Date: 201309
Local time of day: 1801-2400
Location: ZZZZ.ARTCC, FO
Flight conditions: Mixed
Weather elements / visibility: 10
Light: Night
Aircraft: Learjet 35
Operator: Air Taxi
Operating under: Part 135
Flight plan: IFR
Mission: Ambulance
Flight phase: Other Final climb/level off
ATC / advisory: Center ZZZZ
Aircraft component: Engine Air Pneumatic Ducting
Component problem: Malfunctioning
Reporter organization: Air Taxi
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Instructor; Flight Crew Instrument; Flight Crew Multiengine
Human factors: Time Pressure; Troubleshooting; Workload; Physiological - Other
Anomaly: Aircraft Equipment Problem Critical; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Detector: Person Passenger; Person Flight Crew
When detected: In-flight
Result: Flight Crew FLC complied w / Automation / Advisory; Flight Crew Landed in Emergency Condition; Flight Crew Took Evasive Action; Flight Crew Diverted; General Evacuated; General Maintenance Action; General Declared Emergency
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: Climbing through FL360 to FL380; our medical crew notified the flight crew that they smelled smoke in the cabin. I sent the First Officer back to the rear of the cabin (which is also our accessible rear baggage compartment) to investigate for flames; smoke; and an origin. While the First Officer was in the cabin; I leveled the aircraft at FL380 and starting looking for suitable airports in the GPS to land. Upon pulling the rear baggage forward he verified that there was smoke but no visible flames or damage. He could not verify the source other than it was coming from the rear of the cabin. When he arrived back up front; I looked back into the cabin briefly and saw a distinct smoke layer in the rear of the cabin. At this time I was starting to smell the smoke in the cockpit. The First Officer and I had a quick discussion and came to a mutual conclusion that we needed to land immediately. After discussing our landing options we felt a major airport was too far to safely turn back and that a smaller nearby airport was our best option. Although uncontrolled; it had a radio FSS facility; ILS; and more than adequate runway distance. We were about 30-35 west of the larger airport. We notified Center that we had smoke in the cabin and needed to divert. They cleared us to FL290. We initiated a descent at over 6;000 FT/minute. At this point the smoke started to get worse and we declared an emergency and notified ATC we were continuing our descent and asked vector for the ILS and weather/NOTAMS. The weather was a broken layer; I believe; at a few thousand feet and visibility was unrestricted. During the descent I had the First Officer run the Smoke In The Cabin/Cockpit Checklist; [and] then afterward we followed up with normal checklists as they were appropriate. The Abnormal/Emergency Checklists had us turn off the bleed airs one at a time. Each side seemed to slow or stop the smoke from getting worse; but the situation did not improve until we were on the ILS and on our final approach. We intercepted the ILS; and proceeded on the approach as we would in a normal situation. We landed at the divert airport and evacuated the plane with emergency ground crews standing by. No property was damaged and no injuries occurred. After mechanics looked at the aircraft; in the following few days; it was found that in the rear baggage compartment behind the rear baggage compartment wall there was an issue. Normally the bleed air duct has a thermal blanket wrapped around the duct to insulate the hot bleed air duct from the wall of the baggage department. Around the thermal blanket is a second piece of insulation between the wall and the thermal wrap. The thermal wrap had loosened exposing the bleed air duct to direct contact with secondary insulation. The bleed air duct was burning (through heat exchange not flames) the secondary insulation causing it to create smoke. In hind sight; this would explain why turning off the bleed airs; as called by the checklist; did not help to any notable degree. I would think that the smoke lessened due to the throttles being pulled back to idle for the descent; approach; and landing. Even though the smoke was not thick; it definitely was noticeable. As a crew; I believe we made the right decision to safely land before the situation could get worse.




Synopsis: A LR35 developed smoke in the aft cabin which the First Officer found was emanating in rear baggage compartment so an emergency was declared and the flight diverted. Bleed air heated a secondary insulation blanket creating smoke but no fire.