abnormal From UK - England, joined Aug 2007, 81 posts, RR: 0 Posted (5 months 2 weeks 4 hours ago) and read 2994 times:
I don't have pictures but recently saw several CFM 56 engines that had damage to the N1 fan blades. They were curled back at the tips.
The damage occurred during a heavy snow storm. Lots of blowing snow. Temp and dew point were around -5 and -6 degrees Celcius.
The aircraft were taxiing around in that storm for about an hour prior to takeoff and DFDR data shows they were doing engine runups to pretty near 50% about every 10 minutes. The aircraft were deiced.
Is it possible that ice accumulated on the back of the fan blades despite the runups and at application of take off power that ice had enough centrifugal force to curl the fan blade tips? Or is it possible that the ice accumulated in the nacelle barrel behind the heated lip and was dislodged during the initial takeoff run where it flew back to impact the fan blades?
fr8mech From United States of America, joined Sep 2005, 4344 posts, RR: 12 Reply 1, posted (5 months 2 weeks ago) and read 2904 times:
Quoting abnormal (Thread starter): Is it possible that ice accumulated on the back of the fan blades despite the runups and at application of take off power that ice had enough centrifugal force to curl the fan blade tips? Or is it possible that the ice accumulated in the nacelle barrel behind the heated lip and was dislodged during the initial takeoff run where it flew back to impact the fan blades?
More than likely, the engines picked up ice and muck from the taxiway.
But, I have seen CFM engines (among others) accumulate quite a bit of ice when running at idle in the right weather conditions.
abnormal From UK - England, joined Aug 2007, 81 posts, RR: 0 Reply 2, posted (5 months 2 weeks ago) and read 2873 times:
We've kind of ruled that out. There were other airlines operating the same engines in the same lineups and they had no damage. Some of them were also 737s and the engines are even closer to the ground. No damage on any EMBs either.
I'm thinking it has to be something the specific airline does, either in their ground procedures/checks/inspections or in how the engines are operated. The aircraft had overnighted so the engine were stone cold at the start of the day.
fr8mech From United States of America, joined Sep 2005, 4344 posts, RR: 12 Reply 4, posted (5 months 1 week 6 days 6 hours ago) and read 2575 times:
Quoting abnormal (Thread starter): The aircraft were taxiing around in that storm for about an hour prior to takeoff and DFDR data shows they were doing engine runups to pretty near 50% about every 10 minutes. The aircraft were deiced.
You know, we had a similar SOP when taxiing in crappy conditions. I believe it was about 50% for 2 minutes every 10 minutes. As I recall, this wasn't to shed the snow, but to heat up the bleed air so that the NAI was more effective and longer lasting.
Could it be that your folks weren't doing their run-ups long enough to have a lasting effect? At idle, there just may not be enough heat in the bleed air to keep the inlet adequately de-iced.
abnormal From UK - England, joined Aug 2007, 81 posts, RR: 0 Reply 5, posted (5 months 1 week 6 days 6 hours ago) and read 2557 times:
Quoting HAWK21M (Reply 3): Shouldn't basic SOP be to ensure freedom of rotation of the Fan blades prior to startup.
Apparently that is preflight SOP and but during the storm the blades are usually already rotating. What isn't being though is a tactile check of the back of the blades.
Quoting fr8mech (Reply 4): As I recall, this wasn't to shed the snow, but to heat up the bleed air so that the NAI was more effective and longer lasting.
Could it be that your folks weren't doing their run-ups long enough to have a lasting effect? At idle, there just may not be enough heat in the bleed air to keep the inlet adequately de-iced.
That's an interesting idea. The documentation says the runup is intended to shed the ice from the blades but we have seen ice form in a circumferential ring around the inside of the inlet (even up top) a good distance back from the heated lip. That's why one of our suspicions is that the damage from inside the inlet is breaking free and impacting the blades.
Are you suggesting that given the high moisture content of the air and the heavy snow, the anti ice lip might be overwhelmed . That the moisture should be vaporized when it contacts the nacelle lip and and then be sucked back in the airflow? That would imply there shouldn't be any ice in the inlet then. Hmm?
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What I have a hard time believing though is that ice from inside the barrel is breaking free for whatever reason, flowing back to contact the fan and that's curling the tips.
I'm more inclined (though not totally convinced) to think the tip curl has to be from the ice sliding off the back of the blade outwardly while the blade is spinning at 5000 rpm. The ice contacts the raceway at the end of the blades where it meets some resistance to it's outward motion. Instead of flying away it is momentarily stopped or restricted. At that moment, the inertia from a small chunk ice rotating at 5000 rpm must be sufficient enough to then curl the tip backwards.
fr8mech From United States of America, joined Sep 2005, 4344 posts, RR: 12 Reply 6, posted (5 months 1 week 6 days 5 hours ago) and read 2553 times:
Quoting abnormal (Reply 5): At moment, the inertia from a small chunk ice rotating at 5000 rpm must be sufficient enough to then curl the tip backwards.
It is sufficient. We've seen it plently of times.
Quoting abnormal (Reply 5): Are you suggesting that given the high moisture content of the air and the heavy snow, the anti ice lip might be overwhelmed
Yes, at idle an engine just doesn't produce the air necessary to suffiently (de)anti-ice the inlet. I don't have any empirical data, but I recall, that we had some events over the last couple of years, where we had extended taxi times into parking, with high humidity and cold temps, and the blades were coated with ice, though the lip was clean.
We wound up using kerosene heaters (the torpedo type) to de-ice the blades..
abnormal From UK - England, joined Aug 2007, 81 posts, RR: 0 Reply 8, posted (5 months 1 week 5 days 2 hours ago) and read 2257 times:
Would somebody be able to quote the 737 procedures for ice shedding? Is it a momentary runup or do they suggest holding the higher N1 it for a period. How often do they suggest the runup be done?
fr8mech From United States of America, joined Sep 2005, 4344 posts, RR: 12 Reply 9, posted (5 months 1 week 3 days 18 hours ago) and read 2044 times:
We don't operate any CFM equipped aircraft anymore, but I took a look at our B767 AMM and found that it completely contradicts me (or, more likely, I contradict it). On the B767 (CF6 equipped) we are cautioned not to operate at higher power settings for greater than 30 secs in icing conditions. Further, we are to perform the ice shedding procedure (operate at 60% N1 for 30 secs) every 30 minutes. Curious.
Now, reading into the CF6 equipped B744 AMM, we read the same instruction as the B767, but they add:
If there is large ice in the engine inlet cowl, make sure that you operate the engine at approximately 80-85% N2 RPM with the nacelle anti-ice on until the ice melts. if you do not, engine damage can occur when the ice breaks and goes into the engine.
abnormal From UK - England, joined Aug 2007, 81 posts, RR: 0 Reply 10, posted (5 months 1 week 3 days 9 hours ago) and read 1973 times:
They both make sense in terms of ice shedding from the fan blades. The additional 747 Caution for the same engine is curious though. You certainly wouldn't want the ice breaking free and blasting back into the blades when you apply t/o thrust but why only include it in the 767? They must have seen it occur in practice I suppose.
abnormal From UK - England, joined Aug 2007, 81 posts, RR: 0 Reply 14, posted (5 months 2 days 18 hours ago) and read 1485 times:
Just thought I'd follow up with what we have found.
The problem seems to be unique to CFM 56 5A and 5B engines. The blades a pretty thin and weak at the tips.
During extend ground ops in heavy a heavy snowfall a lot of moisture flows back from the heated nacelle lip and freezes in the nacelle inlet barrel. It gets scraped away at the fan face by the constant rotation of the blades and the runups.
As T/O thrust is applied and the engine picks up speed it also puts a higher load on the blades as the take in more air and the blades flex much further forward where they then come into contact with a significant ice accumulation that hadn't been scraped away by the blades during ground run ups.
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