What happened
On May 29, 2017, a Boeing 737-800, registration OK-TSF, operated by Travel service, a.s., was conducting a commercial flight from Warsaw/Babice (EPBC) to Tel Aviv (LLBG). While cruising at FL390 within the Kyiv UIR, the aircraft triggered a Master Caution alert.
Following the alert, the flight crew observed significant oscillations in the cabin differential pressure indicator, with values fluctuating between 8 and 1 psi at least five times. The sudden pressure changes caused both the pilot and co-pilot to experience dizziness, and the crew eventually found it impossible to breathe normally. Both pilots donned oxygen masks, and the captain initiated a rapid descent.
During the descent, the crew manually activated the passenger oxygen masks after being informed by the purser that they had not deployed automatically. The crew prioritized reaching a safe altitude and notifying Air Traffic Control (ATC). The aircraft descended to FL100, where the situation stabilized. The crew declared an emergency, set the transponder to 7700, and diverted to Košice (LZKZ), landing without further complications. One passenger required medical attention due to blood pressure issues, but there were no injuries to the crew or other passengers.
The investigation
The ÚZPLN investigation focused on the cause of the pressure fluctuations and the subsequent emergency. Upon inspection of the aircraft at LZKZ, investigators discovered that the static and dynamic pressure sensors and their associated lines were contaminated with both organic and inorganic materials, including sand, dust, pollen, and insects.
Further technical inspections revealed several component failures:
- The engine number 1 temperature sensor had sustained damage from high temperatures.
- The engine number 1 precooler control valve failed its functional test.
- The engine number 2 high stage valve failed its functional test.
- The cabin altitude and differential pressure indicator was malfunctioning, exhibiting the observed oscillations.
Findings
- The primary cause of the incident was reduced permeability of the static and dynamic pressure systems caused by the accumulation of dust, sand, and organic debris.
- This contamination, combined with existing faults in various pressurization components, triggered the Master Caution alerts and the erratic cabin differential pressure readings.
- The flight crew followed established emergency procedures and successfully managed the rapid descent and diversion.