Southwest engine failure

[FowVay]

Originally Posted By: nthach


In the case of a fan blade that is nicked or cracked, is SOP to have it sent back to a workshop to get it repaired and re-checked for damage via NDT? Or is it scrapped for a new OEM blade? Is there a different procedure for a composite blade like what the GE90/GENX/LEAP uses?


Titanium blades can have minor nicks blended on-wing. The plane would be pulled from service and have the blend accomplished during the down time. Blending of any part would require the nick be blended, acid etched and then FPI'd. this procedure can take several hours.

If blade replacement were necessary then they are always replaced in weight-matched sets. The bad blade is removed and the transverse blade is also removed. Replacement blades are kept in sets for this use so it's faster to replace a blade than to blend one.

As for the composite blades, they all have a metal strip on their leading edge to prevent fiber damage. The blades are made from carbon fiber tape, carbon fiber fabric and a fiberglass fabric ply placed periodically within the layup. The outer ply of carbon tape is also surrounded by fiberglass fabric. This is done to increase impact resistance. While they are very light (GE90-115 blade weighs about 48lbs) and strong they are susceptible to impact damage from ingested FOD. Any fiber damage will necessitate replacement.

 

[SilverFusion2010]

The article I read was pointing at metal fatigue cracking at the blade root being the cause of failure

 

[Shannow]

Virgin replacing "17" blades

https://thenewdaily.com.au/news/national/2018/04/23/virgin-replace-737-engine/

surely some misinterpretation by the reporters.

 

[SEMI_287]

I wonder if this will affect the A320/340 series since they use the CFM56 as well.

 

[FowVay]

Shannow, I would say that Virgin had accomplished the ultrasonic inspection on their fleet that had over 30,000 cycles and found 17 blades to fail the inspection.

The next inspection will be on engines that have 20,000 cycles or more. They have until August to accomplish these inspections.

And ultimately they will inspect the entire fleet of CFM56-7B engines - time frame not announced yet.

 

[FowVay]

Semi, the inspections are for -7 series engines only. I don't know for sure what the A320 family uses but the A340-200/300 use older -5 engines.

Look closely at the fan blades on a -5 CFM and compare them to a -7 CFM. The -5 uses a mid-spar bridge to help reduce blade twist. The -7 blades are full cord with no bridge. They truly are different engines.

Out of all of this I would like to see something go into redesigning the inlet cowl inner acoustic liners. Those things seem to tear apart midway between the fwd/aft bulkheads when massive imbalance is encountered. This can't be an acceptable design.

 

[SEMI_287]

Yeah, the -5 have thinner blades and the -7 have wider blades. I've always checked the inner liners very well on preflights. I never liked that design myself. I've never flown a -5. Only the -3 and -7. The only major difference between the -3 and -5 I'm aware of is the addition of FADEC on the -5.

 

[FowVay]

The difference between a FADEC engine and a PMC is literally night and day.

 

[BusyLittleShop]

My current conjecture of the sequence of events are:

One fan blade separated from the fan disk and was ingested into the engine...

Out of balance Fan Core shakes the engine with no way for the pilot to
stop it in the air stream.

The left inlet separated from the engine and debris damaged the
aircraft fuselage, wing and empennage, and the passenger interior
compartment was penetrated.

No fan blade material will be found to have struck the passenger
window.

 

[Shannow]

Originally Posted By: BusyLittleShop
My current conjecture of the sequence of events are:

One fan blade separated from the fan disk and was ingested into the engine...

Out of balance Fan Core shakes the engine with no way for the pilot to
stop it in the air stream.

The left inlet separated from the engine and debris damaged the
aircraft fuselage, wing and empennage, and the passenger interior
compartment was penetrated.

No fan blade material will be found to have struck the passenger
window.


None for two so far BLS...

Separated blade went forward, as it's "clawing" through the air, and the uncontained forces on the now released moving blade cause it to move both forward (aerodynamic), and radially outward (no centripetal restraining force from the blade root area)

 

[Shannow]

http://aerossurance.com/safety-management/uncontained-cfm56-failure-b737/

 

[BusyLittleShop]

Engineer's test the Fan Core side walls to insure they are strong
enough to contain a separated fan blade... blades must not bust free
and damage the aircraft fuselage, wing or penetrate the passenger
interior compartment. a successful test is one where the light weight
titanium blade separates from the fan disk and is ultimately ingested
back into the engine.

 

[Shannow]

Look at the damage to the adjacent blades...it took off forward.

If the front section of the engine can handle a failed blade, then it won't "shake off" with one blade missing.

(BTW, hands up everyone in the thread who has ever done a forensic investigation of a failed turbine blade)

Given that we are now youtubing rather than engineering...



Interesting in what Rolls Royce reckoned happened, passenger footage of the vibration of both the engine and the plane.

The blade failure engine stand tests use explosives to fail the blade, not fatigue...they test (In my professional opinion) only one aspect of containment.

High cycle fatigue is interesting. Have seen 4 failed blades on a GEC/Parsons Erith turbine (L-2)...each of them finally parted at BDC, when the "G" forces were average +1G.

 

[Nyogtha]

(Raises Hand)

 

[Shannow]

LOL, you know that sinking feeling then.

 

[john_pifer]

A fragment from the rotating assembly that was ejected, had to have struck the window. No other way it would have been broken.

 

[Shannow]

Originally Posted By: john_pifer
A fragment from the rotating assembly that was ejected, had to have struck the window. No other way it would have been broken.


The affected window was a fair way back...

Originally Posted By: Shannow
http://aerossurance.com/safety-management/uncontained-cfm56-failure-b737/

Ignore the hand held piece, and look at the scratch marks behind it.

It was a glancing blow, not a spearing blow.

The 2016 event was nearly as dramatic, but different.

 

[john_pifer]

Originally Posted By: Shannow
Originally Posted By: john_pifer
A fragment from the rotating assembly that was ejected, had to have struck the window. No other way it would have been broken.


The affected window was a fair way back...

Originally Posted By: Shannow
http://aerossurance.com/safety-management/uncontained-cfm56-failure-b737/

Ignore the hand held piece, and look at the scratch marks behind it.

It was a glancing blow, not a spearing blow.

The 2016 event was nearly as dramatic, but different.

Crazy. I wouldn't have thought the airflow would be traveling in that direction, to carry a piece of debris into the fuselage.

 

[BusyLittleShop]

Originally Posted By: john_pifer
A fragment from the rotating assembly that was ejected, had to have struck the window. No other way it would have been broken.


You're not alone thinking the separated blade went forward
aerodynamically and radially outward... however there is mounting
evidence the separated blade was ingested back into the engine and
exited the rear just the way engineers designed it and test proved it...

 

[FowVay]

The fan blade may have been ingested into the engine but upon initial liberation it most likely pulled forward and did the damage to the inlet cowling. The inlet (referred to as the nose cowl) cowling then came apart and caused the fuselage damage and ultimately broke the window.

Those fan blades are pulling forward with amazing pressure. I work the CF6 engine and as the engine became more powerful they had to redesign the blade dovetail area to withstand up to 60,000 pounds of forward pulling torque due to the massive load. Inspection of a fan blade and stage 1 disk on a commercial high time jet engine shows the majority of wear to be on the aft areas. This is because that load is from the fan pulling on the air mass.

Another thing, look closely at the CFM56 picture where the NTSB investigator is looking into the inlet. You can see a blue color in the blade track area. This is a syntactic potting that seals the blade tip to the fan case to reduce leakage. You can see that the liberated blade wiped that entire circumference of potting out of the case. The blade did not just break off and go into the engine. It took a ride around the inlet wiping out everything in its path before heading out.

I feel that this is what damaged/weakened the nose cowl and vibrational forces and high speed air did the rest.