Wednesday, November 27, 2013

High Altitude Ice and Unintended Consequences

I swear that the most important natural law that never gets taught in school is the Law of Unintended Consequences.  Without exception, I see that law rear its head in response to every law, every regulation and even every desire.  It's a topic I've written about many times.

You may have heard this past weekend that Boeing has issued warnings to airline customers with the new 747-8 and the 787 about troubles from high altitude ice crystals.  Over the life of these aircraft, several flights have been aborted due to loss of engine performance caused by ice crystals causing loss of engine thrust.  Both of these aircraft use the new General Electric GEnx ("Genix") engines, among the highest fuel-efficiency engines made. Aviation Week reports:
The advisory, which warns operators of GEnx-1B powered 787-8s and GEnx-2B powered 747-8s to stay at least 50 naut. mi. away from large convective thunderstorms which can generate high altitude ice particles, is expected to be followed shortly by an FAA airworthiness directive. Although five of the six events that have been reported so far involve 747-8 freighters, one recent incident occurred on a 787.
So why the reference to the law of intended consequences?  Engineering is all about trades, and it's almost always the case that designs good at one characteristic are bad at another.  For an example that many people have experienced, have you ever ridden in a flat-bottom Jon boat?  Compared to a boat with a more V-shaped hull, it's more stable in the water, and tips less when passengers move around, but the flat bottom rides extremely rough in a chop compared to the V hull.  Crossing a few miles of windy open lake or bay in a Jon boat will pound you until you think your teeth are coming out.   In the jet engine case, two of the main design trades that give the GEnx higher fuel efficiency make them more susceptible to ice problems. 

There are two problems directly caused by being more fuel efficient.  The biggest is that the engine is larger in diameter than the one it replaces; that helps the bypass ratio, the ratio of intake air that doesn't go into the combustion core area to the air that does, and ultra fuel efficient engines are ultra high bypass ration engines, but being larger exposes more engine frontal area to the icing environment.  Second, more efficiency comes from minimizing wasted heat.  Jet fuel burns at temperatures higher than the melting point of even the most advanced aviation alloys.  As a result, the jet engine designers had to come up with elaborate ways to prevent melting and cool off the hot section of the engine by “bleeding” in air through tiny ducts and pinholes inside the turbine blades.  This makes the engine cooler, but also less efficient; still, they have to do this in order for the engine to survive.  But optimizing the engine to keep the heat in the one area where fuel burns keeps heat out of the big fan blades in front, making them easier to ice. 

The design factors that led to higher efficiency led to more issues with ice getting into the combustion area and interfering with operation. 

Fuel efficiency is probably the biggest issue for the air travel industry.  Fuel prices have risen substantially in the last decade and are not likely to go down substantially, if at all.  Modern aircraft design is all about cutting fuel costs.  The industry sees the problem with icing and is organizing both private and public sector groups to study the problems and find ways around it.


  1. Greybeard, love the way you abbreviate "trade-offs" with "trades", took me a sec to realize what you were talking about.

    Seems to me that even more important that fuel efficiency is safety. If flights cost a bit more but the planes are designed to handle the stringent requirements they are "expected" to encounter, then a major goal is achieved. Promising lower fuel costs and not delivering on the all important safety issue would be business killer for most, I would think.

    To me, the law of "UN-intended" consequences is when someone thinks they have pulled a fast one only to get bitten in the ass later on by something they just didn't happen to think of. With the history of experience and organizational learning that has hopefully happened at companies like Boeing, these kinds of dumbass "trade-offs" should be able to be avoided.

    1. I think the airframe designers think they have the safety part down. If you ever have time to sit and watch the test videos on YouTube, you'll find them bending the wings of planes until they break, aborting a takeoff with the plane way overloaded, and sitting on the ground with the brakes glowing red hot. They shoot things into running engines. All sorts of torture tests to the airframes and systems. Air frames basically don't break anymore.

      Safety engineering has advanced a lot, too, so that now it's a very rare accident that is caused by a design problem. After all, if you know it's a design mistake, you just "stop it" and don't make that mistake again. As a result, most air disasters have a large pilot error component - or at least a large human error component. There was a terrible accident in Germany a few years ago, where the automatic collision avoidance system told one plane to pull up and the other to descend. The controller told both planes to descend, and they flew into each other. Controller error: if they had listened to their TCAS systems, nothing would have happened. (Obligatory pilot joke: what do pilots and air traffic controllers have in common? If you, as a pilot, screw up, you die. If the air traffic controller screws up, you die).

      The icing problem is actually General Electric's problem and the only surprise to me is that it wasn't found in ground testing. The reason it didn't show up in ground testing is probably that no one knew enough about the environment up there to get it into the tests. Unfortunately, there is still a lot we don't know about the atmosphere up at 30 to 50,000 feet. It's really a question of sensors, because these dry crystals of ice don't show up well on weather radar. Of course, weather radar designers are working to improve that, and get better information on the environment.

  2. I had to laugh at the "organizational learning" comment applied to Boeing. I give you the lithium battery. They learned nothing from the loss of a UPS 747 due to a Li battery fire - which was being carried as cargo.

  3. It isn't isn't just the new gen engines with the problem, too. Our manuals for the 757/767 recently started including guidance on engine icing crystals as well. (I'm an instructor on those aircraft at FedEx).

    One thing about is the pilot error thing; many pilots leave the WX radar tilted at current altitude or slightly above. A lot of this icing occurs above areas over heavy precip, so there is some way to predict it as long as the crew actually looks at the weather below and in front of the flight path.

    Been reading your blog for over a year now, love it.

    1. Thanks for the input! I love it when I get to learn new perspectives. And thanks for the kind words on the blog.

      I understand that the ice can show up in dry air if the radar receiver gain is cranked up all the way, it just doesn't tend to be set that way, and (of course) the big guys in the field are working on making radar something the crew doesn't have to mess with. I think HAIC and then clear air turbulence are the next big things to go after for improved safety.

      While putting another system on aircraft is a tough sell, I always thought the answer would be to go to higher frequencies. Aviation weather radars are around 9.50 GHz, while ground based weather radars tend to be around half that frequency. For dust in clear air, and tiny crystals, higher frequencies and shorter wavelengths make sense to me. The trick would be somehow getting it to work in the same transmitter and antenna.