What happened
On June 18, 2004, an Iberia McDonnell Douglas MD-88, registration EC-FLK, was performing a scheduled domestic flight from Madrid-Barajas to Alicante. The aircraft was carrying 151 passengers and 5 crew members.
During the pre-flight inspection, the crew found no anomalies. Due to high ambient temperatures of approximately 32 °C, the crew requested an external air conditioning unit to supplement the Auxiliary Power Unit (APU) for cabin cooling. After the preceding aircraft departed, the crew disconnected the APU and commenced takeoff.
As the aircraft climbed through approximately 3,500 to 4,000 feet, a sweet-smended smoke began to appear in the cockpit. The pilots soon received reports that smoke was also present in the passenger cabin. The crew requested to maintain an altitude of 5,000 feet and initiated an emergency return procedure. The smoke flow ceased once the aircraft leveled off at 5,000 feet, and the pilots decided to land without declaring a formal emergency. The aircraft landed safely at Madrid-Barajas approximately 14 minutes after takeoff, and all occupants disembarked normally.
The investigation
Following the incident, maintenance personnel identified high APU oil consumption, which had migrated into the air conditioning ducts. The investigation focused on the APU, a Garrett gas turbine unit.
Detailed workshop inspections of the APU revealed several issues. While borescope inspections showed no damage to the compressor or turbine blades, significant amounts of oil were found in the compressor area. The investigation identified that the carbon seal on the No. 1 compressor bearing was deteriorated. Additionally, a leak was found in the oil pump between the two pump bodies. A vacuum loss was also noted in the No. 2 turbine bearing cavity, though this leak was determined to discharge into the APU exhaust rather than the air conditioning system.
Findings
- The smoke and sweet odor in the cabin were caused by APU oil saturating the water separator mats within the air conditioning system.
- The primary cause of the oil entering the bleed air system was the deterioration of the No. 1 compressor bearing carbon seal, supplemented by a leak from the APU oil pump.
- A separate leak from the No. 2 turbine bearing had been present since May 2004, contributing to high oil consumption, but this leak only discharged into the exhaust.
- Maintenance records showed a pattern of high APU oil consumption and low oil pressure reports dating back to May 16, 2004, which had been addressed with oil top-ups and minor repairs but had not been traced to the underlying seal failure.