Fuel economy

[radeng]

Once you're up at 30,000+, is there any fuel economy by going slower? For example, if you increased journey time trans-Atlantic by say an hour, would you save a worthwhile amount of fuel on a 747 from LA?

Also, as you get higher, does the air speed need to increase to prevent you losing altitude? As the air is thinner, it would seem logical, but that doesn't mean it happens!

[AerocatS2A]

At any altitude and weight there is a particular speed that will give you best fuel economy. Go faster and you use more fuel, go slower and you also use more fuel. But passenger flying isn't just about saving money through fuel economy, you also need to be on time and balance fuel economy with maintenance requirements, crew duty times and so on. Flying at max range cruise may not be the best option.

As to your second question, there are two types of airspeed that are relevant. True airspeed (TAS) is the actual speed of the aeroplane through the air and indicated airspeed (IAS) is what the airspeed indicator says. The higher you go the bigger the split between true and indicated airspeed. I'm a lowly turbo-prop pilot but using one of those as an example, at 25,000 feet we might have an indicated airspeed of 160 knots but a true airspeed of 240 knots. At sea level true airspeed is the same as indicated airspeed.

Indicated airspeed measures the dynamic air pressure (that is, air pressure due to the forward movement of the aeroplane) entering a small tube normally positioned somewhere around the nose of the aeroplane. Because it measures air pressure, it automatically accounts for the changes in density with altitude.

The upshot of that is that the stall speed remains roughly the same in terms of indicated airspeed regardless of altitude, but you are quite right when you say that the aeroplane needs to fly faster the higher it goes, and that is reflected in the higher true airspeed that you get from a given indicated airspeed at higher altitudes.

I hope that makes sense.

[radeng]

Thanks aerocat.

I thought that as far as duty hours were concerned, airlines wanted the maximum they could possibly screw out of the crew!

[AerocatS2A]

Well sure, and they will try to "roster efficiently", that is, get the most out of the crews :ok:, but there's only so much they can get. If a crew runs out of duty time in flight, it can result in a landing at an alternate destination, flying crews in from elsewhere to take over the flight, flying the original crew back home as passengers on another flight, extra hotel accommodation for the crew and passengers, bad publicity, etc. The point is that fuel economy is only part of the equation and there comes a time where going slower is not going to be cheaper or better for the company even if it does mean getting slightly better fuel economy.