An A320 aircraft was stabilized in final approach in CONF FULL. Autobrake MED was selected. The runway length was 3700 m. Autopilot and autothrust were both engaged. External conditions were combining snow showers and left crosswinds between 16 kt and 22 kt. The runway was reported as contaminated with snow.

① The touchdown was performed nominally at 140 kt ground speed (GS). The spoilers extended, ② MAX REV was applied, and the MED autobrake mode activated. ③ The DECEL light on the MED pushbutton switch transiently turned on and ④ then went OFF shortly after. The ON light remained ON.

⑤ At around 70 kt, the flight crew set the A/SKID & N/W STRG switch to OFF. The PF selected REV IDLE almost simultaneously. The aircraft was slightly diverging from the centerline toward the left edge of the runway.

⑥ The PF applied differential braking and rudder inputs to the right to try to recover the runway centerline. ⑦ The flight crew set the engine thrust levers to IDLE and ⑧ the PF released the brake pedals for some seconds. ⑨ They selected REV MAX again and ⑩ applied maximum pedal braking. The aircraft skidded, deviated from the centerline and ⑪ came to a stop at 90° from the centerline with the nose landing gear out of the runway.

(fig.1) Description of the first event (rudder inputs not illustrated)

Analysis of the flight recorder data showed that the autobrake activated correctly and that the anti-skid function released the brake pressure several times to prevent wheel blockage due to the snow on the runway surface. This resulted in an average deceleration of 0.2 g, below the autobrake MED target of 0.3 g. As a consequence, the DECEL light briefly illuminated at the beginning of the landing roll.

The pilot report confirmed that the LOSS OF BRAKING procedure was applied with no further details provided. Airbus could not identify the reason for application of the LOSS OF BRAKING procedure because analysis of the flight recorder data showed the brake system functioned normally in autobrake normal braking mode until the flight crew switched the A/SKID & N/W STRG switch to OFF. A possible explanation is that the limited deceleration, due to the runway contamination with snow, may have been interpreted as a loss of braking function by the flight crew.

An A321 was stabilized in final approach in CONF FULL. The runway length was 2400 m. The Captain was PF, autobrake MED was selected, autopilot was OFF and autothrust engaged. The airport was subject to heavy rainfall. The ATIS and tower reported WET runway condition. The ENG 2 thrust reverser was inhibited as per MEL item 78-30-01A. The flight crew planned to select REV IDLE during the landing roll. Nominal touchdown and autobrake activation

① The touchdown was performed nominally at 142 kt ground speed, with a 2 kt tailwind. The spoilers extended, ② REV IDLE was selected as planned, and the MED autobrake mode activated. The aircraft began to decelerate and ③ the DECEL light of the MED pushbutton switch illuminated.

The flight crew then felt a sudden deceleration drop. ④ The PM observed that the ON light of the autobrake MED pushbutton switch was illuminated but the DECEL light had extinguished. He announced that the autobrake was lost. ⑤ The PF took over from the autobrake by applying full manual braking.

(fig.2) Description of the second event (rudder inputs not illustrated)

The flight crew could not feel an increase in deceleration and decided five seconds later to apply the LOSS OF BRAKING procedure, ⑥ by setting the A/SKID & N/W STRG switch to OFF while the brake pedals were still fully deflected. This resulted in the instantaneous blocking of the four main landing gear wheels.

The aircraft skidded for 10 seconds and the PF applied right rudder input to maintain the aircraft’s alignment with the runway. The PF then ⑦ set both reversers to REV MAX, whilst maintaining right rudder inputs.

⑧ While maintaining right rudder input, the PF started to apply differential braking by transiently releasing the left hand brake pedal. The aircraft veered to the left. ⑨ The PF applied full right inputs on the nose wheel steering tiller whilst differential braking was maintained. The aircraft quickly deviated toward the left hand edge of the runway.

⑩ The flight crew applied the parking brake and ⑪ the aircraft exited the runway at 16kt before coming to a stop with the aircraft nose positioned at around 90° from the runway axis and 200 m from the end of the runway.

The flight crew set the thrust lever to IDLE and switched off both engines.

Analysis of the flight recorder data showed the autobrake MED activated correctly.

The sudden change in deceleration felt by the flight crew, and confirmed by the analysis of the flight recorder data, was due to the fact that the aircraft entered a portion of the runway with a very degraded runway friction performance, which is probably due to a significant layer of standing water. Performance computation based on the actual aircraft deceleration showed that the runway condition dropped abruptly from an equivalent of GOOD to POOR and remained POOR until the end of the landing roll. This runway condition was below the MED TO POOR condition that would be expected in the case of a runway contamination by standing water and used for landing distance computation (fig.3).

The POOR runway friction caused the anti-skid function to be very active as it released the brakes several times to prevent wheel skidding. This reduced the deceleration rate below 80% of the target rate of the MED autobrake mode. This caused the DECEL light of the MED pushbutton switch to extinguish.

The sudden change in deceleration due to the uneven runway contamination, combined with the significant activation of the anti-skid function when manual braking was applied, led the flight crew to interpret the situation as a loss of braking and inappropriately apply the LOSS OF BRAKING.

The flight crew set the A/SKID & N/W STRG switch to OFF while the PF was pressing the brake pedals. This caused the wheels to lock instantly due to full application of the alternate brake pressure without anti-skid modulation.

The attempt to use the steering tiller during the final seconds of the landing roll was ineffective because the nose wheel steering was unavailable due to the application of the LOSS OF BRAKING procedure.

The LOSS OF BRAKING procedure was introduced to cover an extremely rare failure of the automatic switching to the alternate/emergency braking mode, in cases where the normal braking mode failed. This may happen after a takeover from automatic to manual braking during landing or rejected takeoff. It may also happen during taxi. In-service experience has shown that the application of this procedure has been also efficient to cope with other kinds of braking system malfunction.

There is no equivalent LOSS OF BRAKING procedure on A220 aircraft because it has a different braking system design.

The procedure must only be applied during manual braking, and only if the flight crew does not feel any effect on the deceleration while pressing on the brake pedals.

Applying the LOSS OF BRAKING procedure has non negligeable consequences which depends on the aircraft type:

• Loss of anti-skid function only on A300, A300-600, A310, A340-500/600, A350 and A380 aircraft. This may reduce the braking performance of the aircraft, increase the risk of tyre burst, and potentially lead to runway excursion.
• Loss of both the anti-skid function and the Nose Wheel Steering (NWS) on A320 family, A330 and A340-200/300 aircraft. The braking performance may be reduced as well as the ground handling capabilities. The flight crew has to use the rudder at high speed and differential braking at lower speed for lateral control.

The two case studies described previously highlight the need to pay particular attention when landing on wet or contaminated runways in order to prevent inappropriate application of the LOSS OF BRAKING procedure. It is important that flight crews are aware of the behavior of the aircraft and its braking system when landing on wet or contaminated runways. This will prevent them from interpreting some normal phenomena linked to the conditions of the runway as loss of braking.

The rate of deceleration perceived by the flight crew may be less on a contaminated runway than on a dry runway, particularly in the case of standing water or presence of other contaminants. There is a high probability that the anti-skid function will activate, reducing the perceived rate of deceleration. Even if it is the case, flight crew should be aware that the anti-skid is designed to ensure the optimum deceleration rate adapted for the runway condition.

The second case study showed the level of contamination of a runway, and therefore the runway condition, may vary along its length. This can lead to sudden changes in the rate of deceleration perceived by the flight crew, which may cause them to incorrectly determine they have a loss of braking.

Flight crews should also be aware that a sudden deceleration drop may be felt during the transition from REV MAX to REV IDLE, especially on wet or contaminated runways since the braking efficiency is reduced. This does not mean that the braking capability is lost.

Being prepared to manage the effects of a wet or contaminated runway on the aircraft deceleration prevents any surprise effects during landing that could lead to the flight crew inappropriately applying the LOSS OF BRAKING procedure.

During the arrival briefing:

• Discuss the runway conditions based on the available information
• Discuss the stop margin and the available deceleration means. A drop in deceleration may be felt during the landing roll if the anti-skid activates, and when reversers are selected from REV MAX to REV IDLE, and
• If the weather conditions are expected to change or in the case of significant precipitation at the airport, the flight crew should be prepared for a reduction of the braking performance. The case study n°2 showed us that a reported WET runway can quickly be contaminated with standing water, degrading the braking action from an expected MEDIUM to a MEDIUM TO POOR. The flight crew should therefore consider making a second computation of the landing distance with the worst condition possible.

The Flight Crew Training Standard (FCTS) manual recommends training the LOSS OF BRAKING procedure in the simulator in order to recall and apply the memory items with the associated callouts.

This can be performed at low speed such as during taxi, and also at high speed during landing. It is recommended to perform one of these scenarios with a startle effect situation.

Due to the limitation of the simulator ground model, instructors must not mix a LOSS OF BRAKING with contaminated runway conditions. Combining both situations would not be representative of the real aircraft behavior and would therefore give a negative training scenario. However, the specific behavior of the aircraft on wet and contaminated runways and possible confusion with a loss of braking should be reviewed during the briefing of the simulator session using the LOSS OF BRAKING section of the Flight Crew Techniques Manual (FCTM).