UA flight #328 loses engine over Broomfield CO.

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Easy to have an abundance of caution when a pandemic has created a surplus of airplanes and a company has multiple types (787, 767) to backfill the routes on which that airplane type (777-200A) was operating.

Pretty hard to have an abundance of caution when you operate one airplane type and a lethal flaw is found.

In 1999, the NTSB determined that two fatal (all on board killed) crashes of 737s were due to design flaws in the rudder PCU (the hydraulic actuator for the rudder). The 737 was built with a single rudder and single PCU, unlike 747, 757, 767, 777, even 727. The PCU was at fault in the United 585 crash in COS, which had been blamed on mountain rotor (wind) and in the USAIR 427 crash which had been blamed on pilot error.

There were several incidents involving 737 near loss of control, along with a few crashes at the time that the NTSB made this determination. They had tested the PCU, and found that when cold-soaked, the PCU could throw the rudder to full deflection without flight control input and that the rudder would stay there, no matter what flight control input was made.

Fatal design flaw in an entire fleet of aircraft. Fatal crashes. Just like the MAX, only, the design flaw wasn't discovered until about 8 years later, because the crashes had been blamed on something else at the time.

Still, a fatal flaw in the airplane that put everyone flying it at risk.

So, United grounds all 150 of their 737s. American, Delta, USAIR, Continental - all ground their 737s. All out of caution. Not even an abundance, just basic caution in light of the NTSB discovery.

But, the FAA allows other 737s to keep flying, despite the fatal crashes, these total losses. Hard to call that an "abundance of caution". One airline, in particular, was able to convince the FAA to keep flying the airplane with this flaw in the rudder, as long as they used different flap speeds to enable a crew to counter the possible rudder hard over with aileron. The "crossover" speed is the speed at which change in AOA allows rudder to overpower aileron - so, the fix was to fly above that speed for each flap setting and hope the crew responds quickly if the flawed PCU slams the rudder full stop.

Fatal, serious flight control design flaw in the airplane's rudder, but you guys have three years to get new PCUs installed.

So, the 737 flew, with one airline operating a full flight schedule of 737, despite the design defect and despite the deadly history of that defect.

Where was the "abundance of caution" then? Oh yeah, it cost some airlines a LOT of money, so, sure ground them. But it would've cost that one airline even more, since they only had one airplane type...so, yeah, caution can be waivered and put aside, rather than lose money...in certain cases...when it's convenient to do so...


Thanks for making that clear. ✔️
 
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Apparently the crew pulled the engine fire T handle and both bottles were fired. Obviously the fire warning didn’t go out after the first one so they fired the second one.

Very curious as a pilot to know how long it took for the fire indication to “go out” as the fire was persistent. Also curious to know how less effective the fire suppression system was due to the cowling being ripped off by fan blade damage.

The engineering worked in terms of containing the fan blades but the fan blades caused the cowling to fly off ( Technically a contained engine failure although ).

Getting a flight Deck warning ( plus vibration, loss of thrust ) of a fire that won’t go out after both bottles were used up sure changes the dynamics of the emergency and time to get the aircraft on he ground. That becomes a time critical emergency.

Hopefully most pilots get to see time critical scenarios in the simulator often enough to be caught off guard when things don’t go as planned.
 

MolaKule

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I'm puzzled by the persistent fire as well.

By the way, the oil cooler is more of a fuel warmer than just oil cooler. In very cold conditions, the fuel gets close to gel temperature, and warming it up dissolves any paraffin so that the engine keeps running and the filter and/or fuel control don't get clogged up.
The heat exchanger on jet turbine engines really is synergistic since the fuel is heated well above its wax precip point before injection into the combustor, and at the same time, the lubricating oil is cooled for its trip trip back to the bearing cells.

What is interesting is the fire appears to emanate along and outside the core, and not coming from the internal combustor/turbine area (but we don't know yet if the case was not cracked in this area).

Since the fan and first stage compressor was still freewheeling, it is possible the accessory gearbox was still pumping oil, and any broken oil lines may have been spewing oil over the outside of the combustor/turbine area. The outlet temp of the combustor is on the order of 2500F, about 1028F over the fire point of the 5.1 cSt special polyol ester lubricant.

Now the case temp over the combustor/turbine inlet area is probably below the 2500F temp, but still hot enough to ignite the oil.
 
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MolaKule

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Apparently the crew pulled the engine fire T handle and both bottles were fired. Obviously the fire warning didn’t go out after the first one so they fired the second one.

Very curious as a pilot to know how long it took for the fire indication to “go out” as the fire was persistent. Also curious to know how less effective the fire suppression system was due to the cowling being ripped off by fan blade damage.

The engineering worked in terms of containing the fan blades but the fan blades caused the cowling to fly off ( Technically a contained engine failure although ).

Getting a flight Deck warning ( plus vibration, loss of thrust ) of a fire that won’t go out after both bottles were used up sure changes the dynamics of the emergency and time to get the aircraft on he ground. That becomes a time critical emergency.

Hopefully most pilots get to see time critical scenarios in the simulator often enough to be caught off guard when things don’t go as planned.
For sure the crew is to be credited for the safe landing and proper checklist procedures were followed before landing.

But you bring up a good question: Is the fire suppression system sufficient under these conditions? If I recall correctly, the fire suppression system is designed for suppression of an engine fire with the complete cowling intact.

I think something else contributed to these ongoing flames.

This is one incident in which I am intently awaiting the NTSB report, as I am sure many others are awaiting as well.
 
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Astro14

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Completely agree, @MolaKule.

With the cowling ripped off, the halon (if used, it’s all still speculation at this point) didn’t do anything to suppress the fire. On the other hand, there was plenty of cooling air flow, and the structure of the airplane wasn’t at risk from the heat of the fire.
 
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The heat exchanger on jet turbine engines really is synergistic since the fuel is heated well above its wax precip point before injection into the combustor, and at the same time, the lubricating oil is cooled for its trip trip back to the bearing cells.

What is interesting is the fire appears to emanate along and outside the core, and not coming from the internal combustor/turbine area (but we don't know yet if the case was not cracked in this area).

Since the fan and first stage compressor was still freewheeling, it is possible the accessory gearbox was still pumping oil, and any broken oil lines may have been spewing oil over the outside of the combustor/turbine area. The outlet temp of the combustor is on the order of 2500F, about 1028F over the fire point of the 5.1 cSt special polyol ester lubricant.

Now the case temp over the combustor/turbine inlet area is probably below the 2500F temp, but still hot enough to ignite the oil.
I bet your right, I just can't explain it as well as you.
 
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While not directly related, this thread hijack question comes from reviewing P&W engine (well, general high-bypass turbofan engine; GE looks the same) sketches. CONCEPTUALLY, is a high bypass (fanjet) like a turboprop with the "fan" and "prop" just realized differently? Web efficiency sketches suggest the "prop" is better up to maybe 350-400 kt with the fanjet passing it around there. Is this a tip vortice issue with the shrouded fan minimizing them?
 
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And the B773ER get the GE90’s …

I've been following this story for a while. How's this for a really oddball suggestion? I'm hearing one guy claiming that the only correct way for P&W to address this is to use composite fan blades similar to what's used in GE90 engines and that the titanium alloy blades are no longer acceptable in the application for safety reasons.
 

4WD

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I've been following this story for a while. How's this for a really oddball suggestion? I'm hearing one guy claiming that the only correct way for P&W to address this is to use composite fan blades similar to what's used in GE90 engines and that the titanium alloy blades are no longer acceptable in the application for safety reasons.
Think only GE is using them on the fan … obviously not in the HT/HP sections …

 
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While not directly related, this thread hijack question comes from reviewing P&W engine (well, general high-bypass turbofan engine; GE looks the same) sketches. CONCEPTUALLY, is a high bypass (fanjet) like a turboprop with the "fan" and "prop" just realized differently? Web efficiency sketches suggest the "prop" is better up to maybe 350-400 kt with the fanjet passing it around there. Is this a tip vortice issue with the shrouded fan minimizing them?

GE and P&W had prototype propfans. Kind of like that scene in Back to the Future, "I'm afraid you're just too darn loud".


 
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Think only GE is using them on the fan … obviously not in the HT/HP sections …

P&W and RR have actual experience with them. Not sure if RR had them in production, but look at one of the PW1000G geared turbofans with their composite blades with pitch control. Still - I was under the impression that it's not as simple as just substituting them, especially it's too hot. I'm not sure how many they've got left, but I really doubt they're going to redesign a 30 year old product.

It's my understanding that they're absolutely not suitable in something like a fighter jet where it's going to be too hot jammed inside.

pw_gtf_tech_production_1_738x622.jpg
 

MolaKule

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I bet your right, I just can't explain it as well as you
I only wish I had one tenth of the flight experience that you and Astro 14 possess.

In one of the courses in graduate school, Mechanics and Thermodynamics of Propulsion, we studied the design and performance of almost every jet turbine engine available at the time including the PW4000 and the RR RB211 series among others. We also studied rocket design and propulsion but jet engine design and performance was my primary interest.
 
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Those propfans are crazy. There's nothing to force the two props to turn at the same rpm. Hopefully they turn at least close to the same rpm, otherwise one is in danger of overspeed. Close but not exactly the same would set up a beat frequency that would give everyone a throbbing headache.
 
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I only wish I had one tenth of the flight experience that you and Astro 14 possess.

In one of the courses in graduate school, Mechanics and Thermodynamics of Propulsion, we studied the design and performance of almost every jet turbine engine available at the time including the PW4000 and the RR RB211 series among others. We also studied rocket design and propulsion but jet engine design and performance was my primary interest.
Thanks.

You are a very knowledgeable man and can explain things well. Same with Astro. Good technical writing skills.
 
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I only wish I had one tenth of the flight experience that you and Astro 14 possess.

In one of the courses in graduate school, Mechanics and Thermodynamics of Propulsion, we studied the design and performance of almost every jet turbine engine available at the time including the PW4000 and the RR RB211 series among others. We also studied rocket design and propulsion but jet engine design and performance was my primary interest.

How much did you get into fan blade and turbine blade construction? I was only taking an introductory class about 30 years ago, but I was told our professor (quite a character I might add) was one of the leading authorities in cyclical metal fatigue in the world. I'm pretty sure he's having a field day right now with this fan blade failure unless he's retired. He talked a lot about superalloys for turbine blades and how fatigue results in failure of metals.

  • High-cycle fatigue of turbine engine alloys: Ti-6Al-4V; Ni-base alloys; grain-boundary engineering;
 
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I don't know how experienced you guys are with jet engine maintenance but the fan blade set is considered its own engine module and is handled as such with each overhaul. Fan blade sets are removed and the blade dovetails are lubricated at scheduled intervals. At these intervals the blades undergo a simple visual inspection where they're inspected for nicks and other damage. At engine overhaul the blade set is completely stripped and undergoes several forms of NDT inspection to include eddy current, radiography and fluorescent penetrant. It is after these inspections that the blade set is classified as an overhauled component.

I know I've posted these pictures before but here are some fascinating pics to remind us of how critical it is to keep these machines maintained. Picture (1) is a stage 2 HPT disk from the no. 1 engine of a B767. The disk cracked, tore from the engine, hit the ground and is seen lodged in the no. 2 engine. Picture (2) is another view of the stage 2 disk (GE CF6). Picture (3) is of a high pressure compressor disk from a Pratt 4000 that was being inspected for overhaul. This crack in the dovetail post was caught before the associated blade was able to depart the disk and cause engine damage. The method of inspection for these parts is fluorescent penetrant. Picture (4) is of the CF6 overhaul facility where I used to work. These are all CF6 fan frames for engines being heavy maintenance overhauled.
 

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Those propfans are crazy. There's nothing to force the two props to turn at the same rpm. Hopefully they turn at least close to the same rpm, otherwise one is in danger of overspeed. Close but not exactly the same would set up a beat frequency that would give everyone a throbbing headache.

In a successful attempt at mitigating the resonance of a fan, Eurocopter (now Airbus) has staggered the blades. I always believed that the UDF/propfan could use the same method and get the same positive result.

The 1:00 and 2:00 blade pairs are much closer than the 9:00 and 10:30 pair.

KGnfQ4Z.jpg


Here's our aircraft with the titanium fan blades removed and out for inspection/overhaul. We pull them out every 1000 hours for inspection and blade root lubrication. They get a 4000 hour "rework" and that's what this event was.

lYwZcw6.jpg
 

MolaKule

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How much did you get into fan blade and turbine blade construction? I was only taking an introductory class about 30 years ago, but I was told our professor (quite a character I might add) was one of the leading authorities in cyclical metal fatigue in the world. I'm pretty sure he's having a field day right now with this fan blade failure unless he's retired. He talked a lot about superalloys for turbine blades and how fatigue results in failure of metals.

  • High-cycle fatigue of turbine engine alloys: Ti-6Al-4V; Ni-base alloys; grain-boundary engineering;
Turbine blade construction concentrated on various high-temp metal alloys with ever increasing inlet temperatures, and their cooling methods. At the time, first stage turbine blades were mostly of single crystal metallurgy to improve their strength and resistance to creep under high temps.

For Fan Blades, it was suggested (for efficiency reasons) that it be a wide-chord blade made of a light, but strong metallic core with a carbon fiber overlay, or be made of a titanium honeycomb structure.

In terms of forces, for example, the RR RB211-535E4 fan had a 74" dia., a fan tip speed of 1500 ft/sec. Each 15-lb. blade was subjected to a centrifugal force of 60 tons!
 
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