After a transit of 3 hours and 42 minutes, the flight crew of an A330-300 was ready for an early afternoon, 5-hour return flight to their origin airport.

The takeoff was normal. Five minutes later, while climbing through 10 800 feet, the IHYD G SYS LO PRI and  IHYD G RSVR LO LVLI ECAM alerts triggered followed by the IBRAKES NORM BRK FAULTI and IBRAKES AUTO BRK FAULTI ECAM alerts.

The flight crew performed the ECAM procedures and evaluated the situation with the assistance of an onboard maintenance engineer. They contacted the Maintenance Control Center of the airline, and agreed to continue the flight to the destination airport that had a long runway.

The flight crew checked all related FCOM procedures during cruise, and prepared their approach accordingly: Landing gear gravity extension was necessary, autobrake was not available, and the alternate braking system was to replace the normal braking system.

The aircraft landed in CONF FULL. At touchdown, the PF selected MAX REV and applied full brake pedals. Because no deceleration was felt, the PF applied the LOSS OF BRAKING memory items, that requests the PM to set the A/SKID switch to OFF. The PF then reapplied pedal braking, which was not successful. In the end, the PF applied the parking brake 3 times, and the aircraft finally stopped.

All tires of the main landing gear burst (fig.1). Damage occurred to the main landing gear, the main landing gear doors, the forward water drain mast and the left engine. 

(fig.1) All wheels of the main landing gear burst during the event (images from the investigation board)

Recorder data analysis indicated that when the flight crew performed the pre-flight check of the alternate braking system, the pressure only reached a maximum of 416 psi instead of the expected value between 2200 and 2700 psi. Despite this low pressure value, the flight crew proceeded with the flight without seeking maintenance intervention, allowing the aircraft to take off with an inoperative alternate braking system.

The loss of the normal braking system during flight was caused by the failure of the green hydraulic system. An overpressure inside the pitch trimmer actuator of the right main landing gear bogie resulted in the crushing of its piston rod, which caused an hydraulic leak (fig.2). 

This type of damage on the pitch trimmer actuator can result from shocks caused by a damaged runway at the departure airport.

The combination of the undetected alternate braking system malfunction during the pre-flight checks, with the in-flight failure of the normal braking system, caused the loss of braking during the landing roll. Consequently, the flight crew could only rely on the parking brake to stop the aircraft during the landing roll.

(fig.3) An undetected insufficient pressure of the alternate braking system during the pre-flight checks, combined with a loss of normal braking caused by a leak on the green hydraulic system during flight, are at the origin of the event.

Analysis of the aircraft maintenance records indicates that several malfunctions of the alternate braking system were reported during the four months before the event (fig.4). Three separate flight crew reports of a low brake pressure indication on the right side during the check of the alternate braking system were recorded, including one with a “spongy” right brake pedal. 

Each time either the high-pressure circuit of the alternate braking system at the wheel brakes was bled, or the nitrogen charge pressure of the brake accumulators was adjusted. This did not address the issue. 

Application of the appropriate TSM task in the case of reported low alternate brake pressure (TSM 32-43-00-810-807-A “Brakes - Alternate-Braking Pressures Do Not Get to 2200 psi When Brake Pedals Applied (with Blue Hydraulic System Depressurized)”) would have helped the maintenance personnel to identify and fix the issue.

17 days before the event, maintenance personnel found a leak on the right master cylinder, which was probably the root cause of the previous low alternate braking pressure events on the right side. The cylinder was replaced by a new one.

Investigation revealed the presence of air in the low pressure control circuit of the alternate braking system that caused the insufficient pressure on both sides of the alternate braking system during the event. 

An incorrect servicing of the low pressure control circuit after the installation of the new master cylinder is most probably the cause of the introduction of air in the system. 

(fig.4) Sequence of maintenance actions performed before the event

There are two types of control for the alternate braking system of Airbus aircraft.

A300, A310, A330, A340-200, A340-300, and A320 family aircraft delivered before 2007 are equipped with a hydromechanical system for alternate braking control. 

The brake pedals are mechanically connected to two master cylinders, converting the pedal displacement into a hydraulic pressure that activates the dual valve (fig.5). The dual valve regulates the quantity of hydraulic pressure coming from the blue (A330, A340-200/300 aircraft) or the yellow hydraulic system (A300, A310 and A320 family aircraft) to activate the brake pistons. 

An electrical control of the alternate braking system was first introduced on the A340-500/600 aircraft, and Airbus decided to also progressively equip A320 family aircraft with this enhanced technology from 2005 (the last A320 family aircraft equipped with an hydromechanical control was delivered in December 2007). 

The position of the brake pedals is sent to the Braking & Steering Control Unit (BSCU) and Emergency (or Alternate) Braking Control unit (EBCU or ABCU).

(fig.5) Types of Alternate braking system installed on A300, A310, A320 family, A330 and A340 aircraft

On aircraft equipped with hydromechanical control of the alternate braking system, (the) SOP requests that the flight crew check the alternate braking system:

• During the preliminary cockpit preparation, before the exterior walkaround of the first flight of the day, or
• If there is a flight crew change. 

The test is performed after checking that the brake accumulator(s) pressure is within the green zone on the triple pressure indicator. If this is not the case, the flight crew should switch the blue (A330 & A340-200/300 aircraft) or yellow (A320 family, A300 & A310 aircraft) ELEC PUMP to ON to refill the accumulators.

① The flight crew should first check that the blue (A330 & A340-200/300 aircraft) or yellow (A320 family, A300 & A310 aircraft) electrical pump is OFF. ② Then, they should check that the wheel chocks are in position. ③ The flight crew should set the parking brake to OFF. ④ Fully pressing the brake pedals compresses both master cylinders that send hydraulic pressure to open both sides of the dual valve.

(fig.6) Check of the Alternate Braking System (part 1 of 2)

The brake pedals have an artificial feel that requires some force to activate the brakes. If the flight crew feels that the artificial feel is lost while pressing the brake pedals (brake pedals are “spongy”), maintenance action must be performed. This indicates that there is air in the low pressure control system.

When the brake pedals are fully pressed, ⑤ the brake pressure must be between 2 000 (or 2 200 PSI) and 2 700 PSI on both sides of the triple pressure indicator. If not, maintenance action must be performed. ⑥ The flight crew must then release the brake pedals and ⑦ set the PARK BRK back to ON, so that they can check the brake wear indicators during the exterior walkaround.

(fig.7) Check of the Alternate Braking System (part 2 of 2)

When the flight crew reports a failed check of the alternate system, the appropriate troubleshooting task must be applied, based on the reported symptoms.

If the flight crew reports that one of both pedals feel 'spongy', corresponding to a loss of the artificial feel, one of the following tasks should be applied:

• A320 TSM 32-43-00-810-805-A “Loss of the Pedal Artificial Feel on the Two Sides (Auxiliary Low-Pressure System Failure)”
• A320 TSM 32-43-00-810-806-A “Loss of the Pedal Artificial Feel on the Right Side (Auxiliary Low-Pressure System Failure)”
• A320 TSM 32-43-00-810-807-A “Loss of the Pedal Artificial Feel on the left Side (Auxiliary Low-Pressure System Failure)”
• A330/A340 TSM 32-43-00-810-802-A “Brakes - Loss of the Brake-Pedal Artificial Feel or Spongy Pedals”
• A300/A310 TSM 32-43-00 “Alternate Braking Fault Symptoms”

If the displayed brake pressure was not in the correct range during the test, the following task should be applied:

• A320 TSM 32-43-00-810-805-A “Loss of the Pedal Artificial Feel on the Two Sides (Auxiliary Low-Pressure System Failure)”
• A320 TSM 32-43-00-810-806-A “Loss of the Pedal Artificial Feel on the Right Side (Auxiliary Low-Pressure System Failure)”
• A320 TSM 32-43-00-810-807-A “Loss of the Pedal Artificial Feel on the left Side (Auxiliary Low-Pressure System Failure)”
• A320 TSM 32-44-00-810-801-A “Loss of the Right Yellow Pressure Indication on the Triple Indicator (Electrical Failure)”
• A320 TSM 32-44-00-810-802-A - Loss of the Left Yellow Pressure Indication on the Triple Indicator (Electrical Failure)
• A330/A340 TSM 32-43-00-810-807-A “Brakes - Alternate-Braking Pressures Do Not Get to 2200 psi When Brake Pedals Applied (with Blue Hydraulic System Depressurized)”
• A300/A310 TSM 32-43-00 “Alternate Braking Fault Symptoms”.

Each time one or several components of the low pressure control circuit is replaced, the low pressure control circuit must be serviced to ensure:

• The correct filling of the brake control hydraulic reservoir
• That there is no air in the low pressure control circuit.

The use of an appropriate tool to do the servicing of the low pressure control circuit is essential to ensure correct filling.

Airbus recommends performing the refilling procedure slowly, to ensure that all the air is removed from the low pressure control circuit.

As a general rule, Airbus recommends using an off-aircraft hydraulic source for the refilling of the low pressure control circuit. These devices have a deaeration function to remove air from the hydraulic fluid.

If the fluid is obtained from the green hydraulic reservoir, maintenance personnel must observe at least a one hour waiting period before starting the refilling procedure. During this hour, no systems powered by green hydraulics should be operated.