What happened
On January 25, 2006, a Boeing 737-400, registration OK-YGU, operated by České airlines a.s., was performing a flight from Brussels to Prague (flight CSA633). During the flight, the captain experienced a strong, unpleasant odor in the cockpit, which he described as a cosmetic-type scent. The odor was also noted by the first officer, though at a lower intensity.
As the intensity of the smell increased, the captain experienced eye irritation and physical discomfort. To mitigate the effects, the captain decided to don a crew oxygen mask and set the oxygen supply to 100%. After approximately ten minutes, the captain noted that the eye irritation had subsided and, after confirming the odor was still detectable by the first officer, removed the mask. The flight was completed safely at Prague Ruzyně Airport without any injuries to the crew or the 96 passengers on board.
The investigation
The ÚZPLN investigation examined the aircraft's maintenance history and the technical state of the lavatory systems. Prior to the incident, the aircraft had flown from Prague to Brussels, where the crew had noted a similar, though much weaker, odor. At Brussels, ground technicians inspected the aircraft but could not find a specific fault, though they suspected a frozen water system due to freezing ground temperatures. As a precaution, the forward lavatory was taken out of service under MEL 38-2.
Subsequent technical inspections following the return to Prague revealed a surface leak in the filling hose of the forward lavatory tank, approximately 50 mm from the filling point. This leak had caused disinfectant solution to soak into the soundproofing insulation in the forward fuselage area. The investigation found that air from this contaminated area was being drawn into the cockpit via the equipment cooling system, which provides air to cool the EFIS instrument displays.
Findings
- The primary cause of the incident was the contamination of the air supply for the EFIS equipment cooling system, which distributed the odor into the cockpit.
- This contamination was caused by a leak in the forward lavatory filling hose.
- The intensity of the odor varied depending on the disinfectant solution used during the filling process at different airports; the use of a concentrated disinfectant solution in Brussels led to higher levels of contamination compared to the plain water used in Prague.
- The leak was located in a position that was not detectable without removing access panels to the forward pressurized fuselage area.
- The captain's decision to use the oxygen mask was appropriate and in accordance with non-normal procedures to ensure flight safety.