Investigators have determined that a CityJet-operated Bombardier CRJ900 landed in a tailwind on a contaminated runway, on which it hydroplaned and spun more than 180° before coming to rest.
The crew of the aircraft, conducting a service for SAS to Turku on 25 October last year, had opted to use runway 26 – despite the tailwind – owing to a lack of ILS on the opposite-direction runway 08.
Although the CRJ900 landed at 148kt in the correct zone, the touchdown was relatively hard at 1.95g. The captain had selected reverse thrust immediately after touchdown but the aircraft’s rebound effectively inhibited this system, and the engines remained at reverse idle.
When the nose-gear made runway contact the captain initiated manual braking. But the aircraft’s wheels began to hydroplane and did not spin up to normal rotation speed.
The wheels locked some 5s after landing and the aircraft skidded for 2,050m, almost the entire length of the landing roll, says the Finnish Safety Investigation Authority.
Some 1,200m from touchdown the jet began veering to the right, striking runway edge lights, and almost exiting the runway surface. But it started rotating to the left, still travelling at 42kt even while perpendicular to the runway centreline.
The aircraft continued to spin, eventually coming to rest just 160m from the runway end, having turned through an anti-clockwise arc of 196°.
At the time of the landing the air temperature was around freezing, with a gusting wind from the south-east, and the runway was covered with more than 10mm of slush.
Runway conditions had been assessed 21min before the CRJ900 landed, and an estimate passed to the crew.
But heavy snow was falling and the runway conditions were “changing rapidly”, says the inquiry, as a result of the slush accumulation. No new measurements were carried out before the jet landed, and maintenance personnel had decided to clear the runway only once the aircraft arrived and another flight had departed.
“The runway conditions transmitted to the flightcrew were not representative of the conditions prevailing at the time of landing,” says the investigation.
The tailwind component on landing was around 10kt, close to the maximum, and the aircraft was also near its maximum landing weight.
None of the tyres deflated and the inquiry says this meant the friction between the wheels and the runway was “extremely low” throughout.
It says the crew transmitted a ‘Mayday’ distress call as the aircraft was skidding along the runway and, while none of the 92 occupants was ultimately injured, adds: “The occurrence had potential for a serious or major accident.”
Investigators point out that the logic of the reverse-thrust system on the CRJ900 is such that, if selected prematurely on landing, it cannot be regained without specific pilot actions. The inquiry has recommended that Bombardier provides information to operators regarding this aspect of the thrust-reverser system.