bit faster, but almost all domestic flights are in the .74 to .78 speed range.

    Another factor that adds some time to almost all flights is the congestion at the destination airport. Generally the last 200 miles you’re slowed down and turned and spaced in order to get all of the airplanes spaced properly for limited runway space.

  3. Belgarath says:

    Generally your logic and reasoning still hold, however, just a minor nit on the actual speeds used.

  4. carbonUnit says:

    Hope the family trip to LA was a good time. Condolences to you and the Novella clan about your dad.

    One other way to avoid the sonic boom (and go yet faster!) would be to go suborbital, above the atmosphere. Of course, supersonic and trans-atmospheric craft are presumably much more complicated and expensive beasts, so they might not compete well with conventional Mach .85 aircraft. I have heard that, for long haul flights, the energy required to get above the atmosphere is on par with that of plowing through the atmosphere all those miles.

  5. carbonUnit says:

    Actually, Belgarath, getting to and from the airport, getting through screening, etc. can take as long as a short flight. For destinations up to about an 8-hour drive away, it is sometimes better to drive.

  6. MWSletten says:

    It’s not just aerodynamics, it’s also propulsion technology. Today’s high-bypass turbofan engines, developed partially to reduce noise, are also most efficient at .85 MACH. Interestingly, the B707 was most efficient at higher speeds, because it used turbo jets (more of a pure jet) and low-bypass turbofans. Because the B707 operated at speeds closer to MACH 1, pilots had to closely monitor speed in a decent to ensure they didn’t exceed it.

    So in one respect, we’ve taken a bit of a step back in capability. The older jets, as you note however, are far more expensive to operate, both in maintenance and fuel costs.

  7. Deemer says:

    I am sure I once heard that the supersonic across US landspace was a political reaction to the Concorde consortium beating Boeing to a viable product.

    And that had that ban not arisen, the core market of fast NYC/BOS – LAX/SFO as extensions from London would possibly have been economic at the time.

    Until it’s sad demise I believe that while the marginal operating costs were not profitable, the Concorde service played a significant role in BA’s reputation and marketing.

    Re total travel times, there’s an ongoing European initiative to encourage fast rail developments such that any city-to-city travel sub 4 hours flying becomes faster, more convenient and pleasant by train.

  8. EvanHarper says:

    It’s worth mentioning that the need to arrive at the airport early to be sure you don’t miss your flight, the time-consuming security checks and boarding procedures, and most of all the travel time between the airport and the actual ultimate travel destination — typically some miles away, via inefficient and congested surface transport links — all add up so that the time of a typical flight is only about half the total journey time. And while the engineering & basic physics challenges of supersonic flight are inherently very difficult to solve, there are probably large improvements we could make in security procedure, boarding procedure, mass transit and so on that would not be nearly so challenging and would also reap broader benefits. This is another reason why aggressive R&D efforts to make flights, say, 50% faster are really not very smart bets.

  9. Michael Finfer, MD says:

    Unfortunately, it turned out that the Concorde was not a safe aircraft. One crashed during a demonstration take off at an air show because a ruptured tire caused, I believe, a fuel leak and a fire. If I remember correctly, everyone on board was killed.

    That should not have been possible.

    The entire fleet was retired abruptly after that.

  10. DisplayGeek says:

    One of the stated thesis of the post is that people assume that all progress in technology is linear. We made incredible progress in aviation from the late 1890s to about 1970… then things dramatically slowed down. But what really happened is that we reached a “good enough” and cost effective plateau for commercial aviation and for general aviation too. In fact, both of my airplanes were built in the early ’70s and are still flying.

    But aviation is seeing dramatic changes in the cockpit as it shares in the advances in electronics and are about to see dramatic changes in powerplant and control. I wrote a vignette (it is a prequel to a SciFi novel that I’m currently editing before self-publishing):

    https://candicebrownelliott.wordpress.com/2014/08/20/the-future-in-the-rear-view-mirror/

    The problems listed regarding door-to-door time are more important than wheels-up to touch-down time. Commercial flight will also use these new technologies and smaller aircraft flying to more convenient and smaller aerodromes. The flight times will actually be slower, but the door-to-door times will be very much improved for short to medium flights. Long distance flights will still use the same airports as today, but will not be competing with short-haul flights for runway assignments.

    I also expect that pre-screening of individuals will eventually replace the “security theatre” presently occurring at airports today.

    But, if you want some of the advantages of the above today… hey, as a CFI I (or many thousands like me) will be happy to instruct you to become private pilots.

  11. Survivalist13 says:

    Steve, your statement about drag being proportional to velocity squared is a very superficial analysis of the aerodynamics and when it comes to understanding modern airlines is of little help on its own. That law only holds for a constant drag coefficient which is not true once compressibility becomes important (above Mach 0.3). This is by the by because what is really important for flying is the Breguet range equation, which shows that the lift to drag ratio of your airframe is the important quantity, NOT the drag. Modern airliners use super critical wings (partly supersonic, partly subsonic flow which have very high lift to drag ratios) but only work around Mach 0.8. You also have to remember that drag scales with density so as you go higher density decreases, so it is beneficial to fly high (falls out of the range equation, in fact this observation is what drove Frank Whittle to develop the jet engine). Another surprising factor is that jet engines actually become more efficient at higher speeds as you can use the shock waves to compress the air more efficiently than a compressor.

  12. BillyJoe7 says:

    DisplayGeek,

    “The problems listed regarding door-to-door time are more important than wheels-up to touch-down time”

    My last flight from Melbourne to Hobart took 55 minutes. The car trip from Mooroolbark to Melbourne took 70 minutes, and there was a 45 minute preamble at the airport. I don’t think I need a faster jet.

  13. MosBen says:

    Whatever the practicalities involved in Hyperloop, do the current proposals have the pods (?) going faster than the speed of sound? Is the sonic boom not an issue if the pod is enclosed in a tube which has reduced air pressure?

  14. aubreycohen says:

    You gloss over the Sonic Cruiser. It was supposed to go nearly the speed of sound with the efficiency of the slower airliners available at the time. Airlines, however, preferred a jet that would go the same speed as current airliners, but more efficiently. Thus, the 787. So, even if the efficiency of faster travel can be improved, if that could bring even higher efficiency at current speeds, airlines will undoubtedly prefer that yet again.

  15. Michael Woelk says:

    @Michael Finfer, MD:

    Unfortunately, it turned out that the Concorde was not a safe aircraft. One crashed during a demonstration take off at an air show because a ruptured tire caused, I believe, a fuel leak and a fire. If I remember correctly, everyone on board was killed.

    That is incorrect, the Concorde was an incredibly safe aircraft. In it’s 27 years of service, there has been only one deadly accident and no other incidents that resulted in any kind of injury or hull loss. The incident you’re referring to was Air France Flight 4590 (from Paris to New York City), which was caused by a metallic strip that fell from a Continental Airlines DC-10 that had taken off minutes earlier. This fragment punctured a tyre on Concorde’s left main wheel bogie during take-off. The tyre exploded, and a piece of rubber hit the fuel tank, which caused a fuel leak and led to a fire.

    Unable to gain altitude or speed, the aircraft entered a rapid pitch-up then a violent descent, rolling left and crashing tail-low into the Hôtelissimo Les Relais Bleus Hotel in Gonesse, killing all 100 passengers and 9 crew members on board the flight, and 4 people on the ground.

  16. BillyJoe7 says:

    Michael Woelk,

    “The tyre exploded, and a piece of rubber hit the fuel tank”

    That is incorrect. 😀

    The rubber did not directly puncture any of the fuel tanks. It sent out a pressure shockwave that ruptured the number five fuel tank at the weakest point, just above the undercarriage.

    Either that or Wikipedia is incorrect! 😀

    https://en.wikipedia.org/wiki/Air_France_Flight_4590

  17. SteveA says:

    Michael Finfer, MD

    I think you might be conflating two separate incidents. There was the crash of the Air France Concorde referred to by Michael Woelk; but there was also a crash of a Concorde-like aircraft, the Russian Tupolev Tu-144 (known as the ‘Concordski’) at a French air-show in 1973.

    Deemer: “Until it’s sad demise I believe that while the marginal operating costs were not profitable, the Concorde service played a significant role in BA’s reputation and marketing.”

    I agree. It was a sad day when they stopped the flights. It felt like a step back.

  18. sarah_theviper says:

    A guy at work told me he had went to talk by the French guy who designed the interior (I think it was on the Concorde). He was going for “baby shit brown” with the seat color in order to hide the baby shit.

  19. Nema says:

    I like the 6-hour cross-country flight. It’s enough time to relax and watch the landscape change and mentally transition from one coast to the other.

  20. Michael Woelk says:

    @BillyJoe7:

    Either that or Wikipedia is incorrect!

    That’s pretty funny because that part of my comment was a direct quote from the Concorde’s Wikipedia page. 🙂

    https://en.wikipedia.org/wiki/Concorde#Air_France_Flight_4590

    I’d also like to point out that the phrasing

    (…) a piece of rubber hit the fuel tank, which caused a fuel leak and led to a fire.

    doesn’t necessarily imply that the piece of rubber actually punctured the tank – so it’s technically correct. (the best kind of correct :))

  21. Pete A says:

    Michael Woelk,

    Three things were obvious:
    1) you copied from Wikipedia;
    2) you failed to quote your copy;
    3) you failed to link to the source from which you copied.

  22. Michael Woelk says:

    @Pete A:

    I’m not sure what you’re trying to say. First of all, I never intended to obfuscate the fact that I was quoting Wikipedia – I’d even argue that it wasn’t obvious at all until I pointed it out myself. Second of all, I did it solely because it’s easier than paraphrasing what’s already there – no harm done. I’m not writing a paper here, so there’s no obligation to cite sources or explicitly mark quotes as such. Besides, no false information has been stated and no one claimed any authority, so what’s the problem?

  23. Pete A says:

    “[Michael Woelk] … I’d even argue that it wasn’t obvious at all until I pointed it out myself.”

    It was obvious to me because I was previously familiar with the Wikipedia article from which you copied.

    https://en.wikipedia.org/wiki/Wikipedia:Text_of_Creative_Commons_Attribution-ShareAlike_3.0_Unported_License

  24. Michael Woelk says:

    @Pete A:

    It was obvious to me because I was previously familiar with the Wikipedia article from which you copied.

    Fair enough. I’m an aviation enthusiast and have read the article about flight 4590 several times myself (in fact, I’ve read all articles in Wikipedia’s “List of accidents and incidents involving commercial aircraft” at least twice). That said, I still stand by my point that citing sources in a comment section is unnecessary, and frankly, a bit silly. Unless someone asks where my information comes from or contests the truthfulness of my statement that is.

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