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First a thud, then a hole in jet engine

Discussion in 'AviatorChat.com' started by 2NA, Mar 26, 2009.

  1. 2NA

    2NA F1 World Champ
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    #1 2NA, Mar 26, 2009
    Last edited by a moderator: Sep 7, 2017
    Star Tribune

    Last update: March 26, 2009 - 12:29 AM


    Northwest Airlines passenger Brandon Snetsinger said he was "just flying along above the clouds" when he heard "a thud."

    "I looked out my window and noticed the hole in the engine," said Snetsinger, who on Wednesday recounted his hastily reconfigured trip from the Twin Cities to Phoenix on Monday and can count a photograph he took of the hole among his travel souvenirs.

    Snetsinger, of Greenfield, Minn., said he waited a moment or two after the thud, but "with nobody from the flight crew doing anything different, I called for the flight attendant. She called the captain, and he came back to look.

    "He walked a lot faster back to the cabin than he did to come check it out."

    Flight 121 was diverted to Denver, where its 165 passengers were put on a different plane later that day.

    The airline said the hole was in the left engine inlet of the twin-engine Boeing 757-200.

    "The left engine inlet has been replaced, and the aircraft is back in service," NWA spokeswoman Leslie Parker said Wednesday. The damaged part is being examined by engineers to determine how the hole was made, said Parker, who added that the engine "remained fully functional."

    PAUL WALSH


    It looks like it spit out a compressor blade to me. :eek:

    Probably lucky it didn't happen about 90 degrees higher up.
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  3. Bob Parks

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    If a compressor blade had departed, the nose cowl would have had catastrophic damage quickly followed by the engine going into convulsions and probably departing the airplane. The imbalance and rotational energy of 1000 pounds would have gotten everybody's attention, I think. I would bet on a nut or a small tool being shaken loose. I'm guessing and will be interested to find out what it was.
     
  4. future328driver

    future328driver Formula 3

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    #3 future328driver, Mar 26, 2009
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    Definitely not a compressor blade issue, since that would create catastrophic engine damage.

    The leading edge of the inlet is heated with hot bleed air coming from engine compressor section. Inside the inlet cowling behind the polished metal inlet lip is loop-shaped tube with small holes drilled into it. Bleed air from the engine flows through tubing from the engine compressor to this inlet tube and the heated air exits the holes in the tube and blows onto the backside of the inlet lip, thereby heating it to prevent icing. The tubing from the engine compressor to the engine inlet is not one straight tube - it is a series of tubes connected together by a flow control valve and various flange clamps that hold the tubing together. My guess is that a part of the flange clamp was blown off. Probably a bolt that is common on the types of v-band clamps used in this applications. The clamp is tightened using a screw and bolt and when you tighten the clamp using the screw, it can create a lot of force on the bolt which can break cause the flange to fly off/fly open. I used to design these types of systems for Citation aircraft and had more than one occasion in the pneumatics lab where the band clamp popped open forcefully.
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  5. 2NA

    2NA F1 World Champ
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    I've used those clamps, wouldn't any part there that came off under power at 500 mph go straight down the throat of the beast? Are you suggesting that this loose bolt/nut entered the compressor and was hit "outta the park"?
     
  6. future328driver

    future328driver Formula 3

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    The clamps are not actually exposed to the inlet airflow, so FOD from the clamps would not be carried into the engine by the velocity of the inlet air. All of the anti-icing tubing is plumbed inside the engine cowling outside of the engine itself. If the FOD had been blown off 180 degrees from the damage shown in the picture, then it would have been ejected into the inlet air stream and sucked into the engine.
     
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  8. solofast

    solofast Formula 3

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    #6 solofast, Mar 26, 2009
    Last edited: Mar 26, 2009
    It wasn't anything that was spit out from the rotating hardware. The rotating stuff is surrounded by a containment ring (generally a Kevlar wrap) tha surrounds the fan case. As noted, if anything got thru that the engine would have vibrated like crazy and the pilot would have gotten a high vibration warning.

    Moreover, the damage looks to be forward of the fan, so I doubt that it anything that was rotating. Anything that comes off of a rotor system pretty much stays in the plane of where it was when it was liberated. More likely something like Future 328 driver noted, having to do with the anti-ice system, but I didn't think that that system was pressurized all the time, only when the anti-ice is on. Could have been that it spit out the anti-ice valve or a piece of the plumbing.

    The forward fan case is generally pretty robust, they want to keep the case round so that the tip clearances don't get rubbed out if the case deforms (the early JT9D had a problem with fan case ovalization) and you have to be able to hold onto the energy relased by a liberated blade, so it isn't generally likey that something could get into the inlet from the outside of the fan case. If it is forward of the fan case, all bets are off since the skin up there isn't very thick.
     
  9. Bob Parks

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    That's exactly what I was gonna say...well almost. A liberated fan blade at 7000 RPM would be close to something like a 40MM cannon shell or worse. The way the blades are installed pretty much traps them in place with the inverted Christmas tree base and the matching recesses in the disc. I have never heard of a blade leaving the disc without much of the disc going with it. Having viewed tests where rocks and birds have been passed through the fan, the blades bent and sometimes spit the foreign object back out with the engine going nuts from the vibration but I have never heard of a blade flying off of one of the current generation of engines. Stellite and titanium are awful tough.
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  10. future328driver

    future328driver Formula 3

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    Solofast is right that the anti-ice system is not pressurized all the time. Even when it is pressurized, the pressure in the line would be very low since there is not much need for high-pressure bleed air in this application. That is why my thought is that it was a nut/bolt from one of the band-clamps. They are under a lot of mechanical stress when they are tightened.
     
  11. 2NA

    2NA F1 World Champ
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    I can't believe that a clamp springing open could propel a piece of hardware with enough force to penetrate the shell of the engine AND make enough noise to be heard inside the cabin over the sound of the engine.
     
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