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Why do airlines separate passengers into 1st and 2nd class but do not offer fast or slow connections? Isn't the biggest cost the kerosene? And isn't the cost directly proportional to speed?

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don't forget to accept an answer if you someone gives you a good one. You get an increase in your acceptance rate which makes people more likely to answer your questions in the future. –  TechZen May 14 at 13:16
    
Private jets are an example of what you are asking about. Some can travel near mach. But they are MUCH more expensive than a commercial flight. –  Sponge Bob Oct 13 at 3:56

11 Answers 11

up vote 9 down vote accepted

isn't the cost directly proportional to speed?

No.

  1. Most fuel can be saved by flying higher because the lower air density reduces parasite drag. Provided the trip is long enough for the saving at altitude to offset the cost of getting there.
  2. Aircraft drag has two components. The parasite drag which increases with speed and the induced drag which decreases. So they have a point of minimum drag and for jet airliners it's typically somewhere between 200 and 250 knots indicated. Flying any slower would increase the cost. At the altitude where jets normally fly, the true speed is significantly higher than indicated speed.
  3. For jet engines, the fuel burned is roughly proportional to thrust they generate and at higher thrust lower per unit of thrust. Since power is thrust times speed, this translate to much better efficiency at higher speed. So a jet wants to fly slightly faster than their lowest drag point.
  4. Even if the trip cost is higher, higher speed allows you to make more trips, so the fixed costs get split over more passengers. So it makes sense to fly slightly faster than the lowest fuel burn.

Generally airliners are operated at the point of lowest cost. For long trips where it pays off to climb high, jets are most efficient. For short trips, turboprops are often used that can't climb that high, but are more efficient at lower altitudes and speeds.

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Can you cite a reference for the 250KIAS figure? –  rbp Oct 12 at 22:59
    
I changed 'reduces drag' to 'reduces friction,' and someone changed it back. There are many different causes of drag, and friction is one of them, the one which is alleviated by lower air density. –  rbp Oct 12 at 23:10
    
@rbp: Lower air density reduces more than friction, but indeed not all drag. The most precise without complicating the discussion is probably using 'parasite drag' or 'form drag'. And the end effect is not that the aircraft flies with less total drag, but that it flies faster with given drag. –  Jan Hudec Oct 13 at 5:12
    
@rbp the 250 is the general imposed limit below 10,000 ft in the US, maybe that's where the figure came from. –  Burhan Khalid Oct 13 at 6:01
    
@BurhanKhalid: No, definitely didn't come from that. Probably a little too high too; I recall reading A320 has Vy of 220 KIAS, but not sure where. The actual value varies between aircraft and for given aircraft with weight. And best glide distance speed, which is more relevant here, is higher than Vy (though I wrote minimum drag and that means Vy). –  Jan Hudec Oct 13 at 7:56

Concorde was built for speed. But it was costly too. People did not like it much. Reality was that they didn't want to get somewhere faster by paying more. People love to pay for luxury though. That is the reason different travel classes were introduced since late 1970s.

Cost of fuel and speed are NOT directly related. As you may know, Boeing 787 has increased fuel efficiency (claimed to be the most fuel-efficient), and still reaching higher speeds.

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+1 for mentioning the Concorde, but it was well received by the comparatively few pax who rode on it thanks to the shorter flight time, lower cabin altitude, and good food. And hey, the view at 60000 feet is pretty good too! –  shortstheory May 7 at 8:31
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@shortstheory yes, but they lambasted the narrow, cramped, seats, the lack of legroom, etc. etc. Effectively you were paying first class rates for cattle class seating with business class service. –  jwenting May 7 at 10:57
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If I recall correctly, as concorde was not invented in the US, it was not allowed to fly at full speed over the US, hence limiting it’s usefulness. Also due to time difference, it only worked well for going from the UK to the US, as the other way a full size bed in the aircraft was a better option than having half a night’s sleep in a hotel. –  Ian Ringrose May 7 at 12:24
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@IanRingrose, the ban on over-land supersonic flight wasn't because Concorde was European and not American. It would have applied to all supersonic airliners, and is one of the major reasons the US SST was cancelled. Boeing took a huge hit when they cancelled the 2707. –  egid May 7 at 15:40
    
I flew the Concorde to France, and loved it! The "narrow cabin and lack of legroom" was a myth cited by people who'd never flown to Europe in 2.5 hours –  rbp Oct 12 at 23:00

The optimal speed of the aircraft is set by the manufacturer.

isn't the cost directly proportional to speed?

No, any speed above or below the optimal one will lead to sub-optimal performance of the engine, increasing the costs.

Why airlines do not offer fast or slow connections?

The answer is partial speculation, but the costs do not increase linearly, and I think that the people that could afford such increase in costs (and would buy such a ticket) are not enough to fill standard commercial aircrafts. In fact I think they have their own personal and private business-jet (or something of this sort)

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Good answer. And even with a fast private business jet, cruising at Mach 0.9 really doesn't get you there that much sooner than at Mach 0.75-0.85 (typical airliner range). To see a really significant travel time reduction you have to go supersonic (not currently an option unless you have access to military hardware :) –  dvnrrs May 6 at 19:03
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Private jets reduce your total travel time (door-to-door), not your airport-to-airport gate time. And they can use smaller airports closer to where you want to go, on your schedule. No checkin, baggage or security hassles either. –  paul May 7 at 7:50
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I think the majority of people flying first class (in US Domestic flights) have not even paid for their first class upgrade. Rather, I believe the majority of them have free upgrades granted due to their frequent flier status. –  mah May 7 at 10:49

Providing a fast and a slow "class" for flight tickets would add a lot of overhead to airline operation in many ways - way more than it could be worth it:

To have fast and slow flights, one could

  • Use two kinds of planes, fast and slow:

    • Requires big, slow planes and fast, smaller planes, assuming fewer seats are sold in the fast, expensive class. An airline that has only one size of planes now would require a broad range of types of planes.
    • Cannot handle flights that now use relatively small planes when the fraction of fast-class passengers is low, as for very few it get's just too ineffective to have a separate flight.
      • The potential solution of downgrading the high-price customers to slow flights does not sound useful.
      • Solving it by upgrading the slow-class passengers to a fast flight does not work in general as the fast planes needed otherwise are too small for the higher count of passengers (or would at least make planning even harder).
  • Use one kind of plane, used to fly fast or slow:

    • Needs about twice as many planes compared to now.
    • Makes seat allocation planning harder - need to estimate counts per speed class for planning, which may change depending on external events (like trade fairs at destination being consumer or business-targeted)
    • Will provide not much benefit to the passenger, as the difference between the fast and the slow flight speed is limited by the physics of the planes used
      • The slow speed would need to be the most fuel-effective speed, to support low prices. That is the speed of best L/D ratio - so the slow speed could not be much slower that today's standard speed, as that would cost extra fuel.
      • The fast speed would be limited by the highest speed of the plane allowing acceptable fuel use, and by the power that the engines of the airplane can provide (assuming it's save to fly at that speed).
        Because the fast-class speed is faster than the slow-class speed, it's above the speed of best L/D ratio. For speeds in that range, the induced drag is dominated by the form drag, and the from drag increases with the square of the speed. At some speed, looking at total fuel usage it does no longer make economic sense to go faster, and because of the square relation, I expect the possible speed difference to be "not large enough to make sense" to distinguish two completely different flight classes.


It could work, though, if we take large airships into account, and providing very slow, but very cheap flights, but still faster that water-based ships.

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The problem with the airship is that JFK-LHR will need two days, which means you need six meals, four liters of beverages, a seat and a bed per passenger, which all adds up to the bill. And then, no-one has two days to spare these days. Either they go on a five-days-or-more cruise aboard Queen Mary (which costs way more than a first-class flight, by the way, but then, there won't be a swimming pool aboard your airship), or they just take a plane. –  Alexander Oct 13 at 7:11

To offer different rates for different speed of travel the airline would need to add different aircraft types to their fleet, which would increase their cost of doing business dramatically.
They'd need to have a slow type for the lower price, a medium speed type for the "regular" price, and a fast type for the "high speed rate".
If those even exist in the range/size bracket the airline is planning to introduce that service in, they'd now have to have multiple groups of crews, several more maintenance facilities with crews trained in maintenance of different aircraft types, a vastly increased amount of parking space for storing the aircraft in between missions, massively increased spare parts inventory, etc. etc. etc..
All that would mean the cost of running the scheme would rapidly overshadow the potentially higher income. And at the same time the customers would run away from the lower speed option to take the train of fly with a competitor who offers the same route on a faster aircraft using a more traditional service based pricing policy.

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In thinking about it, the 1st, 2nd and 3rd class distinction originally arose in the time of watercraft travel and during those days, there were also significant distinctions in the speed/cost tradeoffs on various types of vessels. Hence the term, "slow boat to China."

The slowest and cheapest passage was to ride supercargo (literally above the cargo) on the slowest cargo vessels such as barges. The fastest and and sometimes most expensive expensive passages were on packet ships, ships built for raw speed and used to carry mail. They were small, cramped and took a pounding from the sea. People seldom took passage on them if they didn't have to. When you read a historical reference to someone important coming in a packet ship, it would be like flying in a VIP in the rear seat of fighter jet today.

The classes came from accommodations and might have come from the British practice of rating ships themselves, 1st, 2nd and 3rd class. They really only appear in the steamship era when purpose built passenger ships evolved. Ocean liners made a point of their speed, sometimes beating packet ships because of their long hulls, but since even a short trip was matter of weeks accommodations mattered.

So why didn't the air travel industry break down into slow and fast planes? Others have noted the scaling problems with speed vs cost but another major reason is the difficulty in accommodating aircraft flying at different speeds at the same airport.

The length and strength of runway, for example, has to be proportional to the weight and takeoff/landing speed of the aircraft that use it. A slow light plane on long,strong runway is a waste and the opposite impossible. Likewise, it's hard to safely coordinate planes all flying around at different speeds with different safety intervals. That's why we have the distinction between military, commercial and civil aviation airports, each supports a different array of aircraft.

Contrast that with ships in smaller vessels and larger ones skim right by each other (at least at lower harbor speeds.) Harbors used to be crammed with all sizes and speeds of vessels sailing safely by each other and that in turn meant there was an economic niche for both fast/expensive and slow/cheap passage rates.

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There are slow connections which cost less, which is accomplished by having long transit hours.

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A more detailed reasoning and explanation would be appreciated. –  Federico May 8 at 10:54

There is indeed a benefit in flying more slowly, but the aircraft needs to be designed that way to profit from slower speeds. You can see that wing sweep has been slightly reduced in the more modern airliners, but this is also due to better airfoils with higher Mach drag rise onset.

But some things will also cost more when you slow down: You have to pay the crew for more hours, and the aircraft cannot be utilized as often. To transport the same number of passengers requires more aircraft when they fly at a lower speed. The cost versus speed relationship is not linear, but it should have a minimum somewhere below today's operating points. Aircraft with a slower design cruise speed can be lighter and smaller for the same payload, but for now airlines don't have a choice of slow or fast jets; all manufacturers try to fly at least Mach 0.83.

This has a lot to do with marketing: The aircraft with the higher speed will show a shorter travel time for the same distance, so it will show up first in the booking systems of travel agents. Of course, now you will argue that most people book on the internet and will try to get the lowest cost, regardless of travel time. True. But this is not the sort of customer the airlines are after. Their profit comes from people in First and Business class, and these still book predominantly via travel agents. Therefore, Airbus and Boeing try to market their planes as the ones which come first on the Amadeus and Sabre screens.

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You have many good points, but I think some things are not correct. The biggest benefit is flying higher and at the usual cruise altitude the jets actually fly at their best fuel economy speed. –  Jan Hudec May 7 at 8:41
    
Maintenance costs also go up if you put more time on the airframe/engines. –  Lnafziger May 8 at 15:44
    
@Jan Hudec: The main benefit of flying higher comes from the lower temperatures. Once you climb above the tropopause, this effect goes away. What remains is less friction at the same speed, but also less dynamic pressure. And you need that to create enough lift. Most airliners cruise already at their maximum operating altitude (at the given mass), if possible. Less fuel consumption can only be gained from flying slower, and with an optimized airframe for that speed. –  Peter Kämpf May 12 at 12:12
    
@PeterKämpf: Both lift and drag are functions of dynamic pressure and the benefit stems from the fact that the same dynamic pressure with minimal drag is at higher true speed. So the benefit does not go away above tropopause, it increases even faster. Limiting factors are the wave drag when approaching speed of sound (which by the way is lowest in tropopause and increases above as it increases with temperature) and amount of oxygen available for the engines to burn. –  Jan Hudec May 12 at 13:27
    
Oh, come on, Jan, you should know that the benefit only occurs if the plane is not flown at its design point. Flying higher means quickly growing engine demands which allow for nice short take-off runs and fast climbs, but make the airplane less efficient overall. And since temperature is roughly constant at the first few kilometers of the stratosphere, the speed of sound does not increase until you climb much higher. Current engines, runways and aerodynamics combine acceptable take-off and cruise performances at Mach 0.8 - 0.85 at FL 300 - 390. Improving this at the same Mach number is hard! –  Peter Kämpf May 13 at 20:57

Gravity * time aloft.

Because a heavier-than-air craft must expend a constant amount of energy just to stay aloft* per unit of time, it maximizes efficiency in a trip of given length by minimizing the amount of time required to complete that trip. This is of course balanced against other factors, but it appears to dominate within any given set of parameters.

*Of course, there are more and less efficient ways to do this. Rocket-hovering is awful, and flying by airfoil at the best L/D ratio for a particular design is great. Watch an F-14's efficiency go to zero as its constant-altitude airspeed goes to zero. It is possible to waste copious amounts of energy on heat and noise, but you cannot counteract more than gravity's constant pull without rising.

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There is a faster option - it's a case of looking at total travel time - door to door. In which case private jet is faster.

  1. It goes when you want it to - no waiting for the 747 or A380 to go once a day - saving up to 12 hrs on journey
  2. No checkin - minimal formalities (esp if you e.g. head of state) - saving 1hr or more?
  3. It goes direct to the nearest airport the aircraft can access. No going via super hubs like Chicago or Heathrow. Saving 3+ hours.

Flying super fast between hubs doesn't save that much time compared to aircraft flying direct to where you want.

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This is sort of fast -vs- slow in air travel.... I don't think that this is what the question was actually asking about though. –  Lnafziger May 8 at 15:43

At the moment, planes travel at around mach .75 to .95; which is a tradeoff between speed and fuel consumption.

While it is possible to produce faster airplanes, no one does, because significantly faster (=supersonic) planes are not allowed everywhere; apart from that, they need much more fuel.

But to improve time, you don't have to consider using faster planes on some legs, you have to consider the whole trip.

Which may be:

  • 90 minutes from manhattan to airport during rush hour
  • Check in, security check: 90 minutes
  • Six hours on a 747, 3 on a Concorde.
  • 90 minutes waiting for the connecting flight (if you are lucky).
  • Three hours on a 737.
  • Half an hour by cab to your Oslo office.

Which makes your whole trip 14 hours on a normal plane, or 11 hours if you take Concorde, doubling(?) the price. I guess that heli transfers from and to the airports would be better investments than a seat on Concorde, if your company has helipads on the roof.

Overall trip times are why Boeing decided to not supersize their planes even more, and opt for more direct connections instead:

  • 90 minutes from manhattan to airport during rush hour
  • Check in, security check: 90 minutes
  • 7.5 hours on a 787
  • Half an hour by cab to your Oslo office.

And voilà, you are down to 11 hours, without the need for fancy supersonic planes.

By the way: While flying all-Economy is what some low-fare airlines do for a living; no one was ever successful in trying scheduled all-First flights. First is mostly combined with Flex. So, while First is a good thing on top, Economy is the bread-and-butter business which attracts the Flex-First bookers, because Economy is what allows the airline to do seven trips a day and still have a spare seat for the VIP on whichever connection he chooses.

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