Why can’t planes fly near volcanic ash? A (very) brief look at engine failure

Nearly a week into the volcanic ash crisis plaguing swaths of Europe, passengers and airlines alike are starting to tire of the restricted airspace. The haunting cloud drifting thousands of feet above Earth’s surface is often invisible to the naked eye both at ground level and high into the reaches of the troposphere, causing many to wonder how this material could impact a flight. Could all of these microscopic particles of ash really be that big of problem?

Yes, and in many ways.

Large volumes of volcanic ash have an obvious effect on flight performance. Any particulate getting into cooling holes will cause the engine pressure and temperature to increase, dropping efficiency and potentially causing serious issues inside of the engine. This failure mechanisms poses an immediate and large threat to aircraft safety and is the primary situation that airlines are trying to avoid.

But even small volumes — parts per million of the material — can have a long-term detrimental effect on engine performance.

Typical engine combustors operate at extremely high temperatures — hot enough to melt most metals — and the materials used in each component are specially designed to withstand this heat. The single-crystal turbine blades used in the fabrication of commercial engines often are exposed to temperatures well over 2500°F, and because of this are coated with a special Thermal Barrier Coating (TBC) to prevent overheating. In short, the TBCs prevent the turbine blades from melting.Part of what helps the TBCs do their job is their microstructure. Instead of being fully crystalline, solid materials like the compressor blades, most coatings are porous and less dense, preventing them from transferring too much heat. But this also subjects them to infiltration by foreign particles like calcium magnesium alumino silicate (aka CMAS, formed in and near sand particles) or volcanic ash.

Over time, these embedded particles fill in the pores of the TBC, and they remain in the microstructure as the engine gets hot and cold over and over again. Each time the engine heats and cools, this thermal cycling creates strain between the two materials, and like a sealed bottle of water in the freezer, the container eventually will burst. And once the TBC breaks down, heat can flow freely to the compressor blades, potentially melting a section and causing a catastrophic failure.

Depending on the volume of ash or particle ingested, this can happen quickly over several engine cycles or over a long term of repeated use. But the result is the same: failure during operation.

TBC degradation is only one mechanism for long term failure. Engineers also need to consider abrasion, creep and a host of other materials problems that can result from interaction between volcanic ash and highly specialized engine components.

As you can probably guess, this is partially why the European Aviation Safety Agency is being so cautious with easing restrictions on airspace — many of the long term effects of volcanic ash (which varies in composition by geographic location) on engine components are unknown. Only with time, testing and weeks of analysis will the full impact of these materials be know. Until then, we’re going to have to wait for the skies to clear.

Read more about the short term effects of volcanic ash at popsci.

Check out Alaska Airlines’ operating procedure near ash here.

Boeing’s comprehensive study on engine performance in ash clouds can be found here.

Plane Answers: Glide ratios and the most critical phase of flight

Welcome to Gadling’s feature, Plane Answers, where our resident airline pilot, Kent Wien, answers your questions about everything from takeoff to touchdown and beyond. Have a question of your own? Ask away!

Twenty-four hours before the US Airways ditching in the Hudson River, I received this question from Roger:

I have taken lessons in a 182 Cessna and I remember that the glide ratio was very good if there was ever a need to land without power. What is the Glide Ratio for something like a 767, 747 or an MD80 or [a bag of] rocks? Hopefully, I’ll never get to experience it on a commercial airliner in flight.

Tell me you didn’t ask this question and then jump on a flight from New York to Charlotte the next day, Roger!

The truth is, on almost every jet airliner, we use a rule-of-thumb that says it will take three miles to descend 1000 feet at idle. That same rule-of-thumb would work for an airliner that has lost all engines.

So at 3,000 feet, you should be able to glide 9 miles. At 30,000 feet you’ll be able to glide 90 miles. And here’s an interesting twist. A heavy airliner will actually glide farther than a light one because of the added momentum.

This glide ratio is at least 16:1 or 16 feet forward for every foot of altitude lost. This is a ratio right up there with a Schweizer 1-26 glider, and better than most birds or a hang glider in fact.

As for the bag of rocks, well, I’d even prefer to be in an MD-80 than a ‘dead-stick’ bag of rocks. (pilot humor, excuse me folks).
Kylie asks a question, again just hours before the accident:

I’m not sure that the reason I’m so nervous about flying is that I’m not in control, but that I don’t know how “in control” the pilots are if something goes wrong. One of my friends told me that one time he was on a plane and all of a sudden the power goes out, it sounded like the engines stopped, the plane seemed to slow down, and began to drop. However this took place over a few seconds before everything came back on. I’m always worried that if some unexpected problem occurred, and something caused the engine(s) to stop working, is there anything that pilots could do, or is there any back up that would get the engines to work again?

The FAA requires that all transport category aircraft have enough performance to operate on one engine in all phases of flight including just after takeoff. When deciding on the criteria for a two-engine aircraft to cross the Atlantic, they did some studies to calculate the odds of both engines failing. These odds were overwhelmingly small, since each engine is built to have a record of no more than one failure per 30,000 hours or so. So the odds for both engines failing is exceedingly rare, even with the recent bird strike accident.

There have been cases of fuel exhaustion, most recently with an Air Transat A330 that developed a significant fuel leak. That airplane successfully glided to the Azores off the coast of Portugal for a rather successful landing at the airport. The relatively good glide ratio shown above allowed them to travel nearly 100 miles without power.

We also have an extra generator not associated with the engines called the APU that’s available if we need it. This generator can power the entire airplane on its own. Finally, if we lost all three sources of power, on the 757 and 767 that I fly, there’s a small wind generated turbine called a RAT (ram air turbine) that pops out below the airplane and drives a generator to provide power to the flight controls. So double and triple-redundancies are built into every airliner.

Rose asks an equally dramatic question:

I have a brother who is an aeronautical engineer, and he tells me that the first 15 seconds of a flight, (once the wheels have left the ground) is the absolute most volatile because there is no recovery. If something ‘goes wrong’….the aircraft “must fly” before it can land. So he always tells me to count “one-one thousand, two – one thousand….until I reach 15. He says that if I’m ‘still alive’, that my chances of survival increase by 1000%. Is this true?

We actually call it a critical phase of flight. And that’s why the FAA makes us train for just about any scenario during that period.

Fortunately, modern airliners are rather overpowered and an engine failure is rather easily overcome by the thrust of the other engine. I’ve actually flown many small planes that didn’t have the performance with all engines running that a 757 does on one engine.

Ahh, but what about the dreaded dual-engine failure caused by geese you’re surely asking. There have been a number of engine failures that have been caused by a bird strike, but it’s incredibly rare for both engines to be taken out.

Contrary to what I’ve heard reported by some in the media, we do practice two-engine flame outs, at least at my airline. We usually save it as a ‘bonus’ practice at the end of our checkride or training.

Looking at this graph below, the portion after liftoff until the flaps are all the way up represents 5% of all accidents and 14% of the fatalities, interestingly.

Do you have a question about something related to the pointy end of an airplane? Ask Kent and maybe he’ll use it for next Monday’s Plane Answers. Check out his other blog, Cockpit Chronicles and travel along with him at work.