ARCHIVED - Cracking a high altitude mystery
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August 28, 2009— Ottawa, Ontario
A mysterious phenomenon that can cause aircraft engines to shut down during flight has been recreated in the lab by NRC.
The buildup of ice inside a jet engine, long thought to be impossible, has now been associated with at least 14 cases of mid-air engine shutdowns in recent years. Engines in large commercial aircraft are designed to automatically restart in the rare case of a shutdown, and passengers usually have no idea that anything has happened. But until now, no one could show exactly how this type of icing occurs.
For the first time, NRC has recreated the conditions that could cause ice to build up inside an engine at about 36,000 feet. The tests, done in partnership with Boeing and NASA, are a first step towards avoiding similar events in the future.
It's long been unclear how ice could survive inside a jet engine, where temperatures are well above freezing. At the core of the mystery are ice crystals. High concentrations of these crystals typically form in thunderstorms, often in tropical areas. They’re not visible to pilots and don’t show up on aircraft radar.
Crystal icing was recognized as a potential problem in the 1950s, but until recently it wasn’t thought to pose a risk to commercial aircraft. “Before 2003, the industry really didn’t have any indication or awareness that ice crystals could build up inside the core of an operating jet engine,” says Jeanne Mason, a senior specialist engineer for Boeing.
Recent investigations have increased understanding of the role of ice crystals in unexplained engine failures. The hypothesis was that melting ice crystals could be causing interior engine surfaces to cool down to the freezing point, though no one had ever seen this in action.
What NRC did
NRC built a scale model of an engine, and recreated the conditions that an aircraft might experience if encountering ice crystals at high altitude. The test showed that the hypothesis was indeed correct and that in the right conditions, ice can form in the portion of an engine where the air temperature is above freezing.
NRC’s tests showed an accumulation of about 1 kg of ice in five minutes, which in a real aircraft could represent a much larger buildup. If even a small amount of ice were to break free, it could affect engine performance (see box). “It’s within minutes of an aircraft encountering these conditions that the engine may lose power,” says Jim MacLeod, leader of the environmental testing group at the NRC Institute for Aerospace Research (NRC-IAR) in Ottawa.
A larger mystery
Recreating the ice crystals in the lab is the first step toward understanding the issue and finding possible solutions. "We proved that it could stick," says MacLeod. "Now we can study it more closely to learn the fundamental physics."
Many questions remain about the exact conditions in which icing will occur. For example, some engine models have never been affected by ice crystals, while others have experienced multiple incidents. And in some cases, the same engine has had problems in one type of aircraft, but not in another.
"The question is why?" says MacLeod. "If we can, at least, pinpoint the driving criteria, manufacturers may be able to design engines in such a way that they minimize the effect." An alternative may be to come up with a system for warning pilots when they enter a cloud of ice crystals.
NRC researchers will carry out further testing with a consortium of aircraft and engine manufacturers. They are also developing a probe that will be mounted on an aircraft to measure ice crystals in the atmosphere.
How can ice form inside an engine?
Because ice crystals don’t stick to cold surfaces, they bounce harmlessly off the wings and body of an aircraft, which can be as cold as -40 degrees C. “The problem is that the ice crystals get sucked into the engine, where it’s warm,” says Jim MacLeod of NRC-IAR. “The crystals start to melt, and then they pile up and stick to surfaces.”
NRC’s test showed that in the right conditions, ice buildup can happen in the region of an engine’s interior where the temperature is about 20 degrees C. High speed video of the test shows crystals melting on the engine’s interior walls to create a film of moisture that traps more crystals. The accumulation forms a slush that sucks heat out of the metal. “It actually draws the temperature of the metal down to the freezing point,” says MacLeod. “Then everything starts sticking.”
Eventually the ice may get sucked into the engine’s core, where temperatures reach 500 degrees C. The ice flashes instantly to steam, causing a pressure change that disrupts the flow of air in the engine, possibly causing a “flameout.” “A gas turbine engine is a lot like your furnace,” says MacLeod. “There’s a flame inside the engine, and if that flame gets blown out, the engine shuts down.”
This issue is significant with more passenger flights over the Pacific Ocean and other tropical areas, where the storms in which ice crystals form are common.
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