Fan blade gash/dent

[Mach trim]

The other day in a walkaround we found a small gash or dent in one of our fan blades 320/CFM. 2-3 mm with a sharp edge,triangular shape. close to outer perimeter of the fan 4 cm from the outside.

Like a small pebble had impacted.

What are the chances of the dent spreading ?

I would like to find out what the limitation would be for dispatch on a CFM-56 5B A-320 as gash was so small say if we had no maintenance available

Where can I find reading material on this as I would like to learn more about fan inspection and the repercussions of a tiny gash ?


We phoned maintenance as we were away from base and had it filed/repaired, prior to flight.

Cheers

[lomapaseo]

Dents don't spread but cracks do.

The limits in the maintenance manual are based on engine tests and stress measurements. There are vibratory stresses in the running regime that could progress a defect into a crack and fail the blade. These stresses vary by engine model and rating (as the Kegworth incident demonstrated) and also vary by span location on the blade. Thus the dent/nick/gouge limitations in the manual also vary by span location.

OH, and don't forget about the possibility of a bird ingestion on just such a blade. Prior damage doesn't mix well when the blade bites into a bird.

[Piltdown Man]

And, believe it or not, a lump is taken out of the blade opposite - to balance the fan (unless the engineer who told me that was having a laugh!).

[Flying Torquewrench]

Piltdown man,

Don't believe it!! Your engineer definetly was having a laugh. Fan blades are reworked according to the maintenace manual. If vibration increases a read out off the AVM box is required and said AVM box wil produce a list off balance screws which need to be changed to reduce the vibration. Never in my time as an engineer have i taken out a lump off the opposite blade.

Mach Trim,

The only place you will find the information about fan inspection and allowable damage is in the Aircraft Maintenance Manual. Airbus produces this on a cd-rom so maybe you can ask your engineer if you can have a copy. However if you receive a copy be warned that this information is NOT updated and limits can change. So you can use it as a rough guide but NOT to dispatch an aircraft. Always ask somebody with the latest update of the limits to have a look.

Regards, FT

[GotTheTshirt]

Piltdown man your engineer is not having a laugh
Some impact damage is normally allowable : general rule is root third no damage, middle third reasonable damage, outer third is more generous damage and on some engines cropping is allowed and then balancing is required by removing material on opposite side.
All engines are different and the only definitive information is in the MM.

[Flying Torquewrench]

GotTheTshirt,

Very interesting. Just out of curiousity which engines need removing material on the opposite blade for balancing?

Worked a lot with several engine makes and models but never came across this. Just had a look through some MM's and can't find any information about this. By theway i amnot doubting your knowledge.

Piltdown man, my apologies my information was incorrect.

FT.

[NigelOnDraft]

MT The other day in a walkaround we found a small gash or dent in one of our fan blades 320/CFM. 2-3 mm with a sharp edge,triangular shape. close to outer perimeter of the fan 4 cm from the outside.

We phoned maintenance as we were away from base and had it filed/repaired, prior to flight IMHO you did it all just right...

If the damage is "new", no fan blade damage is acceptable to Flt Crew. These Blades are under incredible stress, and "nicks" concentrate stress => nasty failures.

Correctly treated, smoothed out nicks, cropped baldes, balanced blades are not a problem - but it is beyond Flt Crew to assess what is / is not acceptable.

There is a well publicised Harrier Pilot wheelchair bound as a result of an LP Fan Blade failure resulting from a small FoD damage area spreading / developing to blade failure. The blade failure did not just equate to "engine failure", but catastophic with blade taking out Hyd systems etc. - easily equatable to a civil airliner.

In summary your job is done when you spot the damage - well done. Now over to the experts :)

[SeldomFixit]

Bit of confusion here over what was probably a well intentioned response to the question " how do you maintain the balance after you take an apparently rather large lump of metal out of 1 blade". It's not uncommon to change a " pair " of opposing blades when dealing with damage on one. Moment weights and computer generated information used to determine just how you go about it.

[westhawk]

Not to put too fine a point on it, but it is the ENGINE M/M which contains information on acceptable damage limits and repair procedures for engine fans. I wouldn't know what limits or repair schemes might apply for the CFM fan, but blade cropping is not an uncommon practice in general. The opposite blade is often cropped to match the repared blade. My experience with this procedure is limited to the TFE-731 engine. I seem to recall that there were restrictions on how many cropped blades were allowed and their location relative to each other.

Sounds like we have another believer in the value of a thorough pre-flight walkaround! Good job.

Best,

Westhawk

[barit1]

When I worked the Manuals game, the engine MM was prepared by the engine guys as ATA100 Chapters 71,72 etc.

The tinbenders then incorporated these chapters into the AMM.

Thus, while it's engine data, it's issued to the airline as a combined AMM with engine, reverser, APU, landing gear etc. in digital form. (That said, the local engine rep might have a more recent revision of the engine MM in his private stock...)

[oldebloke]

Mach Trim,you seemed to have all the answers supplied..
I might SUGGEST that to avoid some Embarrassment,should you find blade damage on the walkaround(out station)phone home et.Or look in the log to see if the damage is 'historic'(done before)
Some carriers have a page in the AFM of scar damage on the airframe(past dings)
I know of a case where the Captain reported Blade damage, to find out(after the firm had flown the bits and Mechanic to the aircraft),that the blades had been 'trimmed' after an earlier incident.....
cheers..:ok:

[N1 Vibes]

MT

just to add to some of the answers here. The CFM56-5 family blade is a strong bugger. It is a 'solid' blade so can take the sort of ding that you spotted. Now, it it was an RR Trent engine that has 'hollow' blades, made with a sandwich of Titanium. The l/e of the blade is only solid for about 25mm width, so a 2-3mm, 10% of that solid width ding is a little more serious.

lomapaseo

to give you an idea of the stresses on a fan blade, one the size of a P+W JT9, at take-off is about 90 tonnes at the blade root.

westhawk

The opposite blade is often cropped to match the repared blade. My experience with this procedure is limited to the TFE-731 engine

This may be the case in the military world. But, when the bean counter comes and asks me why I cut a lump out of a perfectly good $10,000 fan blade, instead of fitting a longer balance screw next to the damaged/cropped blade, as found on the CFM. What can I say?

oldbloke

very valid point, am in the engine game and have always thought adding significant visible engine blade crops to the 'dent and buckle' chart was a good idea.

[jumbojohn]

Any acceptable damage to an engine blade, dent, crack or piece blended out, should simply be marked around its extent with a big marker pen. Then flight crew on walkround and engineers releasing the aircraft could immediately see if that damage is known about and if any extension to the damage has propagated.

[cwatters]

Just out of interest...

When its windy do you have to stop the engine windmilling to inspect the blades during preflight? How do you do that? Just stick a hand in or?

[lomapaseo]

When its windy do you have to stop the engine windmilling to inspect the blades during preflight? How do you do that? Just stick a hand in or?

You carefully climb in the inlet, not getting anywhere near the spinning blades and then lean against the spinner (surface speed is much lower) and friction drag the fan to a near stopped condition over time.

Once you get it down to about 1-10 rpm you stick your shoe in an pray that the wind doesn't gust.

and N1 Vibes

to give you an idea of the stresses on a fan blade, one the size of a P+W JT9, at take-off is about 90 tonnes at the blade root.

90 tonnes is a force and not a stress, also the worrisom stress is a combination of vibratory high frequency stress coupled with centrifugal stress expressed as lbs-per- sq-in or PSI and plotted on a Goodman diagram.

[westhawk]

N1 Vibes

As I stated in my earlier post, my knowledge of fan blade cropping was derived from personal experience on the Garrett TFE-731 fanjet engines. Even though used on some military applications, the 731 is primarily a civilian engine used on bizjets. I have not worked on CFM engines, so I cannot say what repairs are approved for that engine in it's approved M/M. (or the excerpts installed in an airline manual) Perhaps I did not make this clear enough. The requirement to crop the opposite blade in the case of cropping a blade to remove damaged material was written by engineers tasked with developing repair schemes for foriegn object damage on the 731. The "bean counters" have nothing to do with it and nothing pertinent to say about it. Engineers who have taken factors that would never be imagined by "bean counters" into account designed this repair method so that a damaged blade could be repaired rather than replaced. It is possible that mass balance was only one of the considerations and that simply adding balance weights to solve the radial imbalance did not solve all of the problems encountered when running the fan at max thrust with one cropped blade. It is also possible that removing material from two blades is less costly than buying a new blade to replace the original blade that had repairable tip damage. In any case, the engine maker has the final say as to what constitutes an approved repair. (with the certifying authority's agreement) One could request that the engine builder design and approve a repair more to one's liking if one so desired!

Best regards,

Westhawk

[lomapaseo]

Westhawk

you got it right.

Balance weights work when you are just a tiny amount out of balance (having already installed moment weighted blades in pairs).

When you have large shifts in moment weights (a cropped blade tip on one side, then you can save yourself from boprrowing yet another spare blade from a line station, by simply applying the same maintenace fix to the opposite blade with an approved crop.

As it turns out the very tip corner of these blades is pretty low in stress, )both vibratory and centrifugal) while at the same time being so thin and at the highest speed is the most easiest damaged by FOD or case jamming. Fortunately there is not much performamce loss when cropping off a triangular piece.

[747guy]

oldebloke- ""I might SUGGEST that to avoid some Embarrassment,should you find blade damage on the walkaround(out station)phone home et.Or look in the log to see if the damage is 'historic'(done before)
Some carriers have a page in the AFM of scar damage on the airframe(past dings)
I know of a case where the Captain reported Blade damage, to find out(after the firm had flown the bits and Mechanic to the aircraft),that the blades had been 'trimmed' after an earlier incident.....""


-Some airlines use whats called a "pinwheel" It is pretty much as you decribed, although No pilot would be allowed to make any type of mark on it....simply because most pilots dont know what their looking at.


N1 Vibes- What JT-9 engine are you refering to? as the blades are completely different.