Something on Efficiency

Watching the fuel flow gauges the other day while enroute to north eastern BC, I wondered where, in a world where most everyone is becoming more environmentally conscious, the aircraft that I fly fit in. A glance at the flight plan showed that over the course of our day flying, we would burn approximately 2700 Lbs of fuel, or about 1500 litres. Compared with the typical car, those numbers seem to be disproportionally high. Relatively speaking, for every hour of operation, the Beech 1900 burns about enough gas to fill the tank of my Jetta 7 and a half times. Of course, trying to fit 19 passengers into my Jetta and then driving them 450 nm in an hour and a half, and I would find myself coming up a little short on a way to accomplish this. It makes sense then, that to make any useful comparison between the two, those numbers must be brought down to a common denominator. In reports on vehicle efficiency, it seems as though Litres of fuel consumed per passenger per kilometer is the favored common denominator for making comparisons of this type. For curiosity's sake I have also dug up the performance numbers for the Dash 8 200, CRJ 200, Boeing 777 200, 747 200, and the DC-10 30. Some of the numbers which I arrived at surprised me, others were much as I suspected.

On the highway, my Jetta burns on average about 7 L of fuel for every hour of driving. Commuting around the city, with it's inherent multiple starts and stops the fuel burns get pushed up to somewhere around 10 L for ever hour driven. To come up with a number that roughly averages out highway and city driving, I estimated 8.5 L of fuel consumed per hour. Since most aircraft performance numbers are cited in terms of fuel burned in pounds per hour, I converted my average Jetta fuel burn of 8.5 litres, to 13.6 pounds of fuel burned per hour.

Next 13.6 pounds per hour fuel burn is divided by the 4 passengers my car will hold, to come up with a figure of 3.4 Lbs. of fuel consumed every hour for each passenger on board. This is of course often not the case, as most of the time my car has a maximum of two people in it, but in order to determine the maximum efficiency my car could attain, I assumed a full car load. Next I divided the 3.4 Lbs. of fuel that the Jetta consumes for each passenger by the average speed which I drive, which works out to about 75 km/hour. Now we have a figure of .045 pounds of fuel, or .028 L consumed for every kilometer for each passenger in my car. Stacked against the aircraft I could find performance numbers on, the Jetta fared quite well when full. Take out three passengers though, and the Jetta's fuel burned per kilometer per passenger jumps up to .11L. Driving style has a huge impact on efficiency as well. With my mild mannered grandmother like driving, I can average about 550 km before my 55L fuel tank runs dry. Throw in a few afternoons worth of hard driving, and you can watch your mileage head south proportionally.

The Beech 1900 can hold 19 passengers, 21 people including the crew, will fly at 463 km/h, and burns about 750 pounds of fuel per hour for an average 400 nm trip. Longer flights will increase the efficiency as fuel flows at higher altitudes drop significantly. This helps to offset the fuel consumed to climb the aircraft up to the higher altitude. On shorter flights, often the fuel savings realised at higher altitudes does not justify the amount of fuel required to get the aircraft up to that given altitude. To get around the impact that flights of different lenghts or at altitudes can have on the overall fuel efficiency, I have simply used cruise fuel flows. For those who are interested in determining the efficiency of the overall flight, adding about 7% to enroute fuel burns for taxi, take-off, climb and approach seems to get you in the ballpark.

With a fuel flow of 700 pph, and 21 people on board, the 1900 is burning 33.3 pounds of fuel per hour per person. With a cruise speed of 463 km/h, we will be consuming .071 pounds or .039 L of fuel for each kilometer traveled, for every person on board. This puts the 1900 into the same fuel efficiency category as an Impala sized car in the city, or about 11L/100 km.

The Dash 8 200, DC-10-30 and 747-200 came in with .031, .034 and .037 litres of fuel burned per passenger per kilometer respectively, which would put them all into the realm of

somewhere around the efficiency of a mid-size car.

The CRJ-200 came in at the top of the list with .041 which works out to about the equivalent of 13 L/100km. That puts the RJ somewhere close to what a Ford F-150 will achieve on the highway.

The Boeing 777-200 surprised me the most with it consuming .022 Litres of fuel/km/passenger. Put into automotive terms, it works out to about 6.7 L/100 km, which is almost exactly what a 2009 Toyota Corolla is advertised to achieve on the highway.

To a certain degree, how efficient a vehicle is, is not only dependant on the vehicle design itself, but also on how it is operated. Much like refraining from racing from one set of lights to the next in a car, there are steps which can be taken to reduce fuel consumption in aircraft as well. In the wake of higher fuel prices last summer, many companies, including the one with which I am employed, implemented a reduced fuel burn program, where crews would reduce their cruise speed and correspondingly, reduce how much fuel was consumed on a given flight. The result gave us a average cruise speed which was reduced by 2%, while we saw a reduction in fuel consumption of 12.5%. Not a bad trade off really. A number of years ago several tests were conducted to find the "optimum speed" which would allow passenger cars to travel the furthest distance on the least quantity of fuel. It was discovered that somewhere around 90 km/hr most passenger cars efficiency peaked. Driving any faster, or slower than this speed, would result in more fuel being consumed for a given distance traveled. Environmentally speaking, the damage that may be caused to the world around us by going faster is really quite intangible. Even for those who are concerned about the effect of their actions on the environment, driving 110 on the highway instead of the more efficient 90 has no immediate impact that they can readily see aside from that they will arrive at their destination a few minutes earlier. Even climatologists cannot agree on what impact specifically consuming more fuel will have on the world around us.

At the end of the day, regardless of whether or not I am actually helping the environment, I will aim to drive my car, and while I am at work, fly planes in the most efficient and safest way I can. If it turns out that I helped out in some small way, all the better. If it is discovered that global warming was nothing more than an inevitable upward swing in the temperature of our planet's atmosphere, then at the very least I can smile in knowing that with each time I filled the fuel tank in my car, I saved about enough money to buy myself a popsicle. With the extra time it is going to take me to get to where ever I am heading to, I will also have a little extra time to savour it.

For those who have made it all the way through this post,.