As Avianca Flight 52 circled above Kennedy International Airport on Thursday evening, Matias Moyano Rojas, the flight engineer on the Boeing 707, had to rely on fuel gauges that pilots said are often imprecise.
The cockpit fuel gauges are what investigators have based their calculations on to estimate that the plane had 1,300 gallons of fuel left. But they are still uncertain whether the gauges were accurate. Improper calibration, temperature and even an algae that grows in planes' fuel tanks can cause a gauge to err by more than 150 gallons.
The 707 was 23 years old, and pilots said the gauges on planes of that age are slightly less accurate than those on newer jets. Many airlines account for the possibility of gauge errors when calculating how much fuel to load.
Two Methods to Measure Fuel
Fuel quantity is measured by weight by electrical probes inside each of the 707's seven fuel tanks, two of which hold reserve supplies. The probes are supposed to compensate for the weights of different types of fuel, but they need to be calibrated periodically.
Investigators will look into how the fuel was measured when it was loaded in Colombia and whether the crew relied on the cockpit gauges or used a more accurate method, in which a hollow tube is inserted into the tanks to determine the fuel level. A leak in the fuel system may have thrown off the crew's original fuel calculations.
A type of algae often grows in the fuel tanks of jet planes, clogging the electric probes. Airlines periodically flush the tanks with chemicals to remove the algae, which is more likely to grow in warmer climates.
Unlike newer planes, the 707 has no warning lights to indicate that the plane is running critically low on fuel. The first warning light seen by the flight engineer would have been a low-oil-pressure light. But by then the engine would have begun to flame out.
The fuel gauges on newer planes, like the Boeing 767 and 747-400, have been refined. Unlike the older jets' mechanical gauges, the newest cockpit gauges show the fuel level on a video display, which is slighty less prone to error.
On newer jets, computers monitor the fuel tanks and direct the flow among them. But on older jets flight engineers calculate fuel use hourly.
Flight engineers like the 45-year-old Mr. Moyano, who died in the crash, periodically switch tanks from which the engines are fed, to keep the plane's wings balanced. When the tanks are nearly empty, some fuel - as much as 150 gallons in each tank - is unusable because it is below the level of the pumps that feed the fuel to the engines.
When Flight 52 circled for at least an hour, it burned much more fuel than it had at the higher cruise altitudes, where the engines are more efficient.
