Flight were beginning to take to the skies again after days of air space closures due to ash from an erupting volcano in Iceland.
Here are questions and answers about flying in volcanic ash condition:
Q: Why don't the airlines just fly below the ash?
A: Ash from the main plume, which has varied in altitude from 20,000 to 32,000 feet, is gradually falling to the ground, making flight beneath the clouds somewhat risky.
Q: Why don't the airlines on trans-Atlantic flights fly south to avoid the ash?
A: Because that would be much longer and would require far more fuel. The shortest route across the Atlantic is over the northern Atlantic — where the highest concentration of ash is currently located. Some airliners don't have the range to fly longer distances with a full load of passengers.
Q: What is a safe altitude for airline flights?
A: Anything above 35,000 feet is considered absolutely safe because the ash plume never exceeded that altitude.
Q: What is the standard operating procedure if pilots fly into an ash cloud?
A: Crews are instructed to do an immediate 180 degree descending turn, while throttling back to idle to reduce engine temperatures and decrease the likelihood of damage. Efforts to apply full thrust and quickly climb up above the cloud are likely to end in engine failure because of the sharp increase in temperature.
Q: Why are jet engines so susceptible to damage caused by ash ingestion?
A: Unlike ice and other contaminants that jet engines routinely suck in, volcanic ash can cause serious damage because of the way the intense engine heat interacts with the particles.
The ash adheres to the engine's interior parts, such as the combustion chamber or the turbines, where it melts to form a glassy coating that restricts air flow. It also can clog the rows of tiny cooling holes on the jet's fan blades and cause other damage, leading to overheating and eventual engine failure.
Q: How are jet engines inspected for possible ash damage?
A: The easiest and least intrusive way of monitoring the engine's condition is to do an oil analysis. Borescopes — flexible optical devices usually mounted with a camera — are used to inspect hard-to-reach areas such as the combustion chamber and compressor stages.
The most expensive method is to completely dismantle the engine and strip the combustion chamber and compressors, where most contaminant damage occurs.
Q: How reliable are jet engines in general?
A: Despite their complexity, modern jet engines are considered extremely reliable, with a very low probability of failure and with extremely long periods between regular maintenance checks. Safety statistics show that the average airline pilot will encounter just 0.7 in-flight engine shutdowns during a normal 40-year career.
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