I was on a flight today and the speed was 500 mph but the max speed of the plane was about 600 mph. Noticing that there weren’t any police in the sky to write speeding tickets or even any sky traffic lights, I was wondeing why the plane doesn’t fly at max speed. Does it save fuel, or is there some sort of conspiricy by the air lines to stop me from getting where I need to go on time?
Well, we have real pilots and aero engineers here who, I am sure, will shortly appear to give the definitive answer. But, speaking as a layman, I imagine it’s much the same as the reason you don’t always drive your car at 90mph. You could, probably, given the opportunity, but your fuel economy would probably tank at that speed.
Hence, my guess…fuel economy. It could mean the difference between a profitable flight and an unprofitable one.
First, it does save fuel if the plane flies at less than its maximum speed. Second, the skies are crowded and to put it as vividly as possible, in many cases if it were to go as fast as possible it would eventually bump into the plane in front of it, and that would be very bad.
Ed
I think he was asking about if all planes went faster. The second problem disappears then right?
Not if the plane in front was slower than the plane behind it.
I’d put another vote for fuel economy, plus ambient conditions. A plane can get a very nice speed boost by flying with the jet stream; when it doesn’t, it makes it that much harder to get to top rated speed.
Even if planes flew faster in the air, there’s still only a limited amount of space available at the airports on the ground, and the planes already there can only taxi, unload and reload so fast. It may be that airlines determine recommended flight speeds based partly on when there’s an available landing window at the destination, since it doesn’t do much good to fly cross-country at top speed (probably using up more fuel), only to have to circle the airport (and thereby creating a bigger traffic hazard than if they were just cruising in a straight-line along the main air corridors) for an hour waiting for another plane to leave the gate.
And while it may be possible for everyone to put their heads together and reschedule the takeoff and landing times based on everyone flying at top speed, it then only takes a slight weather change or mechanical fault (in the air or on the ground) for everything to go pear-shaped. Maintaining standard flight speeds at 80% or so of maximum makes the whole system more tolerant of changing conditions while still getting people to their destinations fairly quickly.
First of all, here are some definitions to get started:
Ground speed = the speed of the aircraft over the ground (normally what is displayed on the passenger video screens.) This speed bears no relation to the max air speed of the aircraft. The aircraft is limited by airspeed.
**Air speed **= the speed of the aircraft through the air. This is the speed that makes the aircraft fly (or not) and is the only speed that is of any relevance to the aircraft itself.
**Max operating speed **= the max structural speed of the aircraft. Go faster than this and you’re in test pilot territory.
**Max cruising speed (not an official speed, but probably the most interesting to passengers) **= the max speed the aircraft can achieve in straight and level flight at max continuous power. Depending on the aircraft and conditions this may be equal to the max operating speed, or it may be less.
**Min drag speed **= the airspeed at which the drag created by the aircraft is at a minimum. This is also the speed at which the least thrust, and therefore fuel, is required to stay airborne. It is therefore the speed for best endurance.
So the first question to ask is, where were you getting your information from? 500 mph ground speed or airspeed? What is the source for a max speed of 600 mph?
Aircraft normally fly a bit slower for fuel economy reasons. They can also be required to slow down due to traffic requirements although aircraft that are geographically close together are seperated in altitude to reduce the chance of conflict. So speed restrictions for traffic purposes are normally only imposed during the departure and arrival phases of the flight as ATC try to sequence the aircraft.
More on fuel economy:
As noted above, the speed for best endurance is that which gives the minimum drag and therefore the minimum thrust/fuel requirement. This gives the best fuel economy if your only purpose is to stay airborne. If you actually want to go somewhere then you need the speed that gives the best speed for the lowest fuel flow. This speed is best range and is normally somewhere around 1.32 times the min drag speed.
Because drag increases significantly at speeds approaching the speed of sound, the maximum speed of an airliner (which is normally close to the speed of sound) is not very fuel efficient.
Modern airliners are fitted with a computerised flight management system (FMS.) The FMS is integrated into the aircraft automatics and provides inputs to the autopilot. In a modern FMS a cost index number, which represents what kind of fuel economy the airline wants to operate on, can be entered by the pilots. The FMS will then schedule the autopilot and autothrottle to fly a flight profile (altitudes and speeds etc) that will match the selected cost index.
Missed the edit window.
Disclaimer for the above: I’m not an airline pilot or flight operations person.
Also, remember that at elevation, the speed of sound is slower. Don’t have the numbers in front of me, but it’s definitely below 600 knots at cruising elevation … don’t want to go that fast, I was in a commercial airliner once that tried to break the sound barrier. The “mach buffet” on the wings made the ugliest noise I’ve ever heard …
w.
The speed of sound is related to temperature and therefore indirectly to altitude. As you point out, it is slower at high altitudes (and colder temperatures.) Assuming a standard temperature of -56 C, at an altitude greater than 36000’, the speed of sound is about 575 kts. (Speed of sound = (sqrt temp in degrees kelvin) * 39.)
Another factor is that the aircraft’s wings accelerate the air flow over them and so even at speeds well below Mach 1 there will be areas of local airflow that are supersonic. Therefore an aircraft experiences mach buffet well before the speed of sound. In fact, the whole purpose of swept wings is to reduce the acceleration of air and therefore delay the onset of shockwaves and their associated effects.
At any rate, an airliner is restricted by the speed of sound at altitude more because it can fly faster up there than because the speed of sound reduces.
One more thing. In some controlled airspace and at some altitudes (below 10,000’ in Australia) there are speed restrictions (e.g., 250 kts indicated airspeed below 10,000’.) There may not be police but there are ATCos with radar screens who would have an idea that you were flying excessively fast.
Airlines have schedules to meet. Passengers want them to be on-time, and the Govt publishes rankings of on-time performance of the airlines. So they really try to be on-time.
Scheduling a flight to travel at less than the fastest speed of the airplane allows them to make up time when needed. If they are delayed taking off, or by bad weather on part of the route, they can increase speed a bit and still reach their destination on time. Having that option available is very useful for an airline. (Plus the benefits of better fuel mileage that were mentioned.)
yup. IANAA(nything) but I fly a lot. Often when a flight is delayed, for whatever reason, the captain comes on the PA and says that they have been given a green light to make up the time and the flight will still land on time. That is always good news for us passengers
Simple answer: air resistance increases as the square of the speed. So if you increase the speed from 500mph to 600mph, the drag the engines have to overcome increases by nearly 50%.
(600[sup]2[/sup] / 500[sup]2[/sup] = 1.44, i.e. there is 1.44 times as much drag.)
The old racing joke applies to airplanes, too.
“Speed costs money. How fast do you want to spend?” (Another version says, “How fast do you want to go?”) 
I once heard that between LA and the Bay Area, the air routes were like a highway–with bad traffic!–and that planes often had to wait, or fly slow, just as cars on the ground do.
Does anyone know if this still happens? Flying used to be a great way to travel from L.A. to San Francisco, back when getting on a plane was almost as easy as getting on a bus, but now, with two-hour security holdups, air travel is a lot less convenient.
The jet stream, or any wind for that matter, only affects the speed over the ground - it has nothing to do with any of the rated speeds (speed of the aircraft through the air).
For example, if the jet stream was going east at 100 mph and you were flying west at your best economy setting, your track over the ground would actually be 100 mph less than your airspeed indicator shows, but if you were going east in the stream, you’d track 100 mph faster across the ground. However, the airplane only “feels” its speed through the air, not the speed of the air itself.
It’s been too many years since my flight training - I should be able to explain this better.
I vote for fuel economy. The total drag curve is concave upward with a minimum in the vicinity of 400 mph. The speed for max range is around 500 mph. Flying at the minimum drag speed of 400 mph gives you the least fuel uses/flight hour. However flying at the max range speed gives you the least fuel used/mile of travel. Flying faster than the max range speed greatly increases the fuel used/mile because, as was mentioned above the induced drag increases approximately as the square of the speed. For best fuel economy you want to fly at the speed that gives you the most miles for each gallon of fuel used.
No, that’s fine. Makes perfect sense.