I have been getting very impatient on the flights, when time passes ever so slowly, mainly when I'm flying to the Mrs. I keep checking the time and the map every few minutes and when I notice the speed at which the bird zooms I always wonder, "why are we only cruising at 420 mph ground speed?! The plane is capable of doing over 500... Then why go slower than that??"
I would think going top speed or just below top speed as a flight would be ok flying in open space, thanks to the great job that the controllers do.
Please don't bring in the argument/comparison of cars saying cars of capable of 120,then why do you just drive 65-75. Obviously law and rules and safety.
Are there similar rules to flight paths as our roadways? Do airlines have speed restrictions too? Or is it just to achieve a better fuel economy? Is it something else or a combination of all?
Ever eager to see the glowing face of the beauty awaiting me at the airport
Programs: Qantas WP, LTG , Delta, SPG, Priority Club
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Commercial Flights and top speeds
Two reasons:
1. Ground speed is not the same as air speed. So a plane flying at 500 knots into an 80 knot headwind will have a ground speed of 420 knots.
2. Fuel efficiency. In the same way that flooring the pedal in a car burns a lot more fuel than driving a little slower, the same can be true for planes. So they may run the engines at less than 100% power to burn less fuel. There is probably some calculation they do to get the optimum engine power and speed.
It's routine to hear ATC stating "do not exceed xxx knots" which stops annoying air fender benders..usually on the approach side of congested airspaces on he coasts...
#3 most have a prebooked landing slot.
#4. the airport may not open . early evening flights to lhr can frequently arrive an hour before the airports open.
#5. planes do burn less fuel at lower speeds
#6 as long as they keep their schedule, why would they want to burn fuel to get there early?
#7. the speed shown on your in seat instruments are ground speed. the plane is flying at an air speed. over the atlantic in the winter, the airspeed is frequently 100knts different than ground speed.
#8, they may be taking a short cut, and saving time that way.
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420mph would be quite slow, and I'd be very surprised if you actually observed that number on a regular, repeated basis during cruise at altitude. Unless you only fly one way, against the jet stream, on short flights, on older aircraft.
Efficient cruising speed on most large jets is 510-590mph (older 737s are just under 500, iirc). That is based on engine, aerodynamic, etc., efficiency. Top cruising speed can be 30-40mph faster, but less efficient, burning more fuel per mile.. Vn - do not exceed/max structure design speed is even higher. But as the name implies, you don't want to be there.
Winds can change the ground speed by +/- 200mph or more, but it all tends to even out. I've flown PIT-LAX that has taken as long as 6 hours, and I've come back in under 4.
Speed can also be limited by ATC, as mentioned, but I usually find that occurs for short periods, nearing approach.
flightaware will show the actual speed data - do you have some specific flights we can look at to see what was going on? That would be more accurate than a watch and a map, especially since travel isn't always straightline.
Flight 497 - 7-dec-2012
It barely was keeping up 420-430 range.
Looks like the history has it in the same range, except for a few days around 500 mph...
I'm booking award flights from DUB-BOS. Checking availability on Qantas website, where they list the flight duration on some days as 6h55m, and on other days as 5h55m. Same flight, same time.
I can understand 10-15 minutes difference, but exactly an hour longer/shorter depending on whether you're flying on a Friday or Saturday?
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Quote:
Originally Posted by therealintellectual
Flight 497 - 7-dec-2012
It barely was keeping up 420-430 range.
Looks like the history has it in the same range, except for a few days around 500 mph...
That would fall under the "Unless you only fly one way, against the jet stream," note. Flying west across the US is almost always against the jet stream.
6hrs 1min average westbound, 4hrs 32min eastbound.
Eastbound, ground speed regularly in the 630mph range. The four westbounds I checked were all in the 450mph range. Net out the wind, and that's 540mph air speed across the board, above the documented A320 cruising speed of 511-537. The 320 seems to be one of the slower jets out there at 511.
420mph would be quite slow, and I'd be very surprised if you actually observed that number on a regular, repeated basis during cruise at altitude. Unless you only fly one way, against the jet stream, on short flights, on older aircraft.
Efficient cruising speed on most large jets is 510-590mph (older 737s are just under 500, iirc). That is based on engine, aerodynamic, etc., efficiency. Top cruising speed can be 30-40mph faster, but less efficient, burning more fuel per mile.. Vn - do not exceed/max structure design speed is even higher. But as the name implies, you don't want to be there.
Winds can change the ground speed by +/- 200mph or more, but it all tends to even out. I've flown PIT-LAX that has taken as long as 6 hours, and I've come back in under 4.
Speed can also be limited by ATC, as mentioned, but I usually find that occurs for short periods, nearing approach.
Great explanation.
Generally speaking, flight crews try to fly at cruising speed as much as possible. Cruise on a 747 is at around 70-75% throttle (as fraction of the maximum engine RPM—idle is around 20%). Thus, flying at full throttle would require ~33% more power (and a similar percentage of fuel) while only resulting in an ~8-9% increase in speed.
The autopilot does most of the flying during cruise. The flight crew dials in the airspeed (either on the bank of knobs and displays located on the edge of the sunshield or in the flight computer) at which the autopilot flies at. The ground speed is indicated, but usually not referenced against.
After takeoff and before landing, the aircraft makes adjustments in altitude/speed/heading in increments. At takeoff, the aircraft is at full power (which is why many people compare sound levels to that of an airplane taking off). After takeoff, the aircraft flies faster and higher at the controller's discretion until eventually being allowed to fly at the speed and altitude in the flight plan. The approach is done much the same way, with airplanes very close to the airport flying at speeds of around 180 knots (195 MPH).
One of the benefits of the FAA's next generation ATC system is that airplanes will be able to make smooth transitions in speed and altitude, allowing them to increase fuel efficiency to almost glide into a landing.
Flight 497 - 7-dec-2012
It barely was keeping up 420-430 range.
Looks like the history has it in the same range, except for a few days around 500 mph...
I'm not seeing that. For the last 2h 30min of flight prior to descent, or about half the time at cruising altitude, the plane is in the 460mph-480mph range.
But as others have said, since west-bound flights will encounter headwinds, a 420mph ground speed would be nothing out of the ordinary.
Programs: Qantas WP, LTG , Delta, SPG, Priority Club
Posts: 158
Commercial Flights and top speeds
Flight times (or should i say aircraft airspeeds) were probably slower in the 70s. The longer get times now are due to congestion plus "expectation management": if the 1 hour flight has a scheduled arrival time 1.5 hours from departure, then nobody will complain that it's half an hour late! ;-)
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Quote:
Originally Posted by theassassin
Flight times (or should i say aircraft airspeeds) were probably slower in the 70s. The longer get times now are due to congestion plus "expectation management": if the 1 hour flight has a scheduled arrival time 1.5 hours from departure, then nobody will complain that it's half an hour late! ;-)
That was my initial thought. But surprisingly, a DC-10 cruises at 564-610, faster than almost all the 737-777 and A3x0's I had checked above.
