Narrative:

Myself and a coworker (pilot rated; current and on flying status for type) were on a repositioning flight. I had flown the airplane on several legs the night before and that morning with no abnormalities found. Takeoff; climb; and cruise were initially uneventful. Upon reaching cruising altitude of 16000 ft MSL; I engaged the autoplt and completed the necessary checklist. About 45 mins into cruise flight; we both heard a loud bang that came from the left side of the fueselage aft of the crew member seats and felt a moderate vibration. After looking out the window and scanning the engine instruments; I noticed the left engine torque gauge pegged out at 120% (120% is the highest reading the gauge will allow). Retarding the power lever had no effect on the torque gauge; engine RPM stayed constant; and egt decreased as expected. I thought that I did not have any control of the left engine and not wanting to do further damage to it; I shut down the engine. After completing the shutdown and completing the necessary checklists; I notified ATC of my situation and asked them for the current WX for ZZZ. Most of the airports local to us; where the shutdown happened; were small uncontrolled GA airports without acceptable apches and as snow fell heavily the night before and continued during the morning; I expected the airports to be unacceptable for landing. We were cleared direct to ZZZ and the GPS showed about 19 minutes en route. About the same time I shut the engine down; the cabin press low annunciator illuminated; and I requested a descent. I did not declare an emergency; however; ATC declared us as an emergency aircraft. When I had some time; I looked out of the window at the propeller and noticed a dent in the propeller about 3 inches from the tip on the leading edge of the blade with some paint scratched off of the back of the blade. At this time; we knew that the propeller hit something but were uncertain what it was and thought that on one engine; the remaining engine was just unable to supply sufficient air for pressurization. I was vectored for final at ZZZ following another emergency aircraft. I flew the approach and landed without further incident. I taxied to and shut down on the ramp. Upon completion of the appropriate checklists and securing the airplane; I looked at the left side of the airplane and saw that the outer window was shattered across the span of the window and was missing about half of the outer pane in the center of the window. We deduced that a failure of the outer window blew the fragment into the propeller; caused the vibration and torque malfunction; and resulted in the airplane unable to maintain cabin pressure on one engine with a breach of the cabin seal. I notified company dispatch and my immediate superior of the event and wrote up the problem in the airplane's traveling log. The total time from 'bang' to shutdown was less than 10 seconds. The mu-2B-60 is subject to a window inspection ad; however; I am uncertain how long ago the previous inspection was completed and when the next inspection is due. In retrospect; I should have declared an emergency upon shutdown; however; my company; to the best of my knowledge; does not have a policy as to that effect; and I was satisfied with the performance I was getting from the airplane. The WX was good day VFR; I was flying to a familiar airport; and getting satisfactory performance from the airplane. As well; I was confident in my own flying and command skills and from reading previous mu-2 accident reports where things went bad really quick; did not think I was in an emergency situation. There was not an explosive decompression with the outer window striking the propeller. Perhaps the window was slowly beginning to fail prior and the cabin pressure was increasing slowly in response to the failing component and it was too gradual for my ears to detect. After failure of the window coupled with the bleed air selection to the operating engine; I expect the cabin pressure increased enough to set off the annunciator. Our training emphasizes the importance of minimizing the stresses to the airplane's windows;especially the side windows by: ensuring a within-tolerance fuel balance; proper fueling (not excessively fueling in one side before switching sides); keeping the cabin door closed as often as possible; especially during refueling; and ensuring the main gear struts are serviced regularly and the airplane sits level. I see; however; single-hose refuelings with excessive airplane tilt; open doors before; during; and after refueling; improperly inflated main gear struts on between daily and now and then basis. Our company should implement a formal written policy to implement this portion of our training into everyday use. As far as the window inspection ad is concerned; I am uncertain if it entails anything more than a cursory visual inspection or something more demanding or even if it is limited to certain windows. Irrespective of this; these airplanes are going on 30 yrs old and my company flies them on a nightly basis. These are not owner-flown planes that are lucky to see 100 hours in a year...they are more likely to fly 100 hours in a month. Perhaps the inspection ad should be more frequent or more detailed for operators who fly these airplanes substantially more than what mitsubishi design them for. I'm happy it was an aft window and not a pilot compartment window or windshield. Callback conversation with reporter revealed the following information: the reporter stated this airplane had an airworthiness directive on the cabin windows and must be inspected at or before every 100 hours of flight. The company has a fairly good maintenance program and keeps the repetitive inspections current. Reporter indicated that no emergency was declared by the crew but the center did declare an emergency. Ground inspection revealed one half of the outer pane of the cabin window departed the aircraft and struck one blade of the propeller on the left engine causing a dent. The window inner pane remained intact but is much thinner than the outer pane. This airplane is about 30 years old and the reporter suggests the repetitive inspection time required by the airworthiness directive should be shortened to 50 hours of flight time. The company is accomplishing an engineering change order removing the windows and installing sheet metal plugs which should be the terminating action for the airworthiness directive. The engine was removed and sent out for testing and no results or findings on the engine condition have been received.

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Original NASA ASRS Text

Title: MU2 AT 16;000 FT; EXPERIENCED LEFT SIDE CABIN WINDOW OUTER PANE FAILURE. PARTS FROM THE FAILED OUTER PANE REPORTEDLY DAMAGED THE LEFT ENG AND PROPELLER; LEADING TO AN INFLT ENG SHUTDOWN AND EMER LNDG.

Narrative: MYSELF AND A COWORKER (PLT RATED; CURRENT AND ON FLYING STATUS FOR TYPE) WERE ON A REPOSITIONING FLT. I HAD FLOWN THE AIRPLANE ON SEVERAL LEGS THE NIGHT BEFORE AND THAT MORNING WITH NO ABNORMALITIES FOUND. TKOF; CLB; AND CRUISE WERE INITIALLY UNEVENTFUL. UPON REACHING CRUISING ALT OF 16000 FT MSL; I ENGAGED THE AUTOPLT AND COMPLETED THE NECESSARY CHKLIST. ABOUT 45 MINS INTO CRUISE FLT; WE BOTH HEARD A LOUD BANG THAT CAME FROM THE L SIDE OF THE FUESELAGE AFT OF THE CREW MEMBER SEATS AND FELT A MODERATE VIBRATION. AFTER LOOKING OUT THE WINDOW AND SCANNING THE ENG INSTRUMENTS; I NOTICED THE L ENG TORQUE GAUGE PEGGED OUT AT 120% (120% IS THE HIGHEST READING THE GAUGE WILL ALLOW). RETARDING THE POWER LEVER HAD NO EFFECT ON THE TORQUE GAUGE; ENG RPM STAYED CONSTANT; AND EGT DECREASED AS EXPECTED. I THOUGHT THAT I DID NOT HAVE ANY CTL OF THE L ENG AND NOT WANTING TO DO FURTHER DAMAGE TO IT; I SHUT DOWN THE ENG. AFTER COMPLETING THE SHUTDOWN AND COMPLETING THE NECESSARY CHKLISTS; I NOTIFIED ATC OF MY SIT AND ASKED THEM FOR THE CURRENT WX FOR ZZZ. MOST OF THE AIRPORTS LOCAL TO US; WHERE THE SHUTDOWN HAPPENED; WERE SMALL UNCTLED GA ARPTS WITHOUT ACCEPTABLE APCHES AND AS SNOW FELL HEAVILY THE NIGHT BEFORE AND CONTINUED DURING THE MORNING; I EXPECTED THE ARPTS TO BE UNACCEPTABLE FOR LNDG. WE WERE CLRED DIRECT TO ZZZ AND THE GPS SHOWED ABOUT 19 MINUTES ENRTE. ABOUT THE SAME TIME I SHUT THE ENG DOWN; THE CABIN PRESS LOW ANNUNCIATOR ILLUMINATED; AND I REQUESTED A DSCNT. I DID NOT DECLARE AN EMER; HOWEVER; ATC DECLARED US AS AN EMER ACFT. WHEN I HAD SOME TIME; I LOOKED OUT OF THE WINDOW AT THE PROPELLER AND NOTICED A DENT IN THE PROPELLER ABOUT 3 INCHES FROM THE TIP ON THE LEADING EDGE OF THE BLADE WITH SOME PAINT SCRATCHED OFF OF THE BACK OF THE BLADE. AT THIS TIME; WE KNEW THAT THE PROPELLER HIT SOMETHING BUT WERE UNCERTAIN WHAT IT WAS AND THOUGHT THAT ON ONE ENG; THE REMAINING ENG WAS JUST UNABLE TO SUPPLY SUFFICIENT AIR FOR PRESSURIZATION. I WAS VECTORED FOR FINAL AT ZZZ FOLLOWING ANOTHER EMER ACFT. I FLEW THE APCH AND LANDED WITHOUT FURTHER INCIDENT. I TAXIED TO AND SHUT DOWN ON THE RAMP. UPON COMPLETION OF THE APPROPRIATE CHKLISTS AND SECURING THE AIRPLANE; I LOOKED AT THE L SIDE OF THE AIRPLANE AND SAW THAT THE OUTER WINDOW WAS SHATTERED ACROSS THE SPAN OF THE WINDOW AND WAS MISSING ABOUT HALF OF THE OUTER PANE IN THE CENTER OF THE WINDOW. WE DEDUCED THAT A FAILURE OF THE OUTER WINDOW BLEW THE FRAGMENT INTO THE PROPELLER; CAUSED THE VIBRATION AND TORQUE MALFUNCTION; AND RESULTED IN THE AIRPLANE UNABLE TO MAINTAIN CABIN PRESSURE ON ONE ENG WITH A BREACH OF THE CABIN SEAL. I NOTIFIED COMPANY DISPATCH AND MY IMMEDIATE SUPERIOR OF THE EVENT AND WROTE UP THE PROB IN THE AIRPLANE'S TRAVELING LOG. THE TOTAL TIME FROM 'BANG' TO SHUTDOWN WAS LESS THAN 10 SECONDS. THE MU-2B-60 IS SUBJECT TO A WINDOW INSPECTION AD; HOWEVER; I AM UNCERTAIN HOW LONG AGO THE PREVIOUS INSPECTION WAS COMPLETED AND WHEN THE NEXT INSPECTION IS DUE. IN RETROSPECT; I SHOULD HAVE DECLARED AN EMER UPON SHUTDOWN; HOWEVER; MY COMPANY; TO THE BEST OF MY KNOWLEDGE; DOES NOT HAVE A POLICY AS TO THAT EFFECT; AND I WAS SATISFIED WITH THE PERFORMANCE I WAS GETTING FROM THE AIRPLANE. THE WX WAS GOOD DAY VFR; I WAS FLYING TO A FAMILIAR ARPT; AND GETTING SATISFACTORY PERFORMANCE FROM THE AIRPLANE. AS WELL; I WAS CONFIDENT IN MY OWN FLYING AND COMMAND SKILLS AND FROM READING PREVIOUS MU-2 ACCIDENT RPTS WHERE THINGS WENT BAD REALLY QUICK; DID NOT THINK I WAS IN AN EMER SIT. THERE WAS NOT AN EXPLOSIVE DECOMPRESSION WITH THE OUTER WINDOW STRIKING THE PROPELLER. PERHAPS THE WINDOW WAS SLOWLY BEGINNING TO FAIL PRIOR AND THE CABIN PRESSURE WAS INCREASING SLOWLY IN RESPONSE TO THE FAILING COMPONENT AND IT WAS TOO GRADUAL FOR MY EARS TO DETECT. AFTER FAILURE OF THE WINDOW COUPLED WITH THE BLEED AIR SELECTION TO THE OPERATING ENG; I EXPECT THE CABIN PRESSURE INCREASED ENOUGH TO SET OFF THE ANNUNCIATOR. OUR TRAINING EMPHASIZES THE IMPORTANCE OF MINIMIZING THE STRESSES TO THE AIRPLANE'S WINDOWS;ESPECIALLY THE SIDE WINDOWS BY: ENSURING A WITHIN-TOLERANCE FUEL BALANCE; PROPER FUELING (NOT EXCESSIVELY FUELING IN ONE SIDE BEFORE SWITCHING SIDES); KEEPING THE CABIN DOOR CLOSED AS OFTEN AS POSSIBLE; ESPECIALLY DURING REFUELING; AND ENSURING THE MAIN GEAR STRUTS ARE SERVICED REGULARLY AND THE AIRPLANE SITS LEVEL. I SEE; HOWEVER; SINGLE-HOSE REFUELINGS WITH EXCESSIVE AIRPLANE TILT; OPEN DOORS BEFORE; DURING; AND AFTER REFUELING; IMPROPERLY INFLATED MAIN GEAR STRUTS ON BETWEEN DAILY AND NOW AND THEN BASIS. OUR COMPANY SHOULD IMPLEMENT A FORMAL WRITTEN POLICY TO IMPLEMENT THIS PORTION OF OUR TRAINING INTO EVERYDAY USE. AS FAR AS THE WINDOW INSPECTION AD IS CONCERNED; I AM UNCERTAIN IF IT ENTAILS ANYTHING MORE THAN A CURSORY VISUAL INSPECTION OR SOMETHING MORE DEMANDING OR EVEN IF IT IS LIMITED TO CERTAIN WINDOWS. IRRESPECTIVE OF THIS; THESE AIRPLANES ARE GOING ON 30 YRS OLD AND MY COMPANY FLIES THEM ON A NIGHTLY BASIS. THESE ARE NOT OWNER-FLOWN PLANES THAT ARE LUCKY TO SEE 100 HOURS IN A YEAR...THEY ARE MORE LIKELY TO FLY 100 HOURS IN A MONTH. PERHAPS THE INSPECTION AD SHOULD BE MORE FREQUENT OR MORE DETAILED FOR OPERATORS WHO FLY THESE AIRPLANES SUBSTANTIALLY MORE THAN WHAT MITSUBISHI DESIGN THEM FOR. I'M HAPPY IT WAS AN AFT WINDOW AND NOT A PLT COMPARTMENT WINDOW OR WINDSHIELD. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THIS AIRPLANE HAD AN AIRWORTHINESS DIRECTIVE ON THE CABIN WINDOWS AND MUST BE INSPECTED AT OR BEFORE EVERY 100 HOURS OF FLT. THE COMPANY HAS A FAIRLY GOOD MAINT PROGRAM AND KEEPS THE REPETITIVE INSPECTIONS CURRENT. RPTR INDICATED THAT NO EMER WAS DECLARED BY THE CREW BUT THE CTR DID DECLARE AN EMER. GND INSPECTION REVEALED ONE HALF OF THE OUTER PANE OF THE CABIN WINDOW DEPARTED THE ACFT AND STRUCK ONE BLADE OF THE PROPELLER ON THE L ENG CAUSING A DENT. THE WINDOW INNER PANE REMAINED INTACT BUT IS MUCH THINNER THAN THE OUTER PANE. THIS AIRPLANE IS ABOUT 30 YEARS OLD AND THE RPTR SUGGESTS THE REPETITIVE INSPECTION TIME REQUIRED BY THE AIRWORTHINESS DIRECTIVE SHOULD BE SHORTENED TO 50 HOURS OF FLT TIME. THE COMPANY IS ACCOMPLISHING AN ENGINEERING CHANGE ORDER REMOVING THE WINDOWS AND INSTALLING SHEET METAL PLUGS WHICH SHOULD BE THE TERMINATING ACTION FOR THE AIRWORTHINESS DIRECTIVE. THE ENG WAS REMOVED AND SENT OUT FOR TESTING AND NO RESULTS OR FINDINGS ON THE ENG CONDITION HAVE BEEN RECEIVED.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.