Left engine lower cowling came off descending through FL330.

Attributes
ACN	322049
Time
Date	199511
Day	Mon
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : okk
State Reference	IN
Altitude	msl bound lower : 33000 msl bound upper : 33000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	artcc : zau
Operator	common carrier : air taxi
Make Model Name	Learjet 24
Operating Under FAR Part	Part 135
Flight Phase	descent other
Flight Plan	IFR
Person 1
Affiliation	company : air taxi
Function	flight crew : captain oversight : pic
Qualification	pilot : cfi pilot : atp
Experience	flight time last 90 days : 270 flight time total : 4800 flight time type : 2100
ASRS Report	322049
Person 2
Affiliation	company : air taxi
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
Events
Anomaly	other anomaly other
Independent Detector	other flight crewa
Resolutory Action	none taken : detected after the fact
Consequence	other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

The left engine lower cowling fell off the airplane in-flight. I noticed the cowling missing when I got out of the airplane in okk and performed my postflt inspection. Descending out of 33000 ft into okk we felt a momentary rumble, which is probably when the cowling fell off. The aircraft had flown approximately 14 hours since the last time the cowling had been removed by maintenance. The director of operations was notified immediately upon landing at okk. The reason for the event is unk. Callback conversation with reporter revealed the following information: the reporter stated he was flying a lear 24. There was an airworthiness directive on this model aircraft for this problem. The airworthiness directive note was complied with on this aircraft. The aircraft had flown 12-13 hours during the 24 hour period prior to losing the cowling. The cowling was latched properly and had not been unlatched over the past 24 hours. The spoilers were out and the speed was an estimated .77 mach when the cowl came off. By design the extended spoilers deflect the full force of the wing airflow right at the lost portion of the cowling. The company has about 20 lear jets and about 12 of this model 24. They are flown about 1000 hours per yr. This company has been operating for about 20 yrs with lear jets and has not had any prior problems with the cowling.

Original NASA ASRS Text

Title: L ENG LOWER COWLING CAME OFF DSNDING THROUGH FL330.

Narrative: THE L ENG LOWER COWLING FELL OFF THE AIRPLANE INFLT. I NOTICED THE COWLING MISSING WHEN I GOT OUT OF THE AIRPLANE IN OKK AND PERFORMED MY POSTFLT INSPECTION. DSNDING OUT OF 33000 FT INTO OKK WE FELT A MOMENTARY RUMBLE, WHICH IS PROBABLY WHEN THE COWLING FELL OFF. THE ACFT HAD FLOWN APPROX 14 HRS SINCE THE LAST TIME THE COWLING HAD BEEN REMOVED BY MAINT. THE DIRECTOR OF OPS WAS NOTIFIED IMMEDIATELY UPON LNDG AT OKK. THE REASON FOR THE EVENT IS UNK. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED HE WAS FLYING A LEAR 24. THERE WAS AN AIRWORTHINESS DIRECTIVE ON THIS MODEL ACFT FOR THIS PROB. THE AIRWORTHINESS DIRECTIVE NOTE WAS COMPLIED WITH ON THIS ACFT. THE ACFT HAD FLOWN 12-13 HRS DURING THE 24 HR PERIOD PRIOR TO LOSING THE COWLING. THE COWLING WAS LATCHED PROPERLY AND HAD NOT BEEN UNLATCHED OVER THE PAST 24 HRS. THE SPOILERS WERE OUT AND THE SPD WAS AN ESTIMATED .77 MACH WHEN THE COWL CAME OFF. BY DESIGN THE EXTENDED SPOILERS DEFLECT THE FULL FORCE OF THE WING AIRFLOW RIGHT AT THE LOST PORTION OF THE COWLING. THE COMPANY HAS ABOUT 20 LEAR JETS AND ABOUT 12 OF THIS MODEL 24. THEY ARE FLOWN ABOUT 1000 HRS PER YR. THIS COMPANY HAS BEEN OPERATING FOR ABOUT 20 YRS WITH LEAR JETS AND HAS NOT HAD ANY PRIOR PROBS WITH THE COWLING.

Data retrieved from NASA's ASRS site as of July 2007 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.