What happened
On 28 December 2005, a Boeing 737-73V, registration G-EZKA, was preparing for a scheduled passenger flight from Newcastle to Budapest. Due to significant snow accumulation on the airframe, wings, and tailplane, the aircraft underwent de-icing procedures prior to departure.
Because of performance limitations requiring maximum engine thrust, the flight crew performed a 'No Engine Bleed Air Takeoff'. This configuration required the engine bleed air to be switched off, relying instead on the Auxiliary Power Unit (APU) bleed air to provide air conditioning and pressurisation during the initial climb.
While climbing through approximately 300 feet, the commander noticed a faint odor, which was quickly followed by the appearance of thick black smoke behind the commander's shoulder. The smoke rapidly spread through the cockpit, prompting the crew to don oxygen masks. Simultaneously, cabin crew reported that the cabin air had also become contaminated. The first officer isolated the APU bleed air and switched to engine bleed air, which caused the smoke and fumes to dissipate. The flight crew declared a PAN and returned the aircraft to Newcastle, where passengers were provided with medical attention for eye and throat irritation.
The investigation
The AAIB investigation examined the aircraft's maintenance records and the de-icing process. A review of the CAA’s Mandatory Occurrence Report database identified at least three previous instances of contaminated bleed air on Boeing 7/37 aircraft caused by de-icing fluid entering the APU inlet.
An inspection of the aircraft by maintenance personnel found no mechanical defects in the bleed air or air conditioning systems. The investigation also reviewed manufacturer documentation, which includes a supplementary procedure for 'No Engine Bleed Takeoff and Landing' but does not specifically address the risk of de-icing fluid contamination during such operations.
Findings
- The primary cause of the smoke and fumes was excess de-icing fluid entering the APU air inlet during the de-icing process.
- The ingested fluid entered the hot sections of the APU, generating smoke and odors that were distributed through the aircraft's air conditioning system.
- There were no injuries to the 5 crew members or 128 passengers, and no damage was sustained by the aircraft.