We're not going anywhere in our airliner without fuel - lots of it. Until someone builds a practical electric aircraft, probably quite a few years down the road, we have to burn hydrocarbons to make this thing go.
So let's take a look at what we're burning and how we get it to the engines.
In the airlines we mostly burn what is called Jet A. It's essentially kerosene with various chemicals added to help prevent ice buildup, prevent corrosion and to keep it from gumming up the fuel system.
There are some other types of jet fuels, mostly used by the military:
Jet A-1 is very similar to Jet A but can handle colder temperatures.
Jet B is a naphtha based fuel used in very cold climates.
JP-4 was a mixture of kerosene and gasoline. The Air Force used this stuff for years. It had a very low flash point, meaning it was very explodey. Glad they don't use it any more.
JP-5 is used by the Navy. Has a relatively high flash point. This is very important because fire on a ship is your worst nightmare.
JP-7 was used by the SR-71. It had a very high flash point because of the very high temperatures encountered at Mach 3. No longer in use.
JP-8 This is what the Air Force switched to after getting rid of JP-4. This stuff is essentially the same as Jet A.
So what's it cost? It's a commodity and the price fluctuates from day to day and location to location. As of today the spot price was $2.80/gallon.
We purchase fuel by the gallon but what we really care about is how much it weighs. Jet A weighs roughly 6.7 pounds per gallon. That adds up quick. For every hour of flight in a 757 we'll need roughly 8000 pounds or a bit over 1000 gallons. If we're going a long way, by the time we add in fuel to get to an alternate plus reserves we might be carrying 50,000 pounds of fuel.
That's why we don't carry a bunch of extra gas around. We'd be heavier and have to burn even more fuel just to carry the extra fuel. We carry enough to get from Point A to Point B, and maybe Point C if the weather is bad at Point B - plus 45 minutes of reserve. That was a big adjustment after flying Air Force tankers that usually had enough gas to divert anywhere in the lower 48.
The fuel tanks are normally in the wings and the lower fuselage. For example, the 757 has a fairly simple fuel system with a single tank in each wing plus a center tank between the wings.
Once the fuel gets to the engine it passes through a filter and then through a fuel-oil heat exchanger. The fuel keeps the engines oil cool and the oil warms up the fuel. The fuel then goes through an engine-driven fuel pump and into the fuel control unit (like your car's fuel injection).
Larger aircraft may have a few more fuel tanks. Let's look at an Airbus A300. Not the best picture I realize.
Most airliners are normally refueled from a central point. On a 757 there's a refueling panel on the underside of the right wing. A tanker truck will hook up there and put the specified fuel quantity in the desired tanks. During our pre-flight we make sure that the proper amount of fuel was pumped into the correct tanks. That's obviously not something we leave to chance. Otherwise you could end up like these guys:
From the cockpit this is what we see. This shows a 757-200 with full tanks, 14,600 pounds in each wing plus 46,200 pounds in the center tank. It also shows the temperature of the fuel, which can become important on long flights at high altitude where the temperatures are very cold.
75,000 pounds would let us fly a lot longer than I want to sit in the thing
So let's take a look at the fuel control panel. Like everything else on the 757 it's pretty simple and almost pilot proof.
If we have fuel in the center tank, we turn its pumps on as well. The center tank pumps are called override pumps because they put out more pressure than the boost pumps. The center tank fuel will push its way to the "front of the line" and get burned before the wing tanks.
This way there is always positive fuel pressure. Once the center tank runs dry the pumps will shut off automatically but since the wing tanks are still pumping the engines (and pilots) stay happy.
Normally our fuel panel should look something like this. No lights illuminated means all is well. Mister Boeing likes a dark cockpit.
The plane's computer, or FMS, also tries to keep us honest. When we program our route of flight into the computer we tell it how much fuel we want to get there with. In flight, if it thinks we're going to get there with less fuel than that it will "yell" at us with an INSUFFICIENT FUEL message.
The 757 fuel system is mostly "set it and forget it". Normally all we have to do is turn off the center tank pumps when the tank runs dry. The pumps will have actually shut themselves off automatically, we're really just repositioning the switch to keep them from cycling on and off (there might be a few gallons of fuel sloshing around the tank).
About the only action we may have to take is cross-feeding. Let's suppose one engine is burning a bit more fuel than the other. Over a long flight we'd end up with more fuel in one wing tank than the other. To fix this we can temporarily run both engines from the wing tank that has more fuel.
Cross-feeding is a very simple operation, but if you screw it up you can make matters worse by starving an engine of fuel (bad). The main thing to remember is:
ALWAYS TURN SOMETHING "ON" BEFORE YOU TURN SOMETHING "OFF".
So let's say the right wing tank has more fuel than the left and we want to fix it.
1. Open the cross-feed valve (Turn something on)
2. Turn off the boost pumps in the left wing tank (the one with less fuel)
Now the right tank pumps are feeding both engines. They're quite powerful enough to do that as long as both of them are working. If they're not both working we've got other issues to deal with.
Left pumps are off
Center pumps would probably be off at this point
Once the fuel is balanced we:
1. Turn the left boost pumps on (Always turn something on first)
2. Close the cross-feed valve (Turn something off last)
Sounds pretty simple but at 5:00 AM at the end of a long shift there aren't a whole lot of extra brain cells to go around. At that point I'm down to "Mongo turn something on before Mongo turn something off".
OK, the 757 fuel panel is pretty simple. Let's take a look at something a little fancier like the Airbus A300. It's got a few more bells and whistles but it's also fairly automatic. If everything is working properly it takes care of itself.
The Boeing philosophy, at least on the 757, seems to have been "You don't need to know that. Just turn it on and if a light comes on go to the checklist."
The A300 fuel system worked pretty well most of the time. Occasionally one of the 1970s vintage mechanical relays would stick but it wasn't a big deal normally.
Want to see something really scary? Here's the 727 fuel panel.
I haven't talked about fuel dumping yet. Some planes are capable of dumping fuel in an emergency to lighten the load. The A300 didn't have fuel dump capability and neither does the 757-200. They don't need it. They have plenty of thrust on one engine and can be landed at their maximum weight if need be.
The 727 could dump fuel, and if you lost two engines you would be dumping gas in a hurry because it didn't fly very well on one engine.
Now you might be wondering about the environment impact. At high altitude, it will dissipate in the air and unburned hydrocarbons are not as bad as burned hydrocarbons.
At low altitude, well what can I say. It's better for the planet than crashing a 727 into it.
So what can go wrong with the fuel system?
A leak of course. We'll know we have a leak when we see one tank noticeably lower or when our fuel usage noticeably exceeds what we expected to use. On long flights we make a point of checking actual fuel vs the flight plan at least once per hour. If we detect a leak we're going to land.
Losing a boost pump is no big deal. We've got two in each tank and one is sufficient. If you've learned anything by now it's that we like redundancy. Lose both? Still not a problem. We can cross-feed from the other wing and keep the engine happy.
Lose all the boost pumps? Yeah, could happen if we lost all the generators (the boost pumps run on AC). The engines should still be able to suction feed with the engine-driven fuel pumps. We may have to slow down or go down to keep the engines happy but they should keep running. That's one advantage to having the engines mounted on pods below the wing - gravity is helping get that fuel to the engine.
Another possible problem is a stuck valve. Not much of a problem on the 757 with its simplistic fuel system. On a bigger plane with more fuel tanks it's possible you could end up with "trapped fuel" - you've got fuel in a tank but you can't get at it because the valve won't open. Not something I'd want to learn halfway across the Pacific. That's why when flying KC-135s I always made sure all the tanks would feed prior to heading out over the water.
Of course we all want to save fuel these days. Fuel is a major expense for an airline. We try to fly at our most efficient speed and altitude. We try to fly the most direct route to where we're going. We don't carry as much extra fuel as we used to in order to save weight. We'll even taxi with just one engine running and start the other one just before takeoff.
So how energy efficient are jets anyway? Turns out they're not too bad if used properly. This will take a little math, so bear with me here.
Let's suppose you're going to take a 400 mile trip in your SUV that gets 15 mpg. If you're the only person in the vehicle you're getting 15 passenger-miles to the gallon.
OK, let's look at the 757. On a 400 mile flight it will burn 8000 pounds of fuel, at 6.75 pounds per gallon that's 1185 gallons. This works out to .3375 miles per gallon. Not very good. Obviously a lot worse even than your Ford Canyonero.
Ah, but we can cram 239 people into the 757 (and believe me they'll cram them in today). This gives us 80 passenger-miles to the gallon. So the guy driving 400 miles by himself in the SUV would actually do a lot better fuel wise by getting on a jet.
Of course I'm cheating a bit here. I'm giving the jet a full passenger load and only partly loading the car. But when did you last get on a jet that wasn't fully loaded? I'd wager sometime around 1983.
Now if our SUV driver has his wife and two kids with him, he's getting 60 (4 x 15) passenger-miles to the gallon so the jet still beats him.
Ah, but you're not one of those wasteful SUV drivers. You have a Prius. OK, let's say you get 40 highway mpg. If you're driving by yourself, the jet easily beats your 40 passenger-miles to the gallon. You'd have to at least take your spouse along to tie the jet and you'd need one more person to come out ahead.
Now I've ridden in a Prius (they use them as cabs up in Winnipeg). Not sure I'd want to spend 400 miles in the back seat of one. Of course, compared to the average airline seat today I'd say it's a wash.
My whole point being, on a longer trip a jet is fairly efficient if you fill it up.
So what's the future?
I don't really know. The next generation of engines (geared turbofans) will be even more efficient. They're also currently testing bio-fuels in aircraft and they seem to burn them quite happily. Don't know if we could ever produce bio-fuel in sufficient quantities but that's a whole different argument.
Electrics? I suppose you could spin the fans with electric motors but I don't know if you could get enough energy density out of batteries to go 500 mph for 500 miles. I don't see this any time in the near future. Right now anything powered by electrics is in the ultra-light to light aircraft range.
I suspect we'll be flying around burning hydrocarbons for a few more years. At least we're not flying the smoke-belching gas-guzzling turbojets of yesteryear.
You really don't want to see the fuel panel on this thing.