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United B777 engine failure

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United B777 engine failure

Old 23rd Feb 2021, 08:30
  #181 (permalink)  
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Less Hair

The airplane was not hit by high-energy fragments. Those would have gone straight through. A non-structural fairing was hit by a low-energy fragment. Not something you want, but not a catastrophic (arguably not even "major") failure from the point of view of the aircraft.
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Old 23rd Feb 2021, 09:24
  #182 (permalink)  
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Less Hair

As previously mentioned (but removed by Mods) this WAS CONTAINED. An uncontained failure involves bits flying off radially - and at very high speed - that very much do have the capability to seriously wound an aircraft. The worst that was going to happen in this instance (assuming a competent crew) was a relatively low energy fragment penetrating the pressure hull and causing a decompression. No worse than a blown door or window - although still not pleasant of course. However there is / was significant risk to persons on the ground. In this case they were lucky.
I know this is a rumour forum but please, guys & girls, try to stay calm & accurate!
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Last edited by Gizm0; 23rd Feb 2021 at 09:38. Reason: additional comment
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Old 23rd Feb 2021, 10:38
  #183 (permalink)  
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'Sumwalt (NTSB) said the United incident was not considered an uncontained engine failure because the containment ring contained the parts as they were flying out. There was minor damage to the aircraft body but no structural damage, he said.'
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Old 23rd Feb 2021, 10:38
  #184 (permalink)  
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Looks like the wing got at least scratches.
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Old 23rd Feb 2021, 11:03
  #185 (permalink)  
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probably engine oil, from engine lubrication system,


Looking at the lagging, and insulated coupling I would agree with that observation
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Old 23rd Feb 2021, 11:35
  #186 (permalink)  
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To be fair the Bring Another Engine 146 was for the BAe 146 which had the much less reliable LF502.The damage on the RJ100 LF507 in your example was caused by incorrect maintenance not an unreliable engine,although there have been examples of LF507 turbine failures.
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Old 23rd Feb 2021, 12:16
  #187 (permalink)  
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But didn't the "other" United 777/PW fan blade failure in 2018 happen in cruise, at FL360, over the Pacific ?
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Old 23rd Feb 2021, 12:19
  #188 (permalink)  
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Outside the box

Question for the engines, structures and certification guys:

Instead of planning future engines for x- number of fan blade separation failures and constructing appropriate static containment surrounding the fan disk -

- how would it be to support the blades themselves with an integral peripheral band or cord, providing a centripetal force and opposing the separation loading?

This solution, while increasing the inertia of the rotor could be quite a bit lighter than all the present external measures and would tackle the problem at source tending to prevent separation (and by the way to damp vibration) in the first place and easing certification requirements.

Worth a try?
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Old 23rd Feb 2021, 12:26
  #189 (permalink)  
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"But didn't the "other" United 777/PW fan blade failure in 2018 happen in cruise, at FL360, over the Pacific ?"

Yes, shortly before ToD.
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Old 23rd Feb 2021, 12:27
  #190 (permalink)  
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Less Hair

Possible damage to some bolts and some superficial scratches. This will undergo a very detailed and careful inspection but my first guess would be that the airframe structure was not compromised in any way. Which is precisely the point of the fan containment ring. It worked as designed, and it is there precisely because a fan blade failure is not one of those "extremely improbable" (10^-9 per hour) failures (not expected during the lifetime of the fleet), but merely "extremely remote" (not anticipated during the lifetime of each individual airframe, but may occur a few times during the lifetime of the fleet".
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Old 23rd Feb 2021, 12:28
  #191 (permalink)  
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When I posted this I had only seen the passenger video, which, to me did not show fan blades missing. Since I posted this I have seen the pictures showing 1 blade broken at the root and an adjacent blade missing the outer quarter. I still feel that design peak loads would not have been reached so a failure of the pylon or wing structure leading to a loss of an engine would not have happened in this case.
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Old 23rd Feb 2021, 13:09
  #192 (permalink)  
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Old 23rd Feb 2021, 13:21
  #193 (permalink)  
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I'd guess the wing fairing damage was caused by cowling bits or the inlet ring itself. The slice/slash that goes through the inlet ring is outboard between 2-3 o'clock.
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Old 23rd Feb 2021, 13:31
  #194 (permalink)  
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Sometimes in fan blade failures, the blade flies out the front of the engine due to the aerodynamic force on it. In this case, it appears to have struck the belly fairing. This is very similar to a incident an airline I flew for experienced.
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Old 23rd Feb 2021, 13:35
  #195 (permalink)  
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There are several fundamental problems with that idea: what would hold such a band or cord in place? If it is attached to the blades, it only increases the weight of the blades, and thus increases the force on the blade's attachment point. It would need, in turn, to be anchored to something else, but to what? If you anchor it to the fan disk (the "hub"), it just creates additional structures that could break and fly off, and if you anchor it to the case, you basically have a containment ring, which is just what we already have. Any way you turn it, it makes no sense.

I'm also not sure you have a grasp of the stupendous magnitude of the centripetal force required to make these blades turn in circles. Assuming an effective radius of 0.5 m, a blade weight of 20 kg and 3000 rpm fan speed at takeoff, that would come out to roughly 1 MN ("100 tonnes") per blade! Show me the cord or band that can hold even just a substantial fraction of that.

Last edited by bsieker; 23rd Feb 2021 at 14:18.
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Old 23rd Feb 2021, 13:44
  #196 (permalink)  
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Not exactly the same thing but in Formula 1 the F.I.A. mandated a similar system to stop wheels coming off the cars in a crash. This system uses three separate Zylon tethers each bonded to a different part of the car to provide redundancy if one of those parts fails in a crash.

Despite each tether being designed to take 10x the weight of a car, they still fail fairly often in high speed crashes.
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Old 23rd Feb 2021, 14:05
  #197 (permalink)  
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Dan Winterland

What makes you think it was a part of the fan blade that struck the fairing? Lots of other pieces fell off from the nacelle.

And is that really true about forward ejection? It certainly never looks like that in blade-off tests I have seen. And the maths also doesn't work out:

Even full forward load is tiny compared to the centripetal load, so any forward acceleration would not move it forward any significant amount in the time it would reach (a) the following blade trying to shove it backwards and (b) the containment ring, deflecting it unpredictably. So (b) could possibly deflect it forwards, but the talk about aerodynamic load is a red herring.

Look at the numbers: centripetal load per blade: 1 MN. Maximum forward aerodynamic load: 16 kN (rough numbers: 350 kN of thrust distributed over 22 blades). In the fraction of a second that blade would need to reach the containment ring, it would only accelerate to single-digit m/s forward speed component, and would have perhaps a 50th of a second to move. It wouldn't move more than a few centimetres forward.

TL;DR: yes, blades could be ejected forward, but not from aerodynamic loads, but from being deflected erratically by the containment ring.

Last edited by bsieker; 23rd Feb 2021 at 14:20.
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Old 23rd Feb 2021, 16:04
  #198 (permalink)  
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Have they ruled out bird strike or drone strike ? Or would more blades be damaged from such an event ?
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Old 23rd Feb 2021, 16:21
  #199 (permalink)  
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What I'm hearing is that they found evidence of metal fatigue at the fracture surface of the failed blade - just like on the previous two PW4000/112" FBO events (presumably the trailing blade that failed mid-span due to overload due to impact with debris from the leading blade).

Metal fatigue is pretty obvious when you know what to look for at the facture surface.
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Old 23rd Feb 2021, 16:59
  #200 (permalink)  
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Bear with me for some notes that possibly should be in the Nostalgia section, but as I understand it the fan blades of the P&W engines here are hollow, whereas on other 777 engine types they are solid.

If Boeing care to go back 70 years in their archives they will find that the P&W piston-engined Boeing Stratocruiser also offered a choice of propellers, hollow or solid, and that well into the aircraft's life the hollow ones began to give significant problems from internal fatigue and internal corrosion. Eventually, 10 years into the aircraft's operation, an AD was issued that prohibited the hollow units.

AD 58-19-02, if Boeing would care to look it up.

Discussed on a PPRuNe thread here (post #43 in particular) 12 years ago Propellors on DC6/7, Stratocruiser, Constellation & Britannia - PPRuNe Forums

Last edited by WHBM; 23rd Feb 2021 at 17:10.
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