Cityjet CRJ9 at Turku on Oct 25th 2017, pirouette during landing
Last Update: October 23, 2018 / 11:33:23 GMT/Zulu time
Incident Facts
Date of incident
Oct 25, 2017
Classification
Incident
Airline
Cityjet
Flight number
SK-4236
Departure
Stockholm, Sweden
Destination
Turku, Finland
Aircraft Registration
EI-FPD
Aircraft Type
Canadair CL-600 Regional Jet CRJ-705
ICAO Type Designator
CRJ9
Airport
Turku Airport, Turku
Airport ICAO Code
EFTU
The runway was closed for about 90 minutes for an inspection to ensure no damage to the runway, runway lights or other infrastructure had occurred.
The return flight was cancelled. The occurrence aircraft is still on the ground in Turku about 8 hours after landing.
Passengers reported the aircraft touched down hard and bounced several times until reaching the end of the runway and spinning around.
Emergency services reported the aircraft skidded sideways for some time before coming to a stop. The landscape and runway were completely covered in snow, weather most likely played a role in the occurrence that could have resulted in a more serious accident. Following inspection by emergency services the aircraft was able to taxi to the terminal on own power, the aircraft had not sustained any damage but needs to be examined. Finland's accident investigation board have opened an investigation into the occurrence.
On Oct 23rd 2018 Finland's Onnettomuustutkintakeskus (Safety Investigation Authority SAIF) released their final report releasing following conclusions:
1. Aerodrome maintenance decided to clear the runway after an arriving flight had landed and a departing flight had taken off. The decision was not reviewed even though weather was changing rapidly and continuous snowfall increased the thickness of the slush layer on the runway.
Conclusion: Long-term runway maintenance planning is not possible under rapidly changing weather conditions.
2. The flight crew did not question the weather and runway condition data on which they based their decisions. The flight crew monitored wind data closely.
Conclusion: Some time had passed from the preparation of the previous SNOWTAM. Personnel at the aerodrome knew that snowfall was intensifying
SNOWTAM reliability degrades fast under rapidly changing weather conditions.
3. The airplane was above the maximum performance limited landing weight on landing.
Conclusion: The multi-step procedure required to determine the permitted landing weight using the tables provided for the purpose contributed to an error in weight calculations.
4. At Turku, only runway 26 is ILS-equipped.
Conclusion: The flight crew elected to conduct a tailwind landing on runway 26 because they considered a runway 08 approach impossible due to the lack of ILS. Finland's airports, with the exception of Helsinki-Vantaa, have ILS at one end of the runway only.
5. Landing was firm; therefore, weight on the right main landing gear lightened as the oleo leg extended after touchdown, the extension reaching a point where the airplane systems sensed an airborne condition. The thrust reverser system, although armed, was unavailable after touchdown.
Conclusion: The flight crew was not familiar with reselecting the thrust reverser system in case of it being inhibited.
6. The airplane entered a hydroplaning condition at the moment of touchdown due to the high groundspeed and a slush deposit on the runway.
Conclusion: Anticipating the possibility of hydroplaning enhances situational awareness and prepares for a necessary action if the airplane enters a hydroplaning condition.
7. Hydroplaning prevented the wheels from spinning up to a required speed and therefore the anti-skid system did not activate. The captain's brake application, which was later augmented by simultaneous brake application by the first officer, resulted in the wheels remaining locked until the airplane came to a halt.
Conclusion: The flight crew did not recognize a hydroplaning condition and the fact that the wheels were not rotating.
8. The equation used to determine hydroplaning and hydroplaning speeds was verified in tests in the 1960s. The values derived from the equation do not necessarily correlate with modern aircraft tires, which may enter a hydroplaning condition at lower speeds.
Conclusion: More knowledge, reports, and possibly research will be needed for the reliable determination of the hydroplaning speeds of modern aircraft tires.
9. Since aileron control was not applied throughout the landing roll, the airplane started to veer towards the right edge of the runway, pushed by the wind.
Conclusion: In the CRJ900, the application of aileron control is important in order to maintain airplane control during a crosswind landing.
10. The verification of the prevailing runway conditions began approximately 25 min after the occurrence. Since the conditions were changing due to the snowfall, the runway conditions at the time of the occurrence were, perforce, based on estimates.
Conclusion: It is essential for investigation purposes that the prevailing runway conditions are determined immediately after an occurrence.
11. The controller called the ERC, which alerted rescue units to respond to a minor aircraft accident. Paramedic units were not alerted, and the incident commander was not consulted about the matter.
Conclusion: When a controller makes an emergency call while simultaneously attending to other duties, the processing time will increase and the risk assessment at the ERC will be affected by uncertainties.
The SAIF summarized the sequence of events:
After touchdown, the airplane traveled along the runway at 151 kt (280 km/h) groundspeed. It did not decelerate as anticipated after touchdown. During the landing roll it entered a skid and started to drift towards the right edge of the runway with the nose pointing to the left of track.
Approximately 1,200 m from touchdown, the skidding airplane began to veer to the right towards the edge of the paved area. It impacted and broke five runway edge lights. The minimum distance between the right mainwheel tires and the unpaved area was less than 0.5 m. The airplane then started to rotate to the left. When it was at right angles to the runway heading it was moving at 42 kt (78 km/h) groundspeed. It came to a halt next to the runway centerline 2,050 m from the initial touchdown point, having rotated 196° counter-clockwise from the initial direction of travel. The distance from the final position to the runway end was approximately 160 m.
...
The crew made an initial contact with Turku air traffic control (ATC) approximately 20 min before landing and received the latest information on runway and weather conditions. ATC reported friction as medium, deposits of 2 mm of slush over each third of the runway, and wind from 120° at 16 kt. The captain inquired ATC about runway 26 tailwind component, which was reported as 12 kt.
The crew noted this was above the permitted maximum, but since the tailwind component was close to the limiting value they decided to continue runway 26 instrument landing system (ILS) approach and land, provided the tailwind component would be 10 kt or less. Should the tailwind exceed the maximum they would execute a missed approach and divert to Helsinki.
They therefore requested ATC for an alternative missed approach clearance to 5,000 ft and also prepared to enter a hold. Approximately 8 min before landing, the captain studied the airplane’s performance calculations against the reported conditions. The captain calculated that the maximum performance limited4 landing weight for the prevailing conditions was 36,000 kg. The maximum structural landing weight of the CRJ900 is 34,065 kg. The crew members did not cross-check the calculations and continued the approach as planned.
When the airplane was 500 ft above ground level (AGL), ATC reported wind from 120° at 14 kt and a tailwind component of 10 kt. The captain took control and stated they would land. The captain disengaged the autopilot at 121 ft AGL. The captain crossed the runway threshold at a higher-than-normal descent rate in order to aim the airplane at the correct touchdown point. The airplane crossed the threshold5 at 151 kt indicated airspeed.
Touchdown occurred at a correct point within the aiming point markings at 151 kt groundspeed and 148 kt airspeed. Vertical acceleration at touchdown was 1.95 g. The captain selected full reverse thrust immediately after touchdown. At the same time the spoilers which are increasing aerodynamic braking and reducing the lift were activated. Due to a firm touchdown, weight on the landing gear lightened to such an extent that the airplane systems sensed an airborne condition. The design of the CRJ900’s full authority digital engine control system (FADEC) incorporates a logic that inhibits thrust reverser operation above idle power when the airplane is airborne. Consequently, reverse thrust was unavailable and FADEC commanded the engines to reverse idle. Although full reverse thrust remained selected until the airplane entered the skid, the engines remained at reverse idle.
The captain initiated manual braking upon nosewheel touchdown. However, after touchdown, the wheels started hydroplaning and did not spin up to the normal rotational speed. A function in the anti-skid system, which is designed to prevent wheel locking during brake application, inhibited the system, and the wheels locked after 5 s from touchdown.
The captain steered the airplane at first with the rudder and applied constant upwind, i.e., left aileron. 6 s after touchdown, the captain indicated an inability to control the airplane. The captain released the control wheel and attempted to regain control using nosewheel steering and the rudder. The captain also stated that a FADEC FAULT message had displayed; this resulted in reverse thrust remaining at idle. 13 s from touchdown, the first officer also initiated braking, assuming that the captain was not applying the brakes or the brakes were inoperative. The first officer did not notify the captain of the brake application. The rate of deceleration was low due to the lack of reverse thrust and the fact that the locked wheels were in a hydroplaning condition. Due to the loss of lateral grip, the airplane entered an uncontrolled left yaw 24 s after touchdown. The captain attempted to counter the yaw by applying right rudder until the rudder reached full right deflection.
Approximately 30 s after touchdown, at 2024 h, the first officer transmitted a mayday call and a distress message on the ATC frequency, believing the airplane was skidding and was about to depart the runway. The message overlapped a taxi clearance that the controller was issuing. The first officer repeated mayday and the message 10 s later, and 48 s after touchdown told ATC that the airplane had stopped.
There were no injuries. The aircraft's main tyres received minor damage (wear and impact marks with runway edge lights) and were considered unserviceable. Five runway edge lights were damaged, the runway was NOTAMed closed for 70 minutes.
The SAIF analysed with respect to aerodrome maintenance:
Runway conditions were assessed 21 min before the flight landed, and the estimate was passed to the flight crew. Heavy snow was falling at the aerodrome, and therefore runway conditions were changing rapidly due to slush accumulation. Maintenance personnel did not conduct new measurements before the landing of the flight since they estimated that the conditions were adequate for the landing of the arriving flight and the departure of a subsequent flight. They had decided to clear the runway after the latter flight had departed. Since they did not adjust their actions in view of the changing conditions, the runway conditions transmitted to the flight crew were not representative of the conditions prevailing at the time of landing.
With respect to the landing performance the SAIF analysed:
The captain verified that the tailwind component was within the permitted limits but did not realize that the airplane was above the maximum performance limited landing weight. The estimated landing weight was approximately 33,800–33,980 kg, which was at least 1,600 kg above the maximum performance limited landing weight. This was partly due to the complexity of the performance charts (ergonomic factors could be considered here) and a possible error in the calculations. The captain completed the calculations while managing the final stage of the approach, which may have contributed to the error. The crew members did not cross-check the calculations, even though this could have revealed the discrepancy. This indicates deficiencies in CityJet’s procedures and training.
The SAIF analysed landing and loss of control:
Touchdown occurred at the correct distance from the threshold albeit slightly right of the runway centerline. Despite a stable approach, the airplane started tracking to the right of the runway heading on touchdown, and as a result drifted to the right of the centerline. Furthermore, the wind raised the left wing, and this was not countered with aileron control. The resulting bank angle created a horizontal lift component to the right. On a crosswind landing on a contaminated runway it is particularly important to maintain the airplane track aligned with the runway heading. In addition, in the CRJ900, aileron control shall be used to compensate for any wind-induced banking tendency.
The airplane entered a hydroplaning condition at the moment of touchdown. A tailwind and a higher groundspeed, which was due to the prevailing conditions, contributed to the onset of hydroplaning. It is likely that the flight crew did not realize that the wheels were in a hydroplaning condition, and the aircraft does not have a system that would caution the crew of abnormal deceleration. It is likely that releasing the brakes, repeatedly if necessary, would have spun up the wheels, which in turn would have increased lateral grip. Aircraft systems that use wheel speed information may not operate correctly if the wheels do not rotate.
The captain had taken control on the final approach thereby assuming responsibility for all actions affecting aircraft control. During the initial phase of the landing roll, the captain indicated an inability to control the airplane. The first officer also applied the brakes but did not notify the captain of this, assuming that the captain was not applying pressure on the brake pedals or the brakes had failed. Since the captain continued braking until the airplane came to a halt, the first officer's brake application had no effect, and regaining control was therefore impossible.
CityJet’s OM-B instructs to reduce the brake pressure during a skid until airplane control is regained. Momentary brake release could have caused the wheels to spin up, in which case the anti-skid system would have activated to prevent subsequent wheel locking.
The AFM gives 116 kt as the hydroplaning initiation speed. The correct threshold crossing speed would have been 141 kt (Vref), but due to gusty wind the captain flew the approach at a higher airspeed. Groundspeed on touchdown was 151 kt so the possibility of hydroplaning was significant.
...
Reverse thrust was not available above reverse idle since the airplane systems recorded an airborne condition and the thrust reverser system was inhibited by FADEC. If reverse thrust is selected prematurely during landing it cannot be regained without specific flight crew actions. In order to use reverse thrust, the flight crew should have selected the reversers stowed. This procedure was unknown to the flight crew. Paragraph 2.4.9 of OM-B contains guidance on the use of reverse thrust on landing. No action was carried out in order to reselect the reversers after the premature selection of the system, which had triggered a FADEC FAULT message. The AFM contains instructions after a FADEC FAULT indication during any phase of a flight. This indication may be displayed for several reasons and it has no bearing on a thrust reverser system malfunction in particular.
CityJet’s OM-B contains instructions that should be followed if the airplane starts to skid during a crosswind landing. According to the manual, reverse thrust should be reduced or even canceled, which would improve aircraft control. However, reverse thrust was at idle and therefore unavailable. With reverse thrust available, the wheels would still have entered a hydroplaning condition, but would probably have exited this condition as speed decayed. Reverse thrust is most effective at high speeds.
The captain was facing a challenging situation, in which stress factors included a firm landing, a slippery runway, lateral forces acting on the airplane, and unexpected thrust reverser and braking performance. The situation was conducive to human errors despite an individual’s familiarity with the applicable procedures.
Related NOTAM:
A3108/17 NOTAMN
Q) EFIN/QMRLC/IV/NBO/A /000/999/6031N02216E005
A) EFTU B) 1710251755 C) 1710251900 EST
E) RWY08/26 CLSD
Metars:
EFTU 251920Z AUTO 12013KT 1200 R08/P2000 SN OVC004/// M00/M00 Q1006=
EFTU 251850Z AUTO 12013KT 090V160 1800 SN OVC004/// M00/M00 Q1007=
EFTU 251820Z AUTO 12013KT 090V150 1400 R08/P2000 SN OVC004/// M00/M00 Q1007=
EFTU 251750Z AUTO 12012KT 1700 SN OVC005/// 00/M00 Q1008=
EFTU 251720Z AUTO 13014KT 2100 -SN OVC005/// 00/M00 Q1008=
EFTU 251650Z AUTO 12014KT 090V150 2200 -SN OVC005/// 00/M00 Q1009=
EFTU 251620Z AUTO 12014KT 2000 SN OVC005/// 00/M00 Q1009=
EFTU 251550Z AUTO 13013G24KT 100V160 4100 -SN BKN006/// OVC012/// 01/M00 Q1010=
EFTU 251520Z AUTO 12013KT 090V160 5000 -RASN OVC008/// 01/M00 Q1011=
EFTU 251450Z AUTO 13014KT 090V160 7000 -RA FEW014/// BKN016/// OVC023/// 02/M00 Q1011=
EFTU 251420Z AUTO 13015G25KT 110V170 9999 -RA OVC021/// 03/M01 Q1012=
Aircraft Registration Data
Incident Facts
Date of incident
Oct 25, 2017
Classification
Incident
Airline
Cityjet
Flight number
SK-4236
Departure
Stockholm, Sweden
Destination
Turku, Finland
Aircraft Registration
EI-FPD
Aircraft Type
Canadair CL-600 Regional Jet CRJ-705
ICAO Type Designator
CRJ9
Airport
Turku Airport, Turku
Airport ICAO Code
EFTU
This article is published under license from Avherald.com. © of text by Avherald.com.
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