Pratt & Whitney Jet Engines Failure

Repair and Life Extension of Titanium Alloy Fan Blades in Aircraft Gas Turbines

How technicians will search for weakened fan blades

.."The PW4000 engine has 22 blades, investigators said, one of which was found lodged inside the jet engine's containment ring. Another was found in a soccer field in Broomfield, Colorado.

One of the blades in the plane's right engine broke free at the hub, likely hitting another that was broken mid-span, investigators said Monday. The former shows damage "consistent with metal fatigue," NTSB Chairman Robert Sumwalt said."....

It seems like there should be a way to check these blades for cracks without taking the whole thing apart....What about hooking up some voltage on the end of the blade and looking for eddy current abnormalities...? Giant portable x-ray machine? Pinging the blade for tonal anomaly....vibrating the blades at a range of frequencies....Somebody should be working on a foolproof way of checking these blades with everything in place so that there would be little to no downtime...a roll-up machine that could be wheeled into place...what about a resonant frequency that matched the blades? Maybe they could implant some sensors that would enable this testing...any idea's?

https://www.aircraftspruce.com/catalog/pnpages/09-03052.php?gclid=Cj0KCQiA7NKBBhDBARIsAHbXCB6PXGzqgrwo6ZtwaROet85Ew302oV8f8buuXRFE0gx_yYf2ykiM8AUaAuvFEALw_wcB

https://www.sciencedirect.com/science/article/abs/pii/S1359645415005558

https://www.researchgate.net/publication/339936028_Investigation_of_Rejuvenation_of_Fatigue_Damage_in_Ti-6Al-4V

There are vibration sensors and analyzers that detect frequencies tuned to the rpm of the engine. I don't know how likely they are to detect cracked blades.

https://meggittsensing.com/aerospace/product/engine-vibration-monitoring-systems-for-on-wing-balancing-and-advanced-health-monitoring/

https://www.geaviation.com/bga/services/prognostic-health-management-plus

Externally, you can look at the fan blades, but the turbine blades are inside, and it would be impossible to perform the detailed inspection and testing that are done with engine disassembly.

Question : The story says that the plane was 26 years old, the story doesn't say how old the engine was (?) Any ideas

Although this is probably the original engine sold with the airplane 26 years ago without the complete maintenance records we have no way of knowing if they are the original fans....

https://www.wsj.com/articles/boeing-777-engines-made-by-pratt-whitney-already-faced-new-scrutiny-11614044828

https://en.wikipedia.org/wiki/Turbine_blade

"Most of the commercial airline industry is on what is called “On Condition Maintenance”. You can use a part as long as you want as long as it is within repairable limits and is brought back to serviceable condition. With High Pressure Turbine Blades which operate in a very hot environment, experience with the fleet will allow the airline to set a limit as to the max number of cycles they will allow these blades to operate. Note, the life of the blades is based on cycles, not hours. It’s the thermal transits that impact the life of the blade. Hours operated in a stabilized condition do not have much impact on blade life. Another thing to keep in mind is that once a HPT blade gets close to it’s life limit (soft time) it won’t be reinstalled in a fully overhauled engine. Blades with a similar life will be grouped into a set and installed in an engine that is going through what is called “Heavy Maintenance” , not a full overhaul.

Low Pressure Turbine (LPT) Blades are normally on an on condition basis. Operating temperatures are much lower so it’s not necessary to set a cycle limit."

https://www.quora.com/How-often-are-turbine-blades-in-jet-engines-replaced

"The new directive requires all engine fan blades to be inspected when they have undergone 20,000 takeoff and landing cycles, or by Aug. 31 if already past that threshold."May 1, 2018

https://www.heraldnet.com/nation-world/faa-orders-more-jet-engine-fan-blade-inspections/#:~:text=The%20new%20directive%20requires%20all,if%20already%20past%20that%20threshold.

https://www.flight-mechanic.com/turbine-engine-maintenance/

..."On Monday, the FAA acknowledged that after a Japan Airlines (JAL) PW4000 engine incident in December it had been considering stepping up blade inspections that use thermal acoustic imaging to find signs of metal fatigue."...

..."The Boeing Co 777 plane had flown nearly 3,000 cycles, equivalent to one take-off and landing, which compares to the checks every 6,500 cycles mandated after a separate United engine incident in 2018, said the sources."...

..."Pratt, the maker of the PW4000 engines, advised airlines on Monday to step up checks to every 1,000 cycles, in a bulletin seen by Reuters."....

https://www.reuters.com/article/us-boeing-777-unitedairlines/united-777-plane-flew-fewer-than-half-the-flights-allowed-between-checks-sources-idUSKBN2AP0B7

..."Tuesday’s order, known as an emergency airworthiness directive, requires an inspection process known as thermal acoustic imaging, or TAI, which is used to find cracks. It uses sound energy to create heat, and the resulting images are examined by inspectors, according to the NTSB.

“TAI technology can detect cracks on the interior surfaces of the hollow fan blades, or in areas that cannot be seen during a visual inspection,” the FAA said in its statement."...

https://www.washingtonpost.com/local/trafficandcommuting/faa-order-denver/2021/02/23/3893ce1e-75f3-11eb-948d-19472e683521_story.html

I would like to know what is the minimum cycles that occur between the time the cracks first become detectable till they experience catastrophic failure....this can only be determined through testing...testing every 1000 cycles sounds minimally reasonable, but without knowing the rate of crack detection to catastrophic failure, it means little...Also the age of the blades and any maintenance procedures or treatments that have been done, and flight anomalies such as impacts and vibration levels...I wonder what the number of occurrences of blades breaking in flight are...not many it would seem...I guess the FAA will be looking into all of this...

Another consideration is the metallurgy itself. There are typically variations in each batch of metal and these may react slightly different during machining and heat treatment. So while very similar, no two are exactly alike. Also, the environment plays a part too. Colder, denser air will have different pressures on the blades than warm dry air. Pollution may also have an effect. There are really so many variables that after many years of use you might as well treat each one as unique. The best way, as mentioned earlier, is with sensors monitoring the good condition and then determining that something is changing and needs looked at.

Found this....

...."In one exemplary TAI technique, an object that is being inspected is coupled to an ultrasonic broadband transducer. The ultrasonic broadband transducer is excited so that it couples a broadband acoustic signal into the object, which that introduces broadband sound energy into the object via acoustic waves. As acoustic waves are introduced into the object, the acoustic waves will cause defects in the object to vibrate.

To explain further, defects normally include opposing surfaces or interfaces, such as opposing edges of a crack, that move with respect to and against one another as the object is subjected to acoustic waves. Because the surfaces do not ordinarily vibrate in unison, they will rub against each other, which results in friction between or in the vicinity of the surfaces. Friction between the surfaces generates heat. Defects present in the object will heat up at a greater rate than other defect-free portions of the object, which are only minimally and uniformly heated. As temperature increases in the vicinity of the defect, infrared cameras or sensors can be used to detect defects that are discernable from other defect-free areas.

An infrared (IR) imaging device, such as thermal or infrared imaging camera, can then be used to capture images of the object that can be used to detect variations in temperature or infrared energy. For example, in some implementations, infrared energy can be detected using an IR camera that includes a two dimensional array of infrared detectors (pixels). Each pixel generates a signal that can be processed by an image processor to generate images. High resolution images can be captured by subtracting a background image (taken before acoustic excitation) from image(s) taken after excitation and recording the time derivative of the signal from each pixel instead the signal itself. Defects in the object will appear in the images as brighter areas in contrast to darker background areas that represent defect-free areas of the object.

Thus, thermal acoustic imaging techniques that combine acoustic induced heating with infrared imaging can be used as part of a very efficient, non-destructive defect detection system that can allow defects to be detected without needing to damage the object."...

https://patents.google.com/patent/US9194842B2/en

So you got a machine with 22 of these acoustic transducers and you clamp them to the fan blades(the outer most blades) and look for hotspots with your flir infrared imager...and Bob's yer' Uncle...

Whoops, looks like they already have this system in place, or something like it...

..."It added, "After reviewing the available data and considering other safety factors, the FAA determined that operators must conduct a thermal acoustic image (TAI) inspection of the large titanium fan blades located at the front of each engine. TAI technology can detect cracks on the interior surfaces of the hollow fan blades, or in areas that cannot be seen during a visual inspection.""...

Yahoo news

The biggest problem with "use it 'til it fails" is Maintenance. Because Maint. is the one area the airlines have complete control over so its the one area where they can take shortcuts and not get caught until something bad happens. (kinda like letting Boeing do thier own certifications. smh) Then the business model is to pay off the relatives of the dead and pretend it didnt happen.

Needs a wheel tapper

Del

My brother-in-law flew for the airlines for about 15 years and is now flying a Citation for a small group of owners. Last month he told me that he had to take the plane in for an inspection of specific turbine blades. During the last hot section overhaul, some blades were replaced and some were re-used. The blades have to inspected after a certain number of hours in service.

I thought this was kind of hokey. He said that the people that handle their maintenance would swap engines to keep the airplane flyable, then when they finished the inspection he would return the plane to the maintenance facility and they would re-install the original engine.

Virtually every sensor on the airplane is tracked by the maintenance contractor while it's flying. Pretty wild how they track stuff. He can fly somewhere and check his email after landing and get a message that the number 3 bearing temp was high and needs to be inspected prior to the next flight.

https://apnews.com/article/us-news-transportation-5e9f9b7dbc1515fe5a058e7ce65315f7