Baltic BCS3 at Riga on Mar 8th 2023, runway excursion on landing

Last Update: May 21, 2024 / 17:08:35 GMT/Zulu time

Bookmark this article

Photo of Air Baltic YL-AAP, Airbus A220-300 — Air Baltic YL-AAP, Airbus A220-300 (Photo credit: usf1fan2 / Flickr / License: CC by)

Incident Facts

Date of incident
Mar 8, 2023

Classification
Incident

Cause
Runway excursion on landing

Airline
Air Baltic

Flight number
BT-694

Departure
Paris Charles de Gaulle, France

Destination
Riga, Latvia

Aircraft Registration
YL-AAP

Aircraft Type
Airbus A220-300

ICAO Type Designator
BCS3

An Air Baltic Bombardier C-Series CS-300, registration YL-AAP performing flight BT-694 from Paris Charles de Gaulle (France) to Riga (Latvia) with 89 passengers and 6 crew, landed on Riga's runway 18 at 23:17L (21:17Z), however, the nose gear went off the paved surface after landing. There were no injuries.

The airline stated: "The Latvian Airline AirBaltic confirms that on March 8, 2023 at 21:17 UTC (23:17 local time) during a heavy snowfall its flight BT694 from Paris to Riga performed with Airbus A220-300 (YL-AAP) the nose wheel slid off the hard runway surface after the landing in Riga". The passengers and crew were safely taken to the terminal.

Latvia's TAIIB released their final report concluding the probable causes of the serious incident were:

The runway excursion of the aircraft during landing.

Contributing causes

- Landing on the contaminated runway surface under increasing wind gusts;

- Rapidly changes of the runway surface condition due to the meteorology;

- Uncontrolled sliding of the aircraft on the runway surface;

- Inaccuracy of assessment of the airport runway condition;

- Delay in the apply of the manual braking by the pilot.

Proximate cause

The deficiencies in the operation of the aircraft brake control system.

The TAIIB summarized the sequence of events:

During the landing ground roll and after reducing of airspeed the aircraft deviated to the right of the runway centerline and started skidding. The aircraft came to a complete stop with the nose landing gear wheels approximately 11m from the runway paved surface and 1560m from the THR18 (Threshold of the RWY). Both MGL (Main Landing Gear) remained on runway paved surface. The passengers and the crew of the aircraft were disembarked safely.

The TAIIB analysed with respect to YL-AAP, flight BT-694, Callsign BTI-4HE:

The crew did not report any flight abnormal situation during the approach to the RIX airport. The flight BT1-4HE was informed about the wind change at distance of about 2 nm final from the RWY18. According to the ATIS EVRA message at 21.07 the runway surface was covered with dry snow about 5 mm, the wind speed was 8 kts with wind gusts 19 kts and the wind direction 240⁰.

But at 21.10, according to the constant wind measurements provided by ATC services for the RIX landing the wind gust speed increased to 20.99 kts, and at 21.16.13 the ATC informed the next flight BTI-62D: “wind check 250 degrees 10 kts gusts 21 kts”. The gust speed was close to the maximum recommended crosswind of 20 kts according to the runway condition assessment matrix (RCAM) code "3" in Table II-2-5 of ICAO Doc 9981.

Despite the increased gusts of wind and the actual runway surface condition with the code "3" 4 minutes before the flight ВТ1-4НЕ another airBaltic flight BTI-49E performed an uneventful landing on the RWY18 and the aircraft crew did not provide information about the slippery surface condition on any runway sections.

At 21.17.40, the aircraft A220, call sign BT1-4HE, performed landing on the RWY18 (Landing course ~185⁰). The flight crew calculated the landing characteristics according to the received weather information. The flight crew did not indicate that any limits were exceeded. The estimated required landing distance (RLD) for the aircraft landing was ranged from 1551 m to 1783 m. After landing the aircraft rolled only 1560 m to a complete stop.

An hour after the occurrence, during the inspection of the incident site and assessing the surface condition of the runway and carrying out the necessary measurements the TAIIB investigators together with the responsible representatives of the airport established visually that the surface of the runway was not covered with ice and assessed the slipperiness of the runway surface by shoe scraping (ICAO Circular 355); in some places the RWY18 was covered with dry drifting snow. Consistent with a SNOWTAM issued by the ADE at 22.51, the RWYCC was declared the code "2" for the runway covered with compacted snow; but according to the runway condition assessment matrix (RCAM) the surface description “compacted snow” corresponds to the RWY surface condition with the code "3" (Table II-2-5 of ICAO Doc 9981).

According to the Section 1.12 of the TAIIB investigation report, snow tracks of the main landing gear tires found on the runway surface 97.2 meters long from the runway centerline, could have been caused by sliding of the blocked wheels of the main landing gear.

Possibly the main landing gear wheels were completely blocked after the pilot had rapidly pressed the brakes of the aircraft, subsequently the aircraft began to slide uncontrollably to the right from the RWY centerline.

From 21.17.53 to 21.18.02, according to FDR data the active operation of the anti-skid and auto-braking systems was observed. The active operation of the anti-skid protection and the further wheels locking at the time of the serious incident could happen possibly because there was so much water from the melted snow and runway de-icing fluids that it could freeze and form invisible ice (“black ice”) at an air temperature of -1°C and a surface temperature of -3.6°C degrees. An hour after the serious incident when TAIIB investigators arrived to the incident site, the RWY18 surface condition could have changed under the influence of meteorological conditions at an air temperature of 0°C and a surface temperature of -4.9°C, so it is likely that icing was not detected by the investigators during the inspection of the RWY18 surface.

During the investigation it was established that it remains a partial possibility that the runway surface condition code did not correspond to the code "3" in the anti-skid protection area, and thus, after considering all weather conditions it can be assumed that one of the contributing factors of the serious incident was the possible icing of some runway sections (middle part) on the braking actions of the aircraft in autobraking mode, although RWY was assessed according to ICAO Doc 9981 Runway Condition Assessment Matrix (RCAM).

Flight crew actions during landing conditions

Before the landing of the flight BT1-4HE at the Riga International airport the flight crew received information about wind gusts of 19 kts and the RWYCC "3. The crew checked the fuel on board, because of which it was determined that there was enough fuel for a comfortable landing without rushing to complete a safety landing or to divert to an alternate airport.

Immediately before landing, the flight crew did not receive updated information about increased wind gusts, so the landing parameters were calculated considering the current ATIS-RIGA weather information and the information from ATC about wind change at distance of about 2 nm final from RWY18. The received weather information was within the capabilities of the aircraft (headwind and crosswind components), so the flight crew took a decision to land at the Riga International airport. During the landing approach the flight crew reported of a severe turbulence, but all flight crew actions and weather conditions were within the parameters for a safe landing of the aircraft.

Crew statements

According to the testimony of the aircraft pilot (PF), the landing was carried out exactly along the centerline of the runway, immediately after landing he felt the anti-skid protection working and there was no traction with the surface of the runway, after the aircraft speed decreased, the aircraft began to veer to the right and the pilot felt that he had lost control of the aircraft landing ground roll. According to the co-pilot (PM), the aircraft began to slide and the anti-skid protection was activated at a speed around 60 kts, after that the aircraft started to deviate to the right. The third pilot (PNF) on the jump seat in a function of an observer also confirmed that the aircraft was moving to the right at 50-60 kts and that the PF was trying to correct it.

The FDR data of the anti-skid protection operation

The anti-skid protection activation and aircraft wheels skidding intervals during the landing ground roll are supported by FDR data displayed graphically.

The FDR data displayed graphically reveal that:

- the active operation phase of the anti-skid protection ranges from 11 sec to 23 sec after the aircraft touchdown and at a ground speed of about 60 kts;

- the skidding of the main landing gears wheels is from 14 sec to 23 sec after the aircraft touchdown, until the MLG wheels lock at 23 sec at a ground speed of about 30 kts.

Thus, it can be assumed that after the active operation of the anti-skid protection due to wheels skidding, the decreased wheels rotation speed could be a cause of the divergence of the wheel speed vs aircraft speed and early inhibition of the locked wheel protection and anti-skid function, and the subsequent MLG non-symmetrical wheels blocking, was probably a contributing factor for the pilot loss of control of the aircraft landing ground roll and the aircraft began to veer uncontrollably to the right.

...

The analysis of the recommendations and procedures for the Airbus A220-300 aircraft for landing on a contaminated runway indicates that the procedures for operators of this type of aircraft (until March 8, 2023) did not contain information about possible aircraft wheels blocking after the anti-skid function deactivation during the landing ground roll and about further consequences for the aircraft's controllability.

The Airbus A220 Brake Control System (BCS) operation during landing ground roll

During the 23 seconds of the aircraft landing ground roll to approximately half up the RWY18 the A220 aircraft remained close to the centerline of the runway until the antiskid protection turned off and the aircraft began to deviate to the right from the runway centerline. According to the FDR data and the displayed aircraft heading change plot the aircraft deviation from centerline at a ground speed of about 60 kts was also minimal, approximately +1.50 from the RWY18 true direction.

At 21.18.02, due to the skidding of the aircraft wheels the wheelsrotation speed dropped to less than 10 kts, therefore, the wheel rotation signal, necessary for the operation of the anti-skid system, disappeared. It is possible that the wheels blocking and the crosswind were the primary reasons for the gradual deviation of the aircraft to the right.

As the wheels rotation signals from wheel speed transducers (WST) reduced below 10 kts, the anti-skid protection was de-activated and therefore the aircraft's automatic braking system activated full braking force (increased to ~10400lbs), the wheels blocked completely, and the aircraft began to drift towards the right side of the runway at a ground speed 34 kts.

At 21.18.03, the aircraft pilot pressed the brakes more than 20% trying to maintain the aircraft direction, the autobraking mode was turned off, the MLG wheels were unlocked, and the wheel rotation speed reached the ground speed of the aircraft ~27 kts.

Under the influence of the right crosswind the aircraft continued to drift to the right at a speed of about +3 deg/sec, so at 21.18.07, the pilot of the aircraft, probably trying to prevent the plane from drifting off the runway, applied full braking and the wheels locked again (wheel rotation speed 0 kts). However, the ground speed of the drifting aircraft was ~25 kts and the aircraft slid uncontrollable to the right by inertia for more than 20 degrees, followed by the nose gear leaving the runway surface and coming to a complete stop. The deviation of the aircraft from the runway centerline was ~55 degrees (Heading True 240.6 degrees).

The analysis of the FDR data and the aircraft flight crew actions during the aircraft landing ground roll allows to conclude that the incorrect operation of the aircraft autobrake system when the aircraft was landing on a contaminated runway surface led to the wheels blocking due to the anti-skid protection deactivation. The anti-skid protection deactivation was most likely caused by the difference in wheels speed rotation and the ground speed of the aircraft due to low friction between the runway surface and tire, what probably was the main cause for the pilot's inability to maintain the directional control of the aircraft.

Related NOTAMs:
A0237/23 NOTAMR A0236/23
Q) EVRR/QMRLC/IV/NBO/A /000/999/5655N02358E005
A) EVRA B) 2303082330 C) 2303090030 EST
E) RWY 18/36 CLSD.

A0236/23 NOTAMN
Q) EVRR/QMRLC/IV/NBO/A /000/999/5655N02358E005
A) EVRA B) 2303082153 C) 2303082350 EST
E) RWY 18/36 CLSD.

METARs:
EVRA 082320Z 24006KT 200V290 9999 -SHSN SCT007 SCT025CB BKN049 M02/M03 Q0991 NOSIG=
EVRA 082250Z 24006KT 210V290 9999 -SHSN SCT008 BKN021CB OVC040 M02/M03 Q0991 NOSIG=
EVRA 082220Z 24008KT 210V280 9999 -SHSN SCT008 BKN021CB OVC043 M02/M03 Q0990 RESN NOSIG=
EVRA 082150Z 24010KT 210V290 5000 SHSN SCT007 BKN013CB OVC015 M02/M03 Q0990 TEMPO 3000=
EVRA 082120Z 25010G23KT 200V290 3200 SHSN SCT007 BKN009CB OVC014 M02/M03 Q0989 BECMG 6000 NSW=
EVRA 082050Z 25010KT 200V300 2500 SHSN SCT007 BKN010CB OVC014 M02/M03 Q0989 BECMG 6000 NSW=
EVRA 082020Z 24013G25KT 210V290 4000 SHSN SCT007 BKN010CB OVC016 M02/M03 Q0988 BECMG 6000 NSW=
EVRA 081950Z 23015KT 1400 1300 R18/1700U SHSN SCT005 BKN010CB OVC015 M02/M03 Q0988 TEMPO 0700 +SHSN BKN005=
EVRA 081920Z 23016G31KT 2400 SHSN DRSN SCT012 BKN020CB OVC025 M02/M04 Q0987 TEMPO 0700 +SHSN BKN005=
EVRA 081850Z 23015KT 200V270 7000 -SHSN DRSN SCT021CB OVC028 M01/M05 Q0987 TEMPO 0700 +SHSN BKN005=
EVRA 081820Z 22015G27KT 200V260 9999 -SHSN DRSN SCT024CB BKN032 M01/M05 Q0987 TEMPO 0700 +SHSN BKN005=