Airplane speed
 

Just out of curiosity, and because other members might find also it interesting:

Does the typical commercial passenger jet of a given model have a standard cruising speed which it maintains by default unless there's some reason to go faster or slower? Or to put it another way, how much leeway does the plane have to adjust its speed on a given flight segment up or down from the speed it usually maintains, depending on things like weather conditions or whether it's ahead of schedule or behind?

I'm sure some knowledgeable people on this list could comment. Thanks.

I suspect most airlines have a standard en route mach number to maximize fuel economy. Its like driving a car, you can go balls to the wall and operate at the maximum mach number, but you will lose fuel economy. Also ATC will slow you down when its time to decend into crowded airspace. Most airliners cruise around .78-.80 mach with a maximum operating speed probably around .85 mach. I would hope pilots have some leeway with the airline to shave a few minutes off the flight during delay situations.

Suspect pilot leeway limited. We left an hour late on an 8 hour flight and were told that Pilot could not make up even a minute of the lost time. We actually arrive about 58 minutes late and a few people more than a little mad. I had a 2.5 hour connect (now 1.5 hours) and no problem. Why the pilot could not have picked up 4 or 5 minutes per hour, who knows.

Let's do the math. :)

Say the 8 hour flight has a 7 hour cruise at Mach 0.82 at 35,000 feet, or 473 knots. You fly 3300 nautical miles during the 7 hours.

Now, all you're asking for is a tiny change of 4 minutes per hour, or take about 30 minutes off the 3300 nautical mile distance flying time. OK. Now you need to do about 510 knots to fly the same 3300 nautical miles in 6.5 hours, or Mach 0.89, which is impossible.

edit:
I had the wrong units on some numbers: changed to knots for speed and nautical miles for distance

Headwind and/or tailwind also play into the airspeed. I would imagine that the pilots could not exceed the structural limitations of the airplane and 'fly it apart', but I suppose that would be possible in some conditions.

First, unless you're flying a business jet for someone who doesn't care about fuel I doubt you're flying at Mach .82 as a standard cruising speed. You don't have to shave 30 minutes off a 7 hour journey. Sometimes 15 minutes is all it takes to make a connection :cool:

Headwind and tailwind don't play into airspeed. They have an effect on groundspeed. Winds don't have an effect on the aircraft's max airspeed, all that matters is the relative velocity of the air going past the aircraft.

An aircraft flying at 500 mph airspeed with a tailwind on 100 mph is flying at a groundspeed of 600 mph, conversely a 100 mph headwind translates into a groundspeed of 400 mph.

There's a tradeoff of speed versus fuel burn rate. Normally the fuel burn rate goes up with velocity squared, while the distance traveled only goes up linearly with the velocity. However, the fuel rate goes up even more quickly as one approaches mach 0.9 and above. And of course as the fuel burn rate per distance traveled increases, your range decreases. Also, as the plane burns fuel it gets lighter and can go to higher altitude where it has a bit less air resistance and can cruise with a bit more economy.

So, there are certainly airframe limits (but they must be worried about mostly while descending), and there's a range limit, and there's an economy limit. I imagine that the airline company has limits on the speed because of the economy issue, though the pilot may get permission to adjust by 0.02 mach or so, which might make a difference of (0.02/0.82)*(8 hours)=11 minutes...

They can't really make up 50 minutes on an 8 hour flight...

It depends on the circumstances, but yes they can and more.

How much padding is in the schedule. On some routes there is a substantial pad for delays (hold for connecting pax, expected slow taxi and ATC).

Routing and winds (headwind/tailwind) can make a big difference. Being able to route efficiently for the winds, or take a much shorter alternate flight path from the designated one, can shave a lot of time off. Of course the reverse can also add a lot of time.

Direction of takeoff and landing. On some short routes this can double or half the flight time. Even on longer routes, at some airports it can make a material difference (eg SYD, NRT, LHR, etc).

I have been on flights from HKG-JFK that have left 2 hours late and arrived ontime, due to routing and tailwinds. Conversly Ive left JFK in the winter 2 hours late and arrived 12 hours behind schedule with a tech stop in TPE so YMMV literally..

I should have qualified my statement: they can't make up 50 minutes on an 8 hour flight by putting the pedal to the metal (increase in airspeed). But yes, they can if they have favorable winds (more of a tailwind, less of a headwind than normal), or an especially direct departure or approach pattern - but the first is in the hands of God, and the second is in the hands of air traffic control rather than the pilot.

Real Flight times are not just speed and wind related
 

The BA flight from Stockholm to LHR is listed as approx 2h55m. On a recent flight there was a medical emergency and the flight completed in under two hours, gate to gate.

The optimum cruise speed reached by the Boeing 707 in the late 1950s (ie round about Mach 0.8) has been pretty much followed by all subsequent commercial jet aircraft, there are good aerodynamic reasons for this. You can go a bit faster and use a lot more fuel. If you go slower however there is little if any saving to be made. There are a few slight differences (eg the Boeing 747 is most economical at a slightly faster speed then a 777, giving it a few minutes advantage across the Atlantic), but the difference is not great.

Matters like direct routing into/out of airports, getting the optimum altitude or not, etc, are much more significant in setting journey times.

Airlines know that typical journey times are a scatter, varying from day to day. They will typically publish their arrival time such that they attain a 90% on-time figure, or some equivalent value, based on this typical distribution. Different airlines can publish notably different trip times for the same sectors - for example Ryanair make much of their 20-minute turnrounds, but it is unusual for them not to make the trip in well under the timetable allowance and come in several minutes early.

Not only that, but you also have to think that a plane is not alone in the skies. There are things like minimum separation that will keep you in place wether you can fly faster or not.

That most likely has everything to do with schedule padding and favorable tailwinds, little or nothing to do with flying faster. There is no practical way to make up more than a small fraction of 55 minutes on a flight of that length. Fuel consumption rises very quickly as you speed up from a normal cruise speed toward Mach .9 or so, approaching the "sonic barrier" at Mach 1 - now known not to be a true barrier, but still requiring large amounts of power to cross. (Once you cross it, continuing to fly at supersonic speeds takes less.)

That's a big part of why Boeing's Sonic Cruiser concept was eventually ditched in favor of what became the 787: flying a bit faster, where traffic allows, wasn't worth the added fuel consumption. Much better, from the airlines' point of view, to put the same technology improvements into lower fuel consumption at the speeds we fly at today. (Many FT posters got this right before Boeing figured it out.)

Another factor not mentioned yet is that early jet-powered airliners used turbojet engines, whereas more recent ones use turbofans. Turbofans use less fuel for a given amount of thrust, but work best at slightly lower speeds.

That's probably priority handling from ATC.