Factual Information
On 29 May 2017, a Boeing 737-800, registration OK-TSF, operated by Travel Service, a.s., was conducting a commercial flight from Warsaw/Babice Airport (EPBC) to Tel Aviv (LLBG). The aircraft departed at 16:15 UTC. At 17:02 UTC, while cruising at flight level 390 in the Kyiv UIR, the master caution warning illuminated. The flight crew observed the cabin differential pressure indicator fluctuating between 8 and 1 psi at least five times. Both pilots experienced vertigo and found normal breathing impossible. They donned oxygen masks and initiated an emergency descent. The pilot-in-command announced "RAPID DESCENT" over the cabin intercom. The cabin crew reported that passenger oxygen masks did not deploy automatically; the pilot activated them manually. During descent, conditions stabilized. The crew performed the emergency checklist, declared an emergency to ATC, set transponder code 7700, and diverted to the nearest suitable airport, Košice (LZKZ). After reaching FL100, they informed cabin crew and passengers. Upon switching to Košice Radar frequency, the crew canceled the emergency and set a new transponder code. Approach and landing at LZKZ occurred without further complication at 17:46 UTC.
Aircraft and Crew
The aircraft was a Boeing 737-800, serial number 37360, built in 2009. It had accumulated 21,342 hours and 10,154 cycles. The aircraft was owned by NBB MURRELET CO., LTD and operated by Travel Service. Power was provided by two CFM56-7B26/3 engines. The captain, aged 36, held an ATPL(A) with 3,761 total flight hours, including 3,486 hours on type as pilot-in-command. The first officer, aged 27, held a CPL(A) with 1,117 total flight hours. Both pilots had valid medical certificates and had rested 19 hours 55 minutes prior to duty.
Investigation Findings
Post-incident inspection at Košice revealed contamination in the static and dynamic pressure sensors and associated lines. The contamination consisted of sand, dust, and organic material (pollen, fluff, insects) baked inside the system. Additionally, the following components were found defective:
- Temperature sensor for engine No. 1: damaged by high temperature.
- Precooler control valve for engine No. 1: failed control test.
- High stage valve for engine No. 2: failed control test.
- Cabin altitude and differential pressure indicator: oscillated between 8 and 1 psi.
No injuries to crew or passengers were reported. One passenger required medical assistance after landing due to a blood pressure issue. No structural damage occurred to the aircraft. The aircraft had been repeatedly deployed on missions in areas with elevated levels of organic and inorganic airborne contaminants.
Probable Cause
Contamination of static and dynamic pressure sensors and lines by sand, dust, and organic material (pollen, fluff, insects) baked inside the system.