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View Full Version : ICE SHEDDING PROCEDURES - CFM 56s on A320 and 737(?)


nnc0
23rd Feb 2011, 23:01
Another beautiful day here in wonderland, more snow, freezing rain, and another 5 engines on our A320 fleet damaged due to what we suspect is ice buildup on the fan blades.

A 30 second ice shedding eng runup (70% N1) is required every 30 minutes and prior to takeoff on our A320 fleet and we decrease that interval to 10 minutes if it's freezing precip.

What is it about the CFM 56s that these are the only engines in our fleet that seem to get this damage.

Understandably, it isn't always possible to do a runup in those conditions but then I would expect this to be more of an with other carriers. Apparently it is not.

Can anybody pass along or suggest any special considerations or procedures they use when operating in heavy snow or freezing precip conditions?

Any comments on how the physics of the ice buildup on the blades works?

Prober
23rd Feb 2011, 23:56
I had experience of the RB211 (535C) and many years ago there was trouble in this respect. We used to run up (in severe icing) for 60 secs at 60% N1 every 60 mins. After the trouble and an investigation by RR, this was modified to 10 secs (nominal) at 60% N1 followed by a rapid reduction to IDLE before selecting EPR for T/O. It was the twisting movement of the fan blades caused by the reduction to IDLE which actually delivered the ice shedding. I am now out of the loop but I understand that it is now, once again, no longer deemed necessary to reduce to IDLE before releasing the brakes. Sometimes things get lost with the passage of time. Maybe there is something similar in the history of your engines?
Prober

Green Guard
24th Feb 2011, 00:25
any special considerations or procedures they use when operating in heavy snow or freezing precip conditions?


Heavy snow should not produce any icing on fan blades...

though it may block the PT2 tube and give falce but high EPR readings...

As for "freezing precip conditions" the only safe procedure has been to avoid them.

Air Florida Flight 90 - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Air_Florida_Flight_90)

May I ask which area are you operating from ?

Turbine D
24th Feb 2011, 01:36
It is common in most turbo-fan jet engines to have ice buildup on fan blades during startup and taxi to the runway. It occurs when the air temperature is very near freezing and it is raining. The water quickly freezes when coming in contact with the fan blades as the air mass is cooled with the fan rotating and builds up. It isn't a problem with snow, e.g., below freezing temperatures. The engine run up schedule will normally shed the ice outward to prevent it from entering the core of the engine, having it pass through the fan bypass to prevent compressor damage. The Trent 500, 700 and 800 are susceptible to this as are the CF6-80 series the GE90 and P&W engines. In the Trent 800, ice can build up all the way into the first stages of the IP compressor according to some people.

The real danger, however, is operating in ice fog. Here, the ice buildup is very rock solid due to the fine water particles and not easily shed no matter what you do. It is dangerous as it changes the efficiency of the fan, disturbs the air entering the core and results in less thrust than what may be expected. Also, if it sheds during TO, it can really damage the compressor.

Turbine D

PEI_3721
24th Feb 2011, 02:21
Not a CFM operator, but with an engine which does suffer fan blade icing in freezing fog. Frequent N1 increase on the ground, and if applicable in the air, shed the ice without problems.
However, there have been rare instances of engine damage caused by intake ice shedding if the intake anti-ice system was selected on after icing was encountered – ice already on the intake, and thus the heat acted as a deicer. This caused large lumps of ice broke away, some entering and damaging the engine; the damage being restricted to airborne occurrences.
Alternatively, the intake icing system could be overcome by the conditions – severe icing, or severe for the relatively low heat at low engine rpm, or due to icing system weakness/failure; blocked/leaking system, sticky valves – seen them all.
Thinking out of the box; any chance of sucking up ice/frozen snow lumps during the engine runups with low slung engines.

lomapaseo
24th Feb 2011, 03:46
Engines are very similar in their response to the same weather conditions.

Engine icing comes in different forms depending on your operation and the weather.

Be very careful about grasping general read across knowledge for differing conditions with the same engine model, just as you should be careful in assuming its only a GE problem.

As always your manuals recognize what's important based on how you operate the aircraft. follow them and not necessarily what somebody else does. If you don't understand than ask the manufacturer of the aircraft.

Engine icing comes in many forms and at different operating conditions both from how you operate as well as what is the form of precipitation. The ice protection designs cover most of the inflight stuff, but what happens on the ground is suppose to stay on the ground (sic)

for ground operations

Freezing fog is the worst for fan blade icing but blowing snow is often a problem with spinner icing and core inlet icing. Regardless, the minimizing action on the ground is to spool up the engine and shed the stuff (while still small) by either centrifugals or windage drag aganst the ice blob itself.

Engine temperature does not play a significant part in this for ground operations.

If you see damage in front of the fan blades, it's likely spinner ice. If you see damage only behind the fan blades its likely fan blade ice. If you see damage to a lot of fan blade tips its probably spinner ice. If you see damage internaly in the core it's probably ice that formed behind the fan and in the front of a compressor. If you see random fan blade damage it's probably sheet ice off something in front of an engine (radome, wings, inlet lips, runway, snowbanks, etc.)

If you get an engine surge in icing conditions just after liftoff you better start worrying about all the engines on the aircraft

nnc0
24th Feb 2011, 16:24
Some interesting thoughts. Thanks.

Green Guard
24th Feb 2011, 21:17
Still this one may be most possible explanation for your question..

Thinking out of the box; any chance of sucking up ice/frozen snow lumps during the engine runups with low slung engines.

RTO
24th Feb 2011, 21:51
You must be doing it wrong. We have operated CFM56's in FZFG almost all winter for years without any damage whatsoever.

PantLoad
25th Feb 2011, 13:05
Can you tell us what your company's SOPs are regarding the use of engine A/I?

Flay safe,

PantLoad

nick14
25th Feb 2011, 14:32
Hello all,

I have been trying to find a derivative answer for the operation of engine anti-ice in flight. Our procedures are to switch it on in icing conditions except climb and cruise below -40SAT. my first question is why do we operate it all the time during the decent below -40 but not in the climb or cruise? Does engine thrust have anything to do with it?

Also how long do the cowls take to heat up? My point being is it ok to just flick it on just before entering cloud or should it be on if you expect to enter icing conditions as per the decent checks?

Thanks

PEI_3721
25th Feb 2011, 16:56
Re use of engine anti icing:
There has been a long held industry view that icing other than ‘harmless’ ice crystals do not exist at temperatures below -40C. Recent incidents and atmospheric knowledge has shown this to be incorrect, particularly near large Cbs / cirrus anvil cloud.
Use of AI in the descent may assume that you are going to enter warmer / moist air – airmanship – thinking ahead.
Depending on the type of AI system, engine thrust might affect its capability – type / tech issue, airmanship (type knowledge).
Cowl heating time – also type related. Also consider if cowl heat switches any other engine AI functions – spinner, intake guide vanes … type related.
Note that for some types of cloud, there are warm moist layers just above the cloud top, and some types of severe icing are in the very top of the cloud – be prepared.

RTO
27th Feb 2011, 12:59
Can you tell us what your company's SOPs are regarding the use of engine A/I?
Nothing extraordinary:
GND: below 10 OAT/AIR: TAT below 10, and visible moisture in both cases. However this does not help with ice on the fan.

Our company always checks the fanblades for ice accumulation. And in severe conditions the backside of the blades are almost always contaminated with ice. Leaving the gate like this is a recipe for engine damage. We always have ground crew to heat the fanblades before departing.