February 26th 2018 was a really cold day in Sofia Bulgaria. The temperatures were about -5C to -10C and it was snowing for almost 24 hours. On that day three different Wizz Air Airbus A321 experienced Airspeed indication problems during takeoff. Two of the three continued the takeoff performed the Airspeed Unreliable non-normal checklist and landed back in Sofia airport. The third airplane rejected the takeoff and taxied back
The Bulgarian Investigation Unit Directorate AMRAUD released yesterday a fascinating report (click here to read) that brings to light a phenomenon I was not aware of. Ice Ridges formation.
During the investigation an odd fact was discovered. The three WIZZ airplanes were the only three airplanes to experience such problems. And it is one of the less likeable problems.
The investigators found a few things common for these three flights other than the company. The main one obviously was aircraft type, company, ground handling crews. But they were also the only airplanes that were parked overnight at the airport.
This article is not written for the incidents but rather for the phenomenon.
What the investigators concluded is that the most likely reason for the erroneous airspeed indications is Ice Ridges formed in front of the pitot tube thus disturbing the airflow. Under normal conditions the pitot tubes are mounted on brackets, so they are away from the skin and the turbulent Boundary Layer and ere getting laminar airflow. What the investigators found out is that sometimes ice ridges are formed on the fuselage and can cause turbulence just Infront of the pitot tubes. The result is erroneous airspeed indication.
This ice ridges are basically icicle formed by accretion. (*Accretion is the formation of a shape by the process of adding layers of material. Like icicles and stalactites).
The Ice ridges are speculated to be caused by one of two phenomena. Both involve liquid water. The first would be freezing rain or other cause of dripping water along the side of the fuselage.
The second cause is speculated to be falling snow on a heated windshield melting, dripping down the side of the fuselage and refreezing. The accretion caused these ice ridges to be rather thick.
Most of the documented cases happened on the “first flight of the day” following elongated ground time.
Airbus issued a Bulletin (You can read it here) to all operators and De-Icing outfits. I am not aware of such a document by Boeing or other manufacturers.
All airplane manufacturers have a suggested procedures for elongated ground time in freezing conditions to include reduction of water pipe cracks danger door freezing etc. I just learned one more thing to look for. Even though de-icing is performed usually only after the airplane’s doors are closed.
Stay safe. It is still winter out there.
