Assuming optimum flying conditions (clear visibility, no wind, no other nearby air traffic, etc.) are jets usually flown at their maximum possible speed, or could they be flown faster? Is there a regulatory maximum speed at which a conventional jet must be flown because of safety or fuel usage concerns? Since most cars can be driven faster than what regulations allow I was curious if the same applies to commercial aircraft.
As no one else is biting, I’ll try this one. I’m not an airline pilot, but I do hold a commercial ticket for flying small planes. I’ve never flown a jet.
Having said that, yes, all airplanes are subject to airspeed limitations depending upon their design. There are also optimum speeds at which maximum economy will be achieved, and commercial jets are generally flown in this fashion.
The 747-400 for example, is optimized for flight at .85 Mach (Understanding Flight, Anderson & Eberhardt, 2001, p160). Flying above or below that speed generally results in less efficiency, so they keep it around there from what I understand. There is also a technique called “step climbing” which utilizes altitude changes to attain greater efficiency. Pilot141 will hopefully explain that one when he checks in here.
There is a maximum regulatory speed in the US below 10,000’, which is 250 knots. That can be waived by ATC under certain circumstances, and by the military.
A plane that exceeds its design limits courts structural failure. On the small planes I fly the airspeed indicator is color coded to show various airspeed realms. One that isn’t marked is called “Maneuvering Speed”. This is a speed to stay at or below (at gross weight) in rough air. You can safely exceed this speed in smooth air, and I’ve done so many times. There is also a red line which marks the “Never Exceed Speed”. There is a fudge factor built in, but I consider this speed inviolate.
It seems like I always do this, but: Sorry for the late reply, I just got back from a trip!
For the OP, the answer is no, jets are not usually flown at their maximum speed. At cruise you fly optimum speed, which is less than maximum. How much less depends on many factors, the biggest being airframe design. Going faster than optimum speed burns extra gas.
How much faster could they go? Not a whole lot faster, if you’re using your car as a comparison. Let’s say your car is capable of going 120 mph, but the speed limit is 65 mph. While cruising along (and obeying the law) you are cruising at 54% of your car’s maximum speed capability. Now take an average airliner: it might cruise at .80 Mach and have a MMO (Max Mach Operating) of .86 Mach. This means that the airliner cruises at 93% of it’s maximum speed. So yes, it could go faster but not twice as fast or anything like that. Above 10,000 feet the only speed limit is your jet’s design limit and your fuel available. Most jets climb, cruise and descend as near to the speed limit as possible while staying fuel efficient. Going fast IS the point of a jet, isn’t it?
As for the step climb Gassendi mentioned, it is (once again) done to save fuel. Jet engines are more efficient at higher altitudes, so you always try to climb to the highest possible altitude (this is why on a short flight from Dallas to Austin or New York to Washington you will climb, level off and almost immediately descend: that is the most fuel-efficient profile). Sometimes the airplane is too heavy to climb above a certain altitude (say FL 310). So you cruise at 310 until you burn enough gas (ie reduce your weight enough) to be able to climb to the next altitude. When you can climb to the next higher altitude (FL 350), you do so. If you fly even farther you can climb up to FL 390. It’s called a “step climb” because if you look at the flight profile from the side it looks like a series of steps.
Hope this helps!
Guess it’s time for you to get a laptop with Internet access so you can continue to keep the rest of us informed whether you’re traveling or not! 
Seeing as how I’ve 86ed two laptops in the last four years, I’ll stick with my safe, under the desk computer. 
It is protected from spills (scratch one laptop) and hotel power surges (scratch number two). Although almost every hotel has a business center now, I’ve only used them occassionally. I promise to try to use the hotel computers more often, and at least to do a search of the SDMB!
pilot, you should like this one…
Thanks omni-not ! I’d heard the audio portion of that before, but hadn’t seen it with the “video”.
Fly safe!
So if the FAA caught an aircraft going past its design speed (Vne, or similar) on purpose, could that pilot get in trouble?
An aircraft flying straight & level shouldn’t, by design, be exceeding Vne. In order to do that, for most aircraft, you’d have to be in a steep, possibly unrecoverable dive. By that time, you’d probably have radioed an emergency, or the FAA (more likely ATC) could tell there was an aircraft in distress.
Short answer: no, the FAA aren’t traffic cops. “Yew know how fast yew were goin’ up ‘ere? Yer in a heap o’ trouble, boy”
They do get kinda pissy about people breaking most other regs, like violating airspace, false emergencies, and whatnot. For some reason, can’t get the FAA pages up tonight.
As mentioned earlier, the only speed limit the FAA always cares about is 250 knots below 10,000 feet - and there are exceptions to that rule. There really is only one reason for ATC to care about how fast you go - and that is aircraft separation. While cruising at altitude and a conflict develops (ie you are overtaking another aircraft) ATC might ask you to maintain a certain speed to keep proper separation.
What happens if you speed? First, you will get questioned on your speed when ATC notices an overtake. If you still don’t correct it, ATC will vector you away from the other aircraft and ask again about your speed. By this time you are probably looking at getting violated. A deliberate violation like this will result in the suspension of your license. No license = no flying (which = no pay for us airline types).
So, if ATC assigns you a speed to fly, you fly it. Period.
Other than assigned speeds for separation, ATC doesn’t care one whit about your aircraft’s Vne or anything else - they figure that you are responsible enough to not tear your airplane apart by flying faster than the design speed.
If you exceed Vne the FAA is not your biggest problem
As mentioned before, there is a small fudge factor in there, but if you exceed Vne by a significant amount your control surfaces might lock up, rendering the plane uncontrollable, or “structural damage” may occur, which is a nice way of saying parts will get bent or even start falling off. Or maybe your airplane will simply fold up like an umbrella turning inside-out. When the manual says never exceed this airspeed believe it.
Pilot141 covered the FAA aspect of exceeding the speed limits.
As for other designated “V speeds” - disobeying them will either give you less than perfect performance, or may cause you actual problems with the airplane. For example, exceeding a certain speed in rough air won’t get you trouble with the FAA, but it could result in damage to the airplane (like popped rivets, among other things) due to the effects of turbulence. Flying too slow can also result in loss of lifting force, which means you might not have enough to hold you in the air, which is why, in the regs, when they describe the speed limits under 10,000 feet they give an exception for safety reasons - if your particular airplane needs to exceed 250 knots in order to perform a necessary manuver then you can do so (pretty rare, I’d imagine). Vx is the best angle of climb speed - if you fly at any other speed that angle will be less. The penalty for not flying that speed when asked for (other than criticism for being a sloppy pilot) may be that you don’t clear an obstacle near the end of a runway - again, it’s physics catching up with you (two solid objects can not safely occupy the same space-time coordinates), not the FAA.