A reader asked, “I heard Chinese and Russian air space uses the metric system while most other countries use the imperial system of measurement. Is it alot of trouble converting the different system of measurement? I remember a Air Canada 767 having to make an emergency landing due to running out of fuel from wrong metric conversion. That won’t happen with Cathay?” — Tomcat1

Two great questions that I wanted to answer in my blog, one at a time. The first issue deals with metric flight levels and the second issue deals with metric fuel volume. I’ll answer the fuel question below, and the flight level question is addressed in the previous blog entry here.

Fuel is always an important topic of conversation on the flight deck of today’s commercial airliners. We always want to have enough of that stuff out in the wings while we’re airborne. Fuel can be put onboard an aircraft in several different ways: By weight in pounds or in kilograms, or by Volume, in gallons, liters, or imperial gallons. Many regional airlines in the U.S. don’t have to worry much about this, as all fuel in the U.S. is sold by weight in pounds with the less important amount of gallons somewhere on the fuel receipt. Problems can arise at international carriers, where they visit countries that use different units of measurement. An example would be like one of my flights that starts out in the U.S. where gallons are used, and then travels to Hong Kong, where liters are used and kilos are on the receipt. Getting these figures backwards or messed up can lead to serious consequences.

To keep all this straight, we have a fuel order form that we fill out once we’ve decided how much fuel to put on. Both pilots check the fuel slip before we give it to the fueler. Cathay pilots always order fuel in kilos and the trained, approved aircraft fuelers know this. Once the fueler has put the required fuel on board, he takes the fuel slip we gave him and writes down how many gallons or liters he put on, by taking the reading off his truck.

This is a picture of an excerpt of our fuel slip and shows the math formula we use to double check the fueler’s job. If he fuels us in gallons, he writes the amount of gallons in the gallons section. If he fuels us with liters, he places those digits in the liters section. We multiply the amount of liters times the *specific gravity* of the fuel (fuel changes density depending on its temperature, so the fueler, who knows the specific gravity, gives us that number, something like 0.794) to get our uploaded amount in kilograms. If he used gallons, we first take that value and multiply by 3.785 to convert the gallons to liters, and then multiply by the specific gravity. We end up with the kilos we have in our tanks.

Both pilots check the math, and finally, we check the fuel gauge to see if it agrees with what we ordered. If the fuel slip math and the gauges agree, we are good to go. If the fueler put in wrong numbers or put the numbers on the wrong line, we’d catch the error by crosschecking our math numbers versus the numbers we are seeing on our fuel gauge.

In a famous accident in Gimli, Manitoba, Canada, Air Canada crashed a 767 onto a deserted runway because they got their fuel order wrong. They messed up the whole liters and gallons math. Normally, this would be no big deal, as their math error would be caught by crosschecking the fuel gauge. The trouble that day was that their fuel gauges were not working, and they were using a procedure where fuel was uploaded based on a previous known quantity. This is a perfectly acceptable method, but if it is used, the math better be right! You can read more about that accident here, and the amazing way the captain, a glider pilot as well, glided the aircraft to an injury free landing.

Another famous accident involving fuel, or the lack thereof, was an Air Transat Airbus A330. They developed a fuel leak and didn’t catch it in time and ended up also doing an amazing job to glide the aircraft down to a landing in the Azores. You can read more about that accident here.

We’ve learned from other carrier’s mistakes, and our procedures at Cathay help mitigate fuel problems caused by leaks because we check the fuel once an hour. The flight management computer, or FMC, gives us two readings on fuel: the amount we are calculated to have based on fuel burn, and the totalized amount from fuel sensors in the tanks. If the calculated value and the totalized value differ by a certain margin, we have procedures that help us figure out what the problem is, and how to solve it.

Ultimately, mistakes can and do happen because we are all human and machinery breaks down occasionally. Usually, mistakes come from carelessness or fatigue when running the numbers. Thankfully, fuel issues rarely occur and Cathay has made it a strong focal point to make sure we check and recheck our numbers when it comes to fuel.