so this particular eruption could continue for another year.
Is there anything that could be done to retrofit current commercial airplane engines to be able to fly through current levels of atmospheric ash? I’m imagining some sort of dome over the engine intakes to filter out the ash. I’m also imagining lots of problems with that approach, like being able to handle the airflow required, how fine of ash may need to be blocked, and the dome getting plugged with ash.
My guess would be that it’s well beyond what’s practical for an airliner. Anything resembling an effective filter would restrict airflow sufficiently to reduce power and no doubt efficiency. Developing and testing such devices would be expensive and time-consuming. Fitting them to existing aircraft would be extremely challenging.
And note that engine problems are not the only troubles associated with volcanic ash: there’s also a marked tendency to “sandblast” the airframe, leading to fogged windows and other unpleasantness.
When A modern airliner jet engine enters an ash cloud, what exactly happens? Specifically, what fails in the engine? Small particles clogging up the moving parts? Ash melting and fusing to the extremely hot surfaces? Erosion of critical surfaces?
I’m wondering if instead of keeping the ash out of the engines if it’s possible to build an engine that could ingest it with relatively little harm?
The very short version is two bad things happen to jet engines which ingest ash.
One is plain old sandblasting of high-precision parts. Despite their size and power, the tolerances inside the engine’s air path is measured in thousandths of an inch. Doesnt’t take much erosion to goof that up.
Parts often have coatings applied for things like heat resistance. As the coating is abraded, the heat resistance declines and the part fails, or at least has its life reduced from years to hours, weeks, or months. With engines costing several million dollars each, having to replace them 10 or 100x as often as normal would make flying cost prohibitive.
The second major problem is the ash melts as it passes through the high temperature areas of the engine and the cools and deposits itself permanently on the cooler areas downstream. It doesn’t take much of that to render the engine inoperative. Like a couple minutes worth of flying through dense ash.
Helicopter turboshaft engines use something called a centrifugal particle separator to remove a lot of the larger sand and dust particles found in deserts & such. But the effectiveness goes down as the particle size goes down. The ultra-fine dust common in Afghanistan mostly skips the separator and goes through the engines. The result is a much shortened engine lifespan.
Stratospheric volcanic ash is ultra, ultra fine. All the bigger stuff falls out pretty quickly, like in hours. The really big chunks which are inches across fall out in seconds or minutes. Pretty much anything still aloft any appreciable distrance from the eruption is very very fine.
Add the extra abrasinve cross-section and the melting thing and you have an environment that is really prohibitive for current turbojet / turbofan engines.
This not-very-technical article says:
“…the particles stick to the engine’s hot parts, forming a glasslike coating, and grind up turbines, bearings, and other moving parts, restricting air flow through the turbine.”
The answer appears to be “All of the above.”
Given the reliability required from jet engines, “relatively little” here must be translated as “absolutely no” - and that is going to be very challenging.
Long term, you get abrasive wear that shortens the life of the engine. Short term, the ash can melt and stick to stator and rotor blades, changing their shape and altering their aerodynamic properties to the point where the engine can’t move air properly. Read the account of British Airways Flight 9, a 747 that flew through an invisible ash cloud and killed all four engines. They managed to get the engines restarted after gliding some distance; presumably the parts cooled/contracted and caused the melted-on ash coating to flake off, restoring the correct aerodynamic profiles to the rotor and stator blades.
But as has been noted, the engines are only part of the problem The crew of flight 9 barely managed to get the plane on the ground because the windshield panels were sandblasted so badly they couldn’t see, except for a tiny strip of window that had been protected from the abrasive slipstream by some trim.
What I don’t understand is why they don’t fly at a lower altitude to get out from under the ash cloud. From what I read the ash cloud is mostly between 18,000 to 33,000 feet. It seems to me that flights from London or Paris could fly at 10,000 ft Southwest and be out from under the ash cloud in less than an hour and resume normal cruising altitude for destinations in the Americas and Africa. It isn’t very efficient, but compared to what they are losing sitting on the ground? I also noticed that some flights are allowed to overfly Europe at 36,000 feet to fly over the ash cloud.
I suspect at least part of the issue is liability if something does go bad. Maybe there’s only a .1% chance of catastrophic failure but if it happens, everyone dies. Lawsuits start multiplying like Viagra stoked rabbits and the money grubbing airline is flogged in the court of public opinion for risking people’s lives in the name of profit.
They asked that on the TV earlier: they said that there are several factors, but two main ones. Firstly, the engines are massively less efficient when not at their normal operating altitudes. Secondly, it’s less safe, with aircraft packed more tightly, and less altitude with which to recover in an emergency.
Along these same lines, some of the medium-sized planes like the 757’s and 767’s that are the workhorses of the transatlantic route have barely enough range to make the crossing as is and so any appreciable amount of time spent at lower altitude would probably not be possible.
What Broomstick posted doesn’t answer my question, since it assumes that planes would fly across the Atlantic at 10,000 feet and doesn’t quantify the fuel savings at all. It is hard to get a good answer since there are so many variables.
Based on what I read I doubt that flying for an hour at optimal cruising speed at 10,000 ft would use more than a few gallons of fuel per passenger or a few thousand dollars per flight. After they are out from under the ash cloud, they can ascend to optimal cruising altitude. A few thousand dollars isn’t chump change, but having the plane sit on the ground is probably costing the airline 100,000 dollars in lost revenue. The airlines could even impose a fuel surcharge and the passengers that don’t want to pay it can stay on the ground.
Does anybody actually have the experience at doing the fuel calculations for the relevant Boeing or Airbus equipment?
There’s no “under” – while most airborne particles are fine and carried high, some comparatively coarse ones will be carried lower in the atmosphere (we’re talking colloidal size here, so ‘coarse’ and ‘fine’ are relative, both being ultrafine on almost any other scale. Nor would it be for only an hour.
There is, however, a simple way for an aircraft to move relatively safely through an ash cloud – glide. Engines off and cold, pilot skill used for optimum glide slope to maximize distance at altitude. Also, emergency launch using expendable JATO assist in lieu of non-replaceable engines can be used for, e.g., evacuating people from an island during an eruption. It will be obvious why neither of these procedures is appropriate for commercial flights.
What Broomstick posted doesn’t answer my question, since it assumes that planes would fly across the Atlantic at 10,000 feet and doesn’t quantify the fuel savings at all. It is hard to get a good answer since there are so many variables.
Based on what I read I doubt that flying for an hour at optimal cruising speed at 10,000 ft would use more than a few gallons of fuel per passenger or a few thousand dollars per flight. After they are out from under the ash cloud, they can ascend to optimal cruising altitude. A few thousand dollars isn’t chump change, but having the plane sit on the ground is probably costing the airline 100,000 dollars in lost revenue. The airlines could even impose a fuel surcharge and the passengers that don’t want to pay it can stay on the ground.
Does anybody actually have the experience at doing the fuel calculations for the relevant Boeing or Airbus equipment?
If we’re only talking about fuel load and nothing else, my WAG is that they should have enough to at least get to Gander or Greenland from Europe even if they’re stuck at 10000 ft for the first hour or two. One thing to consider is that all flights are planned such that they always have enough fuel to fly to an alternate at 10000 ft in case it losses cabin pressurization for whatever reasons at any point during the flight. For an oceanic flight, this means that they’re already carrying enough fuel to fly for several hours at low altitude anyway.
Of course, I don’t want to be the one to find out whether it’s really ok to fly under the ash cloud…
Yes. But I am not one of them, hence I did not crunch the numbers. Most of our Doper pilots with relevant experience fly for a living - it typically takes them a bit to find these threads as it is frowned upon to read newsgroups in a working cockpit. Given that the chaos in air travel affects other continents than Europe it is conceivable that they have more pressing concerns, like how the heck to get home, than answering that question quickly. Give them some time and they’ll show up.
I think your cost estimate is off, though - operating something like a B7x7 or Airbus costs thousands of dollars in fuel per minute even with an optimal flight profile. Low level flight is certainly possible but it will cost a lot more than you seem to think it will.
Also, there is no “optimal cruising speed” at 10,000 feet. It just doesn’t exist for a 737 or 747 or Airbus or whatever. Optimal cruising happens at 25,000 to 35,000 feet, that’s how they’re designed.
It might be practical to do low-level flying from the edges of the affected area - that’s probably why some flights are getting in an out of Europe - but the ash cloud is thousands of miles across. Your strategy won’t help countries in the middle of the mess. Also keep in mind that Europe does have mountains taller than 10,000 feet - if you can’t fly over them you’ll have to go around them, in which case you might as well go by road 'cause cars and trucks are an eff of a lot cheaper to run.
Not to mention that while the gliding strategy might save the engines (although some grit will get into them, at least it won’t melt and then stick to the innards) it will still sandblast the hell out of the windows and abrade the paint and get into moving parts and in general still cause damage. Working engines are nice, but so are cockpit windows you can actually see through.
I’m an engineer - not a pilot - but I’ll take a back-of-the-envelope shot.
Standard atmosphere model says density at 10K feet is about 2.4 times what it is at 35K feet. So for the normal cruising speed (~550 MPH?), you’d get 2.4 times the drag if flying at 10K feet.
But the airframe isn’t designed for that much drag; the max permissible speed at 10K feet is probably a lot lower. Form drag goes as the square of speed, so if you lowered your cruise speed to 64% of what it is at 35K feet, you’d see the same drag.
So… with 64% of normal cruising speed, and same engine thrust as normal cruising speed, you’ve basically got 64% of the range you had at 35K feet.
I’ll be curious to hear from a real airliner pilot…